503
ABSORPTION AND FLUORESCENCE STUDIES OF MOLECULES AND CLUSTERS
K. SHOBATAKE, A. HIRAYA, AND K. TABAYASHI
Institute for Molecular Science, Myodaiji, Okazaki 444 Japan
AND
T. IBUKI
Department of Chemistry, Kyoto University of Education, Fushimi, Kyoto 612, Japan
CONTENTS I. Introduction
n. Experimental A. Absorption and Fluorescence Measurements B. Absorption and Fluorescence Measurements of Free Jet Molecules
and Molecular Clusters C. Photofragment Fluorescence Polarization Measurements D. High Resolution Absorption Spectroscopy
IE. Topics A. Photoabsorption and Fluorescence Excitation Spectra
1. Halogenated Methanes 2. Organometallic M(CH3)3 (M = Zn, Cd, Hg) Compounds
B. Spin Conservation Rules l.HNCO 2.HN3andNH3
C. Polarization Studies of Photodissociative Excitation Processes 1. Rotationally Resolved Fluorescence Polarization 2. HCN
D. Deuterium Isotope Effects in Photodissociative Excitation Processes 1. CH3CN and CD3CN 2. CH3OH and CD3OD 3. CHC13 and CDC13
4. The Origin of The Deuterium Isotope Effects E. Two-Electron Excitation Processes F. Fluorescence from Doubly Charged Ions
1. Nitric Oxide 2. Oxygen
G. High Resolution Absorption Spectroscopy H. Absorption and Fluorescence Spectroscopy of Molecular Complexes
and Clusters Formed in Free Jet Expansion
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1. Benzene Clusters 2. Rare Gas Clusters
I. Excimer Formation Reactions Involving Photoexcited Species 1. Gas Cell Experiments 2. Photochemistry of Molecular Complexes Formed in Free Jet Expansion
Acknowledgements References
I. INTRODUCTION
A photoabsorption spectrum provides with the fundamental information on both the ground and excited states of a molecule. The photon energy of a commercially available absorption spectrometry is limited up to -170 nm (7.3 eV) with a nitrogen-purged monochromator. More than 80 to 90 % of oscillator strengths for molecular absorption (photoabsorption cross sections) of valence electrons are concentrated in the vacuum ultraviolet photon energy range of 6 - 50 eV. Therefore many strong and/or sharp photoabsorption bands are observed around the first and second ionization energies of 10 -20 eV.
At the energies below the first photoionization limit photoabsorption occurs to discrete levels of a molecule. Weak and broad photoabsorption bands lying in the low photon energy region are generally referred to as valence states, while higher energy ones with sharp band shapes as Rydberg states which follow the well-known formula
IP-hv = R/(n-8)2, (1)
where IP is the ionization potential of the various manifold, hv the transition energy, R the Rydberg constant for a nucleus with an infinite mass, n the principal quantum number, and 8 the quantum defect.
The electronically excited molecule formed may decompose into neutral and/or charged photofragments. The fragment produced with excess energy can radiate. In this chapter we review some results obtained from gas cell and free jet experiments with the emphasis on the information on the electronic structures and the subsequent photochemical processes of the molecules and molecular complexes excited in the vuv region. Due to low photon intensities available, dynamic information gained on the photochemical processes following molecular excitation in the VUV region has been rather limited, except for the cases in which VUV lasers are used at a few frequencies. The experiments covered by the present review are mostly those in which synchrotron radiation gas discharges are used as light sources. Experimental techniques to measure absorption and fluorescence excitation spectra are mostly applied.
Although synchrotron radiation (SR) and gas discharge light sources do not necessarily give off very strong light compared with VUV lasers, they are still very convenient light sources from the viewpoints of the brightness, stability and tunability in a broad wavelength region. Spectroscopic and photochemical information obtained from the experi-
IP-hv = R/(n-ty, nv = K/{n -, (1)
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ments using SR in the VUV region has recently expanded greatly owing to relatively easy access to newly constructed SR facilities in the world
II. EXPERIMENTAL
A. Absorption and Fluorescence Measurements
Absorption and fluorescence spectra have been routinely measured using SR as a light source. Figure 1 shows the schematic side view of the apparatus for gas cell experiment used on the beam line BL2A of the Ultraviolet Synchrotron Radiation facility (UVSOR) of the Institute for Molecular Science1, with which photoabsorption and fluorescence excitation spectra are measured. In brief, the focused dispersed radiation from a i m Seya-Nami-oka monochromator enters a photoabsorption cell 10.9 cm long through a LiF window. The transmitted photons are detected by a photomultiplier tube (PMT) after conversion to visible light by means of a sodium salicylate coating on the inner or outer side of the exit window. The current from the PMT was measured by a digital picoammeter. The electron beam current in the storage ring, the exciting photon flux, sample pressure, gas temperature of the cell, and the fluorescence intensity are simultaneously monitored and interfaced to a
Figure 1. a) Schematic side view of the apparatus for gas cell absorption and fluorescence experiments on beam line BL2A at UVSOR of the IMS. From left to right, S: source point of SR, FCV: fast-closing valve, Mx: cylindrical first focusing mirror, M2: second concave focusing mirror, S^ entrance slit, G: grating, S2.* exit slit, M3: postfocusing toroidal mirror, Q: spot position (SR-mole-cule interaction region), IP: ion pump, V: shut-off valve, P: turbomolecular pump, b) The expanded side view of the gas cell region. LB: light
buffle.
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personal computer (NEC 9801) via GPIB interface. The computer also calculates photoab-sorption cross sections, assuming that the Lambert-Beer law is applied, and displays fluorescence excitation spectra. The absolute fluorescence cross section is determined by comparing the emission intensity with that of OH(A2Z->X2n) emission following photo-dissociation of H20 at 133.57 nm whose fluorescence cross section is known2
The response of the fluorescence detection system was corrected for by using a standard bromine lamp with a known spectral irradiance. For the time dependent measurements of the fluorescence from the photofragments formed are used the light pulses at 121.6 nm with about 10 ns (FWHM) duration produced by a DC discharge of about 2 Torr of H2.34
B. Absorption and Fluorescence Measurements of Free Jet Molecules and Molecular Clusters
The schematics of an apparatus used for the measurements of absorption and fluorescence spectra of free jet molecules and molecular clusters is shown in Figure 2.5 Instead of the gas cell mentioned above a free jet chamber is connected so that the space occupied by the gas cell is pumped by a liquid N2 buffled diffusion pump (Varian VHS-10). In the chamber a pulsed free jet from a fuel injector crosses the monochromatized SR which vertically enters the free jet chamber through a LiF window. In order to increase the concentration of molecular clusters in the free jet, a diverging nozzle piece (inlet diameter =0.38 mm, oudet diameter =1.1 mm, and 3 mm long) was attached to the fuel injector tip.
The intensity of transparent light is monitored by a photomultiplier tube (Hamamatsu P., Model R585) after converting it with a sodium salicylate coating on the inner wall of the exit window. In this case a photon counting technique was applied because (a) since the free jet is modulated at 10 Hz, a better signal-to-noise ratio is achieved, and (b) even for low intensity light and at higher spectral resolutions, a good signal-to-noise ratio can be obtained. The nozzle for a free jet was opened for 40 msec and closed for 60 msec. The photon counts for the transparent light and fluorescence are accumulated by two sealers for the same duration (30 msec), each activated by a home-made timer for the beam on and off. Emissions from the excited species formed in the photoexcitation of free jet molecules and/or molecular complexes are collected at right angles by a concave mirror or through a lens on the cathode surface of a photomultiplier tube (R585 or R585MGF2) to both the free
Figure 2. Schematics of the apparatus for the measurements of absorption and fluorescence spectra of
free jet molecules and clusters.
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jet axis and the exciting light directions. Filtered fluorescence through a bandpath filter was also detected in order to identify the excited species. An Ortec photon counting system was applied for the accumulation of photon intensities. The beam current in the storage ring as well as the photon counts thus accumulated for 1-4 seconds are interfaced to a personal computer. The transparent light intensities, 70and / , measured at each wavelength were used to calcu-late the relative attenuation (A///<f= (/0-/)//0).
Absorption spectra of isolated molecules in free jets expanded through a round nozzle can be easily measured as along as the absorption cross section is large enough. The criteria for the feasibility of free jet absorption measurement6 with our apparatus is the following: For the stagnation pressure P (in Torr) and the absorption cross section a (in Mb = 1 x 1018
cm2) or molar extinction coefficient e in 1- mol1 cm1.
Po > 104 Torr Mb or Pe > 3 x KFTorr • 1 mol1 cm1, (2)
The corresponding criteria for free jet emission measurement is
Pa Q > lOnTorr-Mb orPe Q > 3 x KPTorrl-mol-1 cm1 (3)
where Q is the quantum yield (in %) for emission. Therefore emission spectra is easier to observe than the absorption spectra by as much as one to three orders of magnitude depending on the quantum yield for emission process. As an example the absorption spectrum of benzene in a free jet formed by expanding the mixed gas in He (benzene partial pressure = 92 Torr and total pressure = 400 Torr) is shown in Figure 3. The unit of
Po > 104 Torr Mb or Pe > 3 x KFTorr 1 mol1 cm1, ► 104 Torr Mb or Pe > 3 x KFTorr 1 mol1 cm1, (2)
PcQ >101TorrMb OTP&Q > 3 x WTorrl-moHcnr1 I > lU'Torr-Mb or/'-f Q, > 5 x 1WTorr-1-mol1 ci (3)
Figure 3. Absorption spectrum of benzene in a free jet expanded through a diverging nozzle (throat diameter = 0.38 mm, and exit diameter =1.16 mm, length = 3.0 mm) at a total stagnation pressure of 400 Torr of He gas mixed with 92 Torr of benzene and at a
stagnation temperature of about 50'C. The spectral bandwidth is 0.065 nm.
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the right-hand ordinate is relative attenuation A///0 and that of the left-hand one is 1-mol-cm1. The absolute value for the molar extinction coefficient e has been determined from the formula (Eq. 4.)
«<*) = - — j — l o g { l - # } (4)
where <c7>obsdis the effective product of molar concentration and path-length, which was determined as <c-l>ohsd= 8.4 x 106 moM^-cm from the observed A///0value of 0.113 at 200.1 nm and the known e value of 6200 1 • mol cm*1.
Fluorescence excitation spectra of rare gas clusters have been measured by Moellers and his coworkers of DESY-HASYLAB. In their experiments dispersed synchrotron radiation is irradiated upon the skimmed beam of rare gas free jets expended at low stagnation temperatures to insure formation of large clusters.7-8 For the detection of fluorescence from the excited atoms, dimers, and clusters, a vuv light converter (sodium salicylate coating on an entrance window of multichannel array detector) was employed. Since the background level of the fluorescence is low, it is feasible to observe the fluorescence excitation spectra, although absorption spectra of skimmed clusters are expected to be difficult due to low number densities.
C. Photofragment Fluorescence Polarization Measurements
Theoretical basis for fluorescence polarization was originally presented by Fano and Macek,9 and introduced by Greene and Zare.10 The relation between the lifetime of the excited molecule and the degree of fragment fluorescence polarization has been derived by Nagata et al.11 It is also discussed by Simons.12*13 An electric dipole excitation process has a cos20 dependence and so does an electric dipole emission process. Therefore the degree of polarization observed in the fragment fluorescence is small (in many cases less than 10% except for the case of CN(B->X) emission polarization of rotationally resolved dispersed fluorescence from C1CN photodissociation.14See §DI.C.l) and thus experimental system should be constructed to make sure that statistical error is small enough.
Figure 4 shows a schematic diagram used for measuring fluorescence polarization of the electronically excited fragments formed from VUV photodissociation.15 This device is installed on the apparatus for absorption and fluorescence measurement of gaseous samples on the beam line BL2A of the UVSOR facility. The monochromatized light enters the gas cell along the z-axis. The polarization of the exciting light is directed in the x-axis. The focused fluorescent light propagated to the direction of y-axis is detected by a photomulti-plier tube (Hamamatsu P., Model R585) through a photoelastic modulator (PEM, Hinds International, Model PEM-80)15and a sheet polarizer polarized in the z-direction. The photoelastic modulator serves as a modulated birefringent media. The modulation frequency was 50 KHz. At the maxima or minima of the retardance in Figure 4b, the polarization direction is rotated by 90 degrees. When the retardance equals zero the direction of polarization is not rotated. The accumulated photon counts for the maxima and minima are recorded as the fluorescence intensity in parallel to the original polarization direction /„ and that for retardance = 0 as that for perpendicular polarization 7±. The degree of polarization P defined as
*<*) = -—JT- l o g { l - # } * < * ) = ■ -f-1 <C-/>ob.d
M0 ■ / . (4)
C. Photofragment Fluorescence Polarization Measurements
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Figure 4. a) Schematics of an apparatus used for fluorescence polarization measurement from the photofragments. L: lens, PEM: photoelastic modulator, P:linear polarizer, Rfilter, PMT: photomultiplier tube, (b) The upper is the retardance vs. time. The gating signals for parallel (counter 1) and perpendicular emission (counter 2) are shown
below.
' = ; = ('■■ - h) Un + II )
(5)
(6)
r _ (3cos2r - 1) (cos2y + 3)' (7)
* - ( = _UJL_I_LLJ_ _ v* ii *■ ± t
"(/„ +2 / ± )
i
(3cos»r - 1) (cos2y + 3)
is often used, but theoretically the anisotropy factor R defined as
is theoretically more convenient since the denominator of the latter is proportional to the total fluorescence photon intensity integrated over all directions and it is easily related to the theoretically calculated quantities15. The degree of polarization P can be related to the average angle y between the absorption and emission transition oscillators by
Consider the cases when electronically excited, emitting diatomic molecules are produced upon photodissociation. The polarization indices depend on a few factors,1113 i.e. (i) if the absorption dipole vector is in the plane of or perpendicular to the molecular plane, (ii) if fluorescence dipole transition vector is parallel or perpendicular to the molecular axis and (Hi) how long the excited reactant molecule survive before dissociation.11 For example when a triatomic molecule is photodissociated forming photofragment AB* which has an emission transition dipole along the molecular axis, P is calculated to be 1/7 when
p
R
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absorption transition dipole is in the (ABC)* molecular plane, and P = -1/3 for P and R branches when the absorption transition dipole is perpendicular to the molecular plane.15*
D. High Resolution Absorption Spectroscopy
High resolution absorption spectra of atoms and simple molecules have been measured using normal incidence monochromators with gratings of 10.7 or 6.65 m radius. Most of the spectra have been photographed using gas discharge light sources. A spectral resolution of 180,000 has been achieved using a 6.65 m normal incidence monochromator by Yoshino, Freeman, and Tanaka.16 However for the measurement of the absolute absorption crosss sections photoelectric detection is necessary. Yoshino, Freeman, and Parkinson17 have succeeded in measuring the absorption cross sections for the (14,0) band of Schumann-Runge system of 0 2 at a resolution of 0.001 nm. They did not succeed in obtaining spectra in the higher energy region than 120 nm due to low intensity level of the discharge light source then available.
In order to compare theoretical absorption cross sections with the observed values higher resolutions should be achieved and thus high resolution monochromators equipped with a photoelectric detector using SR as a light source have been developed at NSLS at Brook-haven and Photon Factory in Tsukuba. Here we briefly describe the apparatus constructed at the Photon Factory using a 6.65 m Off-Plane Eagle spectrograph monochromator (Nickname 6-VOPE) developed by Namioka, Itoh and their coworkers.18 Figure 5 shows an optical system of the 6 VOPE facility, which employs two foregratings, Gl and G2, and a principal grating with 1200 lines/ mm blazed at 550 nm operated at the 6-th or 7-th order.
Figure 5. Optical system of the 6 VOPE facility at the Photon Factory, Tsukuba.
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From the separation between the 338(3/2)! and 31(1(3/2)! lines in the absorption spectrum of Kr I measured photoelectrically, the resolving power of the 6-VOPE facility was estimated to be better than 1.5 x 105.19 The importance of the high resolving power is in the capability of measuring true absorption cross sections which can be compared with theory.
III. TOPICS
A. Photoabsorption and Fluorescence Excitation Spectra
Many a measurement has been made for the absorption and fluorescence excitation study of gaseous samples in the vacuum UV region by Okabe, Simons and Lee, and their coworkers. Absorption and fluorescence spectroscopies are applied for dihalogen molecules, such as Clj,2021 IC1,22 IBr,231^24,25 Many polyatomic molecules excited in the VUV region often undergo photodissociation. Thus photodissociative excitation processes of hydrogen halide (HC1,26 HBr27), H20,228 N20,29 NO^Oa.^HCN^^^ICN^NHa,35
CH4,36 SiH4,37 SiH2Cl2,38 C2H2,39 CF3H,40 CH3CN,41 CH3SH,42 S2C1243 CH3NCS,
CH3NCO, CH3SCN,44 CH3OH,45 and halogenated methanes are important processes, since the products from these processes can be easily identified from the emission spectra.
1. Halogenated Methanes
Photoabsorption and fluorescence excitation spectra of eight halogenated methanes CC14, CCl3Br,i C Q 3 F > CCl2F2/*CHFCl2,CHFBr2,47 CHClBr2, and CHBrCl248 were measured in the 106 - 200 nm region. The grating chamber of the 1 m Seya-Namioka monochromator to disperse synchrotron radiation was isolated from the gas cell by a LiF window which also serves as a filter to cut off higher order light.
1.1. Photoabsorption spectra
The solid curve in Figure 6 shows the photoabsorption cross section of CHFBr247(l Mb
= 10-18 cm2). This molecule contains four different kinds of atoms lying in the first, second and fourth rows of the periodic table. The electronic configuration of the outer shell of CHFBr2 can be expressed as47
(6a,)2(3aM)2(7a,)2(4fl")2(8a,)2(5aM)2.
The 6a! molecular orbital mainly represents the F lone pair character (wF) and the 3a" does the C-Br bonding orbital (ac.Br).The outermost four systems are attributed to bromine lone pairs. The ionization potentials are low and determined by He(I) photoelectron spectro-scopy to be in the order 14.13, 13.50, 12.02, 11.48, 11.34 and 10.94 eV, respectively, for 6a' to 5a".47 The assignment of each photoabsorption band of CHFBr2 appearing in Figure 6 was made utilizing the familiar Rydberg formula Eq. 1, that is, the first broadband at 160 nm (or 62500 cm1) has the term values of 25700 and 29000 cm1, which is equal to IP -hv inEq. 1, with respect to the first and second ionization potentials, respectively.
1. Halogenated Methanes
1.1. Photoabsorption spectra
(6a,)2(3aM)2(7a,)2(4fl")2(8a,)2(5aM)2
HC126 HBr27), HX>,228 NoO,29 N O ^ O ^ H C N ^ I C N ^ N H , ilL 37 SiH.CL 3« CH 39 CF,H 40 CH.CN 41 CJLSH 42 S,Ck43 CH,NC!
CH3NCO, CILSCN,44 CHX)H,45
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Figure 6. Photoabsorption and fluorescence cross sections of CHFBr2.47 Spectral resolutions are 0.2 and 1.0 nm for the photoabsorption and fluorescence excitation
measurements, respectively. The Rydberg transitions are indicated.
They are close to the term value of 27500 ± 900 cm1 for the 5s^P<-Ap5 2PQim (i = 1 1/2,0
1/2) Rydberg transitions of Br(I) atom,49 and thus assigned to be the superimposed 5s terminating Rydberg excitations of 5a" and 8a' orbitals. Employing the similar term value concepts, the sharp bands at 145.2 and 141.2 nm are mainly attributed to the /?-type Rydberg excitations as shown in Figure 6 and summarized in Table I. The quantum defects of the 5s, np and Ad Rydberg levels from the wBrmolecular orbitals given in Table I are deduced to be 2.89 ± 0.06, 2.65 ± 0.09 and 1.37 ± 0.05, respectively, and are in excellent agreement with those of Br atom, that is, 3.06 ± 0.07, 2.54 ± 0.14 and 1.25 ± 0.36 for t heV , np (n = 5,6)- and 6d- terminations, respectively*
The substitution of H atom for F has a large effect on the Rydberg parameters. The 35 and 3/7 term values of fluoroalkanes are -33000 (a = 1.18) and 22400 cm1 (a = 0.79), respectively,50 and in F atom itself they are 37000 (a = 1.28) and 23100 cm1 (a = 0.82)* Thus, the photoabsorption bands at 130.6 and 111.7 nm would be superimposed by the transitions of 6a' to the 3, and 3p Rydberg levels with the reasonable quantum defects of 1.29 and 0.88, respectively, as given in Table I. It is noted that the a value of the 6a\nF) Rydberg excitation is smaller than those of nBr orbital by a factor of ~1.7 since F and Br atoms He on the second and forth row in the periodic table, respectively.
Other halogenated methanes also show the similar photoabsorption features and all absorption bands observed have been consistently assigned to the Rydberg excitations using the term value analyses.1«46"48 Electron energy-loss dipole (e,e) spectroscopy has shown that the steep and monotonous decrease of photoabsorption cross sections of
(i 11/7
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TABLE I Rydberg assignments for CHFBr2.a
Band maximum Rydberg Term value * n 8 [nm) assignment (cm1)
1600 5s 4- 5a" 25700 2.06 2̂ 94 5s <r- 8a' 29000 1.95 3.05
151.0 5s <r- 4a" 26100 2.05 2.95 145.2 5/7 <- 5a" 19400 2.38 2.62
5s <- 7a' 28100 1.98 3.02 141.2 5/7 <- 8a* 20600 2.31 2.69 138.2 4a*<-5a" 15900 2.63 1.37 133.7 4a,<-8a' 16700 2.57 1.43
5/7 <- 4a" 18000 2.48 2.52 130.6b 4d*-4a" 16000 2.62 1.38
5/7 <- 7a' 20400 2.32 2.68 3s<^6a' 37400 1.71 1.29
129.2C 6p <- 5a" 10800 3.18 2.82 122.1b 7/? <- 5a" 6300 4.16 2.84
6p <- 8a' 9560 3.39 2.61 4d<r-ld 15000 2.70 1.30
120.0b 5s <-3a" 25600 2.07 2.93 111.7b 5p <-3a" 19400 2.38 2.62
a. Taken from Ref. 47. n* is the effective principal quantum number and S is the quantum defect
b. Emission band coexists. c. Shoulder band.
chloro-fluorocarbons starts at the excitation energy of around 30 eV and that the decrease of photoabsorption cross section of the valence shell electrons can be expressed by a polynomial of exponential functions such as Aexp(-BE)+Cexp(-D£) in the higher photon energy region where A, B, C and D are constants and E is the exciting photon energy51.
In general, the band intensities of a Rydberg series become weaker for larger principal quantum numbers, being proportional to w3.52 Therefore, it is clear that the Rydberg bands in Figure 6 are superimposed by many valence excitations and thus the spectral profile would appear as a continuous absorption under the low spectral conditions employed, i.e., the sample pressure of -20 mTorr at room temperature and a resolution of 0.2 nm for the lm Seya-Namioka monochromator used.
12. Fluorescence spectra
The emission observed in the CHFBr2 photoexcitation at 121.6 nm (H Lyman-a) was dispersed using a Nikon G250 monochromator as shown in Figure 7. The broad structure is the 2g (n = 0-4) vibrational progression54 and 2? band of CHFOPA"-* PA').47 The sudden disappearance of the emission at v2' = 5 is not due to the predissociation but to the small
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Franck-Condon factors for the electronic transition at V2 ^ 5 because the dissociation limit of the following process lies higher than the CHF^W,^' = 5) level:
CHF(ftA ") -> CF(£2nr) + H(2S1/2) (8)
which is the spin allowed and energetically lowest decomposition path of CHF pi1 A"). The CHF radical is bent in both the upper and lower states, and the AlA" state has a potential barrier higher than 4900 cm1 to become a linear molecule.47- ^ss
CHF(2VT) -> CF(J?2nr) + H(2S1/2) :HF(2VT) -> CF(J?2nr) + H(2S1/2 (8)
Figure 7. Dispersed fluorescence observed in the photolysis of CHFBr2 using H Lyman-a line. The 2g (n = 0-5) and 2° transitions of CHF radical are indicated.
The fluorescence excitation spectrum is shown by the dotted curve in Figure 6. The emission onset lies at -155 nm, which corresponds to
CHFBr2 -> CHF(;*) + 2Br. (9)
As is already mentioned, the absolute fluorescence cross section was determined by comparing the emission intensity with that of OH(A2X+->X2nl) transition for which the fluorescence cross section has been known.2 The maximum fluorescence intensity is small and found to be 1.4 Mb or O = 0.016 at 118 nm. On the contrary, the fluorescence excitation spectrum of CHFC^ shown in Figure 8 gives a large fluorescence quantum yield of 0 = 0.22 at 106 nm,47 although CHFC12 exhibits similar photoabsorption features and has a similar electronic configuration to those of CHFBr2. This large difference in the emission quantum yields is qualitatively explained by the formation of ionic fragment(s) in the photoexcitation of CHFBr2, i.e., above the ionization potential of CHFBr2at 10.94 eV the main part of the excess energy in a molecule would be carried away by the photoelectrons emitted, while the first ionization potential of CHFCl2is 11.92 eV or 104 nm55 and thus, for the wavelength longer than 106 nm (11.7 eV), the whole photon energy can be used in CHFC12 molecule to produce electronically excited radicals with high efficiencies as well as neutral fragments.
CHFBr2 -> CHF(;*) + 2Br. [FBr2 -> CHF(;*) + 2Bi (9)
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The fluorescing species formed in the photolyses of halonomethanes are halogenated methylene radiacals.1'46-48 The collision-free lifetimes of the carbenes are determined to be 58 ± 2, 635 ± 30, 2000 ± 290, 2570 ± 160, and 7010 ± 250 ns for C F ^ A O , CCIFC*^"), CC^O*1^), CHF(AW), and CHCl^^"), respectively.1-46^8 The radiative lifetimes of CF2(A) and CC12(A) have been also determined by laser induced fluorescence spectroscopy in supersonic molecular beams56*58 which agree with the present values. Except for CF2(^) radical which shows a single exponential decay profile, the time decay of the fluorescence intensities is analyzed as a superposition of two components. It has been indicated that the slowly decaying species comes from the fast one through bimolecular collisions.1*46^8 That is, the vibrationally hot, nascent singlet halocarbene radicals are converted to the vibrationally excited triplet state radicals through collisions. In fact the lowest triplet level is predicted by ab initio calculation59 to lie just above the singlet ground state. Because of the transition between triplet and singlet states is optically forbidden, the vibrationally hot triplet radicals formed would be again transferred via collisions to the lower vibrational levels in the electronically excited singlet state1.
Figure 8. Photoabsorption and fluorescence cross sections of CHFC12. The fluorescing species formed in the photolyses of halomethanes are halogenated methylene radicals. 1«46"48
2. Organometallic M(CH3 )3 (M = Zn, Cd, Hg) Compounds
Organometallic compounds usually contain one heavy atom with valence electrons of a large principal quantum number. In lib dimethylmetals, the outermost atomic orbitals of metals are 4s, 5s and 6s for M = Zn, Cd and Hg, respectively. Each of the metal atoms has a compact core and relatively large radii for the outer-most electrons: i.e. for Zn, <r>3sjfd< 0.032 nm and <r>4s = 0.1065 nm; for Cd, <r>4Virfs 0.04 nm and <r>5x= 0.104 nm; for
2. Organometallic M(CH3 )3 (M = Zn, Cd, Hg) Compounds rganometamc M<UH* h (M = z*n, ca, tig) compouno
> < » w * 0-04 cr>^= 0.1065i
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Hg, <r>5sj>4< 0.06 nm and <r>^= 0.107 nm60. The M-C bond lengths are 0.1929, 0.2112 and 0.2094 nm for M = Zn, Cd and Hg, respectively, while that of the C-H in methyl is 0.109 nm.61 These molecular constants suggest that the four electrons in the two M-C bonding orbitals play an important role in the electronic transitions of the molecule at relatively low exciting photon energies.
There have been some confusions in the assignments of the photoabsorption bands and the emitting fragments produced: (1) Gedanken, Robin and Kuebler62 have assigned the absorption bands of Hg(CH3)2in the region of 5 - 31 eV (250 - 40 nm) as Rydberg type transitions, while Chen and Osgood63 have claimed those of M(CH3)2(M = Zn, Cd, Hg) observed at -200 nm are valence type excitations. (2) In the vacuum ultraviolet photolysis of Cd(CH3)2, Yu et al.64 and Amirav, Penner and Bersohn65 have first observed fluorescence from MCH3 (M = Zn, Cd), but they did not detect atomic emissions. On the contrary, Suto, Ye and Lee66 have observed the emission from Cd(I) atom instead of CdCH3 radical.
Figure 9. Photoabsorption and fluorescence cross section of M(CH3)2. (a) Zn (CH3)2., (b) Cd(CH3)2., and (c) Hg(CH3)2.
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2.1. Photoabsorption spectra
Photoabsorption cross sections of dimethyl lib organometallic compounds measured using a gas cell (see Figure 1) are represented by the solid curves in Figure. 9.67 M(CH3)2 molecules belong to the D^ group and valence shell configurations are expressed as <*
(2ey(2ey(3alT(2al^
The outer molecular orbitals of 2a" and 3ax' are mainly of metal-carbon bonding character and the 2e" and 2e* orbitals are of C-H bonding character. The ionization potentials for the 2a "y 3a{> 2e" and 2e' orbitals have, respectively, been determined by photoelectron spectroscopy to be 9.42, 11.32 and 12.4-14.4 eV for Zn(CH3)2; 8.76, 10.56 and 12.2-13.4 eV for Cd(CH3)2; and 9.30, 11.60 and 12.3-14.5 eV for Hg(CH3)2.68 Ionization potentials of 2e" and 2e% lie closely and give rise to a broad band in the photoelectron spectra.68 Adopting these ionization potentials in Eq. 1, the Rydberg assignments of the photoabsorption bands are summarized in Table n. For example, the 4p- and 5s- terminating assignments for the 202 and 140 nm bands of Zn(CH3)2 are justified since their quantum defects of 5 = 1.97 and 2.65 are quite close to those for the Rydberg transitions of np <- 4^1/>1°and ns <r-slS0of Zn(I) atom: 2.08 ± 0.01 and 2.64 ± 0.02, respectively.49 In Cd(CH3)2, the deduced quantum defects for the Rydberg transitions to the np and ns levels are 5 = 2.96 ± 0.06 and 3.62 ± 0.04 as summarized in Table III, respectively, being just comparable to those of 3.05 ± 0.01 and 3.59 ± 0.01 for Cd(I) atom.49 Similarly the Rydberg parameters for Hg(I) atom49 are 5 = 4.03 and 4.64 for the excitations to the np and ns orbitals, respectively. The strong absorption bands commonly observed at around 120 nm are assigned to the Rydberg transitions of the C-H bonding electrons with 2e" and 2e%
symmetries67.
Band Maximum Term value n* 8 Assignment (nm) (cm"1)
212.0 28800 4J(7*<- 7a" 202.0 26500 2.03 1.97 4p<r-2ax" 151.8 25400 2.08 1.92 Ap <-3ai' 140.0 27200 2.01 0.99 3s<-(2e'/2e") 118.0 23300 2.17 0.83 3/x- (2e'/2e") 109.2b 16500 2.58 0.42 3d<-(2e'/2e")
Table II. Rydberg Assignments for Zn(CH3)2.a
a. Taken from Ref. 67. b. Emission band.
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TABLE in . Rydberg assignments for Cd(CH3)2.a
Band maximum Term value n* 8 Assignment (nm) (cm1)
250.0 30600 5OT*^ 2ax" 217.0 24500 2.11 2.89 5/7 «- 2at
n
168.9 11550 3.08 2.92 6/7 *~ 2ax° 166.9b 25300 2.08 2.92 5/7 «- 3 ^ . 150.7 18800 2.42 3.58 6* - 3a{ 137.2b 12300 2.99 3.01 6/7 «- 3af 132.8C 9840 3.34 3.66 7* «- 3a/
29700 1.98 1.02 3s <~ (2e'P.e") 127.2b 6990 3.96 3.04 Ip «- 3 a / 125.0b 5170 3.39 3.61 Ss «- 3a{ 123.8b 4400 5.00 3.00 8/7 - 3a/ 123.4 22200 2.22 0.78 3/7 «- {2e'l2e") 111.2 13300 2.87 0.13 3d *- {le'lle")
a. Taken from Ref. 68. b. Emission band. c. Absorption and emission bands coexist.
22. Fluorescence spectra
Dispersed fluorescence spectrum observed in the Zn(CH3)2 photolysis using H Lyman-a line is depicted in Figure 10. The emission can be assigned as the ZnCH3(32£. -> X2AX) transition. The positions of the main bands in the ZnCH3(A <- X) absorption are indicated by bars in Figure 10, each height of which is proportional to the reported relative absop-tion intensity. From the analysis of the observed bands the following assignments have been made: The energy separation of 1050 cm1 in ZnCH3 is assigned to the CH3 deformation mode.69 The splitting of 650 cm1 is assigned to a CH3 rocking mode, which has been observed at 707 cm 1 in the ground state.61 The 250 cm1 separation is attributed to the 2E3/2 - 2E1I2 level splitting.64 The photon energy of H Lyman-a (121.6 nm or 235 kcal/mol) is enough to produce a Zn(4/?3/>1°) atom:
Zn(CH3)2 -» ZnpPJ + 2CH3, A// = 182 kcal/mol (157 nm). (10)
However, the atomic transition of the Zn(3Px -> XS^) at 307.7 nm has not been observed64-67
The CdCH3(22£, -» £2A0 emission at around 450 nm and Cd(I) atomic lines at 228.8 and 326.1 nm are observed in the excitation of Cd(CH3)2.67 In the case of Hg(CH3)2only the transition of Hg(6p *P° -> 6s2lS0) at 253.7 nm is found in the 200-600 nm range.67
The collision free fluorescence lifetimes of MCH3 were measured to be 47 ± 2 and 62 ± 2 nm for M = Zn and Cd, respectively.67
Fluorescence cross sections are illustrated by the dotted curves in Figure 9. Spectral features are quite different from each other for the CdCH3 radical and Cd atom emissions as
Band maximum Term value n* S Assignment (nm) (cm1)
25O0 30600 5OT* *~2a{ 217.0 24500 2.11 2.89 5p ^ 2 a / ' 168.9 11550 3.08 2.92 6/7 «- 2a{ 166.9b 25300 2.08 2.92 5p «- 3a{ 150.7 18800 2.42 3.58 6s - 3a{ 137.2b 12300 2.99 3.01 6/7 «- 3a/ 132.8C 9840 3.34 3.66 Is *- 3a/
29700 1.98 1.02 3s <- (2e'P.e") 127.2b 6990 3.96 3.04 Ip «- 3 ^ ' 125.0b 5170 3.39 3.61 8* «- 3a{ 123.8b 4400 5.00 3.00 8/7 *- 3a/ 123.4 22200 2.22 0.78 3/7 «- (2e'/2e") 111.2 13300 2.87 0.13 3a* <" {le'/le")
Zn(CH3)2 -» ZnpPJ + 2CH3, A// = 182 kcal/mol (157 nm). (10)
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Figure 10 Dispersed fluorescence observed in the photolysis of Zn(CH3)2 using H Lyman-oc line.
shown in Figure 9b. A new Rydberg series appears in the fluorescence excitation spectrum of Cd(3/y -> lS0) as given in Table III. We can clearly see that the formation of the electronically excited MCH3(^2£.) (M = Zn, Cd) radical is characteristic of the Rydberg excitations of the C-H bonding electron in CH3 group, while that of the M(np *P*) state of M = Cd and Hg is of the M-C bonding electron.
B. Spin Conservation Rules
From the photolyses in the VUV region NH3, HN3, and HNCO are known to form NH* radicals in the electronically excited states. Here we discuss how the spin conservation rules are satisfied or violated in the photodissociative excitation processes.
/. HNCO
The heats of reactions for the following dissociation processes are known based on the Okabe's onset for NH(c!n) formation as:
HNCO -> NH(X3Z") + CO(Xi£), AH = 3.42 ± 0.03 eV (362.5 nm) (1 la) -> H + NCCKJCTU AH = 4.90±0.01 eV (253.0nm) (lib) -> NH^II ) + CO(X>Z), AH = 7.11 ±0.03 eV (174.4nm) (lie) -> NH(dll) + CO(X*Z), AH = 8.79 ± 0.03 eV (141.1 nm) (lid) -> NH(*3Z-) + CO(^n), AH = 9.43 ± 0.03 eV (131.5 nm) (1 le) -> H + NCO(A*Z+), AH = 7.72 ± 0.01 eV (160.6 nm) (1 If) -> H + NCO(£*II), AH = 8.84 ± 0.01 eV (140.3 nm) (1 lg)
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In the vacuum ultraviolet photolysis of HNCO the emissions from NCO(A2Z) and NH^n, A3n) were observed by Okabe70. If the NH(A3n) radiacals are formed by direct photo-dissociation of the electronically excited HNCO molecule according to Eq. (lie), it means that a spin forbidden process has occurred. Although Okabe has observed emission from NH*(A3n) formed, he considered that NH*(A3n) radicals were formed by collision-induced excitation energy transfer between NH(X3£-) and CO(a3n) which could be also formed in the spin allowed photodissociative process (Eq. 1 le). The reason for this conclusion was that the onset for NH*(A3n) formation observed by Okabe was at 131.8 nm which agrees fairly well with the calculated onset from Eq. 1 le.70
Figures 11 shows the absorption spectrum of HNCO. Figures 12-14 illustrate the emission excitation spectra from and the quantum yields for the formation of NCO(A2Z), NH (dri), and NH(A3n) radicals, respectively.71 The emissions from NH(c1n) and NH(A3n) radicals were detected through band-path filters at 325 and 336 nm, respectively. It is noted that the quantum yield for emission from NCO(A2S) predominates almost in the whole wavelength region studied and thus the absorption profile resembles that of NCO(A2Z), except around 110 nm where the quantum yields for the formation of three emitting radicals become comparable. It is interesting to note that spin allowed processes dominate
Figure 11. Absorption spectrum of HNCO in the region 107-180 nm. The pressure of HNCO was 19 mTorr for the measurement between 107 and 140 nm and 48 mTorr at the longer wavelengths longer than 140 nm. The spectral resolution was 0.5 nm (Taken
from Ref. 71).
as long as the channels are energetically open. The spin forbidden process forming NH(i43n) + CO(X!S) has a low quantum yield at a low photon energy, but as the photon energy becomes closer to the ionization potential, 11.6 eV, the quantum yield increases very rapidly.
In the photolysis in the UV region, NH radicals are found to be formed primarily in the singlet states7274. The dissociation energy for HNCO -> HN^A) + CO^Z) has been determined by Spinglanin, Perry, and Chandler (SPC)75 as 41530 ± 150 cm1 (118.7 ± 0.4 kcal mol1) by laser induced fluorescence (LIF) detection of NH(a!A). SPC claim that this value predicts the threshold for forming HN^n) + CO(X!I) to be 72215 ± 150 cm1 or 138.48 ± 0.29 nm.75 However, in agreement with the Okabe's value, Uno et al.71 have found that the onset for NH(c1n) formation from HNCO is 141 ± 1 nm which is longer than the value obtained by SPC. SPC reasoned75 that the emission Okabe70 has observed in the wavelength region longer than their estimated onset might originate from NH(A3n). As is observed in Figure 14, however, the NH(A3n->X3L) emission intensity decreases slowly beyond 140 nm and extends even up to 162 nm, although not shown in Figure 14.
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Figure 12. NCO(A2Z) produced in the photolysis of HNCO. The HNCO pressure was 25 mTorr. (a) Fluorescence excitation spectrum observed at Xobs > 370 nm. (b) Quantum yield for NCO(A2Z+-» X2!!^} emission process (Taken
fromRef. 71).
Figure 13. Emission from NH^ 1!! -> a1 A) Figure 14. Emission from NH(A3n -> produced in the photolysis of HNCO. The HNCO X3!/) produced in the photolysis of HNCO. pressure was 25 mTorr. (a) Fluorescence Excita- The HNCO pressure was 25 m Torr. (a) tion spectrum observed at 325 nm. (b) Quan- Fluorescence excitation spectrum observed at turn yield for NH (clU ->alA) emission process 336 nm. (b) Quantum yield for NH(A3n -»
relative to absorption (Taken from Ref. 71). X3!-) emission process relative to absorption (Taken from Ref. 71).
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On the contrary the NH(c!n -> a1 A) emission was too weak to be observed above 140 nm. This implies that the threshold observed for HN(a!A) + CO^E) formation is higher than the thermochemical threshold probably because the dissociation channel has a non-zero potential barrier. This may be reasonable because HNCO is linear in the ground state but bent in the excited state.
Concerning the question whether NH(A3n) is formed from direct photodissociation Eq. (1 lc) or collision-induced energy transfer:
HN(X3£-) + CO(a3IT) -» HN(A3n) + CO^Z), AH = 7.11 ± 0.03 eV (12)
following direct photodissociation (Eq. lOe), Hikida et al76 have observed the time decays of NH(A3n->X3£-) fluorescence which follow a single exponential function with a decay constant, T ~ 500 ± 40 ns. They did not observe a long enough induction time expected from collision-induced formation of NH(A3n) (Eq. 12) in the sample pressure region studied. Considering the two experimental findings, i.e. (i) a single exponential decay with a decay constant corresponding to collision free NH(A3n->X3E) emission,77*78 and (ii) two different profiles obtained for the fluorescence excitation spectra of NH(A3n ->X35/) and NH(c1n -> a1 A) emission, one is compelled to conclude that the spin conservation rule is violated for the photodissociative excitation processes of HNCO.
2.HN3andNH3
In contrast to photolyses of HNCO, photodissociative excitation processes of HN3 and NH3 in the wavelength region above 105 nm appear to proceed following the spin conservation rule.76-79«80 Okabe has reported that, in the photolysis of HN3 at 121.6 nm, NH(A3n) radical was not formed from the direct dissociation of the electronically excited HN3molecule since no NH(A3n->X3Z) emission was observed at low sample pressures, but from the secondary reactions between the electronically excited triplet N2* molecules resulting from the primary photodissociation:
HN3 + hv -> N2*(T) + NH(X3Z") (13) N2 * + HN3 -> NH(A3n) + 2N2 (X 1 ^ ) (14)
The same conclusion has been reached from the time decay measurements of NH(A3n) emission as a function of sample pressure by Hikida et al.76
Similarly, in the wavelength region above 105 nm, photodissociative excitation of NH3 is known to proceed almost strictly following the spin conservation rule.3581-82 However at the higher energies the breakdown of the rule is found to occur by Wu83 (see Figure 15). Wu used dispersed atomic resonance lines as an exciting light source. Since the ionization potential of NH3 molecule is 10.15 eV (122.2 nm), at all the wavelengths studied ammonia can be ionized and thus the emitting channels can compete with ionization channel. The quantum yields for NH(A3n) formation relative to absorption at 123.9, 118.4, 106.5, 103.7, and 97.6 nm are found to be 2xl0-5, 3xl05, 1.2x10 ,̂ 8.5x10-4, and 1.5xl0-3, respectively.83 The ratio of NH(A3n) formation relative to NH(c1n) is rather small at all wavelengths except for 83.5 nm excitation83. The onset for the spin allowed process:
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Figure 15. High resolution fluorescence spectra in the overlapping region of NH (c !n -> al&) and (A3n -> X3Z) transitions. All spectra were taken with a resolution of 0.2 nm, except in (d), which was taken with a 0.13 nm instrumental bandwidth
(Taken from Ref. 83).
NH3 -> NH(A3n) + 2H, AH = 12.25 eV (101.2 nm) (15)
is thermochemically located at 12.25 eV (101.2 nm) above the molecular ground state, and thus above this onset the cross section for NH(A3n) formation seems to increase steeply.
In contrast to photo-excitation the breakdown of the spin conservation rule has been observed in collision-induced excitation of NH3 by the metastable rare gas atom impact, although the contribution of spin forbidden channels varies depending on the energy level of a metastable atom. For example metastable Ar*(3P2>0) atoms (E(3P2) =11.55 eV, and E(3P0) = 11.77 eV) can produce both NH(A3n) and NH(c!n) with almost comparable fractions84 in the collision energy region between 0.5 and 1.5 eV. In contrast Kr*(3P20) atom (E(3P2) = 9.92 eV, and E(3Po) = 10.56 eV) impact produces primarily NH(A3n) + H2 products M and the {NH(c!n) + Hj} formation channel was not observed, although the spin forbidden process is thermochemically feasible by 0.3 eV. The failure in observing NH(A3n) formation from Kr* + NH3may be due to a higher potential barrier to dissociation than 0.3 eV, since, otherwise, spin conversion is expected to be facilitate more by Kr than Ar owing to the heavy atom effect.
It would be interesting to compare the photodissociation behaviors of the two molecules HNCO and HN3, since the two molecules are isoelectronic to each other. As is already mentioned above, the HNCO molecule readily undergoes the spin forbidden process (Eq. 1 l c ) , while the photodissociation of HN3 in the UV841 and VUV region almost solely leads to formation of spin conserved fragments. However in the lower energy region where HN3 or DN3 is excited by infrared multiphoton excitation or higher order overtone transitions, almost comparable amounts of the dissociation products from both spin conserved and spin forbidden processes have been observed.841* The difference in the photodissociation behaviors between HNCO and HN3 in the VUV region must be due to the degree of the
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curve-crossing of the potential energy surfaces (PES) involved, that is, of the initially excitated state and the dissociative states leading to spin conserving and non-conserving product channels. It is hoped that reliable, ab initio calculations of PES's for HNCO and HN3 be done in the higher (8-12 eV) energy region.
C. Polarization Studies of Photodissociative Excitation Processes
1. Rotationally Resolved Fluorescence Polarization
The studies of photofragment polarization measurements have been extensively carried out by Simons and his coworkers using rare gas discharge light sources. Beautiful experiments of rotationally resolved photofragment alignment has been carried for rotationally resolved emissions of OH(A2Z+-^X2n)12-86 formed from H20 photodissociation and CN (52E+->X2Z+) from C1CN.14 For the latter measurement Guest, O'Halloren and Zare14 used a fluorine laser at 157.6 nm as an excitation light and also a photoelastic modulator followed by a sheet polarizer before a monochromator (Spex 3/4 m) for dispersing the fluorescence. It would be worth to show the rotational state dependence of the fragment fluorescence polarization as is illustrated in Figure 16 along with the dispersed spectrum for the unpolarized emission measured at a magic angle 54.7°. The observed alignment P is converted to the observed quadrupole alignment A2(2)(N\ obs) by the relation9*10
Figure 16. (a) CN(B2Z+-> X2Z+) photofragment fluorescence polarization measured for different rotational transitions in the (0,0) band at 0.03 nm resolution, (b) The normalized CN(B-X) photofragment emission spectrum following C1CN
dissociation at 157.6 nm.
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1 AP
W.^^BCTP 16)
The geometrical factor hM(N',N") accounts for the effect of the final state NM on the fluorescence polarization of the N'-M" transition. The values for the product hM(N'JV") Ajn(N\ obs) for the CN(B->X) emission are also illustrated in Figure 16a. The experiment agrees quite well with the theory9-10-14 for the rotational states studied. Since the geometrical factor hv>(N',N") are known as -//(2/.+3), -(Jr
i+l)/(27i-l), 1 for P, R, and Q branch transitions, respectively, the true alignment A™(N\ obs) for high J limit is about -2/5, which is the value expected for parallel transition moments for both absorption and emission to the reactant and fragment molecular axis and direct photodissociation.9 Simons and Smith have determined the rotational state dependence of the true fluorescence polarization A<2>(W, obs) for OH(A2Z->X2n) transition of the OH(A2Z) fragments from H20 dissociation excited into the BlA{ continuum at 130.4 nm and into the (0,0,0) level of thetf^Rydberg state at 121.6 nm. They find that the photodissociation from the continuum state excited at 130.4 nm gives A<n(N'9 obs) ~ - 0.4 which implies direct dissociation, while that from the Rydberg state at 121.6 nm does A<2>(AT,obs) > - 0.15, implying a longer lifetime for the excited intermediate molecule.
2. HCN
The results mentioned above are straightforward in elucidating the applicability of fragment fluorescence polarization for discussing the dynamics of photodissociation processes. It is hoped that more experiment of this sort will be done in the future. Since fragment fluorescence polarization gives us information on the lifetimes of the photoexcited molecules, especially when the excited state is predissociative, it has been measured against wavelength for HCN (DCN)15and BrCN86. Figure 17 shows the absorption and emission
Figure 17. Absorption spectrum of DCN (Middle), and the cross section (Bottom) and polarization index (Top) for the CN (B2E+-> X2L+) emission against exciting light wavelength Xexc observed in the photolysis of DCN in the 108-125 nm region. The resolution was 0.2 nm and the DCN pressure was 30
mTorr.
1 AP Af\N\ obs) = hi2WfN1 (3^).
1 4/> WHhl'.N") (3-/>) Af\N\ obs) = 16)
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cross sections, and polarization index R for DCN against excitation wavelength in the 108 -125 nm region.
If the transition dipole vector is in the plane of HCN molecule and dissociation is direct, the degree of polarization P is 1/7 in the high; limit, since the emission dipole is perpendicular to the angular momentum vector / Then the anisotropy index R = 2P/(3-P) becomes 1/10. The polarization indices for the excitation in the 110-116 nm region which is assigned to \n-+ 3pa transition are about 0.075, which value is slightly smaller than the limiting value of 0.1 for the in-plane absorption dipole transition and direct dissociation. In the longer wavelength region between 117 and 125 nm, the transition of which is assigned to l;r-> 3sc7, the R value is on the average about 0.04, which implies a longer lifetime of the excited HCN molecule.When the absorption transition moment is in the plane of three atoms, it is expected that the excited state is A state, and the resulting CN(B2I?-*X2I?) emission is expected to exhibit a positive polarization P. On the contrary if the absorption moment is perpendicular to the molecular plane, then the excited state is A" state and the resulting CN(B-X) emission is expected to exhibit a negative polarization index.10' 12>13
The lifetime dependence of the polarization index upon the excited state is clearly shown, especially in the wavelength region between 130 and 145 nm where the upper states are predissociative. The polarization dependence of fragment fluorescence for DCN in this energy region has been measured point by point by Macpherson and Simons.33 Recently Nagata et al. have carried out thorough measurements of the fragment fluorescence polarization against wavelength of the exciting light in the same region as Macpherson and Simons did, but almost continuously. The expanded spectra in the 136 - 142 nm region are shown in Figure 18. It is of interest to note the following findings: (a) The polarization index has a much richer feature than the absorption and emission
spectra, and strong undulations are observed, unlike with the/? values observed in shorter wavelength region below 125 nm, where the R value varies rather slowly with exciting light wavelength,
(b) it becomes negative in some part of the longer wavelength region and a positive polarization feature originating from predissociative states seems to overlap with continuum transition which gives negative polarization,
(c) the peak wavelength for the emission intensity does not necessarily match with that of polarization index, and the peaks of polarization anisotropy are found on the shorter wavelength side of the prominent emission bands which are assigned to the C1 A'(0 v2 0) progression.
(d) the maximum R is about 0.04, and (e) the features observed in addition to (0 v2 0) progression have been assigned to (1 v2
0) and (0 v21) progressions. The observed polarization index Rob5 is the average of the polarization indices Rn from the
contributions of different states as
*«. - f ^ - (17)
where <jn is the cross section for the emission from the n-th state.
I f f . * . 2cr„
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Figure 18. CN(B2£+-> X2Z+) emission intensity (solid line) and polarization index (dots) from CN(B2I+) fragment formed in the photolysis of HCN in the 138-142 nm region.
The occurrence of negative polarization may be reasonably assigned to a contribution of underlying 1A" continuum states. It is speculated that it is not the BlA" state, since the absorption to this state is too weak to be important, but the continuum DlA" state, which has been assigned to start at 139.6 nm.87
The HCN molecule in the DA' state is known to be bent by 39° from linearity and the absorption dipole moment is in the plane87. The rich features observed in the polari-zation index indicate that the polarization index strongly depends on the rotational state of the molecule. The polarization index has a maximum on the higher energy side of the emission maximum. The bent HCN molecule can be regarded as a symmetric top and thus the rotational states can be represented by two quantum numbers (/, K), where / is the total angular momentum and K is the rotational quantum number along the quasi-linear axis. The wavelength dependence of the polarization anisotropy R can be simulated by assuming that (i) the predissociation rates for K = 1 bands are larger by one order of magnitude than those for K = 0 and (ii) the polarization index is dependent on rotational state as shown in Figure 19a. Figure 19b illustrates also the wave number dependence of the rotational transitions.
As is already mentioned above, the wavelength dependence of polarization index tells us a lot about the details of predissociation dynamics. More experimental studies should be carried out in other molecules, since it is expected that more new information on both the characteristics of molecular excited states and their dissociatin dynamics will be obtained from these measurements. One of our future goals of research done on the beam line BL2A at UVSOR is to study more about the dynamics of photochemical reactions applying the technique of fragment fluorescence polarization measurement.
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Figure 19. The wavenumber dependence of a) the rotational line for C-X transition of HCN and b)the polarization index for the CN(B) photofragments formed from excited
HCN(£! A'). The energy origin is the lowest rotational state at each vibronic state.
D. Deuterium Isotope Effects in Photodissociative Excitation Processes
Kinetic isotope effects (KIE) have been observed in many phenomena88. Here we focus our attention on the KIE in photodissociative excitation processes observed when hydrogen atoms are substituted for deuterium atoms, i.e. deuterium isotope effects (DIE). DIE have been known for a long time in molecular dissociation processes and electron bombardment ionization mass spectroscopy88. However the enhancement or reduction of the rates of thermal dissociation or electron bombardment dissociation processes upon deuterium substitution involves many excited energy levels of a molecule, since the initial excited energy levels cannot be specified due to either Boltzmann averaging of the initial state or multi-level excitation inherent in electron bombardment process, except for the cases of chemical activation developed by Rabinowitch and his coworkers88*89. The primary deuterium isotope effects for C2H5-H and C2H5-D fission reactions have been successfully explained in terms of the RRKM theory88*90 by Rabinowitch and Setser.89 From the mass spectral patterns of the fragment ions formed from deuterium isotopic molecules, for example CH^D^or n = 0 to 291«92, it has been concluded that there is a preference in C-H bond fission over C-D bond fission by a factor of 2.4 to 3.
The photodissociative excitation processes are similar to mass fragmentation processes from the viewpoint that electronically excited molecules undergo fragmentation into several available channels. The advantage of the former is that the energy content of the excited
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molecule can be specified. We have observed the enhancement or decrease in the quantum yields for the formation of excited fragments upon deuterium substitution of CH3CN CH3OH, and CHCI3 molecules. The deuterium isotope effects upon the quantum yields for formation of electronically excited fragments will give us some information upon the dissociation rates among several possible channels.
LCH3CNandCD3CN
Figure 20 illustrates the absorption and CN(B2Z+-> X2Z+) emission cross sections against wavelength in the 107-150 nm region for (a) CH3CN and (b) CD3CN.93 From these spectra one finds (i) that the absorption cross sections for the two isotopic compounds have essentially the same features except for vibronic shifts due to deuterium substitution and (ii) that the quantum yield for CN(B) formation
CH3CN + hv -> CN*(B2L+) + CH3, AH = 8.52 eV (145.5 nm) (18a) CD3CN + hv -> CN*(B2Z+) + CD3, AH = 8.55 eV (145.0 nm). (18b)
is larger for deuterium isotopes than the hydrogen compounds, especially in the shorter wavelength region. The quantum yield varies smoothly, and in particular increases with photon energy41. It would be suffice to mention that CD3CN gives larger quantum yield for CN*(B) production by a factor as large as 1.8 at the photon energies up to about 11 eV, but
Figure 20. Absorption (solid line) and CN(B2£+-> X2L+) emission (dotted line) cross sections for (a) CD3CN and (b) CH3CN molecules in the 107 -150 nm region. The spectral resolution was 0.5
nm.
Figure 20. Absorption (solid line) and CN(B2£+-> X2L+) emission (dotted line) cross sections for (a) CD3CN and (b) CH3CN molecules in the 107 -150 nm region. The spectral resolution was 0.5
nm.
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just above the onset of CN*(B) formation (145.5 nm)94 the quantum yields are almost comparable for two isotopic compounds.
The dispersed fluorescence spectra from the photofragments have been measured in the CN(B-X) emission region by Ashold and Simons95 and the emitting species are identified. The vibrational distributions of the CN*(V) fragments determined95 exhibit almost a statistical behavior, i.e. the photoexcited molecules dissociate from a long-lived intermediate through a "loose" transition state. It would be worthy of noting that the vibrational energy contents remaining in CN*(B) fragments are larger for the deuterated compounds, although the difference is small by as much as 10 %.95 The similar trends concerning the vibrational energy content in CN*(B) fragments are obtained for photodissociative excitation processes of HCN/DCN systems for 130.4 and 129.5 nm excitation96 but not for 123.6 nm96. However the quantum yields for CN*(B) fragment formation from HCN/DCN are found almost comparable to each other in the 107-125 nm region15. This means that deuterium isotope effects upon the quantum yield for CN*(B) formation do not exist in the DCN/HCN systems. The similar results, that is, the absence of the deuterium isotope effects on the quantum yields for fragment fluorescence, have been obtained for H J O / D J O systems97.
Figure 21. a) Absorption and b) emission excitation spectra for CH3OH (solid) and CD3OD (dotted) in the 107-165 nm region. The inset in Figure a) shows the magnified emission excitation spectra. The arrows indicate the thermochemical thresholds for the formation of OH* (A22+) and CH30* radicals.
2.CH3OH,andCD3OD
Figure 21 shows absorption and emission excitation spectra for CH3OH and CD3OD in the 107-165 nm region.98 The excited species detected are OH or OD(A2L+) andCH30*
2.CH3OH,andCD3( ,0D
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radicals formed. The absolute absorption and emission cross sections for methanol have been determined by Suto and Lee41. It would be worthy of noting that the emission cross sections for CD3OD is larger than those for CH3OH in the shorter wavelength region 107-130 nm. Although the CH30* can be formed in the photolysis, the dispersed fluorescence spectrum is dominated by the OH*(A) emission bands. The threshold wavelength for fragment emission agrees well with the thermochemical value of OH*(A) formation. Therefore one can safely consider that OH*(A) is the dominant emitting species (Eq. 19).
CH3OH+ hv -> CH3 + OH*(A2Z+), AH = 8.00 eV (155.0 nm) (19) CD3OD + hv -> CD3 + OD*(A2S+), AH = 8.03 eV (154.5 nm) (19)
Measurements were also done for the CH3OD compound and its emission excitation spectrum is found to be very similar to that for CH3OH except for small isotope shifts in transition frequencies. Although the quantum yield for this process is rather small, the dissociative excitation process observed is very similar to that of acetonitrile, i.e. for both compounds a methyl radical and OH* or CN* are formed, where the star means electronically excited species. In the methanol case too, the ratio of the quantum yields for photo-dissociative excitation process for deuterated to non-deuterated isotopic compounds amounts to as large as 2.0.
3.CHCl3andCDCl3
Figure 22 shows (a) the emission excitation spectra for CHC13 and CDC13 and (b) absorption cross section of CHC13 in the 106 - 170 nm region." As expected, the absorption spectra are almost identical for both isotopic compounds, whereas the emission cross section for the CHC13 is larger than that for CDC13 in the energy range above 120 nm. In the longer wavelength region above 120 nm where CHC1* radicals are known to be the
Figure 22. (a) Emission excitation spectra for CHC13 and CDC13 and (b) absorption cross section of CHC13 in the
106 - 170 nm region.
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dominant emitting species, the emission cross sections for both isotopes are comparable. In the shorter wavelength region than 120 nm the H-isotopic compound exhibits larger cross sections than the D-compounds for forming emitting species and thus the isotope effect is normal, whereas that for acetonitrile and methanol is "inverse," since aemis(D) > (^ . (H) . Below 120 nm the emitting species is assigned to CC12*(A) radical, since formation of CC12*04) radicals involves the motion of a hydrogen atom and electronically excited CHCI3 molecule should exhibit faster rate of formation of a CC12*04) radical and HC1 molecule than CDCI3.
4. The Origin of The Deuterium Isotope Effects
The deuterium isotope effects we have observed for the photodissociative excitation processes are normal for CHC13/CDC13 but inverse for CH3CN/CD3CN and CH3OH/ CD3OD. Basically no isotope effects have been observed for H20/D20 and HCN/DCN systems. There are two reasons for observing the isotope effects. First other reactive channels, in which motions of hydrogen atoms are involved, are competing with the photodissociative excitation process. Second, the isotope effects observed are due to the difference in the rates of intramolecular energy transfer such as internal conversion (IC) and/or intersystem crossing (ISC) between the two isotopic compounds, since the overlaps between the vibronic wavefunctions of the the initially prepared state and the accepting mode can differ upon isotopic substitution. The latter has been excluded from the main reason for the effect because both normal and inverse isotope effects have been observed in the present phenomenon. The isotopic substitution will influence only in the same direction either normal or inverse for all the compounds studied if it influences the rate of intramolecular energy transfer process such as IC and/or ISC.
Therefore the most probable reason is the first one, that is, the existence of the competing channels with the dissociative excitation process. For CH3CN/CD3CN systems, the probable channels competing with the dissociative excitation channel (Eq. 18a) are
CH3CN + hv -» CH3 + CN(X2£+); AH0 = 5.04 eV ( 5.07 eV) (20a) CH3 + CN(A2II); AH0=6.17eV (6.20eV) (20b) CH2CN + H; AH0=3.47eV (3.53 eV) (20c) CH2 + HCN; AH0 = 4.43eV (4.49eV) (20d) CHCN + H2; AHo = 3.90 eV (20e)
Motions of a hydrogen atom or atoms are heavily involved in the latter three channels (Eqs. 20c-20e) and thus they are expected to exhibit the deuterium isotope effect. In the parentheses are presented the heats of reaction corresponding to CD3CN reactions. In a similar manner competing channels can be listed in CH3OH/CD3OD systems.
E. Two-Electron Excitation Process
Ab initio potential curve calculations have shown that neutral two-electron excited states of hydrogen molecule H/* lie in the Franck-Condon region at energies beyond 23 eV above
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the ground state.100 The doubly excited repulsive resonance states of H2 may autoionize yielding ions of H2
+, H+ or H+ + H, which may compete with the decomposition channel into two hydrogen atoms in the excited and/or ground state. Leventhal, Robiscore and Lea 101 first reported the velocity spectrum of neutral metastable H(2s) atoms produced by electron impact of H2 in a time-of-flight experiment. Two distinct groups of atoms were detected as shown in Figure 23. The observed threshold for the slow H(2.s) atom production was 18± 2 eV and is interpreted as arising from the transitions to attractive states just above the H(l.s) + H(2.s) dissociation limit. The fast metastables are believed to arise from the states which are Franck-Condon "allowed" by an electron impact. The potential curves of some of these levels are shown in Figure 24.100-102 Doubly excited H2** states have usually been divided into two kinds of Rydberg states, i.e., the Ql and Q2 states. The lower Qi states have the core orbital of the 22%(2pau) state of H / ion and dissociate into H+
+ H"(l.y2) or H(ls) + H(«/) by curve-crossing with one-electron excited states at large internuclear distances. The higher Q2 states with the 2nu (2p*0 core of Hf ion dissociate directly into two excited H atoms. The electronic configuration and dissociation limits of the Qx and Q2 states are summarized in Table IV.103
Figure 23. H(2s) TOF spectrum observed by the electron bombardment at 45 eV (Taken from Ref. 101).
When the Qx and/or Q2 Rydberg states are generated by a direct photoexcitation of H2 molecule, the atomic fragments produced may be in the n > 2 levels. The H(2/?) radiate Lyman-a emission. Balmer-a and p lines will appear from the n = 3 and 4 levels, respectively, with accompanying the Lyman-a as the cascading emission. Glass-Maujean103-104
has measured the emission cross sections of the Lyman-a and Baimer lines using SR ACO ring. Figure 25 shows the emission cross section for H(/a=2) atom obtained after the calibration of the optical system to detect the Lyman-a line. He also calculated the partial photoabsorption cross sections for the known optically allowed double excited states of H^ as shown in Figure 26.104 No fluorescence profile can be partly fitted by the computed g^ l^ l ) cross section curve whose onset is 510 nm. Just above the threshold the emission cross section profile of Lyman-a can nicely be fitted to the computed cross section curve of Qilnu(l) state. In the high photon energy range above 35 eV, the 2H (2pau) of H / which
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Figure 24. Potential energy curves of doubly excited states (H/*) and ionic states (H^) of
Hj (Taken form Ref.102).
Table IV. Electronic states of Ql and <22 and their dissociation limits.8
a. Taken from Ref. 103.
Figure 25. Emission cross section for the H(n = 2) atom production scaled in incident wavelength and in energy. Dotted lines: the calculated dissociation cross section of the (21
1/7M(1) and Q2lnu(l) states; dashed
line: the cascade contribution; full line: the sum of these contributions. (Taken from Ref. 103).
Qj States
^ ( 1 ) (2p<7u 3s<Jg)
%(2) (2pau 3s(jg) 1 / 7 U ( 2 ) ( ^ C T U 4 ^ )
H(U) H(ls)-H ( 1 J ) , H(U) H
+• H(2J) fH(2p) hH(2/7) - H(w=3)
Q2 States
1/TU(1) (2pflu 250-g)
^ C ) ( 2 ^ 3 * ^ ^ ( 4 ) (2/7^4*7^
H ( 2 J ) -H(2s)4 H(3J) + H(2p)4
hH(2/7) H(2/7) H(2p)
•H(n=4)
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dissociates into H(2p) + H+ plays an important role in forming Lyman-a emission.104 The Balmer-a profile is represented by the superposition of the two 1iTM(2) computed cross sections and that of Balmer-p one by the combination of the Q^njQ) and Q2
lnj(4) states.103
By adjusting the amplitude of the contributing cross section curve, the dissociation yields of these states forming the excited H atoms have been estimated to be 52 ± 21% for lnn(l), 45 ± 13% for 177u(2) and 100% (-25%) for *nu(3) of the Qx group.
Figure 26.. Computed photoabsorption cross sections for the optically allowed double excited states of H2. Dotted lines are for the 1Iti states and the
full lines for the lLa states. (Taken from Ref. 104].
It is obvious from Figure 23 and Table IV that both the H atoms produced through the Q2 Rydberg states are excited and can radiate Lyman-a emission. Arai etal.102 have measured coincidence signals between Lyman-a and Lyman-a photons using synchrotron radiation of the Photon Factory (PF) at Tsukuba. Lyman-a photons were detected by two Csl-coated multi-channel plates (Hamamatsu P. Model F1094-21sX) facing to each other, which are aligned such that the direction of photon emission coincides with that of the main polarization of the linearly polarized SR.102 The observed excitation spectrum from the coincidence signals of two Lyman-a photons is shown in Figure 27.102 The peak of the true coincidence has a width of 5 ns (FWHM). The threshold in Figure 27 is just below 29 eV.
Lyman-a radiation can be produced in the dissociation of doubly excited states of H2
through the following processes:
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1 i i i I H(2p) + H(2p) H(2p) + H(3/) H(3/) + H(3/) H(2J) + H(2/?) H(2J) + H(3/)
Ly-a Ly-a Ly-a H(2p) H(2p) + H(2p) Ly-a H(2p) i i i i
Ly-a Ly-a Ly-a Ly-a
(21a) (21b) (21c) (21d) (21e)
Two Lyman-a photons produced from reactions (21a-c) have a time correlation between the two and may be detected as a true coincidence signal. The lifetime of H(2p) is 1.6 ns, while those of H(3J), H(3p) and H(3d) are 160, 5.4 and 15.6 ns, respectively.105 Therefore, in the true coincidence spectrum (not shown here), the Lyman-a emission produced through cascading reactions (21b) and (21c) should provide with a broader component which in fact was not observed. Thus, the strongest coincidence band is assigned to process (21a), the precursor of which is believed to be the fii'AXl) Rydberg state.102
Figure 27. Excitation spectrum obtained from Lyman-a and Lyman-a coincidence experiments. .Taken from
Ref. 102.)
F. Fluorescence from Doubly Charged Ions
Multiply charged molecular cations are generally unstable due to Coulombic repulsion and decompose into small fragment ions. In a special case that the chemical bond energy is large enough to exceed that of the electronic repulsion, a local minimum appears in the potential curves of the multiply charged ion. Only three doubly charged molecular ions,
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N22 + ,1 0 6-1 0 9 N O 2 V 1 0 « U 1 a n d 02
2 + 1 1 2>1 1 3 are known to give emissions. The fluorescence from the N2
2+ (DlI,+ -> Xx20 transition was first observed by Carroll in 1958 by using a glow discharge.106'107 Rotational analyses of the N2
2+ emission bands were carried out by Cossart and Launay109in 1985. However, fluorescence from the N2
2+(DlI,+) ion has not been observed in photoexcitation. Emissions from NO2* and 02
2+ molecular ions were measured using synchrotron radiation.
1. Nitric Oxide
The existence of stable N02+ was predicted by theoretical calculations: Hurley's calculations in 1962110 predict that the N O 2 ^ 2 ^ -»X2^) emission would appear at around 260 nm since the B2I? state has a potential minimum. The calculations of Thulstrup et al. in 1974m however predicted the B22? state would be dissociative and thus would give no fluorescence. The presence of parent N02+ ion was confirmed in the determination of partial photoionization cross sections for formations of N+, 0+, NO+ and NO2* from the NO molecules excited by photons114 and electrons.115 Latest ab initio calculated potential energy curves for low-lying states of NO2* are shown in Figure 28.116
Figure 28. Potential energy curves for low-lying 22? (JC, B, C) and 2J7(X, B, 3) states of N02 +
(Adopted from Ref. 116).
In 1986 Besnard, et al.117 tried to observe the N02+(B2I^ -» X22?) emission by using a photoion-fluorescence coincidence (PIFCO) technique at ACO synchrotron radiation facility. They detected the PIFCO signals between the total fluorescence in the region of 110-360 nm and the mass of N02+( m/e =15). The PIFCO profile is similar to that of doubly excited H/*. The onset of the B22? state was determined to be 38.6 ±0.1 eV. The
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N02+(B22? -> X2^) emission spectrum was observed at hv> 42.5 eV. After the success of this PIFCO experiment, the rotational analysis of a dispersed fluorescence from an electric discharge of NO confirmed the N02+(B22^ -> X2^) electronic transition. The molecular parameters obtained are: T^cnr1) for the B2I? state is 38995.06 cm1, that is, the band origin is 256.3 nm and the energy level of the B2Z? state is 43.4 eV; re's (nm) for the X2^ andfl2^states are 0.10926 and0.10963 nm, respectively.118
The A2n state of NO2* has been predicted to lie above the ground state by 6700 - 11000 cm1 from several calculations.118 By detecting the PIFCO signals between the N02+ions and fluorescence in the wavelength range of 650 - 900 nm, Besnard, Hellner and Dujardin119 observed the A2i7-> X2!? emission at the exciting photon energies higher than 40.4 ± 2 eV, which falls just into the middle of the ground and B states. However, the emission intensity observed is very weak since the the spectral range of the fluorescence is unfavorable for detection with photomultiplier tube or photographic plates. In addition, the A2n -» X2^ emission is considered to be spread over a wide spectral range with many and weak vibrational intensities as expected from the potential curves shown in Figure 28. Thus, the details of the dispersed fluorescence have not yet been reported.
2. Oxygen
The fluorescence from the 022+ dication was discovered by exciting 0 2 molecule in the
soft X-ray region at NSLS of the Brookhaven National Laboratory.112-113 The schematic diagram of the apparatus used is shown in Figure 29.120 The intensity of x-ray is converted to visible light by a phosphor P31 scintillator. The fluorescence excitation, ion yield and
Figure 29. Schematic diagram of the apparatus for soft X-ray spectroscopy. (a) Io monitor; (b) S3N4 window; (c) ion chamber; (d) I monitor; (e) side photomultiplier. (Taken from Ref. 120].
photoabsorption spectra are depicted in Figure 30 in the photon energy range of 500 - 700 eV.113The broad band centered around 580 eV is the characteristic feature in the fluorescence excitation spectrum, suggesting the existence of a shake-up or shake-off anomaly. Figure 31 is the dispersed fluorescence spectra observed in the excitations at the jc-resonance (531 eV), a-resonance (542 eV), anomaly at 580 eV, and the continuum of
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Figure 30. Soft X-ray spectra of oxygen near Figure 31. Dispersed fluorescence of oxygen the oxygen K edge. From top to bottom: the excited by soft x-ray. The x-ray energy from top total optical luminescence yield, the total ion to bottom: 531 (7C-resonance), 542 (a-resonance), current, and the absorption coefficient. (Taken 580 (shake-up or shake-off anomaly), and 800 eV
from Ref. 113) (continuum) (Taken from Ref. 113).
the oxygen K edge (800 eV).113 The progression at hv> 500 nm in Figure 31 is assigned as the bA& -> (fnu transition of 02
+ molecular ion.121 Emission in the 380 - 500 nm region is attributed to the 02
2+(£3/7g -> A32 d+) transition for the following reasons:113 ( i) The
average splitting of the bands is 1350 cm1 and does not match any known fluorescence of 02
+ ion nor that of an atomic species O or 0+ , or some molecular species 0 2 , 0 3 or 03+; (ii)
the electronic energy levels of the A3^+, B3ng or #32£, and C3/^ of 022+ dication lie at 4.0
± 0.3,122 7 ± 1, and 12 ± 1 eV above the ground state.123 The energy gap of the unknown bands of~2.8eV matches the difference of the B and A states of 02
2+, i.e., 3 ± 1 eV. Thus, the transition energies and selection rules suggest that the 380 - 500 nm bands originate from the 02
2+(£3i7g -» A3!*) transition. From the monochromatically separated fluorescence excitation spectra of the 02
2+ions (which are not shown here)113 one can see that the relative emission intensity of the 02
2+
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dication observed at 440 nm is stronger than that of 02+ ion observed at 565 nm when
excited at the a-resonance (542 eV) and presumably at the anomaly (580 eV), as is clearly seen from the dispersed fluorescence shown in Figure 31. This observation means that one K-shell electron is excited to the antibonding a-orbital of 0 2 at the a-resonance. Although the relaxation processes of such a highly excited molecule are not well understood, one can speculate that, at the 580 eV anomaly, a core electron is excited to the antibonding a-orbital and a valence electron of oxygen is promoted to another valence orbital or to the continuum. At the rc-resonance the 02
+ emission is slightly enhanced relative to the ion yield, suggesting that the resonance Auger decay is favorable for 02
+ ion formation but not for 02
2+ dication, the emission intensity for which is reduced at the it-resonance.
G. High Resolution Absorption Spectroscopy
As is described in §II-D, photoelectric detection of transparent light intensity is essential for estimating absolute values of absorption cross sections. Figure 32 illustrates the photo-absorption cross sections of the Kr atom in the photoionization threshold region measured with a resolving power of 1.5 x 105.19*124 Since there are 6p electrons in the rare gas atoms except He atom, there are five Rydberg series in Kr, three series converging to 4p5(2P3/2) ion state and two converging to 4p5(2P3/2). Above the first ionization potential converging to 2P3/2, typical Fano profiles can be observed, which give rise to due to autoionization. The photoabsorption cross section of rare gas atom between 2P3/2 and 2P1/2 is represented by the Fano formula 125>126by
°-°~ (i + es2 ) + *.d ( 1 + ei) +<*, (22)
where e has a periodic energy scale and expressed by
eL , " " [ « <*■**■>] for L = s and d. (23)
In Eq. (22) qs and qd are asymmetric parameters corresponding to ns and nd', aasand aad are absorption cross sections leading into continuum 2P3/2 state and interacting with s and d state, and q, is non-resonant cross section. In Eq. (23) vL and /^ are quantum defects and WL is a parameter related with the width and v1/2 = [Ry/(Im - £)]1/2 where Ry is the Rydberg constant, Im is an ionization potential converging to P1/2 state, and E is an energy. Figure 33 illustrate the photoabsorption cross sections of the (8s' and 6d') line pairs of Kr atom.125 Open circles are the experimental data and the solid curves are the theoretical cross sections fitted to the quantum defect theory with the MQDT parameters.125*126
Table V summarizes quantum defects /^'s and width parameters W '̂s for the ns' and nd' resonances of the Kr atom.19 One finds that agreement between the observed and calculated parameters is fairy good. It is noted that the Ws parameter starts to increase sharply with n at n = 14, which is probably due to the lack of high enough a resolution.
Careful measurements on the absolute absorption cross sections for other simple gaseous atom and molecules have been made, i.e. Ne,129 H2,130 and CO.131 In the case of gaseous molecules such as CO and N2, the overlapping congestion of many rotational lines and
(es+ <?.) . T ( ed + <7d ) °*T ■oi,
(1 + ed2) ■ d + £ s 2 )
<7 = <T„- (22)
(23) tan [* (vL + nL )]
WL %.= for L = s and d.
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vibronic progressions in the room temperature gases become a serious problem. A much simpler absorption spectrum of cooled N2 molecule expanded in a free, jet has been observed.132 In the future free jet techniques will be applied more frequendy to the studies of absorption spectrum in the ionization threshold region.
Figure 32. Photoabsorption cross sections of the Kr atom in the photo-ionization threshold region measured with the resolving power of 1.5 x 105. The pressure ranged from 2 x 10*5 to 4 x 10'-4 Torr. The uncertainty of the absolute cross section is 5% (Taken from Ref. 19).
Figure 33. Photoabsorption cross sections of the (8s' and 6d') line pairs of Kr atom. Open circles are the experimental data and the solid curves are the theoretical cross sections fitted to the quantum defect theory (Taken
from Ref. 19).
Figure 32 . Photoabsorption cross sections of the Kr atom in the photo-ionization threshold region measured with the resolving power of 1.5 x 105. The pressure ranged from 2 x 10*5 to 4 x 10'-4 Torr. The uncertainty of the absolute cross section is 5% (Taken from Ref. 19).
Figure 33. Photoabsorption cross sections of the (8s' and 6d') line pairs of Kr atom. Open circles are the experimental data and the solid curves are the theoretical cross sections fitted to the quantum defect theory (Taken
from Ref. 19).
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For high resolution spectroscopy in the VUV region from 200 to 70 nm, VUV lasers have become available.133 Although the photon intensity from VUV lasers is not stable compared with those from synchrotron radiation, VUV lasers will become very powerful if resolutions higher than SR by one to three orders of magnitude are possible.
Table V. Quantum defects ^L's and width parameters WL 's for the ns' and nd' resonances of Kr atom.
Ms M d Ws Wd
" s i * at 13/2 0.0978 0.2205 O014 0.152
~~&s' 6cf 0.0998 0.2231 0.0104 0.155 9s' 7d' 0.0971 0.2313 0.0102 0.181 10s' 8d* 0.0959 0.2351 0.0110 0.176 11s' 9d' 0.0946 0.2402 0.0108 0.191 12s' 10d' 0.0940 0.2375 0.0112 0.191 13s' lid' 0.0934 0.2413 0.0111 0.203 14s' 12d' 0.0933 0.2429 0.0136 0.205 15s' 13d' 0.0933 0.2396 0.0152 0.202 16s' 14tf 0.0929 0.2413 0.0177 0.206 17s' 15d' 0.0988 0.2420 0.0180 0.198 18s' 16d' 0.0984 0.2535 0.0214 0.215 19s' 17d' 0.1008 0.2552 0.0250 0.211 20s' 8d' 0.0988 0.2550 0.0236 0.226
SE» at 11/2 0.0866 0.2187 0.015 0.170
AIb at 11/2 0.0729 0.2061 0.014 0.239
a) Deduced from semi-empirical (SE) calculations by Aymar, Robaux, and Thomas (Ref. 127).
b) Deduced from ab initio (AI) calculations by Johnson, Cheung, Haung, andDoumeuf(Ref. 128).
H. Absorption and Fluorescence Spectroscopy of Molecular Complexes and Clusters Formed in Free Jet Expansion
The direct measurements of absorption spectra of cooled molecules in a free jet have been extensively carried out by Vaida.134 As is described in §II-B, an absorption spectrum of cooled molecules expanded in a free jet can be routinely measured in the VUV region using synchrotron radiation as a light source, partly because the light intensity from SR is quite stable. However it is considered to be a difficult task to measure absorption spectra of molecular complexes or molecular clusters in the gaseous state in the vacuum UV region, especially when they are composed of different molecules. The most convenient technique to produce molecular complexes is a supersonic expansion of gas mixtures through a nozzle into vacuum. Here we show some results obtained for the direct measurements of absorption spectra of molecular complexes.
Ms Md Ws wd SE* at 13/2 0.0978 0.2205 0.014 0.152
"8? 6cf 0.0998 0.2231 0.0104 0.155 9s' 7d' 0.0971 0.2313 0.0102 0.181 10s' 8d* 0.0959 0.2351 0.0110 0.176 11s' 9d' 0.0946 0.2402 0.0108 0.191 12s' 10d' 0.0940 0.2375 0.0112 0.191 13s' lid' 0.0934 0.2413 0.0111 0.203 14s' 12d' 0.0933 0.2429 0.0136 0.205 15s' 13d' 0.0933 0.2396 0.0152 0.202 16s' 14d' 0.0929 0.2413 0.0177 0.206 17s' 15d' 0.0988 0.2420 0.0180 0.198 18s' 16d' 0.0984 0.2535 0.0214 0.215 19s' 17d' 0.1008 0.2552 0.0250 0.211 20s' 8tf 0.0988 0.2550 0.0236 0.226
SE* at 11/2 0.0866 0.2187 0.015 0.170
AIb at 11/2 0.0729 0.2061 0.014 0.239
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1. Benzene Clusters
The absorption spectrum of cooled benzene in a free jet is already shown in Figure 3 which was observed under the conditions that the molecular temperature is not cooled enough so that benzene dimers or clusters are not formed.5 From this spectrum new assignments of the vibronic bands in the S2 state have been made. A shoulder band at 205.45 nm is assigned to the origin of the S2 state. The absorption coefficient of benzene in a free jet is determined against wavelength by scaling the observed absorbance with that at the reference wavelength of 200.1 nm where the molar extinction coefficient of benzene is determined as e= 6200 liter mol1 cm in the gas phase. The broad band peaked at 178 nm is assigned to the S3 state.
As the partial pressure of the carrier gas is increased new bands appear in addition to the monomer bands.135 Figure 34 illustrates absorbance Log10(V/) against wavelength for benzene expanded in free jets with different carrier gases at a stagnation pressure of 760 Torr along with the pure free jet spectrum (Figure 34a) in the 205-185 nm region. In order to obtain the absorption spectrum due to only clusters, the monomer contribution has been subtracted from each composite spectrum after multiplying an appropriate scaling factor so
Figure 34. (a) Absorption spectrum of pure benzene expanded in a jet at a stagnation pressure of 107 Torr, and those of benzene seeded in (b) He, (c) Ar, (d) Xe as a carrier gas at a stagnation pressure of 760 Torr. The nozzle temperature was kept at 20 ± 1°C and the spectral
bandwidth was 0.2 nm.
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that the sharp Rydberg band in a monomer at 179.0 nm disappears in the cluster spectrum, as shown in Figure 35.
All of the three difference spectra show common features, i.e. rather broad peaks at 207 and 186 nm, the former of which is assigned to the benzene dimer in the second excited state S2 and the latter to the third excited state S3. These new bands are assigned to benzene dimers because their features observed for different carrier gases are essentially identical and thus they are not assigned to benzene-rage gas complexes. The intensities of the new bands are quite sensitive to the stagnation conditions, that is, when the stagnation temperature is raised to 70°C the new band feature disappears even if the stagnation pressure is kept at 760 Torr and using Xe as a carrier gas. Under the similar stagnation conditions of free jets the mass-selected spectrum of benzene dimer against the exciting wavelength has been obtained in the 215-219 nm region using multiphoton ionization time-of-flight mass spectroscopy (MPI TOFMS) by Shinohara and Nishi.136 The onset for the S2 band observed in the direct measurement (at 218 nm) agrees quite well with that of benzene dimer ion observed using MPI TOFMS.
It is interesting to compare the red shifts of the peak positions of the two bands with those of thin film of benzene.137 The spectral shift of the S2 state in the solid film is about 2100 cm1, which is about the same as that in the cluster. On the contrary, that of the S3
Figure 35. (a) The absorption spectra of pure benzene in a free jet and the difference spectra obtained by subtracting that of benzene monomer (a) from those of Figures 34 b, c, and d with appropriate scaling factors for the monomer
spectrum.
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state in the film (« 4900 cm*1) is about twice as large as that in the cluster. Although not shown here, the fluorescence excitation spectra of benzene clusters have been observed in the VUV region.138 Unlike with benzene monomer in which intramolecular relaxation (such as intersystem crossing or internal conversion) rates are higher in the higher excited states than Sxand thus no appreciable fluorescence is observed,139 clusters show larger quantum yields for fluorescence than the monomers. The origin of the longer lifetimes of the excited states of benzene clusters is under study.
2. Rare Gas Clusters
The last decade has seen a rapid expansion of research on physics and chemistry of a variety of clusters. The experimental techniques employed includes laser multiphoton ionization and fluorescence, as well as electron bombardment ionization mass spectro-metry. Using these techniques however it is usually difficult to characterize clusters in question, especially molecular clusters which are bound with weak van der Waals forces, because excited or ionized clusters easily undergo fragmentation. Therefore for molecular clusters it is important to observe their absorption spectra, since from absorption spectra one can obtain information on the electronic structure of both ground and excited states of the clusters as they are. Woermer et al. obtained fluorescence excitation spectra of Xe clusters with varied average cluster sizes (see Figure 36).8
Figure 36 shows the fluorescence excitation spectra of Xe clusters formed under various supersonic free jet conditions.8 Woermer et al. claim that the fluorescence excitation spectra obtained can be considered as absorption spectra since the spectrum obtained for the largest cluster size of N =2000 is very similar to the one obtained for the solid Xe film. The average size of the clusters in the beam has been estimated from the empirical scaling law of cluster sizes found by Hagena and Overt140, Buck et al.141, and Farges et al.142 As the average size of the clusters become larger, new band features characteristic to solid Xe, i.e. exciton bands, are observed. We have measured direct absorption spectra of Xe clusters formed in free jets,143 which agrees fairly well with fluorescence excitation spectra shown in Figure 36 except for small difference in relative intensities.
Disregarding the minor corrections the fluorescence excitation spectra can be used as absorption spectra when one wishes to discuss the band energies.8 The sharp bands at 8.34 (147.0), 9.57 (129.6), 10.04 (119.2), 10.59 eV (117.0 nm) are atomic resonance lines assigned to the excitations to 6s(3/2)p 6s(l/2)p 5d(3/2)p and 7s(3/2)j states, respectively. The shoulder bands at 8.36 (148.3) and 8.47 eV (146.4 nm) are well-known dimer transitions from the ground state to the bound 0*-state and the repulsive lu-state which are correlated with 6s(3/2)j atomic state. A band at 9.52 eV (130.2 nm) is also assigned to Xe2 dimer. Strong bands at 8.37 (148.1) and 9.56 eV (129.7 nm) are assigned to clusters since their intensities increase with increasing the stagnation pressure. The broad feature ranging from 9.4 to 10.8 eV should be assigned to cluster transitions probably because a large number of forbidden transitions between 9.7 and 10.6 eV can become allowed upon cluster formation. Small clusters are reasonably transparent in the region between 8.6 and 9.4 eV, but as the pressure is raised, i.e. the cluster size become larger, broad absorption becomes visible. For average cluster sizes larger than about 150, two bands become apparent at 9.12 and 9.19 eV . These bands correspond to exciton transitions at 9.07 and 9.19 eV,
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respectively, observed for annealed solid films.144146 The spectral profile observed in the solid Xe film illustrated in Figure 36 (l)147 is somewhat different from that in Figure 36 (k) for average cluster size of about 570. The former resembles more to that obtained from the direct absorption measurement shown in the following section (see Figure 39a),143
especially in the shorter wavelength region. In fact compared with the direct absorption spectra emission spectra tend to give lower quantum efficiencies for fluorescence as the wavelength becomes shorter, in spite of the claim by Woermer et al. that the fluorescence quantum yield is constant with exciting wavelength.8
Figure 36. Fluorescence excitation spectra (Aobsd = 112-300 nm) of Xe beams for stagnation pressures P0 ranging from (a) 0.2 bar to (k) 2.5 bar in comparison with (1) an absorption spectrum of a thin film (d = 38 A) of solid Xe. The average cluster sizes estimated from the scaling law are also shown for each pressure P0- The positions of the excitons are also indicated
in the figure. T = 200 K, nozzle diameter d = 80 Jim. Spectral resolution was 0.25 nm.8
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I. Excimer Formation Reactions Involving Photoexcited Species
Electronically excited atoms and molecules are known to have quite different reactivities from those of the ground state. Excimer formation reactions have been extensively studied for metastable rare gas atoms, because of the importance of their roles played in generation of rare gas excimer lasers widely used. Metastable rare gas atoms Rg are known to react with dihalogens or halogen containing molecules via a harpooning mechanism148
Rg(3P0t2) + RX -> Rg+X- + R. (24)
in a similar manner to the reactions of alkali atoms with halogen molecules. When molecules are excited by photoexcitation, the initially prepared states are optically allowed and different from the metastable states. Since optically prepared states are shorter lived than metastable states and thus it is harder to study chemical reactions with photoexcited molecules. On the other hand, molecular excited states can be more readily prepared by photoexcitation and therefore the knowledge on their reaction behaviors has grown since synchrotron radiation and VUV lasers became available. Zimmerer and his collaborators149,
150 who use SR as a light sources at HASYLAB, DESY and ACO and Donovan and his coworkers151-152 have been studying the secondary photochemical processes of atoms and molecules excited by photoexcitation using synchrotron radiation. The important findings revealed by these studies are (i) that photoexcited diatomic molecules can undergo chemical reactions forming rare gas-halide excimers from the general formula:
RX* + Rg -> Rg+X- + R, (Rg = Ar, Kr, Xe ) (25)
where RX is either a dihalogen molecule or hydrogen molecule,153 in addition to (ii) the harpooning reactions similar to the reactions in Eq. 24:148
Rg*+ RX -> Rg+X- + R, (Rg = Ar, Kr, Xe) (26)
where Rg* represents an optically excited rare gas atom. Other important routes which excited atoms and molecules undergo are quenching processes such as collision-induced surface hopping process:
d a ^ Z I ) + Rg-> OJpnj + Rg. (27)
Synchrotron radiation is not the only light source to study chemical reactions involving atoms and molecules excited above 6 eV, but multiphoton excitation (MPE) processes are often applied in relation to the studies of the photo-induced reaction or laser-assisted association initiated by Setser and his coworkers.154 In this case similar processes (Eqs. 25, 26) can occur. However note that two photon excitation can produce different excited states from those by single photon excitation.1540 The results obtained by MPE techniques will be discussed in §1.2. in relation to the photochemistry of Xe-Cl2 molecular complex .
1. Gas Cell Experiments
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The gas cell photochemistry of Rg/Cl2 system was first studied using synchrotron radiation by Castex et al.155*156 and a series of excimer formation reactions have been studied, from which rate constants of excimer formation, quenching and collisional mixing of the excimer states have been determined.149"153 For the summary of the kinetic studies of excited states produced by synchrotron radiation see the review article by Zimmerer (Ref. 150). The dependence of the excimer formation rate upon molecular vibronic states has been determined for the reaction149
Cl2*(2!z;,v) + Ar -> Ar+C1(B,C) + Cl. (28)
As far as the excimer formation rates are concerned the excited rare gas atoms seem to produce more excimers than the excited chlorine atoms do upon reaction with rare gas atoms. However more studies should be made to determine reaction rates for each process, although some fluorescence decay measurements have been carried out.
Rg + H2 + hv (Rg = He, Ne, Ar, Kr, Xe)
The ArH(B2IT -> X2Z+) emission has been observed by Jones in a discharge of gas mixture of H2 and Ar.157 Similar emission was expected from HeH* excimer from theoretical calculations,158*159 The excited states are predicted to be bound, whereas the ground state potential is of repulsive nature. Figure 37 illustrates the potential energy curves for the selected states of HeH and H2. Moeller, Beland, and Zimmerer observed fluorescence in the 200-500 nm region when dispersed synchrotron radiation is irradiated into a gas mixture of rare gas and H2 through an In film of 100 nm thick.153-160
The excitation spectrum of near-UV fluorescence (180 - 500 nm) produced in the mixtures of 0.3 Torr K^ and 10 to 20 Torr of rare gas are shown in Figure 38 along with
Figure 37. Potential energy curves for selected states of HeH and H2. Data are taken from Refs. 159, 161. The excitation of H2 is indicated by an arrow. The horizontal line denotes the reaction pathway. The HeH transitions which are responsible for the broad continuum are indicated by arrows.
(Taken from Ref. 160.)
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that from pure H2 at 5 Torr (the strong band at 95.8 nm is assigned to fluorescence from N2 impurity). The fluorescence has been assigned to RgH(£2n and/or A2Z -> X2L+) originated from the reaction of electronically excited H / molecule with rare gas atom.
H2* (ft V u or Oil, ) + Rg -> RgH* + H (29)
Assuming that the onset for the emission corresponds to the threshold energy for forming HeH(Z?2n) excimers, the dissociation energy of HeH (B2YI) is determined to be 2.19 ± 0.03 eV from the difference between He+H(2P) +H and the onset energy, which energy agrees well with the theoretical values of 2.20 eV159 and 1.94 eV.158 In the similar manner the dissociation energies for other rare gas-hydride excimers have been determined as NeH*(1.96 ± 0.03 eV), ArH*(3.11 ± 0.03 eV), KrH*(3.19 ± 0.05 eV), and XeH*( >3.5 eV).160
The total cross section, crf, for the reaction
H ^ C ' n ^ v ^ ) + He -> HeH(52n->X2E+) + H (30)
has been roughly estimated as crf = 1.6± lA by comparing the intensities of reactant H2* and product HeH fluorescence. The reaction system H2(C!nu,) + He has one more electron of a 2p (A = 1) character than the H2
+ + He reaction system, retaining the 2p (A = 1) character after the reaction. The H2
+ + He reaction is endoergic and thus it is well known
Figure 38. Excitation spectra of near-UV and visible fluorescence of pure H2 (5Torr) and H2 /rare gas mixtures. Band heads of the H2 transition XlL+ ->
Bll£ or C!riu are indicated.
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that the reaction cross section varies very much with the collision energy and vibrational energy level of the H2
+ ion. It is interesting to note that the cross section af = 1.6+1 A for reaction (29) is of comparable order with that for H2+(v=2)+ He -> HeH++ H, a ~ 0.5 A at low collision energies.162
The radiative decay constant of the fluorescence from NeH* excimer formed from H2 (C1nu,v'=2) + Ne reaction depends on excitation energy, which indicates that two different excited states emit. However no fluorescence for RgH(A-»X) transition (Rg= He, Ar, Kr, and Xe) from RgB*(A) radicals has been observed probably due to fast predissociation. Since the first observation of continuous158 and discrete163 emission spectra from excited HeH* molecules rich experimental and theoretical information has been accumulated on the excited HeH* molecule as well as other rare gas hydride molecules.164
2. Photochemistry of Molecular Complexes Formed in Free Jet Expansion
Spectroscopy of molecular complexes has been extensively studied from the view points of clarifying the problems of intermolecular energy transfer processes and solvent effect upon relaxation dynamics of electronically excited molecules.165 Photochemical studies of van der Waals molecules have been made by laser induced fluorescence spectroscopy.166
The most well-studied systems are the complexes containing a Hg atom since photosensitized reactions by Hg atoms are well known. The relation between structure of a van der Waals molecule involving a mercury atom, such as Hg-H2
167 or Hg-X2,168 and its reactivity has been rather extensively studied.
In the VUV region not much work has been done except for 193 nm laser excitation of molecular complexes present in the gas cell.169171 Two-photon excitation studies of rare gas-dihalogen Xe-X2 complexes formed in free jet expansion have been carried out by a French group for X = Br172and Cl.173,174 In the case of Xe-Cl2 complex the exciation spectra for the formation of XeCl*(B) and XeCl* (C) excimers174 are found to be different from each other, i.e. the spectrum for XeCl* (C) formation exhibits a very broad band, while that for XeCr(B) a finer structure, as shown in Figure 39. Boivineau et al. have examined emission intensity dependence upon laser power, stagnation pressure of mixed gas, and laser-irradiated position of the free jet in order to confirm that the observed emissions originate from two-photon excitation of 1:1 complex of Xe and Cl2. The formations of XeCl*(B,C) excimers are explained in terms of stepwise excitation processes ofXe-Cl2 complex as follows:
hv hv Xe-Cl2 -> X e - C U m j -> Xt-C\2(lUg) -> Xe+Cl2" -> Xe-Cf(B, C) + Cl (31)
and hv
Xe-Cl2 -> Xe-Cl2(l1nu) ->Xe-Cl (X) + Cl (32a)
hv Xe-Cl(X) -> Xe-C1*(B). (32b)
hv hv Xe-Cl2 -> Xe-Cl2(l1nu) -> Xt-C\2(lUg) -> Xe+Cl2" -> Xe-Cf(B, C) + Cl
hv Xe-Cl2 -> Xe-d2(l1nu) ->Xe-Cl(X) + Cl
hv Xe-Cl(X) -> Xe-C1*(B).
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Figure 39. Two-photon fluorescence excitation spectra observed at two wavelengths (a) Xobsd = 345 and (b) Xohsd = 308 nm of Xe...Cl2 complex formed in a free jet expansion of Xe(0.5%) and Cl2 (0.5%) in He. The stagnation pressure p0 is 18 atm and xlD = 30, where x is the distance between the nozzle exit and irradiation point and D (= 200 Jim) the nozzle diameter. Spectral resolution is 0.5 cm'1 (Taken from Ref.
174).
The explanations given by Boivineau et al. for the origins of the two different features174
are not necessarily straight-forward. Although the fluorescence excitation spectra have been observed, the absorption spectra corresponding to the excitation processes have not been obtained and thus there is some uncertainty concerning from which and to which state molecular complexes are excited by the second photon. Therefore it was not possible to estimate what is the quantum yield for excimer formation channel from the excited molecular complexes. From the dispersed fluorescence spectrum, however, an important implication has been given as to the mechanism for intramolecular reaction of two-photon excited Xe-Cl2 complex, that is, the highest vibrational level of XeCl* excimer is only v' = 10.173 This is in sharp contrast with the case of metastable Xe* atom collisions with Cl2, where the excimers formed are vibrationally excited up to levels as high as v' ~ 100, since a product molecule is formed while the Xe-Cl distance is still long.
In order to obtain some information on the excited states of Rg—Cl2 complexes and also on the quantum yield for excimer formation channels, we have made direct measurement of the absorption spectra of what we believe are due to Xe—Cl2143,175 and Kr—Cl2176
complexes formed in a free jet expansion as well as the observations of their fluorescence excitation spectra. For this experiment the same free jet apparatus has been used as the one applied to the spectral measurement of benzene clusters. Figure 40a illustrates the
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absorption spectrum of a free jet of pure Xe at a stagnation pressure p0 = 440 Torr and a temperature of T0 = - 6°C. The I* parameter used for estimating the average cluster size is given by the formula:160
* K D p0 f~~. 1 - T2.29 » \JJ)
where K is a condensation parameter (K = 5500 for Xe), p0 the stagnation pressure in mbar, and T0 the stagnation temperature, r* is determined as I* = 1393. Disregarding the nozzle shape one can determine the average cluster size as large as 280 from the empirical relation between the r* value and average cluster size.160 When a small amount (5.7 %) of Cl2 gas is added to the Xe flow, dramatic changes in the absorption spectrum are observed as shown in Figure 40b. They are 1) a dramatic reduction in the intensities of cluster bands, denoted as P, a band appearing around 148 nm and a broad feature spanning the region from 132 to 115 nm, 2) the appearance of a broad, strong absorption feature from 145 to 120 nm which is assigned as absorption by Xe—Cl2 complex, 3) the appearance of sharp and broad bands below 140 nm, which are assigned to absorption by free Cl2 molecule, and 4) a slight increase in the Xe monomer (denoted as M) intensities.
In order to extract the broad absorption feature which is assigned to Xe—Cl2 complex, a similar procedure applied to the separation of benzene cluster bands has been taken (see
Figure 40. Absorption spectra of free jets of (a) pure Xe at a stagnation pressure p0 = 440 Torr, and (b) a gas mixture of 5.7% Cl2 in Xe atp0 = 550 Torr, at a stagnation temperature of T0 = -6.0°C. The spectral resolution is 1.5 A. M, D, and P denote monomer (atomic), dimer, and
cluster absorption, respectively.
KD0Mp0 T2.29 r*=̂ (33)
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§III-H-1). That is, the complex band is obtained after subtracting the absorption spectrum of the Cl2 free jet from that of Xe/Cl2 free jet by multiplying an appropriate scaling factor so that the sharp Cl2 bands disappear in the final spectra. In fact in this experiment Ne gas was used as a carrier gas to avoid formation of larger clusters, such as Xen and or Xen-Cl2 (n >2), and yet to effectively cool the free jet. Figure 41 illustrates the separated absorption spectra for Xe-Cl2 vdW complex thus obtained under various stagnation conditions.175 In these spectra we find two broad bands peaked at around 143 and 129 nm. Although the spectra are not shown here, similar absorption profiles have been observed for the Kr/Cl2 free jet, which are also assigned as Kr-Cl2 complex.176 This is probably because the transition is mostiy due to Cl2 moiety rather than Xe or Kr.
Figure 41. Absorption spectra of Xe-Cl2 vdW complex in free jets of 02/Xe/Ne gas mixtures under various gas mixing ratios and stagnation conditions, a) [C1J: [Xe]: [Ne] = 1.0: 3.4: 14.5, p0= 560 Torr, and T0.= 26.4°C; b) [CIJ: [Xe]: [Ne] = 1.0: 3.4: 14..6, p0= 530 Torr, and T0.= 9.0°C; c) [CIJ: [Xe]: [Ne] = 1.0: 10.1:4.8, pQ= 590 Torr, and 70.=
8.0°C. The hatched bands are due to Xen (n> 2). The spectral resolution is 0.3 nm.
Figure 42 illustrates the corresponding fluorescence excitation spectra, observed when photons emitted above 180 nm are detected.143 Although the abscissas are different between the Xen cluster bands shown in Figure 36 and Figure 42a, one can see that the absorption and fluorescence profiles for the pure Xe free jet are very similar to each other other, especially when the spectra for the comparable average cluster sizes N ~ 300 are compared. The bands assigned to atomic (M) and dimer (D) bands in absorption are not observed in fluorescence excitation spectrum in Figure 42a, since the PMT detector (HP R585) applied is not sensitive to Xe atomic fluorescence in the VUV region (naturally the atomic emissions in the VUV region are detected when a PMT with an MgF2 window, HP model R585MGF2, is used).176
Our interest resides in the fluorescence excitation spectrum obtained for the free jet of Xe/Cl2 mixture, i.e. Figure 42b. Comparing the fluorescence excitation spectrum in Figure
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40b for a pure Xe free jet with Figure 42b, for which a gas mixture of 5.7 % Cl2 in Xe is expanded, one notes the following points: (i) Emissions from excited Xe atom (M), Cl2 molecule and Xe2 (D) are pronounced very
much. The emissions corresponding to the transitions by Xe atom and Xe2 dimer are reasonably assigned to the excimer formation reaction of the excited Xe* atom with Cl2, which proceeds in the supersonic flow:
Xe*+ Cl2 -» Xe+Cl- (B, C)+ Cl. (34)
(ii) Emission intensities corresponding to the broad absorption feature spanning the region from 145 to 120 nm are quite low in spite of its strong absorption.
A similar trend has been observed for the Kr—Cl2 vdW complex.175 Even when the emissions are detected by a photomultiplier (HP model R585MGF2) which is sensitive in the 125 - 650 nm region, the general feature does not change. Therefore one has to conclude that the electronically excited (Rg—Cl2 )* vdW complex is very easily quenched, probably ending up mostly in non-emitting product channels, such as Xe + 2C1 after curve-crossing to the repulsive states.
Although the intensity of emission from the photoexcited vdW complex is low, one can observe fluorescence excitation spectra in the excitation wavelength region, in particular between 145 and 135 nm (see in Figure 43). Figure 43a is observed when the emissions
Figure 42. Fluorescence excitation spectra of free jets of (a) pure Xe and (b) a gas mixture of 5.7% Cl2 in Xe expanded under the same stagnation conditions as described in Figure 40. The photons emitted in the wavelength region from 650 nm to 180 nm are detected by HP PMT
(Model R585).
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Figure 43. Fluorescence excitation spectra of free jets of Cl2 in Xe under two photodetection conditions. A Hamamatsu PMT model R585MGF2 is used, a) Spectrum obtained without a filter, that is A,obgd = 125 - 650 nm. Stagnation condition; the gas mixing ratio is 4.6% Cl2 in Xe,p0= 265 Torr and 7b = 1°C. b) Spectrum obtained through a band-pass filter when emissions in the 7^isd = 230 -400 nm region are detected. Stagnation conditions; the gas mixing ratio is 2.4% Cl2
in Xe, p0 = 265 Torr and 7b = 2°C.
from all possible emitting species, such as excited Xe* and Cl2*, and XeCF(B,C) excimers, are detected in the A^a = 125 - 650 nm region. In fact the intensity level of emission in the 145 - 140 nm region is low. The quantum yields for formation of emitting species are found to be less than 5% in the broad range of excitation wavelength. Since we noted a weak emission band at 142.5 nm in Figure 42b and 43a, measurements have been made in this range for a longer counting time using a band-pass filter which transmits emissions in the X„bgd = 230 - 400 nm region as shown in Figure 43b. We find a vibrational structure with a spacing of about 640 cm1, which structure may be assigned as the transition to either the excited Xe-Cl-Cl*(v') state of Rydberg character or the ion-pair [Xe+—Cl2] state. Recently Bieler and Janda have determined the structure of the Xe—Cl2 complex in the ground electronic state to be T-shaped,177 as for HeCl2, NeCl2, and ArCl2 complexes.178
One of the reasons why the electronically excited Rg-Cl 2 vdW complex does not efficiendy form rare gas halide (Rg+Cl)* excimers may be that (i) the optically excited state has a [Rg--Cr-Cl] type of ion-pair character, keeping the T-shaped conformation, and (ii) thus, to form (Rg+Cl*)* excimer with an ion-pair character, two steps of internal conversions from [Rg-Cl+-Cl] to [Rg+--Cl-Cl] and then to (Rg+Cl)*+Cl should take place.
Since the Cl2 molecule has many excited states in the 6.6-11 eV region,20149179 the excitation energy of the Rg-Cl2 vdW complex may be easily transferred or absorbed by Cl2 moiety and then the complex will end up in fragmentation. On the other hand fairly
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extensive studies have been made on photodissociation dynamics of halogen molecules doped in rare gas matrices by Apkarian and his coworkers.180184 It has been shown that, when F2 molecule is excited to the repulsive state of the first excited state, it can be easily decomposed in Ar matrix with almost a unit quantum yield.180,181 Fajardo and Apkarian have reported that two-photon excitation of dihalogen X2 in Xe matrix induces hapooning reactions:
Xe + X2 + 2hv -> [Xe+X2 ■] -> Xe+X + X, (35)
and then the ion-pair product Xe+X" forms triatomic xenon halide exciplexes Xe2+X", which
provide emission spectra centered at 390, 480, 775 nm for X = Br, Cl, and F, respectively.182,183 We have measured absorption and fluorescence excitation spectra of Cl2 doped Xe clusters, Xen—Cl2, by adding a small amount of Cl2 (1.2%) to pure Xe flow.185 The absorption spectrum is almost identical with that of pure Xe clusters formed under the similar jet conditions, whereas the fluorescence intensity levels for the Cl2 doped cluster bands are found to be very low compared with those of pure Xe cluster bands, indicating that photoexcitation energy is efficiently quenched by chlorine impurity in a cluster. Guertler et al. have reported fluorescence excitation spectra of Cl2 in Ar and Ne matrices with lifetime measurements.186 Examining all these results, we have a feeling that it would be worthwhile to observe both absorption and fluorescence excitation spectra as well as dispersed emission spectra to obtain information on the quantum yield for each channel, if experimentally possible. As to the VUV photochemistry of 1:1 Rg--dihalogen vdW complexes, probably F2 and C1F molecules should be used as a moiety of vdW complexes, instead of Cl2, since energy levels of their excited states are high. Then intramolecular reactions inside the electronically excited rare gas-dihalogen (Rg-XY) complexes may lead to more efficient excimer formation.
As long as SR is used as a light source, it is experimentally difficult to detect very low number-density of non-emitting Cl atoms formed from photodecomposition of Xe-Cl2 vdW complex. Therefore it would be worthwhile to study this system using lasers, since it is expected that more detailed information will be attained on the dynamics of the chemical reaction.
Since synchrotron light sources became widely accessible, the knowledge on the vacuum UV photochemistry of molecules and molecular clusters has expanded very much. However there are still a lot to be learned about the detailed outcomes of the electronically excited molecules, not to mention molecular complexes and clusters, in terms of multi-potential energy surfaces, especially by using VUV lasers.
Acknowledgements
The authors thank the staff of UVSOR facility for the great support in conducting research described in the present review. They greatly acknowledge fruitful collaborative research with Prof. K. Kuchitsu (now at Nagaoka Univ. of Tech.), Prof. T. Kondow and Dr. T. Nagata of the Univ. of Tokyo, Prof. R. J. Donovan of Univ. of Edinburgh, Prof. I. Tokue of Niigata University, and Prof. C. Y. Ng of Iowa State University.
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References
1. T. Ibuki, N. Takahashi, A. Hiraya, and K. Shobatake, / . Chem. Phys., 85 (1986) 5717.
2. L. C. Lee, L. Oren, E. Phillips, and D. L. Judge, / . Phys., B l l (1978) 47. 3. T. Ibuki and N. Sugita, / . Chem. Phys., 79 (1983) 5392. 4. T. Ibuki and N. Sugita, / . Chem. Phys., 80 (1984) 4625. 5. A. Hiraya and K. Shobatake, / . Chem. Phys. (1991) (in press). 6. A. Hiraya and K. Shobatake, Activity Report of UVSOR, 1986, 20. 7. J. Stapelfeldt, J. Woermer, and J. Moeller, Phys. Rev. Lett., 62 (1989) 98. 8. J. Woermer, V. Guzielski, J. Stapelfeldt, and T. Moeller, Chem. Phys. Lett., 159
(1989) 321. 9. U. Fano, and J. H. Macek, Rev. Mod. Phys., 45 (1973) 553.
10. Greene and R. N. Zare, Ann. Rev. Phys. Chem., 33 (1982) 119. 11. T. Nagata, T. Kondow, K. Kuchitsu, G. W. Loge, and R. N. Zare, Mol. Phys., 50
(19 83) 49. 12. J. P. Simons, / . Phys. Chem., 88 (1984) 187. 13. J. P. Simons, / . Phys. Chem., 91 (1987) 5378. 14. J. A. Guest, M. A. O'Halloran, and R. N. Zare, Chem. Phys. Lett., 103 (1984)
261. 15. T. Nagata, T. Kondow, K. Kuchitsu, K. Kanda, A. Hiraya, and K. Shobatake (to
be published). 15a. (a) M. T. Macpherson, J. P. Simons, Mol. Phys., 38 (1979) 2049; (b) see also S.
R. Leone, Advan. Chem. Phys., 50 (1982) 255. 16. K. Yoshino, D. E. Freeman, and Y. Tanaka, / . Mol. Spectrosc, 76 (1979) 153. 17. K. Yoshino, D. E. Freeman, and W. H. Parkinson, Appl. Opt., 19 (1980) 66. 18. a) K. Ito and T. Namioka, Rev. Sci. Instrum., 60 (1989) 1573; b) K. Ito, K.
Maeda, Y. Morioka, and T. Namioka, Appl. Opt., 28 (1989) 1813. 19. K. Maeda, K. Ueda, K. Ito, and T. Namioka, Phys. Scripta, 41 (1990) 464. 20. T. Moeller, B. Jordan, P. Guertler, G. Zimmerer, D. Haaks, J. Le Calve, and M. C.
Castex, Chem. Phys., 76 (1983) 295. 21. L. C. Lee and M. Suto, and K. Y. Tang, / . Chem. Phys., 84 (1986) 5277. 22. E. Kerr, M. MacDonald, R. J. Donovan, J. P. T. Wilkinson, D. Shaw, I. Munro, / .
Photochem., 31 (1985) 1. 23. R. J. Donovan, G. Gilbert, M. MacDonald, I. Munro, S. Shaw, and G. R. Grant,
Chem. Phys. Lett., 109 (1984) 379. 24. R. J. Donovan, B. V. O'Grady, K. Shobatake, and A. Hiraya, Chem. Phys. Lett.,
122 (1985) 612. 25. A. Hiraya, K. Shobatake, R. J. Donovan, and A. Hopkirk, / . Chem. Phys., 88 (19
88) 52. 26. J. B. Nee, M. Suto, and L. C. Lee, / . Chem. Phys., 85 (1986) 719. 27. J. B. Nee, M. Suto, and L. C. Lee, J. Chem. Phys., 83 (1985) 2001. 28. L. C. Lee,/. Chem. Phys., 72 (1980) 4334. 29. L. C. Lee and M. Suto, / . Chem. Phys., 80 (1984) 4718. 30. L. C. Lee and C. C. Liang, J. Chem. Phys., 16 (1982) 4462. 31. L. C. Lee, G. Black, R. L. Sharpies, and T. G. Slanger, / . Chem. Phys., 73 (1980)
256. 32. L. C. Lee, / . Chem. Phys., 72 (1980) 6414. 33. M. T. Macpherson and J. P. Simons, / . Chem. Soc. Faraday 2, 74 (1978) 1965. 34. A. Mele and H. Okabe, / . Chem. Phys., 51 (1969) 4798. 35. M. Suto and L. C. Lee, J. Chem. Phys. 78 (1983) 4515. 36. L. C. Lee and C. C. Chiang, J. Chem. Phys., 78 (1983) 688. 37. M. Suto and L. C. Lee, / . Chem. Phys., 84 (1986) 1160.
Vac
uum
Ultr
avio
let P
hoto
ioni
zatio
n an
d Ph
otod
isso
ciat
ion
of M
olec
ules
and
Clu
ster
s D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/31
/14.
For
per
sona
l use
onl
y.
558
38. N. Washida, Y. Matsumi, T. Hayashi, T. Ibuki, A. Hiraya, K. Shobatake, / . Chem. Phys., 83 (1985) 2769.
39. M. Suto and L. C. Lee, / . Chem. Phys., 80 (1984) 4824. 40. M. Suto and L. C. Lee, / . Chem. Phys., 79 (1983) 1127. 41. M. Suto and L. C. Lee, / . Geophys. Res., 90 (1985) 13037. 42. T. Tokue, A. Hiraya, and K. Shobatake, Chem. Phys., 116 (1987) 449. 43. T. Tokue, A. Hiraya, and K. Shobatake, Chem. Phys. Lett., 153 (1988) 346. 44. T. Tokue, A. Hiraya, and K. Shobatake, Chem. Phys., Ill (1987) 315. 45. J. B. Nee, M. Suto, and L. C. Lee, Chem. Phys., 98 (1985) 147. 46. T. Ibuki, A. Hiraya, and K. Shobatake, / . Chem. Phys., 90, (1989) 6290. 47. T. Ibuki, A. Hiraya, K. Shobatake, Y. Matsumi, and M. Kawasaki, J. Chem.
Phys., 92, (1990) 4277. 48. T. Ibuki, A. Hiraya, and K. Shobatake, (unpublished results). 49. C. E. Moore, Atomic Energy Levels, Nat. Stand. Ref. Data Ser. 35 (Nat. Bur.
Stand. U.S. GPO, Washington DC, 1971). 50. M. B. Robin, Higher Excited States of Polyatomic Molecules (Academic Press, New
York), Vol.1 (1974), Vol.2 (1975), and Vol.3 (1985). 51. W. Zhang, G. Cooper, T. Ibuki, and C. E. Brion, Chem. Phys., 137 391 (1989). 52. J. Berkowitz, Photoabsorption, Photoionization, and Photoelectron Spectroscopy
(Academic, New York, 1979). 53. J. A. Merer and D. N. Travis, Can. J. Phys., 44 (1966) 1541. 54. R. I. Patel, G. W. Stewart, K. Casleton, J. C. Gole, and J. R. Lombardi, Chem.
Phys., 52 (1980)461. 55. I. Novak, T. Cvitas, L. Klansinc, and H. Guesten, / . Chem. Soc. Faraday 2, 77
(1981) 2049. 56. D. S. King, P. K. Shenk, and J. C. Stephenson, / . Mol. Spectrosc, 78 (1979) 1. 57. H. Hack and W. Langel, / . Phys. Chem., 87 (1983) 3462. 58. M. Suzuki and G. Inoue, private communication. 59. M. T. Ngugen, M. C. Kerins, F. A. Hegartz, and N. J. Fitzpatrick, Chem. Phys.
Lett., Ill (1985) 295. 60. J. C. Slater, Quantum Theory of Matter (McGraw-Hill, New York, 1968). 61. A. M. W. Bakke, / . Mol. Spectrosc, 41 (1972) 1. 62. A. Gedanken, M. B. Robin, and N. A. Kuebler, Inorg. Chem., 20 (1981) 3340. 63. C. J. Chen and R. M. Osgood, / . Chem. Phys., 81 (1984) 327. 64. C. F. Yu, F. Youngs, K. Tsukiyama, and R. Bersohn, / . Chem. Phys., 85 (1986)
1382. 65. A. Amirav, A. Penner, and R. Bersohn, / . Chem. Phys., 90 (1989) 5232. 66. M. Suto, C. Ye, and L. C. Lee, / . Chem. Phys., 89 (1989) 160. 67. T. Ibuki, A. Hiraya, and K. Shobatake, / . Chem. Phys., 92 (1990) 2797. 68. J. C. Tse, G. M. Bancroft, and D. K. Creber, / . Chem. Phys., 74 (1981) 2097 and
references therein. 69. P. J. Young, P. K. Gosavi, J. Conner, O. P. Strauz, and H. E. Gunning, / . Chem.
Phys., 58 (1973) 5280. 70. H. Okabe, / . Chem. Phys., 53 (1970) 3507. 71. K. Uno, T. Hikida, A. Hiraya, and K. Shobatake, Chem. Phys. Lett., 166 (1990)
475. 72. W. S. Drozdoski, A. P. Baronavski, and J. R. McDonald, Chem. Phys. Lett., 64
(1979) 421. 73. G. T. Fujimoto, M. E. Umstead, and M. C. Lin, Chem. Phys., 65 (1982) 197. 74. N. Presser, Y. F. Zhu, and R. J. Gordon, / . Phys. Chem., 91 (1987) 4383. 75. T. A. Spiglamin, R. A. Perry, and D. W. Chandler, / . Chem. Phys., 90 (1986)
6184. 76. T. Hikida, M. Maruyama, Y. Saito, and Y. Mori, Chem. Phys., Ill (1988) 63.
Vac
uum
Ultr
avio
let P
hoto
ioni
zatio
n an
d Ph
otod
isso
ciat
ion
of M
olec
ules
and
Clu
ster
s D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/31
/14.
For
per
sona
l use
onl
y.
559
77. H. K. Haak and F. Stuhl, / . Phys. Chem., 88 (1984) 3627. 78. H. Umemoto, K. Kikuma, S. Tsunashima, and S. Sato, Chem. Phys., 120 (1988)
461. 79. H. Okabe, / . Chem. Phys., 49 (1968) 2726. 80. H. Okabe, Photochemistry of Small Molecules (Wiley, New York, 1978). 81. H. Okabe and M. Lenzi, / . Chem. Phys., 47 (1967) 5241. 82. N. Washida, G. Inoue, M. Suzuki, and O. Kajimoto, Chem. Phys. Lett., 114
(1985) 274. 83. C. Y. R. Wu, / . Chem. Phys., 86 (1987) 5584. 84. K. Tabayashi and K. Shobatake, / . Chem. Phys., 84 (1986) 4930. 84a. a) K. -H. Gericke, R. Theinl, and F. J. Comes, Chem. Phys. Lett., 164 (1989)
605; b) K.-H. Gericke, R. Theinl, and F. J. Comes, / . Chem. Phys., 92 (1990) 6548.
84b. a) B. R. Foy, M. P. Casassa, J. C. Stephenson, and D. S. King, / . Chem. Phys., 89 (1988) 698; b) J. C. Stephenson, M. P. Casassa, and D. S. King, / . Chem. Phys., 89 (1988) 1378.
85. J. P. Simons and A. J. Smith, Chem. Phys. Lett., 97 (1983) 1. 86. K. Kanda, T. Nagata, T. Kondow, K. Kuchitsu, A. Hiraya, and K. Shobatake
(unpublished results). 87. G. Herzberg, Molecular Spectra and Molecular Structure. III. Electronic Spectra
and Electronic Structure of Polyatomic Molecules (Van Nostrand, New York, 1966) p. 493.
88. P. J. Robinson and K. A. Holbrook, Unimolecular Reactions (Wiley-Interscience, New York, 1972) Chapt. 9.
89. B. S. Rabinowitch and D. W. Setser, Adv. Photochem., 3 (1964) 1. 90. R. A. Marcus, / . Chem. Phys., 20 (1952) 359. 91. J. Delfosse and J. A. Hippie, Jr., Phys. Rev. 54 (1935) 1060. 92. M. W. Evans, N. Bauer, and J. Y. Beach, / . Chem. Phys., 14 (1946) 701. 93. K. Shobatake, A. Hiraya, S. Ohshima, Y. Matsumoto, and K. Tabayashi (unpub
lished results). 94. H. Okabe and V. H. Dibeler, / . Chem. Phys., 59 (1972) 2430. 95. M. N. R. Ashold and J. P. Simons, / . Chem. Soc. Faraday 2, 74 (1978) 1263. 96. M. T. Macpherson and J. P. Simons, Chem. Phys. Lett., 51 (1977) 261. 97. L. C. Lee and M. Suto, Chem. Phys., 110 (1986) 161. 98. A. Hiraya and K. Shobatake, IMS Ann. Rev., 1986, (unpublished results). 99. A. Hiraya, T. Ibuki, and K. Shobatake (to be published).
100. S. L. Guberman, / . Chem. Phys., 78 (1983) 1404. 101. M. Leventhal, R. T. Robiscoe, and K. R. Lea, Phys. Rev., 158 (1967) 49. 102. S. Arai, T. Kamosaki, M. Ukai, K. Shinsaka, Y. Hatano, T. Ito, H. Koizumi, A.
Yagishita, K. Ito, and K. Tanaka, / . Chem. Phys., 88 (1988)3016. 103. M. Glass-Maujean, / . Chem. Phys., 89 (1988) 2839. 104. M. Glass-Maujean, / . Chem. Phys., 85 (1986) 4830. 105. H. Bethe and E. E. Salpeter, in Handbuch der Physik, ed. by S. Flugge (Springer.,
Berlin, 1957), vol. 35, pp.355. 106. P. K. Carroll, Can. J. Phys., 36 (1958) 1585. 107. P. K. Carroll and A. C. Hurley, / . Chem. Phys., 35 (1961) 2247. 108. D. Cossart, F. Launary, J. M. Robbe, and G. Gandara, / . Mol. Spectrosc, 113
(1985) 142. 109. D. Cossart and F. Launary, / . Mol. Spectrosc, 113 (1985) 159. 110. A. C. Hurley, / . Mol. Spectrosc, 9 (1962) 18. 111. P. W. Thulstrup, E. W. Thulstrup, A. Andersen, and Y. Ohrn, / . Chem. Phys., 60
(1974) 3975. 112. K. Tohji, D. M. Hanson, and B. X. Yang, / . Chem. Phys., 85 (1986) 7492.
Vac
uum
Ultr
avio
let P
hoto
ioni
zatio
n an
d Ph
otod
isso
ciat
ion
of M
olec
ules
and
Clu
ster
s D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/31
/14.
For
per
sona
l use
onl
y.
560
113. B. X. Yang, D. M. Hanson, and K. Tohji, / . Chem. Phys., 89 (1986) 1215. 114. J. A. R. Samson, T. Masuoka, and P. N. Pareek, / . Chem. Phys., 83 (1985) 5531. 115. Y. Iida, F. Carnovale, S. Daviel, and C. E. Brion, Chem. Phys., 105 (1986) 211. 116. D. L. Cooper, Chem. Phys. Lett., 132 (1986) 377. 117. M. J. Besnard, L. Hellner, Y. Malinovich, and G. Dujardin. / . Chem. Phys., 85,
(1986) 1316. 118. D. Cossart, M. Bonneaw, and J. M. Robbe, / . Mol. Spectrosc, 125 (1987) 413. 119. M. J. Besnard, L. Hellner, and G. Dujardin, / . Chem. Phys., 89 (1988) 6554. 120. B. X. Yang, J. Kirz, Y. H. Kao, and T. K. Sham, Nucl. Instrum. Meth. A, 246
(1986) 523. 121. P. K. Huber and G. Herzberg, Molecular Spectra and Molecular Structure IV. Con
stants of Diatomic Molecules (Van Nostrand, New York, 1979). 122. N. R. Daly and R. E. Powell, Proc. Roy. Soc, 90 (1967) 629. 123. J. Appell, J. Dump, F. C. Fehsenfeld, and P. Fournier, / . Phys., B 6 (1973) 197. 124. K. Ueda, K. Maeda, K. Ito, and T. Namioka, / . Phys., B 22 (1989) L481. 125. U. Fano and J. C. Cooper, Rev. Mod. Phys., 40 (1968) 441. 126. a) K. Ueda, / . Opt. Soc. Am., B 4 (1986) 424; b) K. Ueda, Phys. Rev. A, 35
(1987) 2484. 127. M. Aymar, M. Robaux, and C. Thomas, / . Phys. B, 14, (1981) 4255. 128. W. R. Johnson, K. T. Cheung, K.-N. Haung, and M. Le Doumeuf, Phys. Rev.
A, 22, 989 (1980). 129. K. Ito, K. Ueda, T. Namioka, K. Yoshino, and Y. Morioka, / . Opt. Soc. Am. B, 5
(1988) 2006. 130. K. Ito, and T. Namioka, Rev. Sci. Instrum., 60 (1989) 1573. 131. G. Stark, K. Yoshino, P. L. Smith, K. Ito, and W. H. Parkinson, Astrophys. J.,
(in press). 132. K. Ito, Hoshako (J. Jpn. Soc. Synchro. Rod. Res.), 3 (1990) 11. 133. W. E. Ernst, T. P. Softleu, and R. N. Zare, Phys. Rev. A, 37 (1988) 4172. 134. V. Vaida, Ace. Chem. Res., 19 (1986) 114. 135. A. Hiraya and K. Shobatake, Chem. Phys. Lett., 178 (1991) 543. 136. H. Shinohara and N. Nishi, Chem. Phys., 129 (1987) 1786. 137. a) E. Pantos and T. D. S. Hamilton, Chem. Phys. Lett., 17 (1972) 588; b) E.
Pantos, A. M. Taleb, T. D. S. Hamilton, and I. H. Munro, Mol. Phys., 25 (1973) 1263; c) A. Brillante, C. Taliani, and C. Zauli, Mol. Phys., 25 (1973) 1263.
138. A. Hiraya and K. Shobatake, Collected Abstrcts of Molecular Structure Meeting (in Japanese), 1990, p. 81.
139. a) J. P. Reilly and K. L. Compa, / . Chem. Phys., 73 (1980) 5469; b) T. A. Gregory, F. Hirayama, and S. Lipsky, / . Chem. Phys. 58 (1973) 4697.
140. a) O. F. Hagena and W. Obert, / . Chem. Phys., 56 (1972) 1793; b) O. F. Hagena, Z. Phys.,D4 (1987)275.
141. U. Buck and H. Meyer, Surf Sci., 156 (1985) 275. 142. J. Farges, M. F. de Feraudy, B. Raoult and G. Torchet, / . Chem. Phys., 84
(1986) 3491. 143. K. Tabayashi, A. Hiraya, and K. Shobatake, UVSOR Activity Report. 1989, p. 6. 144. G. Baldini, Phys. Rev., 128 (1962) 1562. 145. G. Zimmerer, in Excited-State Spectroscopy in Solid, Ed. by U. M. Grassano and
N. Terzi (North-Holland, Amsterdam, 1987) p.37. 146. N. Schwentner, E.-E. Koch and J. Jortner, Electronic Excitations in Condensed Rare
Gases, (Springer-Verlag, Berlin, 1985). 147. V. Saile, Ph. D. Thesis, University of Munich (1976). 148. See for example a) M. F. Golde and A. Kvaran, / . Chem. Phys., 72 (1980) 442;
b) M. F. Golde and B. A. Thrush, Chem. Phys. Lett., 29 (1974) 486; c) K. Tamagake, J. H. Kolts, and D. W. Setser, / . Chem. Phys., 71 (1979) 1264.
Vac
uum
Ultr
avio
let P
hoto
ioni
zatio
n an
d Ph
otod
isso
ciat
ion
of M
olec
ules
and
Clu
ster
s D
ownl
oade
d fr
om w
ww
.wor
ldsc
ient
ific
.com
by U
NIV
ER
SIT
Y O
F B
IRM
ING
HA
M L
IBR
AR
Y -
IN
FOR
MA
TIO
N S
ER
VIC
ES
on 0
8/31
/14.
For
per
sona
l use
onl
y.
561
149. T. Moeller, B. Jordan, G. Zimmerer, D. Haaks, J. Le Calve, and M. -C. Castex, Z. Phys., D4 (1986) 73.
150. G. Zimmerer, in Photophysics and Photochemistry above 6 eV, Ed. by F. Lahmani (Elsevier, Amsterdam, 1985) p.p. 357.
151. B. V. O'Grady and R. J. Donovan, Chem. Phys. Lett., Ill (1985) 503. 152. J. P.T.Wilkinson, E. A. Kerr, K. P. Lawley, R. J. Donovan, D. Shaw, A.
Hopkirk, and I. Munro, Chem. Phys. Lett., 130(1986)213. 153. T. Moeller, M. Beland, and G. Zimmerer, Phys. Rev. Lett., 55 (1985) 2145. 154. See for example a) G. Inoue, J. K. Ku, and D. W. Setser, / . Chem. Phys., 80
(1984) 6006; b) J. K. Ku, G. Inoue, and D. W. Setser, / . Phys. Chem., 87 (1983) 2989; c) D. W. Setser and J. Ku, in Photophysics and Photochemistry above 6 eV, Ed. by F. Lahmani (Elsevier, Amsterdam, 1985) p.p. 621.
155. a) M. C. Castex, J. Le Calve, D. Haaks, B. Jordan, and G. Zimmerer, Chem. Phys. Lett.f 70 (1980) 106.
156. J. Le Calve, M. C. Castex, D. Haaks, B. Jordan, and G. Zimmerer, Nuov. Cimen-to, 63B (1981) 265.
157. J. W. C. Jones, J. Mol. Spectrosc, 36 (1970) 488. 158. H. H. Michels and F. E. Harris, / . Chem. Phys., 39 (1963) 1464. 159. G. Theodorakopoulos, S. C. Farantos, R. J. Buencker, and S. D. Peyerimhoff, / .
Phys., 817(1984) 1453. 160. T. Moeller, M. Beland, and G. Zimmerer, Chem. Phys. Lett., 136 (1987) 551. 161. T. E. Sharp, At. Data , 1 (1971) 119. 162. T. Turner, O. Dutuit, and Y. T. Lee, / . Chem. Phys., 81 (1984) 3475. 163. W. Ketterle, H. Figger, and H. Walther, Phys. Rev. Lett., 55 (1985) 2941. 164. W. Ketterle, / . Chem. Phys., 93 (1990) 6929, and references therein. 165. See references in a) M. Ito, T. Ebata, and N. Mikami, Ann. Rev. Phys. Chem., 39
(1988) 123; b) N. Nakashima and K. Yoshihara, / . Chem. Phys., 93 (1988) 7763. 166. C. Jouvet, M. Boivineau, M. C. Duval, and B. Soep, / . Phys. Chem., 91 (1987)
5416. 167. W. H. Breckenridge, C. Jouvet, and B. Soep, / . Chem. Phys., 84 (1986) 1443. 168. a) C. Jouvet and B. Soep, Chem. Phys. Lett., 96 (1983) 426; b) C. Jouvet and B.
Soep, / . Phys. (Les Ulis, Fr.), 46, Cl (1985) 313 . 169. V. S. Dubbov, Y. E. Lapsker, A. N. Samoilova, and A. N. Gurvich, Chem. Phys.
Lett., 83 (1981) 518. 170. H. P. Grieneisen, X.-J. Hu, K. L. Kompa, Chem. Phys. Lett., 82 (1981) 421. 171. B. E. Wilcomb and R. J. Burnham, / . Chem. Phys., 74 (1981) 6784. 172. M. Boivineau, J. Le Calve, M.C. Castex, and C. Jouvet, J. Chem. Phys., 84 (19
84) 4712. 173. M. Boivineau, J. Le Calve, M. C. Castex, and C. Jouvet, Chem. Phys. Lett., 128
(1986) 528. 174. M. Boivineau, J. Le Calve, M. C. Castex, and C. Jouvet, Chem. Phys. Lett., 130
(1986) 208. 175. K. Tabayashi, A. Hiraya, and K. Shobatake, UVSOR Activity Report. 1989, p. 8. 176. K. Tabayashi, A. Hiraya, and K. Shobatake (unpublished results). 177. C. R. Bieler and K. C. Janda, / . Am. Chem. Soc, 111 (1990) 2033. 178. a) K. C. Janda, Adv. Chem. Phys., 60 (1985) 201; b) D. D. Evard, J. I. Cline, and
K. C. Janda, / . Chem. Phys., 88 (1988) 5433, and references therein. 179. S. D. Peyerimhoff and R. J. Bunker, Chem. Phys., 57 (1981) 279. 180. J. Feld, H. Kunttu, and V. A. Apkarian, / . Chem. Phys., 93 (1990) 1009. 181. H. Kunttu, J. Feld, R. Alimi, and V. A. Apkarian, / . Chem. Phys., 92 (1990)
4856. 182. M. E. Fajardo and V. A. Apkarian, / . Chem. Phys., 85 (1988) 5660 183. M. E. Fajardo and V. A. Apkarian, / . Chem. Phys., 89 (1989) 4102. 184. R. Alimi, A. Brokman, and R. B. Gerber, / . Chem. Phys., 91 (1989) 1611.
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185. K. Tabayashi, A. Hiraya, and K. Shobatake, UVSOR Activity Report,. 1990 (to be published).
186. P. Guertler, H. Kunz, and J. Le Calve, / . Chem. Phys., 91 (1989) 6020.
Permissions of Using Figures
Achnowledgements are made to the authors and publishers who permitted the present authors to reproduce the figures used in this chapter.
The following figures are reproduced by the permissions by the American Institute of Physics and the authors: Figure 5 from Ref. 18a [K. Ito and T. Namioka, Rev. Sci. Instrum., 60 (1989) 1573]; Figures 6, 7, and 8 from Ref. 47 [T. Ibuki, A. Hiraya, K. Shobatake, Y. Matsumi, and M. Kawasaki, / . Chem. Phys., 92, (1990) 4277], Figures 9 and 10 from Ref. 67 [T. Ibuki, A. Hiraya, and K. Shobatake, ibid., 92 (1990) 2797], Figure 15 from Ref. 83 [C. Y. R. Wu, ibid., 86 (1987) 5584], Figures 24 and 27 [S. Arai, T. Kamosaki, M. Ukai, K. Shinsaka, Y. Hatano, T. Ito, H. Koizumi, A. Yagishita, K. Ito, and K. Tanaka, ibid., 88 (1988) 3016], Figure 25 from Ref. 103 [ M. Glass-Maujean, ibid., 89 (1988) 2839], Figure 26 from Ref. 104. [M. Glass-Maujean, ibid., 85 (1986) 4830.], Figures 30 and 31 from Ref. 113 [B. X. Yang, D. M. Hanson, and K. Tohji, ibid., 89 (1986) 1215].
The following figures are reproduced by the permissions by Elsevier Science Publishers and the authors: Figures 11-14 from Ref. 71 [K. Uno, T. Hikida, A. Hiraya, and K. Shobatake, Chem. Phys. Lett., 166 (1990) 475], Figure 16 from Ref. 14 [J. A. Guest, M. A. O'Halloran, and R. N. Zare, ibid., 103 (1984) 261], Figure 28 from Ref. 116 [D. L. Cooper, ibid., 132 (1986) 377], Figure 29 from Ref. 120 [ B. X. Yang, J. Kirz, Y. H. Kao, and T. K. Sham, Nucl. Instrum. Meth. A, 246 (1986) 523], Figures 34 and 35 from Ref. 135 [A. Hiraya and K. Shobatake, Chem. Phys. Lett., 178 (1991) 543], Figure 36 from Ref. 8 [J. Woermer, V. Guzielski, J. Stapelfeldt, and T. Moeller, Chem. Phys. Lett., 159 (1989) 321], Figures 37 and 38 from Ref. 160 [T. Moeller, M. Beland, and G. Zimmerer, ibid., 136 (1987) 551], and Figure 39 from Ref. 174 [M. Boivineau, J. Le Calve, M. C. Castex, and C. Jouvet, ibid., 130 (1986) 208].
Figures 32 and 33 are reproduced by the permissions by Royal Sweedish Academy of Sciences and the authors from Ref. 19 [K. Maeda, K. Ueda, K. Ito, and T. Namioka, Phys. Scripta, 41 (1990) 464].
Figure 23 is reproduced from a part of Fig. 2 of Ref. 101 [M. Leventhal, R. T. Robiscoe, and K. R. Lea, Phys. Rev., 158 (1967) 49] by the permission of the authors.
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