application of mpi photoblectbon spectroscopy to excited molecules. use of tunable vuv laser...
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Journal of Electron Spectroscopy and Related Phenomena, 51 (1990) 383-396 EleevierSciencePublishersB.V.,Amsterdam-PrintedinTheNetberlands
383
APPLICATION OF MPI PHOTOELECTRON SPECTROSCOPY TO EXCITED MOLECULES.
USE OF TUNABLE VUV LASER RADIATION
K. Kimura, M. Takahashi, K. Okuyama and I. Plazibat'
Institute for Molecular Science, Okazaki, 444 Japan
('Present address: Department of Natural and Mathematical Science, University of Split, Nikole Tesle 12, 58000 Split, Yugoslavia)
SUMMAEY
In this paper we demonstrate the applicability of VW pulsed laser radiation to photoelectron spectroscopy of electronically excited states as well as ground states. The VW lasers used here are based on: 1) four-wave sum mixing in Mg vapor giving continuous tunability from 140 to 160 nm; 2) third harmonic generation in Xe and Mg vapor (118.2 and 143.6 nm, respectively); and 3) an ArF excimer laser (193 nm). Two-color MPI experiments with these VW lasers were successfully carried out for a few test samples, such as NO, aniline, and pyrazine, clearly indicating that these VW pulsed lasers are promising future light sources.
1. INTRODUCTION
1.1 Photoelectron Spectroscopy
Since the pioneering work of Siegbahn et al.[l] and Turner et
a1.[21 in the 1960's, photoelectron spectroscopy has been
developed as one of main spectroscopic techniques of chemical
analysis in molecular and material sciences. During the last two
decades, much information about electronic structure of atoms,
molecules and materials in the ground state as well as about ionic
states has been obtained using VUV and X-ray photoelectron
spectroscopy.
Ionization potential data obtained from ordinary ground-state
photoelectron spectra have been very useful in interpreting
excited-state photoelectron spectra in the most laser multiphoton
photoelectron experiments. For example, a handbook published by
Kimura 131, which contains He1 photoelectron spectra and
ionization potentials of many fundamental organic molecules and
their ab initio assignments, has provided basic information on
low-lying ionic states necessary for interpretation MPI
photoelectron spectra.
036%2048/90/%03.50 0 199OElsevierSciencePublishersB.V.
384
1.2 MPI Photoelectron Spectroscopy
In atoms and molecules, multiphoton ionization (MPI) is
remarkably enhanced at special laser wavelengths where optical
resonances take place. Such resonance photoionization provides
high-resolution molecular MPI (ion-current) spectroscopy, as
developed by Johnson [4]. Since 1980, photoelectron spectroscopy
of resonant MPI has been developed with nanosecond W/visible
lasers to detect photoelectron spectra of excited-state molecules
in this laboratory [5,6] and others [7,8]. Studies so far
published on excited-state photoelectron spectroscopy have been
reviewed by Kimura [9,10], by Compton and Miller [ll], and by
Pratt et al. 1121. Furthermore, laser photoelectron spectroscopy
has been developed also for single-photon ionization of negative
ions [13].
1.3 Advantages of VW Lasers
Most photoelectron studies of resonant MPI have so far been
carried out with visible/UV pulse lasers [g-12]. In measurements
of excited-state photoelectron spectra, it is important to ionize
any excited states by single photons. If two or more photons are
absorbed by a specific excited state to be studied, further
resonance may occur with higher excited states, making spectral
interpretation difficult.
Therefore, single-photon excitation and ionization are
essential in the following cases, as shown in Fig. 1: Namely, (a)
single-photon ionization of a resonant low-lying excited state;
u+
n
. . . . .:. . . . ‘. . . :: . .
. .
ii
1 %I
(a) (b)
.::.. . . . . . . a.
. .
. .
. . . .
,. . . k . . . .
L % (4
Fig. 1. Schematic energy level diagram relevant to one- and two-photon ionization processes using VW laser radiation in photoelectron spectroscopy: (a) ionization from a low-lying excited state, (b) excitation to a highly excited state followed by ionization, (c) ionization to a higher ionic state, and (d) one-photon ionization from the ground state.
335
(b) single-photon excitation to a resonant highly excited state,
followed by ionization; (c) single-photon ionization producing a
high ionic state from a resonant excited state; and (d) single-
photon ionization of a ground state molecule. Use of VUV laser
radiation therefore has a unique advantage in laser MPI
photoelectron spectroscopy.
Photoelectron spectra of resonant ionization give direct
information about ionization transitions between resonant excited
states and the final ionic states. In other words, excited states
and ionic states are correlated through ionization transitions.
Such excited-state photoelectron spectroscopy can be applied to
various non-radiative excited states in addition to radiative
excited states. Furthermore, photoionization from an excited
state is always in competition with deactivation processes such as
relaxation and dissociation, so that new information about dynamic
behavior of excited states may also be deduced from time-dependent
photoelectron measurements.
1.4 Tunable VUV Lasers
Since pioneering experiments of Harris and Miles 1141 and
Hodgson et al. [151, four-wave sum mixing in metal vapors has been
used to generate tunable coherent VUV laser radiation over broad
regions. The mechanism of four-wave sum mixing is schematically
shown in Fig. 2 in the case of Mg vapor. Metal vapors such as Mg,
Cd, Hg, etc. are efficient nonlinear media to produce coherent VUV
Fig. 2. Energy-level diagram of Mg atom, showing the level of two-photon resonance at which the Wt of four-wave sum mixing takes place. The energy region of ionization continuun and broad autoionizing level associated with four-wavemixingisindicatedbythedottedarea.
386
radiation by four-wave sum mixing (%UV = 201 + @2)1 with high
conversion efficiency 114-161. In Fig. 2, the two-photon
resonance level (3s3d 'D) of Mg atom is indicated as an example.
A technique for generating four-wave sum mixing of metal vapors
has been well developed [17], and many applications to molecular
spectroscopy have been published [18-201.
So far, in MPI photoelectron studies of excited states,
UV/visible lasers have been mostly used, and essentially no VUV
lasers have been used, except for a few cases mentioned in this
paper. Hence, the use of tunable coherent VUV radiation will
further develop MPI photoelectron spectroscopy of excited states.
In the present paper, we want to demonstrate several new
applications of VUV laser radiation to MPI photoelectron
spectroscopy, as well as to illustrate that tunable VUV pulse
lasers should be very important for future studies of molecular
photoelectrongpectroscopy.
2. RXPERIMRNTAL
2.1 Tunable VUV Laser by Four-Wave Sum Mixing in Mg Vapor
A heat pipe oven for Mg vapor was designed and constructed to
generate tunable VUV radiation in the present work. Figure 3
shows a schematic drawing of our heat pipe, which consists of a
horizontal tube containing Mg vapor and a vertical tube containing
Na vapor. The concept and the design of a heat pipe and its use
High-r water
Fig. 3. A heat-pipe oven for Mg vapor to prod- tunable VW laser radiation, consisting of two pipes; the horizontal one containing Mg vapor and the vertical one containing Na vapor. A metal mesh is used as a wick for metal transport.
387
Plate
Plate
Fig. 4. Schematic diagram of the overall laser system producing tunable VW radiation. A dye laser pumped by a XeCl excimer laser is used for @I, and the Nd-YAG pumped dye laser is used for I, both lasers being synchronized.
in spectroscopy has been discussed by Vidal [21,22].
Helium was mixed with Mg vapor in the horizontal tube (He:
200 Torr; Mg: 20 Torr) to attain phase-matching conditions in Mg-
He. In this heat pipe, Na was initially heated at about 800°C,
and then Mg was heated by heat transfer from Na vapor.
Our laser system for generating tunable VUV radiation by
four-wave mixing in metal vapor is schematically shown in Fig. 4.
A Nd-YAG laser pumped dye laser system (Quanta-Ray DCR-lA, PDL-1,
WEX; 10Hz) was used for m2, while a XeCl excimer laser (Lambda
Physik EMG50; 10 Hz) pumped dye laser (Lambda Physik FL2002) was
employed for 'P'I (430.82 nm; Coumarine 480). The two lasers "I
and u2 were synchronized.
Using the above laser conditions, we obtained tunable VUV
radiation with continuous tunability in the region 120-174 nm (10
ns, 10 Hz). The resulting photon flux was evaluated to be 8~10~~
photons/pulse from the intensity measurements with a solar blind
photomultiplier, assuming that the reflectivities of the MgF2
plate and the Cu surface used are 3 % and 100 %, respectively.
From the photon flux, the conversion efficiency was estimated to
be larger than 0.2 %. The resolution of the tunable VUV laser
radiation is 0.1-1.0 cm -1 , depending on incident lasers.
2.2 Third Harmonic Generation in Xenon
Xenon gas is also known as a nonlinear medium for generating
VUV laser radiation by third harmonic generation (THG), 'P'VUV = 3@I
1231. In the present work, we used a gas cell of Xe (100 mTorr;
20 cm long) to generate VUV laser radiation of LVUV = 118.2 nm
388
eVUV (118.2 m)
Pulse Nozzle
Xe Gas Cell f? I‘ : : : :
II 01 (354.7 In) Fig. 5. Schematic drawing of a Xe gas cell (100 mTorr), which produces the 118.2~nm THG radiation, and a molecular beam crossing with the WV laser radiation. Quartz and LiF lenses (f = 100 urn) are used for the windows, and the Xe gas is irradiated with the third harmonic of the Nd-YAG fundamental.
(10.49eV) by tripling the third harmonic (l.1 = 354.7 nm) of the
Nd-YAG fundamental (1064 nm); that is, the 9th harmonic generation
of the 1064 nm fundamental. This method is similar to that
reported by McCann et al. [24].
A schematic drawing of our Xe cell and VUV laser crossing a
molecular beam is shown in Fig 5. The visible laser beam was
introduced through a quartz lens (f = 100 mm) which is the input
window of the Xe cell, while the VUV laser radiation thus
generated was taken out through a LiF lens (f = 100 mm) which is
the output window.
2.3 Photoelectron Apparatus and Measurements
Our molecular-beam photoelectron apparatus has been described
in detail elsewhere [5,6,91. A gas sample was introduced through
a pulsed nozzle into the ionization region. Ion-current
measurements were first carried out for a given gas sample as a
function of laser wavelength, showing a series of peaks which
correspond to electronically excited states (vibrationally and
rotationally resolved). Then, at individual ion-current peaks,
photoelectron kinetic energy measurements were carried out with a
time-of-flight (TOF) electron analyzer (28 cm in length).
Photoelectron spectra were recorded by accumulating photoelectron
signals for at least a few thousand laser shots (10 Hz), using a
transient recorder (Biomation 6500), and then transferred into a
microcomputer data acquisition system.
389
3. APPLICATIONS
3.1 Single-Photon Ionization of Aniline by THG in Mg Vapor
The ionization potential of aniline, Iv = 8.00 eV [25], is
lower than the single-photon energy of the THG of Mg (kvUV q 143.6
nm; 8.63 eV) which is generated at 11 = 431 nm. Figure 6 shows a
photoelectron spectrum of aniline which was obtained by the
nanosecond VUV radiation of the THG of Mg, indicating several
vibrational peaks (shown by arrows) on the first ionization band.
The vibrational structure of the first band in Fig. 6 is
essentially the same as that of an available He1 photoelectron
spectrum of aniline [25]. In Fig. 6, a sharp peak due to
scattered VUV light is also shown on the left side of the band.
The important point to be noted here is that photoelectron
spectra of ground-state molecules can be obtained by such VUV
pulse laser radiation. Therefore, this technique is applicable to
short-lived transient species or photofragment species, by
combining with a photolysis laser. There are many He1
photoelectron studies for simple free radicals, as earlier
reviewed by Dyke et al. [261. However, additional information
about transient species will be obtained from the laser
photoelectron experiments of a flash-photolysis type with VUV
laser radiation.
scattered VW Light
I
II, I
1.0 0.6 0.4 0.2
Photoelectron Energy' (eV)
..-.
@VW (143.6 nm)
Aniline
Fig. 6. The photoelectron spsctrun of aniline , obtained by the 143.6-nm pulsed laser (THG of Mg vapor), showing vibrational structure. A peak due to scattered VW light also appears.
3.2 Ionization of Rydberg F2A State of NO by Tunable VUV
In the present work, we selected the Rydberg F2A state (v'=l)
of NO as the first example of our application of four-wave mixing
in the wavelength region lVUV = 155.2-155.6 nm, using kl = 430.82
nm and L2 = 555.4 - 560.7 nm. As a result, we were able to detect
a two-color ion-current spectrum which shows several rotational
peaks of the v '=l vibrational level of the Rydberg F2A state [271,
as shown in Fig. 7(a).
In order to confirm the nature of resonance of the Rydberg
F2A state (v'=l), we measured a photoelectron spectrum at the main
ion-current peak. The resulting photoelectron spectrum is shown
in Fig. 7(b), indicating a sharp ionization peak at an energy of
0.67 eV, which can be assigned to the v+=l level of the NO+ ion.
This clearly indicates that Av=O ionization takes place from the
v'=l level of the Rydberg F2A state of NO to the v+=l level of the
NO+ XlE+ ion. (Here, the Rydberg F2A state was ,.ionized by 02
rather than @I, because u2 was much stronger.)
The rotational structure in Fig. 7(a) was analyzed in terms
of the rotational constant, the assignments of the rotational
peaks being also shown in Fig. 7(a). The resulting rotational
constant is essentially the same as that determined from an
available high-resolution absorption spectrum 1281. It should be
mentioned here that tunable four-wave mixing in Mg vapor has been
used for the first time in the present work, and it is a very
promising VUV laser source for photoelectron spectroscopy.
(a)
(b)
v+=1
Pll Rll
155.5 155.4 155.3 155.2
Wavelength (a) 5 2 1 0.5 0.1
Fhotoelectron&ergy (eV)
Fig. 7. (a) A two-color ion-current spectrum of NO in the Rydberg F2A state (v'=l), obtained by tunable four-wave mixing radiation in Mg vapor. (b) A photoelectron spectnsn observed at the main ion-current peak, indicating a single peak due to Av=O ionization transition.
3.3 Ionization of the Rydberg N2A State of NO by Tunable VUV
Strong electronic interaction between the Rydberg N2A v'=O
and valence Bt2A (v'- -7) states of NO has been studied in detail by
Dressler and Miescher [291 and by Jungen [301 with high-resolution
absorption and emission spectroscopy. In the present experiments,
391
the NO molecule was excited to the Rydberg N'A (v'=O) state by VUV
radiation from four-wave mixing in Mg (%Uv = 2q + @3), and the
excited state was then ionized by c+. The resulting two-color
ion-current spectrum of the Rydberg N2A v'=O state of NO is shown
in Fig. 8, observed in the wavelength region 147.88 - 147.97 nm.
This spectrum indicates several rotational peaks which can be
explained in terms of rotational progressions Ql and Rl.
RI I
j-112
A=--
147.95 147.90
Wavelength (nm)
No
Fig. 8. A two-color ion-current spectm of the Rydberg N2A v'=O state of obtainedusingtunableVWrad.iation fromMgvapor. Theupperourve shows intensity of the WV radiation.
No, the
3.4 Ionization of the Valence B"A State of NO by Tunable VUV
A two-color ion-current spectrum associated with the v'=7
level of the valence B12A state was obtained here in the
wavelength region 147.67-147.75 nm with tunable VUV from Mg vapor,
as is shown in Fig. 9, indicating several rotational peaks. As
mentioned before, it is known that the valence B"A state at the
v'=7 level interacts to a considerable extent with the Rydberg N2A
state at the v'=O level 1303. (Here again, the excited state B"A
was ionized by p2). These rotational peaks have been interpreted
in terms of two rotational progressions Q21 and RZlr as indicated
on the peaks in Fig. 9.
The results deduced from rotational analysis of the spectra
in Fig. 9 are consistent with those of Jungen [3Ol. Time-resolved
fluorescence measurements at several vibrational levels (v'nl-8)
of the valence B"A state of NO have been carried out by Banic et
392
Fig. 9. A two-color ion-current spectm of No, obtained by WV excitation to the valence H'2A v'=7 state by light fran four-wave mixing in Mg vapor. The upper curve she? the intensity of the VW radiation.
al. [28], using four-wave mixing in Mg vapor. Such a tunable VUV
laser should be an important light source for MPI spectroscopy as
well as for absorption and fluorescence spectroscopy. One of the
advantages of tunable VUV laser radiation is that highly excited
states can be produced by single photons from the ground state
(see Fig. lb), thus simplifying the excitation process compared
with multiphoton processes.
3.5 Single-Photon Ionization of NO by THG in Xe
Third harmonic generation in Xe gas produces coherent VUV
radiation at 118.2 nm (10.48 eV). Its single photon energy is
greater than the adiabatic ionization potential of NO molecule, I,
= 9.26 eV [313. Therefore, in the present work we decided to
measure a photoelectron spectrum of NO by 118.2-nm VUV radiation,
with the experimental arrangements mentioned before (Fig. 5).
Figure 10 shows a photoelectron spectrum of NO thus obtained,
indicating four vibrational peaks (v+=O-3).
Although the photoelectron vibrational structure in Fig. 10
is essentially the same as that of available He1 photoelectron
spectra [2,31], the important point to be noted is that such a
simple Xe VUV laser source makes it possible to measure
photoelectron spectra of ground-state molecules. This suggests
that even for short-lived transient species it may be possible to
detect photoelectron spectra with the 118.2-nm radiation, It
should also be mentioned that observation of such photoelectron
393
v+=2
v+=1 I <. . . .
NO+ ::::
v+=3
I
NO - 5 2 1 0.5 0.2
Photoelectron energy (eV)
:. ‘.‘. ._. . . . . . c
@WV
(118.2 run)
Fig. 10. A photoelectron spectm of NC in the ground state, obtained by 118.2~nm laser radiation, showing well-resolved vibrational peaks of the first ionization bend.
vibrational structure of NO provides useful energy calibraticn.
3.6 Photoelectron
Photoelectron measurements of organic molecules in their
excited triplet states are of considerable interest, since no
photoelectron studies have been reported. Laser MPI may be a new
technique to explore triplet manifolds of organic molecules.
Recently, the triplet pyrazine molecule has been studied by means
of a one-color (lt2) MPI technique by Villa et al. [321 and by
Turner et al. [333. However, in one-color MPI experiments, there
would be another possibility of resonance at a higher excited
state, making spectral interpretation confusing.
In the present work, we were able to obtain a photoelectron
spectrum originating from the triplet Tl 0' state for the first
time from two-color experiments [34], as is shown in Fig. 11.
Combining a XeCl excimer laser pumped dye laser (al) and a 193-nm
ArF excimer laser (02), we directly excited jet-cooled pyrazine
molecule to Tl 0' by ml, and then ionized by @2. The spectrum in
Fig. 11 has been interpreted in terms of the vibrational
frequencies of the ion produced by photoionization from Tl:
3B3u(nz*). Vibrational assignments shown in Fig. 11 have been
made on the basis of the available information on the ion [351.
394
5 2 1 0.5 0.2 0.1
Pfi0tOde0tron Eherg~ (ev)
Tl oo -r
+ u1 (372.86 nm)
I
Pyrazine
Fig. 11. A photoelectron spectrum of pyrazine in the lowest triplet state (Tl OO). Pyrazine molecules were directly excited to T1 from the ground state by the first laser radiation (372.86 nm), then ionized by the second, ,193-nm ArF excimer laser radiation.
4, Conclusion
As demonstrated in the present work, THG and four-wave sum
mixing in rare gases and metal vapors are promising coherent VUV
sources for future application of excited-state photoelectron
spectroscopy. Use of the VUV laser radiation in photoelectron
spectroscopy is necessary, especially for studying highly excited
states, low-lying excited states, higher ionic states (including
core excitation), and even for ground-state transient species.
Several applications of MPI photoelectron spectroscopy to
photophysical and photochemical processes have been developed in
this laboratory [lo]; 1) selective photoionization, 2) one-photon
forbidden, two-photon allowed excited states, 3) autoionization,
4) intramolecular electronic and vibrational relaxation, 5)
electronic states of photofragments, and 6) excited triplet
states. In order to further extend this technique to these
subjects, we need various VUV laser sources including those
mentioned in this paper.
The Xe gas cell to generate VUV radiation by tripling the
output of a YAG-pumped dye laser is easy to handle, and should be
useful for photoelectron spectroscopy, although its tunability is
only within a bandwidth ofwl cm-' [36]. A high pressure H2 Raman
shifter to generate far-VW radiation is also useful for MPI
spectroscopy. For example, in our previous MPI studies [37,38],
we have used 192.2-nm radiation (the 8th anti-Stokes line of the
395
2nd harmonic of Nd-YAG) and 199.8-nm radiation (the 3rd anti-
Stokes line of the 4th harmonic of Nd-YAG laser).
Threshold photoelectron measurements in MPI experiments are
especially important for studying ionic states in very high
resolution spectroscopy [391. Use of a tunable VUV laser such as
four-wave mixing of Mg vapor in threshold photoelectron
spectroscopy will make it possible to study higher ionic states at
high resolution.
ACKNOWLEDGEMENTS
We would like to thank Professor K. Shobatake for letting us
use the XeCl excimer laser for the generation of four-wave mixing
in Mg vapor. We would also like to thank Professor J. H. D. Eland
and Professor E. I. von Nagy-Felsobuki, IMS Visiting Professors,
for reading the manuscript and their kind discussion and advice.
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