Twocolor threshold photoionization of jetcooled aniline: Vibrationally selectiveautoionizationJames Hager, Mark A. Smith, and Stephen C. Wallace Citation: The Journal of Chemical Physics 84, 6771 (1986); doi: 10.1063/1.450680 View online: http://dx.doi.org/10.1063/1.450680 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/84/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Two-color resonantly enhanced multiphoton ionization and zero-kinetic-energy photoelectronspectroscopy of jet-cooled indan J. Chem. Phys. 122, 244302 (2005); 10.1063/1.1938927 Observation of torsional motion in the groundstate cation of jetcooled tolane by twocolor thresholdphotoelectron spectroscopy J. Chem. Phys. 97, 1649 (1992); 10.1063/1.463153 Autoionizing Rydberg structure observed in the vibrationally selective, twocolor, threshold photoionizationspectrum of jetcooled aniline J. Chem. Phys. 83, 4820 (1985); 10.1063/1.449010 Two color photoionization spectroscopy of jet cooled aniline: Vibrational frequencies of the aniline X̃2 B 1radical cation J. Chem. Phys. 80, 3097 (1984); 10.1063/1.447124 Twocolor photoionization of naphthalene and benzene at threshold J. Chem. Phys. 75, 2118 (1981); 10.1063/1.442315

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Two-color threshold photoionization of jet-cooled aniline: Vibrationally selective autoionization

James Hager, Mark A. Smith,a) and Stephen C. Wallace Department of Chemistry, University of Toronto, Toronto, Ontario Canada M5S IAI

(Received 6 February 1986; accepted 14 March 1986)

We report the re~ul~s o~investiga~ions of the autoionization of jet-cooled aniline-h7 and -d7• Using two-color p~~tOlomzatton techmques, we have observed autoionization of high lying Rydberg levels contammg quanta of the nontotally symmetric vibrational modes lOb, I, and 15. No autoionization is observed for Rydberg states containing only totally symmetric vibrational excitation. In all of the spectra in which autoionization is found, the mechanism is vibrational­electronic coupling (vibrational autoionization) with a characteristic Av = - 1 propensity. An upper limit for the autoionization rate of 2 X 1012

S -I has been determined from linewidth measurements. Possible explanations are presented and discussed for the activity of only nontotally symmetric vibrational modes in the autoionization process.

I. INTRODUCTION

The study of high lying Rydberg states of aromatic sys­tems has been of considerable interest to spectroscopists for many years. 1 It has been only recently however that, by com­bining supersonic expansion technology and intense laser light sources, detailed spectroscopic and dynamic informa­tion has been obtained for Rydberg states of molecules such as benzene1 and toluene.3 Of particular interest is that Ryd­berg states lying above the adiabatic ionization potential can couple with the ionization continuum and autoionize.4 A Rydberg level at such energies is often considered to consist of an excited ion core with various amounts of electronic, vibrational, and rotational energy and a Rydberg electron. Transfer of energy from the excited core to the Rydberg elec­tron often leads to autoionization. The most comon mecha­nism for autoionization of a polyatomic molecule is vibra­tional autoionization.s This process involves conversion of vibrational energy of the ion core to additional excitation of the Rydberg electron producing an ion with less vibrational excitation than that characteristic of the Rydberg state. In the present paper, we report the results of a comprehensive investigation of autoionization of high lying Rydberg states of the aniline molecule prepared by a two-color excitation process. By preparing an intermediate vibronie level with one laser and tuning the second laser near the ionization threshold, we have identified ions produced by a vibrational autoionization mechanism.

In addition to ion intensity due to autoionization, pho­toionization spectra contain substantial intensity from direct ionization processes. Direct ionization is characterized by immediate ejection of an electron upon the absorption of an ionizing photon.4 At threshold, photoionization spectra dis­playing direct ionization can be described by a step function corresponding to the ionization onset with a relatively slow variation in ion production as the ionizing light is tuned to higher energies.6 One may also observe subsequent steps cor­responding to the production of an ion in higher vibrational states. For direct ionization of aniline, we? and others8 have

a) Permanent address: Department of Chemistry, University of Arizona, Tucson, Arizona.

shown that the Franck-Condon factors for ionizing transi­tions originating from the IBl excited state show a strong propensity for Av = 0 transitions. In the present study, our results show the same Av = 0 propensity for the excitation from I Bl vibronic levels to the highly excited Rydberg states. This fact has proven to be very important in the analysis of the autoionization mechanism.

We have reported9 preliminary results demonstrating vibrational autoionization in the two-color threshold pho­toionization spectra of jet-cooled aniline. The two-color technique is extremely useful for investigations of vibration­al autoionization. In this experimental scheme,lo the pump laser selects a specific vibrational level of the first excited singlet state eB1 ) and the second laser further excites the molecules to energies near the first ionization potential. Thus, one has access to molecular transitions lying in the vacuum ultraviolet. By exciting different intermediate vi­bronie states and then photoionizing from these prepared states, one is not limited to a single set of Franck-Condon factors for the ionization step. Furthermore, one can investi­gate the role of specific vibrational modes in the ionization process.

In this paper we present photoionization efficiency (PIE) spectra of jet-cooled aniline and discuss the assign­ments of the direct and autoionization signal. In general, we observe vibrational autoionization only in two-color spectra corresponding to ionization from vibronic levels character­ized by quanta of nontotally symmetric vibrations. From these spectra, we have been able to assign four Rydberg se­ries, all bands of which are located within one ionic ground state vibrational quantum of the direct ionization threshold. The results are discussed within the framework of a simple model of vibrational autoionization based on the breakdown of the Bom-Oppenheimer approximation.

II. EXPERIMENTAL DETAILS

The supersonic beam/quadrupole mass spectrometer employed for these two-color photoionization studies has been described in detail in previous publicationsY I Conse­quently, only a brief description will be given here. The inter-

J. Chern. Phys. 84 (12), 15 June 1986 0021-9606/86/126771-10$02.10 @ 1986 American Institute of Physics 6771

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6772 Hager, Smith, and Wallace: Photoionization of aniline

mediate vibronic level was prepared using the frequency doubled light from a Nd:YAG pumped dye laser (Quanta Ray-IA and PDL-t). The linewidth of the UV light was found to be approximately 0.2 cm - 1 with pulse energy of > 4 mJ which was reduced with neutral density filters to approx­imately 100-200 pJ in order to minimize one-color ioniza­tion signal. For the ionization laser, we have used a XeCl (Lumonics 860-T) pumped dye laser (Lumonics EPD-330) operating on the UV dyes DMQ, TMQ, or PTP. This laser produced pulses of 3-8 mJ, 7 ns in duration, with a band­width of approximately 0.2 cm -I. The interaction region in the supersonic jet is defined by the spatial overlap of the two counterpropagating lasers. Due to the high ionization effi­ciency of aniline, only slight focusing of the laser beams is required. Temporal overlap of the lasers, power normaliza­tion, and data acquisition were done as earlier published.7

•11

In certain instances it was necessary to prepare the inter­mediate IB2 1evel by excitation of a hot transition. This was accomplished by reducing the backing pressure of the expan­sion, and in some cases, saturating the hot transition.

The ions produced in the supersonic expansion are ac­celerated into the quadrupole mass spectrometer with a very small voltage « 1 V /cm) where they are focused and ana­lyzed. This small accelerating voltage was found not to affect the position of the direct ionization thresholds reported here. In several spectra we have investigated, larger electric fields (1-4 V /cm) in the laser interaction region were found to result in broadened autoionization peaks as well as an in­crease in a two-color ionization background immediately above the aniline adiabatic ionization potential. Conse­quently, the voltage on the accelerating plate was main­tained at < 1 V /cm for all the spectra reported here.

In these experiments, the identity of the ionized mole­cule was determined initially with the quadrupole mass fil­ter. In each case only the parent aniline-h7 or -d7 cation was observed with no fragmentation evident. This allows a more sensitive technique to be used for recording PIE spectra, namely using the quadrupole as a total ion accelerating re­gion by turning off the ac fields associated with the mass spectrometer. All band positions and ionization thresholds

(0) 0-0

(b)

62.000 62,500

Two-color Energy/cm-I

reported here are in vacuum wave numbers. Experimental uncertainties are obtained from the deviation from the aver­age value of at least three spectra.

The aniline samples used in these studies were obtained commercially and used without further purification. Ani­line-h7 (99 + %) was obtained from Aldrich and the fully deuterated sample (98%) was obtained from MSD Iso­topes.

III. RESULTS A. Anlllne-h7

When aniline is ionized from the band origin of the A ( 1 B2 ) electronic level, one observes a single sharp ioniza­tion onset corresponding to the adiabatic ionization poten­tial measured7 to be at 62 265 ± 18 cm - I. Figure 1 (a) dis­plays such a PIE spectrum. As one can see from this figure, there is no resolvable pre- or post-threshold structure. This is characteristic of an ionization process dominated by direct ionization. Indeed, the PIE spectra of most of the vibronic bands we have examined show similar behavior. Figure 1 (b) illustrates the spectrum obtained when the intermediate lev­el corresponds to the oo~ vibronic band. Here, the ionization onset is shifted from that obtained for the 0-0 PIE spectrum by an energy equivalent to the value of the one quantum excitation of the 00 mode in the ground ionic state (545 ± 10 cm -1). From this spectrum and others we have reported,7

one can see that there is a strong propensity for au = 0 ioniz­ing transitions from the IB2 vibronic levels which has en­abled us to determine vibrational frequencies of the ground state of the aniline radical cation. Our earlier study7 also indicated that, for most of the bands investigated, direct ioni­zation is the dominant process reflecting the similarity of the neutral I B2 and ground ionic state potential surfaces. In fact, direct ionization is the only observable process for photoion­ization from IB2 vibrational modes: 00, 12, 1, 13, and their overtones and intercombinations. The present results, how­ever, show conclusively that this is not the case for ionization from many other IB2 vibrational levels. The normal modes of interest in this investigation are displayed in Fig. 2.

63,000

FIG. 1. The two-color PIE spectra ob­tained withAl fixed at (a) and IB2 origin and (b) the 6a~ transition.

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Hager, Smith, and Wallace: Photoionization of aniline 6773

~~~ $

~ * 0 cS FIG. 2. Pictorial representation of select­

ed aniline normal modes in the C 2. point group. The inversion mode includes some movement of the ring atoms which is not shown in the figure.

°1 °2 bl

1. Inversion mode; I

Figure 3 shows the PIE spectra we have obtained when aniline is photoionized from the I 1, I ~, and I ~ I B2 transi­tions. In these spectra there are two distinct photoionization processes: one marked by the direct ionization onset, similar to that illustrated for the 0-0 and 6a~ bands, and one charac­terized by sharp band structure. We shall consider the direct ionization thresholds first. The ionization onsets exhibiting step-function behavior are located at 655 ± 10 cm -I [I:, Fig. 3(a)], 1320 ± 15 cm- I [I~, Fig. 3(b)], and 1980 ± 15 cm - I [I ~ , Fig. 3 ( c )] above the aniline adiabatic I. P. These values are in excellent agreement with the recently deter­mined inversion mode intervals in the ionic ground state. Using MPI photoelectron techniques, Meek et al.s have measured the first three members of the aniline radical ca­tion inversion mode progression to be 645, 1315, and 1960 cm-I, respectively. Thus, the av = 0 propensity for the di­rect ionization process is maintained for low energy excita­tion in the inversion normal mode despite the large changes in vibrational frequency in going from the excited neutral (I~ = 0 + 760 cm- I

) to the ground state ion. In addition, the aniline radical cation is characterized by a harmonic po­tential for the inversion normal mode through v = 3 com­pared to a quartic potential (IB2 state) and a double mini­mum potential (X state) for the two lowest energy electronic states ofthe neutral molecule. 12,13

62,:DO 63,CXYJ 63.:DO Two-color Energy/em-I

b2

The band structure in each of the two-color PIE spectra displayed in Fig. 3 is identified9 with the autoionization of highly excited states of neutral aniline, in this case, Rydberg states with high principle quantum numbers. Assignment of the Rydberg bands was carried out using the Rydberg equa­tion

E= I.P. -R(n - c5) -2.

From analysis of the bands in the I I PIE spectrum, we have been able to assign four Rydberg series with quantum defects of c5 = 0.78 ± 0.04 (n = 14-21), c5 = 0.45 ± 0.02 (n = 13-25), c5 = 0.41 ± 0.02 (n = 13-17), and c5 = 0.22 ± 0.02 (n = 13-24) each converging to the I.P. associated with the radical cation with the same number of quanta in the inver­sion mode as that of the intermediate state.

The values of these quantum defects implyl that the c5 = 0.78 series belongs to an s series, the c5 = 0.45 and c5 = 0.41 bands to p series, and the c5 = 0.22 bands to a d series. The only other investigation of aniline Rydberg states that we are aware of is that of Fuke and Nagakural4 who have reported series quantum defects of 0.53 and 0.19 based on observation of n = 3-5. Table I provides the band posi­tions for the Rydberg series observed in the PIE spectra of the I I band and Table II is a summary of the quantum de­fects observed for the Rydberg series in other PIE spectra we have measured.

Examination of each of the three spectra in Fig. 3 shows

64,CXYJ

FIG. 3. The two-color PIE spectra ob­tained with A,I fixed at (a) the n transi­tion. (b) the I~ transition, and (c) the n transition.

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6774 Hager, Smith, and Wallace: Photoionization of aniline

TABLE I. Band positions and assignments of autoionization features ob­served in the PIE spectrum of the /: I B2 transition.

Observed (calculated) band positions/cm- I

n 8=0.22 8 = 0.41 8 = 0.45 8 = 0.78

13 62251(62248) 62230(62228) 62224(62222) 14 62343(62342) 62329(62326) 62324(62322) 62295(62292) 15 62418(62418) 62400(62404) 62400(62402) 62378(62377) 16 62480(62479) 62469(62468) 62463(62466) 62445(62446) 17 62530(62530) 62520(62521) 62512(62519) 62502(62503) 18 62570(62573) 62565(62564) 62550(62550) 19 62610(62609) 62602(62601) 62586(62589) 20 62640(62640) 62631(62633) 62620(62623) 21 62669(62666) 62660(62660) 62650(62652) 22 62690(62689) 62684(62684) 23 62709(62709) 62706(62704) 24 62726(62726) 62724(62722) 25 62738(62738)

that this structure begins at approximately the same energy relative to the direct ionization onset ( - 660 cm -1) regard­less of the number of inversion quanta characterizing the intermediate IB2 state. In addition, one should note that all of the Rydberg structure lies within the ionization contin­uum. Using the value of an ionic ground state inversion mode quantum of -655 cm-I, one can identify the begin­ning of the autoionization bands with the energy corre­sponding to one quantum less excitation in the ion than that produced by direct ionization. This implies that the autoion­ization process shown in these spectra are characterized by a Av = - 1 propensity. Thisis in accord with Berry'sl5 vibra­tional autoionization propensity rule for harmonic oscillator behavior.

We have also investigated the photoionization behavior of intermediate vibronic states that are combination vibra­tions of inversion and totally symmetric modes. Figure 4 displays the PIE spectrum of the 601

11 and the 601/2 IB2

levels. The direct ionization thresholds in these two spectra again correspond to Av = 0 ionizing transitions and give the values of the 6a(v = 1 )/(v = 1) and 6a(v = 1 )/(v = 2) ex­citations in the ground state of the radical cation (1512 and 2045 cm-I, respectively). In addition, there is some rather weak autoionization structure which begins at approximate­ly - 660 cm -1 with respect to the direct ionization thresh­olds. This interval is much closer to the ionic ground state value of one quantum ofinversion than 60 (545 cm -1). Con­sequently, we have assigned this structure to vibrational autoionization in which the propensity is to lose one quan-

TABLE II. Summary of quantum defect values for aniline PIE spectra.

IB2 Level Quantum defect

60 1/

2

602/

2

11[2

lOb 2

601 lOb 2

152

0.22 ± 0.02, 0.41 ± 0.02, 0.45 ± 0.03, 0.78 ± 0.04 0.21 ± 0.02, 0.41 ± 0.02, 0.46 ± 0.03, 0.80 ± 0.04 0.22 ± 0.03, 0.40 ± 0.03, 0.43 ± 0.02, 0.47 ± 0.03, 0.81 ± 0.03 0.20 ± 0.02, 0.45 ± 0.03 0.21 ± 0.03, 0.46 ± 0.03 0.22 ± 0.03, 0.42 ± 0.03 0.38 ±0.04 0.40 ± 0.04 0.22 ± 0.03, 0.40 ± 0.03, 0.46 ± 0.04

tum of the inversion mode rather than the 60 mode. Similar results have been obtained for the combination

vibration 1112 which shows very weak autoionizing structure commencing at approximately 650 cm -1 below the direct ionization threshold as is shown in Fig. 5. This is again close to the value of one quantum of the inversion mode in the ground ionic state (in the ionic ground state 11 = 837 ± 15 cm -1). Photoionization of several other inversion combina­tion modes failed to reveal any autoionization structure. The 12112 and the 60212 vibronic levels were found to be charac­terized by multiple thresholds devoid of any reproducible band structure.

We have made a survey of the linewidths of the autoion­ization structure in these spectra. Unfortunately, because of the relatively high principle quantum numbers (n = -14-30) and the four different Rydberg series present, there are only a few examples ofisolated spectral bands which contain no contribution from adjacent transitions. Typical linewidths are found to be approximately 9-10 cm -1 which is considerably larger than the < 1 cm - 1 band width of the ionizing laser or the 4 cm -1 rotational band contour in the IB2 intermediate state. If these linewidths are due solely to lifetime broadening, we can place an upper limit on the rate of autoionization of approximately 2 X 1012 S -1.

2. Out-of-plane modes; 16a and 10b

Figure 6 provides examples of the PIE spectra obtained for the 16a~ (0 + 352 cm- I

) and lOb~ (0 + 348 cm- 1)

vibronic excitations. 16 In Fig. 6 (a) one can see that for 16a~, direct ionization is the dominant photoionization mecha­nism. However, there are two distinct ionization thresholds: one at + 361 ± 10 cm -I and the other at + 712 ± 15 cm - I

with respect to the aniline adiabatic J.P. This is in contrast to the results for most of the PIE spectra we have investigated which typically exhibit a very strong propensity for Av = 0 direct ionization. Here, the Franck-Condon factors for the Av = 0 and Av = 1 processes are comparable. In addition, there is no evidence for any autoionization in the PIE spectra of the 16a~ band. Since there is only a minimal frequency change in the 160 mode upon ionization, we suggest that the high intensity of the t::.v = 1 direct ionization process is due to a shift in the origin of this normal coordinate in the ion with respect to that of the IB2 state.

In Fig. 6 (b), the PIE spectrum of the lOb ~ vibronic transition is presented. As was observed in the case of the PIE spectra of the inversion progression (v' = 1,2,3), there is both direct ionization and autoionization. The direct threshold was measued to be at 352 ± 10 cm -I above the aniline adiabatic I.P.; this interval being assigned to the two quanta excitation ofthe lOb mode in the ground state of the radical cation.

At energies between the adiabatic and vertical I.P.'s there is again band structure indicating autoionization. Here, the structure commences approximately 180 cm- 1

above the adiabatic I.P. or at the energy corresponding to the one quantum excitation of lOb in the ionic ground state. Thus, the vibrational autoionization propensity for t::.v = - 1 is maintained for photoionization from the lOb 2

level.

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Hager, Smith, and Wallace: Photoionization of aniline 6775

(0)

63,000

(b)

63,500

Two-color Energy /cm- I

Investigation of the combination band 606 lOb ~ has also revealed signs of vibrational autoionization [Fig. 6 ( c) ] . The direct ionization threshold was found to be 877 + 15 cm- I above the adiabatic J.P. This value correspondsto a ll.v = 0 ionizing transition from the 60 11 Ob 2 excited state lev­el. Weak autoionization structure begins approximately 170 cm -I to lower energy with respect to the direct ionization onset. This energy is about that expected for the ground ionic level 6a(v = 1)10b(v = 1), and consequently the oberva­tion of a ll.v = - 1 vibrational autoionization process. Note that, in this example of vibrational autoionization, the core preferentially gives up a quantum of lOb rather than 60.

3. In-plsne mode; 15

We have also observed autoionization in the two-color PIE spectrum of aniline photoionized from the 152 level. The normal mode 15 is of b2 symmetry in the C 2V point group and involves tangential motion ofthe amino substituent rela­tive to the phenyl ring as shown in Fig. 2. The PIE spectrum

63,500 64,000 Two-color Energy / cm- I

64,000

FIG. 4. The two-color PIE spectra ob­tained withAl fixed at (a) the6a~ n tran­sition and (b) the 6a~ I~ transition.

obtained upon photoionization from this level is shown in Fig. 7. This spectrum again displays both direct ionization and autoionization. The direct ionization threshold in this spectrum is located 724 ± 15 cm - I above the aniline adiaba­tic J.P. Assignment of this threshold is made difficult by the nature of the structural changes between the neutral excited and ionic ground states. Calculations suggest that removal of an electron from the highest occupied biorbital intro­duces a degree of double bond character in the N-C bond. 17

The strengthening of this bond leads to the pronounced in­crease in the frequency of the inversion mode upon pho­toionization. Since the N-C bond strength is an important factor in determining the frequency of the 15 mode, one also expects a similar increase in this frequency in the ionic ground state compared with neutral electronic states. Using this argument, we assign the 724 cm -I interval in the PIE spectrum to the one quantum excitation of 15 in the ground electronic state of the aniline radical cation. Accordingly, the direct ionization threshold corresponds to the ionizing

64,500

FIG. 5. The two-color PIE spectra ob­tained withAl fixed at the 1~ 1~ transition.

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6776 Hager, Smith, and Wallace: Photoionization of aniline

(0)

62,500 63,000 Two-color Energy / cm- I

(b)

FIG. 6. The two-color PIE spectra ob­tained with A I fixed at the (a) the 16a~ band, (b) the IDb~ band, and (c) the 6ab 1 Db ~ transition.

(c) I 2 600 lObo

I I 62,400 62,600 62f3(XJ 63,000 63,200

Two- color Energy/cm- I

transition from 15\.:4' lB2 ) to 15 1(X+ 2BI). The autoionizing structure in this spectrum is found to

begin at the adiabatic I.P. If the assignment of the direct ionization threshold is correct, the vibrational autoioniza­tion in this PIE spectrum is characterized by Ilu = - 1.

B. Anlllne-d,.

We have also investigated the PIE spectra of several A state vibrational levels of aniline-d7 in order to confirm the assignments of the autoionizing Rydberg series observed in the undeuterated molecule. As an example, we present the two-color PIE spectrum of the I: excitation in Fig. 8. The direct ionization threshold in this spectrum is located at 62 673 ± 10 em -I, or 465 em -I above the aniline-d7 adiaba­tic J.P. (62208 ± 10 em-I). This interval corresponds to the single quantum excitation of the inversion mode in the ground state of the aniline-d7 radical cation. There is, in ad­dition to the direct ionization pathway, also significant ion intensity in the autoionizing Rydberg band structure

between the adiabatic and vertical ionization potentials. Analysis of this structure using the Rydberg formula allows us to identify the quantum defects of each series. As was observed for the undeuterated molecule, the most intense series is characterized by 8 = 0.44 ± 0.02. Other observed Rydberg series have quantum defects of8 = 0.80 ± 0.04 and 8 = 0.22 ± 0.03 in good agreement with the results for ani­line-h7• Table III provides a summary of the band positions and assignments of the Rydberg levels observed in the PIE spectrum of the I: band of aniline-d7•

Photoionization from the I 2 and 1 3 excited state vibra­tions results in almost exclusive direct ionization signal. Us­ing the fact that for direct ionization we see almost dominat­ing Ilu = 0 processes, these spectra have allowed us to determine values for the inversion mode progression (u = 1-3) in the ground state of fully deuterated aniline. These val­ues are 940 ± 15 em-I (u = 2) and 1400 ± 15 cm- 1

(u = 3). Combining these measurments with the interval for I( u = 1), one can see that the inversion potential of aniline-

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Hager, Smith, and Wallace: Photoionization of aniline 6777

62,000 I I I

62,500 Two-color Energy / cm- I

d7 radical cation is quite harmonic at lower energies consis­tent with a planar geometry. 7,8

C. Search for collision-Induced autolonlzatlon

Under much higher sensitivity, we have investigated possible contributions to the autoionization signal from col­lision-induced sources. All of the autoionization intensity reported above is seen within the ionization continuum, and only one vibrational quantum lower in energy than the direct ionization onset. Ito and co-workers,18 however, have re­cently observed autoionizing Rydberg structure at energies lower than the adiabatic I.P. of jet-cooled trans-stilbene. They explain this result in terms of autoionization of high­lying Rydberg states of the neutral molecule induced by the

I I 62,200 62,400 62.600

Two-color Energy / cm- I

63,000

FIG. 7. The two-color PIE spectrum ob­tained with ,.1.1 fixed at the 1 S~ transition.

very low energy collisions taking place in their free-jet ex­pansion. We have also searched for such phenomena in the two-color photoionization spectra of aniline-h7 and -d7 ion­ized from the 1 B2 electronic state origin. Considering the ll.v = 0 propensity for direct ionization, any autoionization observed in these spectra would be below the adiabatic I.P. and likely due to collisional processes. We have carefully searched for such behavior in our aniline spectra but have not been able to observe collision-induced autoionization. This has been found to be the case independent of variations in the nozzle position and backing pressure behind the ex­pansion orifice. This, together with the fact that only specific vibrational modes are found to exhibit autoionization, con­firms that this process occurs by a "collision-free" mecha­nism.

FIG. 8. The two-color PIE spectrum ob­tained with ,.1.1 fixed at the I: transition of

aniline-d7•

62.800

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6778 Hager, Smith, and Wallace: Photoionization of aniline

TABLE III. Band positions and assignments of autoionization features in the PIE spectrum of the I l I B2 transition of aniJine-d7 •

Observed (calculated) band positions/ cm - I

n {j=0.22 {j = 0.44 {j = 0.80

16 62222(62219) 17 62285(62283) 62273(62272) 62257(62256) 18 62327(62326) 62317(62317) 62300(62303) 19 62364(62362) 62353(62354) 62341(62342) 20 62392(62393) 62387(62386) 62376(62376) 21 62419(62419) 62412(62413) 22 62441(62442) 62437(62437) 23 62462(62462) 62459(62457) 24 62479(62479) 62476(62475)

IV. DISCUSSION

In the previous section, we identified four Rydberg se­ries converging to ionization thresholds corresponding to vibrationally excited levels in the ground electronic state of the aniline radical cation. Our studies show that when ani­line is photoionized near threshold from the] I, ] 2, ] 3, 60 I] I, 6a l

]2, 11]2, lOb 2, 60 1 lOb 2, and 152vibroniclevels, autoioniz­ing structure is observed between the adiabatic and vertical ionization potentials. In all these cases, the vibrational auto­ionization is characterized by a au = - 1 propensity. We have not observed autoionization in the PIE spectra involv­ing solely totally symmetric vibrations 60, 1, 12, or 13. Thus, it appears that the presence of the nontotally symmetric modes lOb,], and 15 in the highly excited Rydberg states has a profound influence on the competition between direct ioni­zation and autoionization processes. This involvement of specific normal modes in the autoionization process is quite unlike the recent observations of Ito and co-workers in their investigations of the higher excited states of the tertiary amines DABCOl9 and ABCO.20 These workers have dem­onstrated the rich autoionizing structure in the two-color photoionization spectroscopy of all major SI vibronic transi­tions of these "Rydberg molecules".

Two step, one-color photoionization of aniline from the IB2 excited state has been demonstrated to produce ion cur­rents in the 10-9 range.21 This is due to the extremely large absorption cross section for the ionization step, which is re­ported to be as large,22 or larger,21 than that characterizing the 1B2 band origin 0-4.5 Mb). This coupled with the fact that the potential surfaces of the keB 1 ) and the 1ctB2 )

electronic states are quite similar, suggests that direct ioniza­tion is the dominant process leading to efficient two step ionization. Our two-color studies show that this is indeed the case in photoionization from totally symmetric 1B2 vibra­tional levels. However, for photoionization from vibronic levels containing quanta of modes lOb,], or 15 significant ion intensity is produced by a vibrational autoionization mechanism.

There are several requirements for the observation of vibrational autoionization in photoionization studies. 15,24 The high lying Rydberg states must be optically accessible with reasonable transition probabilities and Franck-Con­don factors. In addition, the rate of autoionization must be

greater than, or comparable to, the rates of other processes of the neutral molecule, i.e., reradiation, predissociation, and nonradiative deexcitation. If these conditions are not ful­filled, autoionization will likely not be observed. The results presented here for aniline photoionization show that auto­ionization is observable from several specificSl vibronic lev­els but not others.

Vibrational autoionization occurs through the coupling of electronic and vibrational motion, and therefore, repre­sents a breakdown of the Bom-Oppenheimer approxima­tion. Within the Bom-Oppenheimer limit, the term in the Hamiltonian leading to this dynamic coupling is the nuclear momentum operator. 15 Considering a particular vibrational level of a particular electronic state, the Bom-Oppenheimer approximation to the wave function is t/J(q,Q)cp(Q). For coupling due to a single vibrational mode Q the matrix ele­ment ofthe major perturbation can be written asl5,23,24

a a (t/Jf(q,Q) I (CPf(Q) I aQ ICPi (Q» aQ It/Ji (q,Q» , (1)

where t/J(q,Q) is the electronic wave function and cp(Q) is the vibrational wave function within the adiabatic Bom­Oppenheimer basis. The initial states of interest in this expression are the high lying Rydberg states and the final states are levels of the ionic ground state. Neglecting higher order vibronic effects, this matrix element can be rewritten as 15,24

a a (t/Jf(q,Q) I aQ It/Ji (q,Q» (CPf(Q) I aQ ICPi (Q» . (2)

Considering the vibrational part of this matrix element, one can see that for two harmonic potentials, the properties of Hermite polynomials lead to the au = - 1 propensity for autoionization. Deviations from au = - 1 behavior are usually considered to be due to anharmonicity effects.25 Ex­amination of the aniline PIE spectra which display autoion­ization shows that the av = - 1 propensity is upheld for all of the intermediate states. From these results, we may infer that the potentials for the normal modes lOb,] and 15 of the ion and the Rydberg levels are harmonic in the energy region we are probing.

If the potentials of the high-lying Rydberg state and the positive ion are similar to each other, then the electronic matrix element can be transformed int023

1 a - - (t/Jf(q,Q) I aQ U(q,Q) It/Ji (q,Q» ,

Eif (3)

where E if is the energy gained by the electron during autoion­ization and U(q,Q) is the total electrostatic interaction, or effective potential felt by the Rydberg electron. For calcula­tions of the autoionization rates of the hydrogen molecule, U(q,Q) is expanded in spherical harmonics with only the spherically symmetric part retained for the calculation. 15

There are several possible explanations for the fact that we do not observe autoionization for vibronic levels contain­ing only quanta of6a, 1, 12, and 13. It is conceivable that the transition probabilities and/or Franck-Condon factors for transitions from the intermediate levels such as 601, II, 121, etc. directly to the ion are much greater than those to the high lying Rydberg states. We do in fact observe extremely

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Hager, Smith, and Wallace: Photoionization of aniline 6779

high two-color photoionization efficiencies for ionization from these levels. For intermediate levels containing quanta of lOb, I, or 15, optical excitation ofthe Rydberg states may be more favorable compared with transitions directly to the ion. It is interesting to note at this point that all three of these normal modes involve deformation (out-of-plane or tangen­tial) of the C-N bond of substantial amplitude. It is this bond that undergoes the most significant change upon ioni­zation, developing double bond character in the ion. I7 This coupled with the fact that the molecular orbital from which the electron is being removed is partially localized on the C­N bond 11 may lead to a greater probablity for excitation to the high lying Rydberg states rather than directly to the ion for these nontotally symmetric modes.

Another possibility is that the autoionization rate for the Rydberg states reached from an intermediate state such as 601 and 11 is less than that required to compete with other processes of the neutral molecule such as predissociation, radiative, and nonradiative transitions. Assuming that these deactivation processes of the neutral are approximately in­dependent of the particular vibration excited, this would im­ply that the magnitude of the electronic matrix element (3) is strongly dependent on the vibrational motion of the core, and that the nontotally symmetric modes lOb, I, and 15, as well as several overtones and combinations, are character­ized by substantially greater autoionization rates than for the totally symmetric vibrations.

Our studies of combination vibrations involving modes lOb or I with contributions from a totally symmetric mode (60 or 1) provide indirect information regarding this point. In these instances, the optically excited high energy Rydberg states contain quanta of both totally symmetric and nonto­tally symmetric modes. Here, there is the possibility of ob­serving autoionization induced by the totally symmetric m9de, the nontotally symmetric mode, or both. This proves to be an excellent opportunity to investigate the role of spe­cific vibrational modes in the electronic matrix element (3). The PIE spectra of these combination modes show that, upon autoionization, the core loses exclusively a single quan­tum of lOb or I rather than a quantum of a totally symmetric vibration. This indicates that the modes lOb and I are more effective than the totally symmetric modes in coupling the high lying Rydberg states with the ionization continuum.

It is of some interest to compare our photoionization spectra with several predictions of vibrational autoioniza­tion theory. As one excites Rydberg levels at high energies, it is expected that the intensity and breadth of the observed autoionization bands decrease1S,23,24 as n-3• This is due to a smaller probability for photoabsorption and for vibrational autoionization as a function of increasing energy. We do, in general, observe a rather dramatic decrease in the intensity of the autoionization bands as the laser is tuned to higher energies. Typically, this decrease is indeed found to go as approximately n-3

• The decrease in linewidth is not ob­served in our studies due to the fact that there is significant spectral overlap of the bands of the four series at the energy at which this effect is expected, i.e., that corresponding to high n Rydberg bands.

One should note however that the complexity of the real

full molecule dynamics for a large polyatomic species such as aniline necessitates our consideration of the problem in terms of the coupling of a single Rydberg series and one continuum. Of course, this overlooks the multichannel char­acter of such high energy excitation of polyatomic mole­cules.26

V. CONCLUSIONS

The intense vibrational autoionization signal produced upon optical excitation of high lying aniline Rydberg states with specific vibrational excitation is a demonstration of the strong nuclear coordinate dependence of such processes. Our results put an upper limit on the aniline vibrational autoionization rate of 2 X 1012

S -I. In addition, several pre­dictions for vibrational autoionization such as the av = - I propensity and the n- 3 dependence of the ion signal have been born out in the present work. The most dramatic result however is that excitation of the nontotally symmetric modes I, lOb, or 15 is required for the observation of vibra­tional autoionization. This suggests that these vibrations serve as "promoting modes" for the aniline vibrational auto­ionization process.

There is a need for further experimental and theoretical investigations of the phenomenon. Experimental studies currently underway in our laboratory include the measure­ment of absolute ionization cross sections of aniline two­color photoionized near threshold. In addition, comparison of the present spectra with studies in which only threshold electrons are produced should show the contribution of autoionization to the total ion signal throughout the spec­tra.27

Two-color fluorescence-dip spectroscopyl8-20 enables the observation of Rydberg levels at energies below the adia­batic ionization potential. This will aid in studies of interac­tions between different Rydberg series at high energies.

Development of model potentials of polyatomic Ryd­berg levels will be useful in understanding the effective po­tential of the ionic core on the Rydberg electron. At the present time, no such model potentials without adjustable parameters are available.28

Finally, we have undertaken vibronic coupling calcula­tions in order to determine whether a simple picture of the interaction between a single vibrationally excited Rydberg state and a continuum can account for the autoionization of Rydberg states characterized by specific vibrational excita­tion.

Interaction of a bound state with a continuum is an im­portant area of research in the dynamics of polyatomic mole­cules and vibrational autoionization represents an excellent example of such dynamic coupling. The results and analysis presented here represent a first step toward the understand­ing of vibrational autoionization in "vibration-rich" aroma­tic molecules.

ACKNOWLEDGEMENT

This research was supported by the National Sciences and Engineering Research Council of Canada.

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6780 Hager, Smith, and Wallace: Photoionization of aniline

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