J. Phys. Chem. 1993,97, 9582-9586
9582
Effects of Intermediate Dissociation in the Two-Photon Threshold Photoionization of CHJI Yi-Fei Zhu and Edward R. Grant' Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received: May 12, 1993; I n Final Form: July 6, 1993'
In the threshold photoionization spectrum of CH31obtained by one-color two-photon excitation, intermediate resonance with the CH3 I dissociative continuum manifests itself in an expanded Franck-Condon envelope of accessible cation vibrational states together with the production and accompanying near-threshold twophoton photoionization of atomic iodine. The spectrum of the atomic photoproduct shows a new system of iodine autoionizing resonances converging to the 3P1state of I+. Two series are identified, [3Pl]npand [3Pl]nf. Both are distinguished by narrow line widths and high two-photon cross sections, which combine to produce distinctive signatures for these states in which electrons produced by autoionization are trapped for delayed detection in the space charge of resonantly produced I+.
+
Introduction Zero-electron-kinetic-energyor ZEKE methods for the highresolution discrimination of threshold photoelectrons show increasing promise as an important new spectroscopic tool for characterizingcation rovibrationalstructure and measuring statetestate photoionization cross sections.' ZEKE techniquesisolate transitions to successive ion internal states by taking advantage of the long field-free lifetimes of Rydberg states of very high principalquantum number that lie just below each corresponding ionization threshold.2 For certain systems, the same field-free conditions also reveal long-lived autoionizing states of lower principal quantum number.3 For molecules that have sharp intermediate states, near-threshold structure is resolved with high discrimination using vibronic photoselection via double resonance,"O while vacuum UV11-13 and nonresonant two-photon excitationl"l6 extend high-resolution threshold photoionization methods to virtually all gas-phase molecules. The case of nonresonant two-photon threshold photoionization is a particularly interesting one. In polyatomic molecules, one commonly encounters dissociativecontinua at energies halfway to ionization limits, and among examples studied thus far, results have shown that first-photon resonance with such continua can widen the Franck-Condon envelopeof accessiblecationvibrational states." In this respect, the vibrational structure associated with twophoton threshold photoionization mirrors the vibrational intensity behavior observed by us nearly ten years ago for two-photon transitions through dissociative continua to lower-lying Rydberg states of N02.17 A formalism established in that work allows one to understand Rydberg spectra observed under both circumstances in terms of second-photon transitions to manifolds of discrete states from first-photon prepared superpositions of scattering states which have very broad Franck-Condon character in bases of either the originating or final electronic state. In the case of NOz, it was shown that the character of this superposition and thus its instantaneous Franck-Condon profile for transitions to the manifold of discrete states can be affected by the power of the laser field driving the two-photon transition. Though they can be expected, similar effects have yet to be demonstrated for two-photon threshold photoionization. Methyl iodide is a system which is well-known for its photochemistryin the ultraviolet.l8 It is widely used as a source of both methyl radicals and iodine atoms for spectroscopicstudy. The dynamics of methyl iodide separation on excited-state potential energy surfaces lying midway to its ionization potential Abstract published in Advance ACS Abstracts, September 1, 1993.
0022-365419312097-9582$04.00/0
have been the subject of detailed theoretical inve~tigationl~ and provide the basis for a time-dependent formalism within which for this system in particular we can visualizethe dramatic influence of intermediate dissociation on the ultraviolet resonance Raman spectrum.20 Using methyl iodide as a source of iodine atoms, Pratt has recorded high-resolution spectra of p and f Rydberg series converging to the I+ 3P2ground state.2l Extending this work by incorporating a step of double resonance, the Argonne group has also been able to resolve autoionizing s and d series converging tothe higher 1D~stateofI+.22 Ineachofthesecases, theionization limits are relatively well isolated, and series are straightforwardly assigned. More complex and as yet uncharacterized is the region immediately above the 3P2threshold, where series converging to the nearly adjacent 3 Pand ~ 3P1limits are perturbed by discretediscrete interactions and decay by autoionization into the 3P2 continuum. In the present work we examine the two-photon threshold photoionization of methyl iodide. We find that, compared with the vacuum UV one-photon ZEKE spectrum of the same transitions, dissociative intermediate resonancein the two-photon threshold photoionization spectrum has the expected effect of transferring intensity to extended vibrational progressions, particularly in coordinatesinvolving C-I separation. We alsoobserve the two-photon threshold photoionization structure of atomic iodine photoproduced by intermediate dissociation, including, under conditions of field-freephotoexcitation,the first observation of a distinct set of odd-parity long-lived spin-orbit autoionizing states converging to the 3P1excited state of I+.
Experimental Section The Purdue ZEKE apparatus has been described in detail elsewhere.3.6 Briefly, a skimmed pulsed molecular beam of CH31/ CD31seeded for convenience in H2/D2 enters the longitudinally mounted spectrometer through a grid in the first plate of a threeelement ion-optics assembly. Voltages applied to the first two plates determine the field in the ionization region. A 1 V mm-l field between the second plate and the entrance to a grounded, magnetically shielded flight tube accelerates extracted electrons 25 cm to a multichannel plate detector. Laser light in the region from 255 to 230 nm for two-photon ionization is supplied by the output of a Lambda Physik EMG 202 MSC/FL2002 excimer-pumped dye laser doubled in &barium borate. The UV light is focused into the spectrometer by a 30-cm focal length quartz lens. The laser energy is approximately 10 mJ per pulse in the visible region. Near-threshold Rydberg states are field-ionized by application of a 3-rs delayed pulsed field of 1.4 V cm-I. To isolate ZEKE 0 1993 American Chemical Society
Two-Photon Threshold Photoionization of CH31
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9583
1 ' "
I
M
I
1
I
I
1
I 81000
82000
85000
Figure 1. Two-photon threshold photoionization spectrum of CH31in the region of the 2El/2 ionization limit, showing resonances associated with transitions to the cation vibrational ground state near 82 OOO cm-l together with a progressionof levelsexcited in theC-I stretchingvibration, v3.
Also evident in the spectrum are two-photon ZEKE transitions from
0.0
82ooO
Results Figure 1 shows the one-color two-photon threshold photoionization spectrumof CHJ in the neighborhood of its 2El/2ioni~tion limit. Apparent is a resonance corresponding to the vibrational origin at the established 2El/2ionization potential, together with higher vibrational structure, which can be assigned, as indicated by the ladder, to a 474-cm-1 progression in the C-I stretching mode, 4. Also evident in this 400-mV DC reverse bias spectrum are strong resonances which we find at the same positions in two-photon ZEKE scans of CD31. As discussed below, this invariant structure is readily assigned to the nonresonant twophoton threshold photoionization of atomic iodine. For comparison, Figure 2 shows the one-photon threshold photoionization spectrum of CH31 obtained with vacuum UV laser radiation. At higher frequency, the sharp iodine threshold photoionization structure is absent, and nearer the 2EI/zorigin, excited vibrational structure cannot be detected: Within our signal-to-noise level the vacuum UV threshold photoionization spectrum looks like the conventional He I photoelectron spectrum.24 Figure 3 shows the two-photon threshold photoionization spectrumof methyl iodide recorded under zero DC field conditions with a 3-ps delayed pulse field of 1.4 V cm-1. Dominating the delayed spectrum under these conditionsare strong features that can be recognized as discrete atomic resonances by their laserlimited widths and identical positions in spectra recorded for both CHJ and CD& A subset of these lines persists at lower signal-to-noise with pulsed-field delay times as long as 4-5 1 s .
82800
82400
83200
WV Laser Frequency (cm-l)
Figure 2. One-photon VUV laser threshold photoionization spectrum of CH31, obtained by frequency tripling U V laser radiation in Kr.
the 2P1/2 state of atomic iodine to the and 'PIlimits of I+, and just above 84 000 cm-1, from the I 2P3/2 state to the 3P2 limit. resonances without recording long-lived autoionizing structure, we apply a DC retarding field (reverse bias) of 400 mV cm-1. Alternatively, to obtain a more complex delayed spectrum including lower principal-quantum-number autoionizing states of atomic iodine, we operate under conditions of nominal zero DC bias. The residual field in this case is estimated to be less than 10 mV cm-l as monitored at the detector by the transit time distribution of slow electrons through the extraction region. For vacuum UV threshold photoionization, we fit the spectrometer with a 60cm long tripling cell containing 40 Torr of Kr gas. Input laser light from 368 to 360 nm is focused into the cell by a 20-cm focal length quartz lens. Vacuum UV output is focused into the spectrometer by a MgF2 lens with a focal length of 15.5 cm at a wavelength of 122nm.23 Laser wavelengths are calibrated by a Burleigh Model 4500 pulsed wavemeter and converted to vacuum wavenumbers.
t
I
i
jl h
7"
.
I
,
.
7 m
80000
82000
84wo
Twd%otonFrequency (cm-l)
Figure 3. Two-photon spectrum of CHsI observed under conditions of zero-field excitation with 3 ps delayed 1.4 V cm-' electron extraction, showing structure assigned to atomic iodine autoionizingRydberg series converging to the 'PI state of I+.
Interference by prompt electrons, however, prevents us from isolating signal from long-lived resonances with pulsed-fielddelays shorter than 3 ps. Figure 4 compares a portion of this zero DC field spectrum with thecation-detected I+photoionization spectrum, which arises from the two-photon ionization of photoproduced iodine atoms in a manner analogous to that operating in the experiments of Pratt and co-workers.21,22The resonances in common show that the atomic structure in the delayedelectronspectrum is coincident with intense resonant atomic photoion production. At lower laser intensities, no structure can be discerned in the delayed pulsedfield ionization spectrum, even though the cation spectrum still shows iodine atomic line structure. The ionization potentials of atomic iodine are well-known, and these sharp resonancescan be assigned immediatelyto transitions originating from neutral I 2P1/2 to autoionizing Rydberg series converging to the 3P1state of the cation. The positions of atomic features found in threshold photoionization spectra of CHJ and CD31are summarized in Table I.
Discussion Effect of Dissociative IntermediateResonance on Intensities in the Threshold Photoionization Spectrum of CH& The ground state of CH31+is formed by removing an electron from the iodinecentered 5p+ nonbonding orbital. The spin-orbit splitting of the resulting 5pu3 configuration yields substates 2E3/2and 2E1/2 separated by about 5000 cm-1. Ionization, however, leaves the
9584 The Journal of Physical Chemistry, Vol. 97,No. 38, 1993
82Mx)
82400
82800
Two-Photon Frequency (cm-')
Figure 4. Two-photon scans of CHJ comparing spectrum obtained with zero-field excitation and 3 delayed electron extraction (top) with the prompt I+ cation-detected two-photon absorption spectrum (bottom).
TABLE I: Assignment of Iodine Autoionizing Resonances Detected at Long Delay Times in the Pulsed-Field Extracted Two-Photon Threshold Photoionization Spectnuo of CHJ principal transition quantum auantum no. energv (cm-1) defect assignment I = 1 Series Converging to I+ 'PI 5 78411 0.477 5P [111/2 78 422
6
7 8 9
10 11 4 5 6 7
0.472
78 5 i i 0.401 80 036 0.582 80 052 0.571 80 117 0.523 80 122 0.520 80 176 0.478 0.639 81 063 81 460 0.623 81 759 0.431 81 860 0.315 81 922 82 206 0.638 0.473 82 266 0.454 82 273 0.367 82 303 82 341 0.254 82 569 0.463 82 607 0.317 82 786 0.469 82 809 0.345 I = 3 Series Con!rerging to I+ 3 P ~ 76 784 0.038 0.014 76 868 0.042 79 311 0.019 79 352 0.048 80 677 80 702 0.024 81 501 0.054 81 516
0.03 1
8
82 037 82 048
9
82 400 82 415
0.054 0.029 0.065 0.017
electron density in bonding orbitals virtually unchanged. As a result, one expects potential surfaces for the cation and neutral molecule to be parallel and Franck-Condon factors to strongly favor Av = 0 photoionizing transitions. This expectation is confirmed both by the conventional He I photoelectron spectrum and by the vacuum UV ZEKE threshold photoionization spectrum reported here. The two-photon threshold photoionization spectrum on the other hand shows substantial intensity in progressions of transitions
Zhu and Grant to cation vibrationally excited states, particularly in modes involving relative C-I moti0n.2~ This aspect of the threshold photoionization resembles the progressions of transitions to vibrationally excited levelsof the ground state seen in the resonance Raman spectrum of this continuum.2o Those experiments in which the properties of the intermediate continuum are sampled by its emission spectrum have been interpreted in terms of dissociative wavepacket dynamics. Here we have a stimulated process, and it is important to include the laser field in describing the progress to ionization. An appropriate formalism has been developed by us for the entirely analogous process observed in two-photon transitions through an intermediate dissociative continuum to the discrete 3pu22,+ Rydberg system of NO2.'' There we showed that the two-photon discretediscrete transition can be viewed as the second-photon photopromotion of a superposition of scattering states produced by first-photon excitation. The properties of this superposition depend on the conditions of the laser field. In particular, its breadth depends on the power of the driving field. As an effect of this, we observe in NO2 that the distribution of intensities across the envelope of discrete vibrational states accessible in second-photon absorption varies with laser power. Our present dynamic range is insufficient to study the power dependence of vibrational intensities in the two-photon ZEKE spectrum of CHJ, but we would expect to find for higher powers (broader intermediate superposition) that the distribution of intensities would tend toward that of the vertical vacuum UV threshold photoionization spectrum.26 By the sametoken, a timeresolved probe of intermediate wavepacket dynamics by subpicosecond pump-probe threshold photoionization should show directly the evolution of the ZEKE spectrum from vertical to Franck-Condon-extended. Such effects have been demonstrated by means of time-of-flight photoelectron spectroscopy by Reilly and co-workersZ7and using ZEKE methods by Knee and coworkers.28 Threshold Photoionization of Atomic Iodine. In addition to ZEKE structure that can be assigned to thresholds for forming ground-state and excited molecular cations, we find strong moderately sharp ZEKE resonances a t 76 692,83 135, 83 778, and 84 295 cm-l in the spectra of both CH31and CD31. In contrast to atomic line structure discussed below, these resonances persist with the application of a DC reverse bias in the range from 400 mV to 1 V cm-'. The positions of these resonances with respect to those of the ground 'P3/2 and excited 2P1/2 iodine atoms immediately identify this structure as arising from the threshold photoionizationofiodine. Thepeakspositionedat 76 692,83 135, 83 778 cm-1 correspond to ZEKE transitions from I 2PI/2 to the 3P2,3Po,and 3P1ionization limits, respecti~ely.~~ The resonance a t 84 295 cm-' reflects the transition from I 2P3/zto the 3P2limit. D ~ ~ ~ Y ~ ~ S ~ N C ~ Ut~[Tllnpand[~P&fR~dbergSerie~ WASS~~DHI of Atomic Iodine. With zero DC bias, the delayed pulsed-field ionization spectrum exhibits a dense, obviously atomic line structure. Because the 1.4 V cm-l pulsed field applied to extract ZEKE electrons field ionizes only Rydberg electrons bound by no more than 7 cm-I, the delayed electrons associated with these resonances must be the product of autoionization, detected upon application of the pulsed field. Examination of the positions of these sharp lines with respect to the ionization limits of atomic iodine shows that nearly all such structure arises from two-photon transitions which originate in the excited 2P1/2state of atomic iodine and terminate in Rydberg series of odd I converging to the 3P1third ionization threshold. On the basis of quantum defects and spin-orbit splitting patterns, we assign these series as iodine [3Pl]np and [3Pl]nf, as indicated in Table I. This JJ angular momentum convention should be the appropriate one for labeling series in this case because splittings among the substatesproduced by the coupling of a relatively high Rydberg
Two-Photon Threshold Photoionization of CH31 electron to the I+ 5p4 core are much smaller than the splittings between the spin-orbit states of the cation. In this coupling scheme,29 the total angular momentum of the I+ ion core (Jc) combines with the orbital angular momentum of the Rydberg electron (I) to give the resultant K. The spin of the Rydberg electron then couples to K to form the total angular momentum J. For the ground 'P configuration of the I+ core, the resulting states are labeled as in Table I, (3Prc)n l [ q J . From this perspective,we find that the f-series are well defined, returning patterns of consistent splittings and quantum defects near zero. The series that we label p is weaker and apparently perturbed near n = 7. Using spin splittings of states of equal K as a guide, we find that the term energies of the p-series, ordered for lower principal quantum numbers, [2]3/2,5/2,[111/2,3/2r [011/2, appear to invert to fit the order established throughout for the f-series. This may be either a local perturbation or, more likely, a transition in angular momentum coupling limit for the more penetrating series. Because I 2P!/2, is a major photofragment of methyl iodide around 240 nm, it is not surprising to find two-photon transitions to iodine atom Rydberg states originating from this level of the ground state. More surprising is the presence of signal corresponding to these resonances at long delay. Two factors control the lifetimes of these states which we detect by delayed electron extraction, spontaneous autoionization, and radiative decay. The autoionization of atomic iodine has been investigated by Berkowitz and co-workers using ionization-detected one-photon absorption.30 These experiments have resolved s- and d-like Rydberg series converging to the higher ionization limits of the 5s2 5p4configurations. The widths observed for these resonances are broader than the instrumental spectroscopicresolution. Pratt and co-workers have applied 2 1 double-resonancetechniques to study ns and nd series converging to the ID2 limit.22 These transitions show relatively large widths for low n (n = 15), but the widths decrease with increasingn. Nevertheless,all resonances imply lifetimes for even-parity lD2-core Rydberg states in the picosecond regime, in agreement with theory.29 In a core-coupling scheme intermediate between LS and j j , the 3Pl state of I+ is unique among those derived from the 5sz 5p4 core configuration. 3 P is~ strongly coupled with 50,with the result that 3Po is energy inverted 639 cm-l below 3P1,while 3P2 is mixed to a comparable degree with ID2. Specific interactions between discrete Rydberg states and Rydberg-continuum couplings can be expected for all core configurations in a Jcl coupling basis, but the number of terms remains smallest for [3P~]nlstates. Nevertheless, autoionization lifetimes of microseconds seem unlikely across the complete spectrum of series converging to this limit. Furthermore, though interferences arising from configuration interaction can be expected to modulate transition moments,31 emission lifetimes from [3Pl]np and [3P~]nfseries to lower lying [3Pl]nsand [3Pt]nd states should fall generally in the sub-microsecond regimea2* Thus, we believe that electrons associated with atomic resonances detected at long delay times in zero field should be regarded as products of relatively prompt autoionization. What distinguishes these resonances and probably accounts for their appearance in the delayed spectrum is the concentration of oscillator strength in the very narrow lines associated with the relatively weak coupling of [3Pl]np and [SPllnf Rydberg states with the 3P2 continuum. Such narrow line widths produce very high cross sectionson resonance, with correspondingly high cation densities. Under zero-field conditions, space charge, such as that produced by sharply resonant autoionization, is known to trap electrons, which can be extracted microseconds later by a delayed pulsed field.32 As a result, in this case the delayed electron spectrum reproduces the prompt ionization spectrum of sharp resonances. Shorter-lived autoionization resonances do not register in the
+
The Journal of Physical Chemistry, Vo1. 97, No. 38, 1993 9585 delayed spectrum because broadened lines yield insufficientcation density at any given wavelength to trap electrons.
Conclusions Comparing the one-color two-photon ZEKE spectrum of CH31 with that obtained by vacuum UV laser threshold photoionization, we find that intermediate resonance with the dissociative continuum encountered in two-photon excitation broadens the Franck-Condon envelope of accessible ion vibrational states. Intermediate dissociation produces atomic iodine, and at laser frequencies near the two-photon ionization threshold for CH31, we observe the two-photon spectrum of a new system of iodine atomic autoionizing resonances converging to the 'P1 state of I+. Two series are identified, [3Pl]np and [3Pl]nf. Both are distinguished by narrow line widths and high two-photon cross sections,which combine to produce distinctivesignatures for these states in which electrons produced by autoionization are trapped in the space charge of resonantly produced I+.
Acknowledgment. This work was supported by the National Science Foundation (Grant CHE-9307131). WethankI. Fischer and K. Miiller-Dethlefs for stimulating discussions and communication of results prior to publication. E.R.G gratefully acknowledges the Alexander von Humboldt Foundation for financial support and the Institut fiir Physikalische und Theoretische Chemie der Technischen Universitat Miinchen for its warm hospitality over the course of a visit during which this manuscript was completed. References and Notes (1) Miiller-Dethlefs, K.; Sander, M.; Schlag, E. W. 2.Nuturforsch., A 1984,39,1089. Miiller-Dethlefs, K.;Schlag, E. W. Annu. Rev. Phys. Chem. 1991, 42, 109. (2) Chupka, W. A. J. Chem. Phys. 1993, 98,4520. (3) Haber, K. S.; Jiang, Y.; Bryant, G. P.; Grant, E. R.; Lefevbre-Brion, H. Phys. Rev. A 1991,44, R5331. (4) Reiser, G.; Habenicht, W.; Miiller-Dethlefs,K.;Schlag, E. W. Chem. Phys. Lett. 1988, 152, 119. ( 5 ) Habenicht, W.; Reiser, G.; Miiller-Dethlefs, K.J. Chem.Phys. 1991, 95. - 4809. ,- - (6) Bryant, G. P.; Jiang, Y.; Martin, M.; Grant, E. R. J . Phys. Chem. 1992. 96. 6875. (7) Harrington, J. E.; Weisshaar, J. C. J . Chem. Phys. 1990, 93, 854. (8) Zhu, L.; Johnson, P. J . Chem. Phys. 1991.94, 5769. Hillenbrand, S.;Zhu, L.; Johnson, P. J. Chem. Phys. 1991, 95, 2237. (9) Zhang, X.;Smith, J. M.; Knee,J. L. J. Chem. Phys. 1992,97,2843. (10) Takahashi, M.; Ozeki, H.; Kimura, K.Chem. Phys. Lerf. 1991,181, 255. (11) Tonkyn, R. G.; White, M. G. Rev. Sci. Instrum. 1989, 60, 1245. Tonkyn, R. G.; White, M. G. J . Chem. Phys. 1991,95, 5582. Wiedmann, R.; Grant, E. R.; Tonkyn, R. G.; White, M. G. J. Chem. Phys. 1991,95,746. Tonkyn, R. G.; Wiedmann, R.; Grant, E. R.; White, M. G. J . Chem. Phys. 1991. 95, 7033. (12) Merkt, F.; Softley, T. P. J . Chem. Phys. 1992,96,4149. Merkt, F.; Softley, T. P. Phys. Rev. A 1992, 96, 302. (13) Kong, V.; Rodgers, D.; Hepburn, J. W. Chem. Phys. Lett. 1993,203, 497. (14) Fischer, I.;Strobel, A.; Staecker, J.; Niedner-Schatteburg, G.; MullerDethlefs, K.; Bondybey, V. E. J. Chem. Phys. 1992.96, 7171. Strobel, A.;
Fischer, I.; Staecker, J.; Niedner-Schatteburg. G.; Miiller-Dethlefs, K.; Bondybey, V. E. J. Chem. Phys. 1992, 97,2332. (15) Fischer, I.; Lochschmidt, A.; Strobel, A.; Niedner-Schatteburg, G.; Miiller-Dethlefs, K.; Bondybey, V. E. Chem. Phys. Lett. 1993, 202, 542. (16) Fischer, I.; Lochschmidt, A.; Strobel, A.; Niedner-Schatteburg, G.; MBller-Dethlefs, K.; Bondybey, V. E. J. Chem. Phys. 1993, 98, 3592. (17) Bigio, L.;Grant, E.R.J. Chem. Phys. 1985, 83, 5361, 5369. (18) See for example: Sparks, R. K.;Shobatake, K.; Carlson, L. R.; Lee, Y.T.J. Chem.Phys. 1981,75,3838. Hermann, H. W.; Leone, S. R. J. Chem. Phys. 1982, 76, 4759,4766. (19) Shapiro, M.; Bersohn, R. J. Chem. Phys. 1980,73,3810. Johnson, B. R.; Kinsey, J. L.; Shapiro, M.J . Chem. Phys. 1988,88, 3147. Lee, S. Y.; Heller, E.J. J . Chem. Phys. 1982,76,3035. Guo, H.; Schatz, G. C . J. Phys. Chem. 1991, 95, 3091. Guo, H. J. Chem. Phys. 1992, 96,6629. Rist, C.; Alexander, M. H. J. Chem. Phys. 1993, 98,6196. (20) Imre, D.; Kinsey, J. L.; Sinha, A,; Krenos, J. J . Phys. Chem. 1984, 88, 3956. (21) Pratt, S. T. Phys. Rev. A 1985, 32, 928.
9586
(22) Pratt, S. T.; Dehmer, P. M.; Dehmer, J. L. Chem. Phys. Lett. 1986,
-I26. - -,12. - -.
Zhu and Grant
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
(23) Karlsson, L.; Jadrny, R.; Mattsson, L.; Chau, F. T.; Siegbahn, K. Phys. Scr. 1977, 16, 225. (24) Hilbig, R.; Wallenstein, R. IEEE J. Quuntum Electron. 1981, 17, 1566. (25) After completing this work, we learned of similar experiments by the Munich group of V. E. Bondybey. Their spectra offer a wider view of progressions of CHJ+ vibrations in the two-photon thrahold photoionization of CH31 which support the conclusions drawn here about the influence of intermediate dissociative resonance on discretbdiscrete Franck-Condon factors. The v3 vibrational frequency observed in our spectrum of CHI!+ agrees reasonably with the value of 478 cm-1 obtained from their analysis. See: Strobel, A.; Lochschmidt, A.; Fischer, I.; Niedner-Schatteburg. G.; Bondybey, V. E. J. Chem. Phys. 1993, 99,733.
(26) Such a power dependence may account for differences between our two-photon threshold photoionization spectrum and the longer progressions observed by Strobel et al.," which were recorded under softer focusing conditions. (27) Song, X.;Wilkerson, C. W., Jr.; Lucia, J.; Pauls, S.;Reilly, J. P. Chem. Phys. Lett. 1990,174,377. (28) Smith, J. M.; Labhminarayan, C.; Knee,J. L. J. Chem. Phys. 1990, 93, 4475. (29) Minnhagen, L. Ark. Fys. 1962, 21,415. (30) Berkowitz, J.; Batson, C. H.; Goodman, G. L. Phys. Reo. A 1981, 24, 149. (31) Cowan,R. D. The TheoryofAtomicSfructureundSpcctru;Univenity of California Press: Berkeley, CA, 1981; pp 4 3 2 4 . (32) Weisshaar, J. C.; Grant, E. R.; MiIller-Dethlefs, K. Unpublished
results.
