J. Phys. Chem. 1994, 98, 3501-3505
3501
Ion Dip Spectroscopy of Benzene van der Waals Clusters 0. Kratzschmar, H.L. Selzle, and E. W. Scblag' Institut f i r Physikalische und Theoretische Chemie, Technischen Universitbt Miinchen, Lichtenbergstrasse 4, 0-85747 Garching, Germany Received: December I , 19938
Mass-selected ion dip experiments were performed to study ground-state vibrational frequencies of benzene clusters. It is shown here that vibrationally excited states are accessible which are well above the dissociation limit of the cluster. In the benzene trimer also van der Waals modes in the electronic state could be observed by stimulated emission pumping; however, only transitions with Au = 0, -1 in the van der Waals modes were found.
Introduction Van der Waals (vdW) clusters are the object of intense research, and hence much information about cluster structure, spectroscopy, and dynamicsexists. Most of these studiesdeal with electronically excited clusters which arise from fluorescence and ionization detected experiments. Here the main problem in all cluster experiments is that neutral molecular clusters, except for a few special cases like reneutralization,' cannot be produced in a sizeselected manner. The supersonic jet, which is normally used for their generation, always produces a size distribution. Thus, measurements of the electronicground state from laser-induced fluorescence or IR-laser experiments produce spectra which are difficult or impossibleto assign, if no simultaneousmass detection is applied. One solution to this problem is ion dip spectroscopy (IDS), a method which combinesstimulatedemission pumping (SEP) with multiphoton ionization. SEP is a double-resonancetechnique in which a molecule is resonantly excited by a first laser pulse (pump) to an electronically excited state. With a second tunable laser (dump) transitions to the electronic ground state are induced at proper wavelengths by stimulated emission. This then decreases the population of the excited state, which can be monitored by observing the total fluorescence or multiphoton ionizationsignal. While scanning the dump laser, dips in the fluorescence (fluorescence dips) or ionization signal (ion dips) can be observed. The fluorescence method implies knowledge of the excitation spectrum of a given cluster before selecting the proper pump transition, which has to be spectroscopically well separated from transitions of other clusters to obtain size selectivity. Ionization detection together with a mass spectrometer easily allows size determination, and hence IDS gives immediate information on the ground state of a size-selected cluster. IDS was developed by Cooper et al. to identify higher electronicstates of 12.2 Later, Suzuki et al. used the power dependence of the ion dip intensity for measuring intramolecular vibrational redistribution (IVR) rates in tilb bene.^ Vibrational predissociation rates of NH3 were reported by Xie et aL4 This technique can also be applied to weakly bound vdW clusters; here, Frye et al. measured fluorescence dips of the glyoxal.-Ar5 and glyoxal.-Ar2 vdW complex: and Stanley et al.7 measured ion dips of phenylacetylene-NH3. Our first measurementsof benzenes produced rotationally resolved ion dip spectra which resulted in a width of about 1 cm-1 for the transition to the SO62 vibrational state. Corresponding ion dip measurements were also performed for the fluorobenzene-Ar ~ o m p l e x .From ~ this it could be seen that the IDS is well-suited for the investigation of intramolecular vibrations in the ground state well above the dissociation energy of the complex. This
* Abstract
published in Aduuncc ACS Absrrucrs, March 1, 1994.
0022-365419412098-3501$04.50/0
techniqueis not limited to the study of intramolecularfrequencies in clusters in the electronic ground state but can also be used to study intermolecular vdW modes. Venturo et al.1° have shown from nonlinear Raman spectroscopy the Occurrence of vdW modes in the spectrum for benzene-noble gas clusters and also for the benzene dimer." In our experiments we perform initial measurements on larger clusters of benzene up to the benzene tetramer. Experimental Section The experimental setup was described previously? and only a short description is given here. The jet apparatus consists of a nozzle chamber and a main chamber to which a reflection timeof-flight mass spectrometer is attached. The supersonic jet is produced by seeding benzene-hs at 273 Kin helium with a backing pressure of 5 bar and expanding the mixture through a piezodriven pulsed nozzle (100-ps fwhm) with a 0.2-mm-diameter orifice into the vacuum. For the experiments with benzene-Ar clusters He with 10% Ar was used for the driving gas. The jet is skimmed 3 cm downstream by a skimmer with a 1-mm opening and enters the ion optics 8 cm downstream of the nozzle. Two separate dye lasers (Quanta Ray PDL-l), each pumped by a Nd:YAG laser (Quanta Ray DCR-1A), are used. With the second harmonic ofthe first dye laser (Coumarin 307) the resonant excitation to the intermediate SI state is performed. The massselected signal from photoionization with a second photon of this laser can be used to position this laser to a fixed transition of the desired intermediatestate of a given cluster. The second harmonic of theother dyelaser is used for ionization and stimulated emission. The bandwidth of both lasers is 0.3 cm-I in the UV. Both laser intensities are measured with photodiodes and used for normalization of the spectra. Wavelength calibration was performed by simultaneously recording a iodine emission spectrum. From ionizing the clusters in a static electric field the onecolor and two-color signals can be separated by applying a time delay between the two laser pulses. The two ion packets originating from the same point in space have different starting times and therefore appear as two separated peaks in the mass spectrum. With this technique it is possible to separate one- and two-color contributions and obtain a pure two-color signal. The optimal time delay between the two laser pulses depends on mass resolution and the lifetime of the intermediate state. In this experiment the ionizing laser is electronicallydelayed 43 ns by a digital delay generator (DG 535, Stanford Research). Both laser beams enter the main chamber coaxially from opposite directions. Two 220-mm-focallength lenses are used to combine the two laser beams in a common focus on the center of the supersonic beam. The ions produced are extracted with a two@ 1994 American Chemical Society
3502 The Journal of Physical Chemistry, Vol. 98, No. 13, 1994
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cm -1 Figure 1. Ion dip spectra of the benzene-Ar complex. (a) Excitation spectrum of the 6; vibrational band. Three vdW modes a, b, and c are observed. (b) Ion dip signal from the intermediate main 6; peak versus the frequency differenceof the pump and probe lasers in the range of the 6; transition. (c) Ion dip signal from the intermediate main 6; peak versus the frequencydifferenceof the pump and probe lasers in the range of the 6; transition. stage ion optic into the reflectron time-of-flight mass spectrometer. The total drift length is 2.2 m, and a resolution of m/Am of 5000 is achieved. The pressure in the mass spectrometer is below 5.0 X lo-' mbar during operation. The mass spectra are recorded by a transient digitizer (Tektronix RTD 7 lo), averaged, and then digitally integrated by a microcomputer (FORCE Target 32), which also controls the experiment and records the integrated laser intensities and renormalizes the spectra.
Results Benzene-Ar. As the first benzene cluster, we examined the benzene-Ar complex, which has been the object of several studies.I0J2-l6 In this complex the argon atom is centered on the benzene molecule, conserving the 6-fold symmetryaxis of benzene. Therefore, the SI00 transition is forbidden as in the bare benzene molecule, and the 61 state is the lowest possible intermediate state with sufficient oscillator strength to perform ion dip experiments. Figure l a shows a one-color 6; excitation spectrum near the 6; band of the benzene-Ar complex, which is red-shifted by 21 cm-I I 3 , l 4 relative to the monomer spectrum. The small features around the 6; transition, which are marked by a star in the spectrum, result from the fragmentation of higher clusters, mainly from the benzene-Ar2 complex. The remaining peaks a t 3 1,40, and 61 cm-I are assigned to vdW modes.13 For obtaining ion dip spectra themain peakat 38 586cm-1 was usedas theintermediate state.
Figure 2. Excitation spectra of the benzene trimer. (a) The 0; band. A and B represent the intramolecular 0; transition, and C to F represent additionallyexcited vdW modeswith comparableintensityto the molecular transition. (b) The 6; band. A and B reprcsent the intramolecular 6; transition, and C to F represent additionallyexcited vdW modm with
comparable intensity to the molecular transition. Here the lines are broader compared to the 0; band, and also a broad background is observed. When recording ion dip spectra, the second laser has to be scanned above theionization threshold of the benzens-Ar complex of 74 404 cm-I.I7 For the S16I intermediate state the necessary energy amounts to about 35 820 cm-'. This energy, subtracted from the SI 6' energy, limits the highest accessible vibrational energy to 2760 cm-1 in the ground state for SEP transitions. The selection rule Au6 = A1 for benzene leads to 6: and 6Al; as possible transitions for the stimulated emission in this energy range. Figure 1b shows the spectrum with stimulated emission to the 62 intramolecular ground-state vibration, and Figure IC shows the corresponding spectrum for 6011. Both spectra show sharp dips with depths of 20-3096. The line widths are comparable to that of the 6; transition, although the intramolecular vibrational energies in the ground state for both transitions are higher than the ground-state binding energy of about 460 cm-I,** There are no indications for vdW modes within the limit of our signal-tonoise ratio. BenzeneTrimer. A more complex system than the benzene-Ar complex is the benzene trimer. In this cluster the 0; transition is allowed, and strong vdW modes can be seen in both the 0; and 6; spectra. The 0; region is shown in Figure 2a. The 0; band is split by 1.6 cm-1, and several groups of peaks are observed showing a similar splitting and are assigned to vdW modes.*99*0 From the first group of peaks two intermolecular frequencies of 19.2 and 22.8 cm-1 can be obtained for the electronically excited state. In Figure 2b the 6; spectrum is shown. The 6; band here is split by 3.6 cm-I, and again intense vdW modes are found. In this case the larger splitting is a result of removing the v6 degeneracy. From the spectrum vdW frequencies of 19.1 and 23.0 cm-1 are obtained. The values are the same as those from the 0; spectrum within the experimental error. Ion dip exper-
Ion Dip Spectroscopy of Benzene vdW Clusters
The Journal of Physical Chemistry, Vol. 98, No. 13, 1994 3503
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Figure 4. vdW mode ion dip spectra for the benzene trimer. Shown here is the ion dip signal versus the difference of the fixed frequency pump laser and scanned frequency of the probe laser. The different scans A to E originate from using the correspondingintermediate statesof Figure 2. A and B represent ion dip spectra using the intramolecular 0; transition for the intermediatestate. For C, D, and E levels with additional
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Figure 3. Ion dip spectra of the benzene trimer. Shown is the ion dip signal versusthe differenceof the fixed frequencypump laser and scanned frequency of the probe laser. (a) Ion dip signal from the intermediate 0; peak, The 6: transition shows a splitting of 1.2 cm-I. (b) Ion dip signal from the intermediate 0; peak. The transitions to the Fermi resonance pair 6ll1/81 are separated by 18.2 cm-I. (c) Ion dip signal from the intermediate0; peak. The transitions to the Fermi resonance pair 61l2/8111 are separated by 25.6 cm-l.
iments were performed by exciting either the 00 or the 6l level. In the 0; region vdW modes were also used as intermediate states. Exciting the @ Band. In benzene the selection rules for transitions between s1 and S,J allow transitions with Av6 = fl. Exciting the SI00 band as the intermediate-state transitions to the ground state with 61 and 611, final vibrational states will be allowed. In benzene the SO6111 state undergoes a well-known Fermi resonance. with the SO8lvibrational state. The coupling constant is on the order of 18.2cm-1.2122Similar Fermi resonances are observed for the higher members of both progressions.23 In the benzene trimer the ionization potential is lowered by 5350 cm-1 24c~mpared to the bare benzene molecule, and this increases the accessible energy range in the ground state for SEP to about 6700 cm-I. In Figure 3a the ion dip spectrum in the 6: region of the trimer is shown. Two sharp dips with an intensity ratio of 4 5 can be observed. The splitting of 1.2 cm-1 is much smaller than in the excited state, where the splitting of the 6; transition is 3.6 cm-I. From the spectrum two values of 608.5 and 607.2 cm-1 can be obtained for the ground-state v6 frequency, which are in good agreement with the monomer value of 608.1 cm-1.25 Two additional results are very important: (i) No vdW modes were detected, although they are very strong in the excitation spectrum and the signal-to-noise ratio of the dip spectrum seems to be sufficiently high. (ii) The stimulated transitions are very
sharp, which is in contrast to the dispersed fluorescence studies of Langridge-Smith et a1.,26 where a line width of 200 cm-1 for the spontaneous emission was reported. Figure 3b now shows the 6;1?/8y ion dip spectrum of the trimer. Two dips with a separation of 18.5 cm-I are found. The resulting ground-state frequencies are 1588.7 cm-' (a) and 1607.2 cm-I (b). This values are about 3 cm-l lower than the corresponding benzene frequencies of 1591.3 and 1609.5 cm-l.22 In Figure 3c the 6y1!/8yl;ion dip spectrum is shown. Again, two sharp dips are observed while no vdW modes could be found within the limit of the present experimental conditions. The separation of 25.6 cm-1 is in good agreement with the monomer value of 26 cm-l.23 It should be noted that the intramolecular vibrational energy is about 2600 cm-I in this case, which is 1000 cm-l higher than the dissociation energy of the trimer of about 1620 cm-I.24 Exciting vdW Modes in the Benzene Trimer. While the excitation spectra show strong vdW modes, in all of the ion dip spectra shown so far no dips could be detected which could be assigned to transitions to vdW modes. This puzzling result showing no vdW modes for the SEP experiment is in contrast to the observability in the excitation spectra. We therefore tried the reverse experiment starting from an excited vdW mode in the electronically excited state. As intermediate state levels we chose peaks C to E of the excitation spectrum of Figure 2a. In Figure 4 the results of these experiments are presented. The spectral region shown is the transition to the 61 vibrational band in the ground electronic state. As shown before, using peaks A and B of the 0; spectrum corresponding to purely intramolecular excitation no dips for vdW mode excitation could be found. However, if the first vdW mode in SI(C) is excited, two dips are observed. The one at lower energy arises from the SEP to the 61 intramolecular state (Av,dW = -I), and the second one arises from additionally exciting a vdW mode with a frequency of 19.5 cm-I (Av,dW = 0). The peaks also show a small splitting of about
3504 The Journal of Physical Chemistry, Vol. 98, No. 13, 1994
Krfitzschmar et al.
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cm -1 Figure 5. vdW mode ion dip spectrum for the benzene trimer. Shown here is the ion dip signal versus the difference of the fixed frequency pump laser and scanned frequency of the probe laser. For the resonant intermediate state the peak C of Figure 2a is used. Here also the SEP signal shows transitions to vdW modes in vibrational band of the Fermi resonance pair 6111/81. Peak c arises from pcak b from additional excitation of a vdW mode of 19.5 cm-I. The vdW peak of transition a is hidden below peak b.
1cm-I. No transition tostatesinvolving highervdW modes ( b d w 1 1) could be observed. We also performed the ion dip experiment for peaks D and E of Figure 2a. Here a similar result was found: one transition to the purely intramolecular 61 mode and one to a state with an additional excitation of a vdW mode of about 19 cm-I. It is interesting to note that no dips were observed using peak F of Figure 4 as the intermediate state. The SEP experiment with dumping to the 6111/81states was also performed using peak C of Figure 2a as the intermediate state. Here the separation of the two transitions to the Fermi resonance pair of 18.6 cm-I is nearly identical to the vdW mode of 19.5 cm-I. The spectrum of Figure 5 shows the superposition of the ion dip signal from dumping to the intramolecularvibrations and the states with additional vdW mode excitation. Peak c corresponding to b d w = 0 represents the additional vdW mode excited state of peak b (AuvdW= -1). The vdW mode transition belonging to peak a is hidden below peak b. It is interesting to note that in thiscaseagain no transitionswith b d w 1 1 populating higher vdW modes could be observed. Exciting the Trimer 6; Band. In addition to the 0; as intermediate state, we also tried to measure dip spectra with excitation of the 6: band of the trimer. Allowed transitions to the ground electronic state are the progressions 6il: and 6A1:. As in the case of the benzenwAr complex, the transitions 6: and 6Al: were studied, but no dips could be found. However, IDRSR experiments of Henson et al.,27.28where the 6;l: transition was used for probing the population of the SI11 state of the dimer, showed that this molecular transition has sufficiently large oscillator strength and should be observable. The failure of the ion dip experiment therefore is probably due to fast IVR in the SIstate relative to the delay time of 40 ns used for the probing laser. This assumption is further supported by measurements of Langridge-Smith et a1.26 They measured dispersed fluorescence spectra of the trimer after exciting the OOor 61 state and compared them with the corresponding monomer spectra. In benzene the selection rule h u g = f l is very strong. Therefore, after the 00 excitation only the progression 6y1: is supposed to be found, and if the 6l state is excited, only progressions 6i1: and 6:l0 should a probe observed. In the trimer spectrum, however, the 611: gression was found independently from the excitation of either the 00 or the 6l state. The 6i1: could not be detected in either case. This showed that from exciting the SI 6l state an emission spectrum was obtained, which was characteristic of the Oo excitation. This and the observed line width of 200 cm-1 were explained by fast IVR processes. Their finding of a fast IVR
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process can now well be the reason for the absence of IDS spectra when exciting the S2 6l vibrational band. The difference here between the measurement of dispersed fluorescenceand the SEP is given by the fact that in the first case a time-integrated signal is observed whereas in the second case real time information can be obtained. The measurements here show that for the 6l excitation a fast IVR process on a time scale shorter than 40 ns has occurred, which is manifested in the absence of the SEP signal. This fast processes will also be the reason for the broadening and the background observed in the excitation spectrum of the SI6l band. Ion Dip Spectra of the Benzene Tetramer. The measurements of SEP spectra for benzene clusters could also be extended to the benzene tetramer. Due to the low abundance in the jet only the strong 0; transition was used to prepare the intermediate state. From this, two vibrational bands in the ground state were probed. Figure 6a shows the spectrum of the6; transition. Two dips with a splitting of 1.5 cm-l are observed, but no additional vdW mode excitation could be detected. The resulting ground-state frequencies are 608.5 and 607.0 cm-l, which are in good agreement with the results for the trimer and with the monomer frequency. The 6?1:/8y spectrum in Figure 6b is similar to the trimer spectrum. From the observed two dips the frequencies for the Fermi resonance pair of 1585.1 cm-l (a) and 1607.3 cm-I (b) are obtained. As in the case of the benzene trimer, these frequencies are about 3 cm-1 lower than the corresponding monomer frequencies. Dimer. While it was possible to obtain ion dips in the case of trimer and tetramer, all experiments with the dimer failed. This result is consistent with the experiments of Hopkins et al.,19 who could not observe a fluorescenceemissionof thedimer. LangridgeSmith et alez6also reported a very low fluorescence quantum yield. The benzene dimer is an interesting case as it is known that the excitation spectrum is the sum of the spectra of three different conformationalisomers.29Only the strongest absorption band belonging to the absorption in the benzene forming the stem of the T-shaped conformer was used in our search for ion dip signals. The structure of the dimer is supposed to be very and geometric changes in the excited state could be the reason for the absence of the ion dip spectra.
Ion Dip Spectroscopy of Benzene vdW Clusters
Discussion In the stimulated ion dip spectroscopy the observability of the spectra depends on the oscillator strength for the probing transition to the ground state relative to the ionization probability from the intermediate state level. It also depends strongly on the time evolution of the intermediate state as the probe laser has to be delayed for the separation of one and two color signals. In the case of the benzene-.Ar complex the weak oscillator strength found in the excitation spectrum for the vdW modes probably should also apply for the SEP and is most probably the reason for not detecting ion dip transitions for the vdW modes within our experimental limits. In summary, the results of our ion dip experiments for the benzene clusters are the following: (i) For the benzene trimer and tetramer dips were found when exciting the 0; band. (ii) For the dimer no ion dips could be obtained, nor for 6; excitation of the trimer. The absence of ion dip signals in the second case is in agreement with the fluorescence measurements of Hopkins et al.19 and Landridge-Smith et a1.26 Both groups found an abnormally low fluorescence quantum yield for the dimer. Langridge-Smith et a1.26 succeeded in obtaining dispersed fluorescence spectra of the trimer when exciting both the SI00 and SI61 levels. Here the wavelength-resolved spectra were nearly the same for the 0; and 6; excitation, which was explained by a fast IVR process. The extreme width of 200 cm-1 of the emission from the SI00 state had to be explained from relaxation to an excimer state. The sharp dips we found in our stimulated emission spectra are a striking difference to the spontaneous emission spectra for the”0; excitation mentioned above. A possible explanation in the dimer case can be derived from ab initio calculation of Hobza et al.,3O which showed that the dimer has a floppy structure and conformational changes can occur without or at least with a very low barrier. If the rate for changing the geometry is very high, and the different conformations have Franck-Condon factors only to distinct ground-state geometries, then the transition intensity can spread over a wide range, as it is in the case of IVR. The trimer, however, seems to be more rigid, as it can be concluded from the quite different excitation spectra. Therefore, the rate of conformationalchange is probably much slower. Our results show that on the time scale of 40 ns no significant change occurs when the SI ground vibrational state is excited. From the fact that dips do not show any splitting for the 6111/81 Fermi resonance pair, it can be concluded that there is no site splitting in the trimer. Therefore, the ground-state trimer structure should be cyclic with equivalent, nondistinguishable benzene molecules. The observed splitting of the 6; dip is then only due to the elimination of the degeneracy of the Vg vibration in the cluster. To explain the absence of ion dips for the 6; excitation, a fast IVR process in the excited state has to be assumed. In a cyclic structure the in-plane V6 mode can couple to the intermolecular modes and give rise to a relaxation on a time scale faster than the 40-ns delay of the probing laser. In the case of the benzene tetramer the lack of a splitting of the 6111/81 Fermi resonance pair indicates a geometry for the tetramer, in which all benzene molecules are nearly equivalent. This result is in contrast to measurementsof the power dependence of the IR photodissociation of small benzene clusters which lead to the conclusion that the benzene molecules are nonequivalent.31 Our experiments support more the theoretically calculated structure32where a tetrahedral arrangement with two pairs of nearly equivalent molecules is found.
Conclusion Ion dip spectroscopy, particularly when coupled with massselected detection, is a direct method to obtain information on
The Journal of Physical Chemistry, Vol. 98, No. 13, 1994 3505 vibrational states in the ground state of molecular clusters. The weak coupling of the intramolecular modes to the intermolecular vdW modes also permits the study of vibrational levels above the dissociation energy of the cluster. In our experiment we could demonstrate this method for the study of molecular vibrations in the ground state of benzene clusters which indicate a structure for the benzene trimer with three equivalent molecules. We see three bands in the ground state of the benzene trimer with mass selective detection. These show the signature of the vl, V 6 , and U S ground-state vibrations and are all accessed from the vibrationless level of the first electronic excited state. There was no ion dip signal from the Vg excited state, presumably due to IVR in accord with the previous results. The fluxional character of the benzene dimer prevent this speciesfrom been observed as well. Similarly for the tetramer we see the mass-selected signature of VI, V 6 , and US ground-state vibrations. We also could directly excite vdW modes in the electronic ground state from SEP. Here for benzene trimer the interesting propensity of Au,dW = 0,-1 for stimulated emission tovdW modes was observed. This represents the first vibrational assignments of the mass-selected benzene trimer and tetramer in the ground state and direct evidence for the symmetric structure of the trimer.
Acknowledgment. Financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References and Notes (1) Amold, M.; Kowalski, J.; zu Putlitz, G.; Stehlin, T.; TrHger, F. Z. Phys. A 1985,322, 179. (2) Cooper, D. E.; Klimcak, C. M.; Wessel, J. E. Phys. Rev. Let?. 1981, 46,324. (3) Suzuki, T.;Mikami, N.; Ito, M. J. Phys. Chem. 1986,90, 6431. (4) Xie, J.; Sha, G.; Zhang, X.; Zhang, C. Chem. Phys. Le??.1986,124, 99. (5) Frye, D.; Arias, P.; Dai, H.-L. J. Chem. Phys. 1988,88,7240. (6) Frye, D.;Lapierre, L.; Dai, H.-L. J . Op?.Sa.Am.B 1990,7, 1905. (7) Stanley, R. J.; Castleman, Jr., A. W.J . Chem. Phys. 1990,92,5770. ( 8 ) KrBtzschmar, 0.; Selzle, H. L.; Schlag, E. W. Z . Naturforsch. 1988, 43A,765. (9) Bader, H.; KrHtzschmar, 0.;Selzle, H. L.; Schlag, E. W. Z. Narurforsch. 1989, 44A, 1215. (10)Venturo, V. A.; Felker, P . M. J . Chem. Phys. 1993,97,4882. (11) Venturo, V. A.; Felker, P . M. J. Chem. Phys. 1993,99,748. (12) Stephenson, T. A.; Rice, S. A. J. Chem. Phys. 1984,81, 1083. (13) Menapace, J. A.;Bernstein, E. R. J . Phys. Chem. 1987,91,2533. (14) Weber, Th.;von Bergen, A.; Riedle, E.; Neusser, H. J. J. Chem. Phys. 1990,92,90. (15) Hobza, P.; Selzle,H. L.;Schlag, E. W.J. Chem.Phys. 1991,95,391. (16) Kim, H.-Y.; Cole, M. W.J. Chem. Phys. 1989,90,6055. (17) Fung, K. H.; Henke, W. E.; Hays, T. R.; Selzle, H. L.; Schlag, E. W. J. Phys. Chem. 1981,85,3560. (18) Krause, H.; Neusser, H. J. J. Chem. Phys. 1993, 99, 6278. (19) Hopkins, J. B.; Powers, D.E.; Smalley, R. E. J . Phys. Chem. 1981, 85. 3739. (20) B h s e n , K. 0.; Lin, S. H.; Selzle, H. L.; Schlag, E. W. J. Chem. Phys. 1989,90,1299. (21) Fischer, G.Chem. Phys. Le??.1978,56, 186. (22) Esherick, P.; Owyoung, A.; Pliva, J. J . Chem. Phys. 1985,83,3311. (23) Knight, A. E. W.: Parmenter.. C. S.:. Schuvler. . . M. W. J . Am. Chem. Si.1975,97, 1993. (24) Krause, H.; Ernstberger, B.; Neusser, H. J. Chem. Phys. Le??.1991, 184,41 1. (25) Stephenson, T. A,; Radloff, P. L.; Rice, S. A. J. Chem. Phys. 1984, 81, 1060. (26) LangridgeSmith,P. R. R.; Brumbaugh, D. V.; Haynam, C. A.; Levy, D. H. J. Phys. Chem. 1981,85,3742. (27) Henson, B. F.; Hartland, G. V.; Venturo, V. A.; Felker, P. M. J . Chem. Phys. 1989,91, 2751. (28) Henson, B. F.; Hartland, G. V.; Venturo, V. A,; Hertz, R. A.; Felker, P. M. Chem. Phys. Le??.1991, 176,91. (29) Scherzer, W.; KrBtmhmar, 0.; Selzle, H. L.;Schlag, E. W.2. Naturforsch. A 1992, 47A, 1248. (30) Hobza, P.; Selzle, H. L.; Schlag, E. W. J . Chem. Phys. 1990,98, 5893. (31) de Meijere, A.; Huisken, F. J . Chem. Phys. 1990,92,5826. (32) Van de Waal, B. W. Chem. Phys. Le??.1986,123,69.
