4882
J. Phys. Chem. 1993,97,4882-4886
Nonlinear Raman Spectroscopy of Ground-State Intermolecular Vibrations in Benzene Complexes Vincent A. Venturo and Peter M. Feker' Department of Chemistry and Biochemistry, University of California, Los Angles, California 90024- I569 Received: March IO, 1993; In Final Form: March 30, 1993
We report the use of mass-selective, ionization-detected stimulated Raman spectroscopies in the study of groundstate intermolecular vibrational resonances in van der Waals complexes. Results are reported for one-to-one complexes of benzene isotopomers with Ar, Kr, and Nz. For these species a single Raman band is observed at 33.4,35.2, and 37.2 cm-I, respectively. All of these bands exhibit rotational contours that are several cm-I in width and without a dominant qQ branch. They also exhibit polarization ratios indicative of nontotally symmetric vibrational transitions. They are shown to correlate with SIvibrational intervals of slightly lower frequency. And, they exhibit red shifts of 2-3 cm-l upon perdeuteration of the benzene moiety. The bands are assigned as transitions involving the van der Waals bending mode in each of the species and are most likely fundamentals of the bend.
Introduction One of the most direct probes of intermolecularpotential energy surfacesis the spectroscopyof intermolecularvibrationsin weakly bound molecular complexes (see ref 1 and references therein). Understanding the characteristics of these van der Waals (vdW) vibrationsis also essential to the descriptionof vibrational energy flow and predissociation dynamics in such species.2 Several approaches have been successfully employed for the spectroscopy of vdW vibrational modes. Fluorescence excitation,3-I1 resonantly enhanced multiphoton ioni~ation,5~~I-l6 and hole-burning spectroscopies17 have provided information primarily about the vdW structure in excited electronic states. Spectroscopiesbased on dispersed f l u o r e s ~ e n c e , ~stimulated -~J~ emission,Is and direct absorption in the infrared1J9have been employed in the study ofground-stateintervals. Todate, however, very little work has been reported on the Raman spectroscopy of vdW vibrations in weakly bound complexes.20 This lack of Raman-based studies is unfortunate given the complementarity between such studies and those based on the spectroscopies cited above. In several recent reports2' we have described the application of mass-selective, ionization-detected stimulated Raman spectroscopies (IDSRS) to the study of intramolecular vibrational resonances in molecular clusters. Mass-selective IDSRS schemes are double-resonancespectroscopies that involve monitoring by mass-selectiveresonantly enhanced multiphoton ionization (REMPI) the vibrational-state population changes induced by stimulated Raman scattering.21aJ2The IDSRS methods are both sensitive and species-selective, Moreover, they permit the measurement of Raman spectra at subwavenumber resolution throughout the vibrational fundamental region with a single-laser system. Thus, they are well-suited to the vibrational spectroscopy of molecular clusters in supersonicmolecular beams and might be expected to be particularly useful in the study of low-frequency transitions in such species. In this letter, we report the first application of mass-selective IDSRSto the study of Raman transitions between vdW vibrational states in weakly bound complexes. Spectra are reported for oneto-one complexes of the form h6-benzene-X and d6-benzene-X, where X is Ar, Kr, and N2. The results have several notable characteristics. Each of these species displays only oneprominent vdW Raman band. Each of these bands is several cm-' broad, indicating the presence of nontrivial rotational structure, and has the polarizationratio expected for a nontotallysymmetric Raman band. Each band also occurs at a frequency consistent with a vdW bending-mode resonance and exhibits a significant (-5-
10%)shift to the red upon perdeuteration of the benzene moiety. Finally, each correlates with a lower-frequencyvdW vibrational interval in the SIexcited-state manifold of the relevant complex. The Raman bands are assigned as transitions involving the vdW bending mode, with the bend fundamental being the most likely assignment.
Experimental Section Spectra were measured by two variants of IDSRS: massselective versions of ionization-loss stimulated Raman spectroscopy (ILSRS) and ionization-gain stimulated Raman spectroscopy (IGSRS). The characteristics of these methods have been discussed elsewhere in detail.21"22 Briefly, ILSRS is a holeburning spectroscopy in which a stimulated Raman transition burns a population hole that is then sampled by mass-selective REMPL23 Raman resonances appear as depletions in the ion signal as a function of the Raman frequency. ISGRS is similar to ILSRS except that the REMPI probe is tuned so as to photoionize those species that are excited in stimulated Raman transition^.^^ Raman resonances appear as increasesin the massselected ion signal as a function of the Raman frequency. The apparatus used for the application of ILSRS and IGSRS to low-frequency vdW transitions was slightly different from that used in our previous IDSRS studies.21g22In particular, the laser system consisted of two, synchronously triggered, pulsed Nd: YAG lasers and threedye lasers (rather than the twoused before). Two of the dye lasers (rhodamine 590 in methanol as dye in both) were pumped by the frequency-doubledoutput of one of the Nd: YAG lasers. The two outputs from these dye lasers provided the two stimulated-Raman excitation fields; one of the dye-laser frequencieswas set to a fixed value (wI)and the other's ( w 2 ) was scanned in the course of an experiment. The relative (linear) polarization of the two dye lasers was adjustable by means of a prism polarization rotator placed in the path of the w2 laser. This degree-of-freedom allowed the measurement of Raman polarization ratios-the integrated intensity of a Raman band with the polarization of W I perpendicular to that of w2 divided by the integrated intensity with the two polarizations parallel to one another (Le., IL/Z11).25 The third dye laser (coumarin 500 in methanol as dye) was pumped by the frequency-tripled output of the second Nd:YAG (whose firing was delayed by about 10 ns fromthatofthefirst Nd:YAG). Thefrequency-doubledoutput of the third dye laser was used as the REMPI probe pulse ( ~ 3 ) . After combining the wl,w2, and w3 pulse trains on beam splitters the excitation fields were focused into the ion acceleration region of a time-of-flight mass spectrometer (TOFMS) where they
0022-3654/93/2091-4882S04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97,No. 19, I993 4883
Letters
1 00
I
03
03
- 00
(a)
(b)
Figure 1. Level diagrams depicting the IDSRS schemes pertinent to this letter: (a) ionization-lossstimulated Raman spectroscopy (ILSRS) add (b) ionization-gain stimulated Raman spectroscopy (IGSRS). In both schemes a two-color ( w l , w 2 ) pulse drives a stimulated Raman transition from the zero-point level of the ground electronic state (00) to an excited vdW vibrational level (XJ.In (a) this process is probed by ionizing through the 6; band of the complex, which produces a loss in ion signal when WI-2 is resonant. In (b) it is probed by ionizing through the 46; band of the complex, which produces a gain in ion signal for resonant w1-2.
w 30
35
40
45
50
Raman Shift (cm.')
Figure 2. Mass-selective ILSRS spectra showing the vdW resonances intersected a pulsed, skimmed supersonic molecular beam. At observed in this work. Spectra were also obtained for the benzeneJ6Kr species. These are not discernibly different from the 84Krtrace shown. the sample the w1 and w2 pulses each had energies of about 15 We were not able to measure a spectrum for &-ben~ene-~~Kr because mJ. The energy of the w3 pulse was several tenths of a millijoule. the species has the same mass as the perdeuterated benzene dimer. The molecular beam was generated by a commercialmolecular beam valve with a 0.5 mm diameter orifice and -200-ps opening or IGSRS may be difficult or impossible when the 6; and time. To form benzene-X complexes, a mixture of helium (70q 6 ; vibronic bands occur at the same frequency. In such a case 100 psig), benzene (-0.2%), and X (5-10% for Ar and Nz,2% w 3 probes both for Raman-induced ion loss and ion gain, which for Kr) expanded through the valve orifice. The expansion was can cancel one another in the overall ion signal. In the case of skimmed several cm downstream by a conical skimmer having high-frequency Raman rcsonances(greater than severalhundred a 2-mm-diameter orifice, after which it passed into the differcm-I) such spectral overlap of vibronicbands does not necessarily entially pumped TOFMS. Photoions created by the interaction preclude the successful application of ILSRS to weakly bound of the laser fields and the molecular beam were accelerated at species because processes involving the fragmentation of Ramanright angles to the laser- and molecular-beam axes and were excited species can still produce Raman-dependent depletionsof detected by a dual microchannel-plate detector. The output of mass-selected However, the low energies associated the detector was amplified and then averaged by a boxcar with vdW vibrational transitions cannot be relied on to produce integrator. An oscilloscopewas used to monitor the mass spectrum much, if any, increase in fragmentation of vdW-excited species and to set the boxcar gate to the ion mass of interest. The boxcar relative to that of vibrationally unexcited species. The point is output was dumped to a computer as a function of w2 to produce that for IDSRS methods to reveal a vdW vibrational resonance, an ILSRS or IGSRS spectrum (depending on the value of 0 3 there generally must be a shift between the relevant vibrational employed). Spectral resolution, as determined by the bandwidths frequency in the ground-state manifold and the corresponding ofthewl andozlasers,wasabout 0.3 cm-I. Theabsolutefrequency frequency in the excited-state that is the intermediate in the scale of a given spectrum (as fixed by the absolute value of WI) REMPI process. Further, it is desirable to tune w3 to a region was established by scanning wz both above and below WI: Any of the vibronic spectrum where only sharp, isolatedvibronic bands single Raman resonance appears once in each of these regions at are present. positions symmetrically placed about 0 cm-I. For each of the species studied survey Raman spectra were RaultS first measured by the mass-selective ILSRS method, Figure la. (ILSRS is much better suited to this purpose than IGSRS i s s e e Figure 2 shows mass-selective ILSRS spectra of the benzene-X refs 21a and 22.) In all these experiments w3 was tuned to the complexes. For each of the complexes studied we have thus far benzene-localized SI+SO 6; vibronicband of the c o m p l e ~ . ' ~ J ~ >found ~ ~ only one Raman band in the 0-50-cm-1 region of the When a Raman band was found by ILSRS (e.g., the X, 00 spectrum. As shown in Figure 2 and listed in Table I, these band depicted in the level diagram of Figure la) subsequentscans bands fall in the range 30-38 cm-I. Several points are noteworthy of w2 were performed with w j set to various values in the region about these Raman features. First, the bands have appreciable of the 61,band. The purpose of these experiments was to locate widths (2-4 cm-l) due to underlying rotational structure. They the position of thex6;vibronic band of the species: With 0 3 set are not dominated by a single, narrow sQ-branch, unlike most of to this band, the X, 00stimulated Raman resonance produces the intramolecular resonances that we have characterized thus an ionization-gain signal (see Figure lb). By locating q 6 ; far with IDSRS.21 Second, measurements of the polarization bands in this way, one is able to correlate the SOvibrational ratios of the bands for the -Ar and -Kr complexes (we have not resonancesobserved by ILSRS with vdW vibrational intervals in yet been able to make this measurement for the NZcomplex) the SIstate. yield values that within experimental error are consistent with the value expected for anisotropicRaman transitions-Le., 0.75.2s One may note from the processes depicted in Figure 1 that the Third, despite the rotational broadening that exceeds our observation of even a strong X,-Oo Raman resonanceby ILSRS
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Letters
4884 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
TABLE I: Characteristics of vdW Bands Observed by Mass-Selective IDSRS complex band center (cm-'). U L /4l) 33.4 30.5 35.2 35.2 37.2 35.4
benzene- Ar d6-benzene-Ar benzeneJ4Kr benzene-86Kr benzene-N2 d6-benzene-N2
0.76
0.74
+
From Gaussian fits of band contours in both the W I - w2 > 0 and W I < 0 spectral regions. Uncertaintiesare fO.l cm-I. The polarization ratio of a band, as described in the text. These were measured under intensity conditions for which the stimulated Raman transitions were not saturated. Account was also taken of the polarization dependence of intensities at the sample due to the polarization dependences of combining beam splitters. Uncertainties are kO.05. A missing entry indicates that the polarization ratio was not measured.
-w2
I
A
Benzene-Ar
'L Ben~ene-~~Kr
Benzene-N,
I
I
I
28
30
32
I
I
I
36 38 Raman Shift (cm.') 34
Several features of the IDSRS results on this species strongly point to an assignment of the observed 33.4-cm-I Raman band to a transition in which the vdW bending-mode quantum number changes, First, there is the observed rotational band contour, which lacks any discernible 4 Q branch structure. Such structure would be expected for the fundamental or any overtone of the vdW stretch, since such an AI A I transition is induced by either one or both of the polarizability spherical tensor elements aio)and ai2).28 On the other hand, the fundamental of the vdW bend (El +AI) is induced by tho ay,)polarizability elements and will be absent AK = 0 features such as a 4 Q branch. Similarly, one of the two components of the first overtone of the bend is an E2 A t transition (the other is A I A I ) induced by the & polarizability elements. It, too, will be absent AK = 0 structure. Second, there is the observed polarization ratio of -0.75. A totally symmetric Raman transition will have a polarization ratio of less, often substantially less, than 0.75. A nontotally symmetric transition has a polarization ratio equal to 0.75.25 The experimental ratio strongly suggests, although does not definitively prove, that a nontotally symmetric transition-one involving the bend-is that which obtains. Third, the substantial effect of perdeuteration on the frequency of the band is further evidence that it arises from the vdW bend. The change in the pseudodiatomic reduced mass of the complex upon deuteration leads one to predict a deuteration frequency shift of only about -1% for thevdW stretch. An estimateof theeffect ofdeuteration on the bend frequency can be had by examining the deuteration change in l/&,29 where I , is the principal moment of inertia of the benzene moiety about an in-plane axis. This quantity decreases almost 9% upon perdeuteration, consistent with the --9% deuteration shift of the Raman band. Having identified the observed benzene-Ar band as one involving the vdW bend, it remains to assign it more specifically to the fundamental or an overtone. The evidence is strongest in favor of the fundamental. There are three points in favor of this assignment. First there is the correlation of the Raman band with an SIvibrational interval that is several cm-1 smaller (see the IGSRS results of Figure 3). Such an SIvdW interval has, in fact, been observed independently at 30.9 cm-I to the blue of the complex's 6; band by mass-selective REMPI.13 Given the IGSRS results and the similar frequencies of these SOand SI intervals, it is quite reasonable to take the two to have the same assignment. Now, the SIinterval has previously been assigned" to the first overtone of the vdW bend. This assignment was made on the basis of observed band intensities, consideration of pointgroup selection rules, and comparison with normal-mode calculations of the vdW frequencies. However, very recent analysis of the rotational structure associated with the 6; 30.9 cm-l vibronic band leads to the conclusion that theSl interval is actually thefundamental of the bend.32 (Notably, permutation-inversion symmetry considerations are fully consistent with such an a~signment.~~) As stated above, this assignment should also apply to the 33.4-cm-I Raman band. Second, there are the results of numerically exact, threedimensional quantum calculations of the SOvdW level structure of benzene-Ar. Such calculations, employing an atom-atom pairwise-additive Lennard-Jones form for the intermolecular potential surface, had previously located the bend fundamental at 21.4cm-l and theE2componentoftheovertoneat 41.0cm-1,27 neither values of which match the Raman results very well. This disagreement is, perhaps, not so surprising given that the LennardJones potential was derived from thermodynamic data pertaining to the interaction of Ar with graphite surfaces (as pointed out in ref 34). Recently, however, the use of ab initio derived potential s ~ r f a c e s in 3 ~3-D quantum calculations33 places the bend fundamental at either 25.5 or 30.2 cm-', depending on how the ab initio results are fit to obtain the p ~ t e n t i a l Clearly, .~~ these new calculations indicate that the Raman band be assigned as the
I
40
42
Figure 3. Mass-selective IGSRS spectra for the vdW resonances observed in this work. For the traces from top to bottom w3 was tuned to the red of the 6; band of the pertinent complex by 3.0, 3.7, 3.5 cm-I. Each of the spectra shown has an appreciable, constant background contribution from REMPI of vibrationally unexcited species through the red tail of the 6; band. This background signal has been subtracted from the spectra shown.
apparatus resolution, the resonances produce very appreciable ILSRS depletions (5-15%). Indeed, we have found by experiments in which the intensities of the W I and w2 fields are varied that under ordinary circumstances (Le., with intensity parameters like those described in the preceding section) the observed Raman transitions are driven very close to saturation. This fact is strong evidence that the pertinent Raman resonances are strong ones. Fourth, the bands exhibit red shifts of 5-10%upon perdeuteration of the benzenemoiety. Finally, each of the Raman bands produces an ionization-gain signal when w3 is tuned slightly to the red (- 3 cm-I) of the pertinent 6; band. IGSRS spectra that prove this point are shown in Figure 3. One concludes that each of the Raman bands observed by ILSRS is associated with an St vibrational frequency that is slightly smaller than the SOinterval.
Discussion The major issue that one seeks to address in regard to the results presented in the previous section is the nature of the vdW Raman resonances that have been observed. It is useful to begin with a consideration of benzene-Ar. This species is known to have C6"point-group symmetry and an equilibrium structure in which the Ar atom is above the center of the benzene ring.26As such, the vdW vibrational modes of the complex consist of a nondegenerate stretch (AI symmetry) and a doubly degenerate bend (El symmetry).I3J7
+
+
+
Letters bend fundamental. Notably, these same calculations predict the vdW stretch fundamental of benzene-Ar to be quite close to the experimentally derived SIvalue.l3,26c Third, the Raman results themselves suggest that assignment to the bend fundamental is most appropriate. In particular, we see no evidence for appreciable Raman intensity at one-half 33.4 cm-I, or anywhere else below 50 cm-1. If the 33.4-cm-1 band were the E2 bend overtonecomponent,then one might also expect to observe significant intensity for the bend fundamental and the AI overtone component. (Both of these latter two bands are Raman-allowed in the c6"point group.) On the other hand, one can readily rationalize weak overtones in the presence of an intense fundamentalsince this is the situation that is found in most Raman spectra.36 We turn now to the assignment of the Raman bands observed for the benzene-Kr and benzene-N2 species. Both of these complexes have geometries analogous to that of benzene-Ar. Benzene-Kr has the Kr atom directly above the center of the benzene ring and has Cs,symmetry.z6bBenzenaN2 has a structure in which the center-of-mass of the N2 moiety is directly above the benzene ring and in which thevibrationallyaveraged symmetry is apparently c 6 u due to free internal rotation of the N2 moiety about the benzene c6 axis.37 Given these structures, one expects bend and stretch vdW vibrational modes in the Kr and NZ complexes that are analogous to those in benzene-Ar (although, of course, there will be two additional intermolecular modes in the NZ complex). The fact that only one appreciably intense Raman band is observed for each of these complexes in the vdW region and that the bands for the different complexes are all fairly close together in frequency strongly suggests that all of the bands are of the same type. This interpretation is supported by the similarityof the benzene-Ar results to thecorrespondingresults on the Kr and N2 complexes in respect to band contour, polarization ratio, deuteration effect,38and correlation with a lower frequency SIinterval. Taken all together, the evidence supports the assignment of the 35.2-cm-I benzene-Kr band and the 37.2-cm-I benzene-N2 Raman band as the vdW bend fundamentals in the respective species. Given the assignment of the observed Raman bands, there are two features of them that are rather surprising. The first is the singularly high intensity (in the vdW region) of the observed bands. Just why such singularly high intensity should be associated with the bend fundamental is not clear. It is possible that other significantly strong vdW Raman bands are present but not observable by IDSRS because their frequencies do not shift appreciably upon excitation to the SImanifold (see the end of the Experimental Section). Still, not only are the observed bands the only ones present, their absolute Raman strength is such that they are readily saturated under our experimental conditions. We have no ready rationalization for this behavior. It may arise, in part, from the tilting of the benzene moiety during the bending vibration (tilting that compensates for the angular momentum of the Ar). Since the in-plane and out-of-planestatic polarizability components of benzene are significantly different,39 this tilting might be expected to modulate the complex's polarizability tensor significantly. In effect, the rotational Raman strength of benzene monomer might account for at least some of the Raman strength of thevdW bend fundamental. On the other hand, it is also possible that complicated processes involving modulation of the electron clouds of the complexing partners during the bending vibration are the source of the Raman intensity. Ultimately, a calculation of Raman activity in the vdW spectral region will probably be required to shed light on what appears to be anomalousintensity behavior. It will be interesting in future IDSRS experiments on other aromatic-rare gas complexes to examine whether the behavior is a general phenomenon. The second, somewhat surprising aspect of the results reported above is the apparent shift to the red of the vdW bending mode
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4885 frequenciesin benzene-Ar, -Kr, and -Nzsubsequent to excitation from the Somanifold to the SI(61) manifold. This is unexpected since the intermolecular bonding in all three of these species increases in strength upon such excitation.13~14,26A simpleminded scaling of vdW vibrational intervals with intermolecular bond strength would predict the vdW bend frequencyto increase rather than decrease upon vibronic excitation of these complexes. Evidently,such thinking fails, even in a qualitative sense, for the complexes pertinent herein. There are several possible reasons for this, including the inherently complex nature of vdW vibrationallevel excitation-inducedchanges in inertial parameters (e.g., Zx of the benzene moiety increases significantly in the SIstate41),or the possibility of stiffer bending potentials in theSomanifold. Whatever the sourceoftheobserved behavior, one hopes that it will eventually serve as a significant piece of information in the theoretical elucidationof how vibronic excitation changes the intermolecular potential energy surfaces of these complexes. In conclusion, we have presented results of the first application of mass-selective IDSRS methods to the study of vdW vibrational bands in weakly bound complexes. The results clearly demonstrate that these methods can contribute in significant ways to the study of these low-frequency resonances. Analogous studies of other weakly bound complexes, including isotopomersof benzene dimer, are underway and will be reported in the near future.
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