J. Phys. Chem. 1994, 98, 71 16-7122
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ARTICLES Crossed Beam Rovibrational Energy Transfer from S1 Glyoxal. 2. Uniform Scattering Characteristics from the Initial Levels Oo, 5l, and 8' Brian D. Gilbert and Charles S. Parmenter. Department of Chemistry, Indiana University, Bloomington. Indiana 47405 Douglas J. Krajnovich IBM Almaden Research Center, K661803, 650 Harry Road, San Jose, California 95120 Received: April 13, 1994"
Results are reported for an extension to higher initial states of crossed molecular beam inelastic scattering from 'A, trans-glyoxal (CHOCHO) by He collisions with 95-meV (770-cm-I) center-of-mass collision energy. A laser pump prepares glyoxal in the initial level US' = 509 cm-I or vg' = 735 cm-l with high rotational selectivity about the top axis; K' = 0 for us' and K' = 0-3 for vg'. A dispersed fluorescence probe detects the final states of rotationally and of rovibrationally inelastic scattering with resolution of AK transitions. Semiquantitative relative cross sections are obtained for the state-to-state scattering. The rovibrational channels show the same extreme selectivity that is observed for scattering from lower levels. Only a single quantum change in the lowest frequency mode, the CHO-CHO torsion v7' = 233 cm-I, may be securely assigned. Scattering to much closer vibrational levels is not competitive. The state-resolved inelastic scattering characteristics of the three levels Oo, 5l, and 8l are now seen to be identical to within experimental uncertainty. The similarity involves both the competition between the rotational and rovibrational scattering channels as seen in the relative cross sections and the distribution of cross sections for the various hK processes within these channels. The experimental results are in agreement with the three-dimensional quantal scattering predictions of Kroes, Rettschnick, and Clary.
Introduction When used with crossed molecular beam inelastic scattering, the laser pump-dispersed fluorescenceprobe technique has been an effectiveway to study state-to-statevibrationalenergy transfer. In its first applications, it gave an extensive view of the stateresolved transfer from various initial levels in the BO: state of 12.14 Those data have subsequently been used for comparisons with quantal scattering cal~ulations.5~~ More recently, the method has been applied to glyoxal (CHOCHO), a planar molecule with 12 vibrational mode^.^-^ In this case the techniqueadditionally resolves rotational energy transfer. Glyoxal is a near prolate toplo (K = 4 - 9 9 ) , and the laser pump may prepare initialvibrational states with zero angular momentum about the top axis (K'= 0). Both rotationally and rovibrationally inelastic scattering then populate a distribution of higher K'states that may be individually resolved with the fluorescence probe. In this report, we are concerned with additionalglyoxal inelastic scattering. We present the rotational and rovibrational energytransfer results from two initial levels involving new fundamentals in higher energy regions than probed in previous studies. To place the experiments in perspective, consider the energy level diagram of Figure 1. Our initial experiments799 concerned scattering from H2 by glyoxal initially pumped to the levels (OO, K'= 0) and (72, K'= 0). The full report on that work is paper 1 of this series.7 Semiquantitative cross sections with AK resolutionwere obtained for rotationally inelastic scattering. We have subsequently measured more than 130 quantitative stateto-state cross sections for inelastic scattering by H2 and He from glyoxal in these initial states." As an illustration, the observed
* To whom correspondence should be addressed. *Abstract published in Aduance ACS Abstracts, July 1, 1994. 0022-365419412098-7116$04.50/0
rotational and rovibrational channels for scattering by glyoxal (00, K' = 0) are schematically displayed in Figure 1. The data from both initial levels have been used as benchmarksfor extensive three-dimensionalquantal scattering calculations carried out for these experiments.I2-I5 We now extend the study to the scattering from SIglyoxal levels (5l, K' = 0) (Cvib = 509 cm-I) and (W, K' = &3) (fhb = 735 cm-I) by He with 95-meV (770-cm-1) center-of-masscollision energy Em. As we climb the SIglyoxal vibrational energy ladder to the initial levels 51 and 8I, we begin to exploreseveralquestions about the rotationally and rovibrationallyinelastic scattering that arose from the results of Oo and 72 ~cattering.~-~Jl Those experiments showed that the vibrationally inelastic scattering was dominated by AUT= f l transitions, where the CHO-CHO torsion v7' = 233 cm-1 is the lowest frequency mode. No other vibrationally inelastic channels were observed. As we explore the scattering from higher regions where the vibrational level density is beginning to grow, we wish to discover whether or not vibrationally inelastic scattering still remains so selective. Both 51 and 8' glyoxal have nearby vibrational levels. For example, the 61 level is only 20 cm-1 away, and the 517I combination level is only 7 cm-I away from the initial level 81. We wish to discover whether vibrationally inelastic scattering populates these levels. We are also interested in the 51 and 81 levels because they are the first vibirationally excited initial levels we have studied that do not contain activity in v7, the mode so active in vibrationally inelastic scattering. Fully three-dimensionalquantal scattering calculations14have already predicted the scattering for He in collision with both glyoxal (51, K' = 0) and (81, K' = 0-3) at our Ecm. These predictions will be compared to the experimental results. We shall be interested particularly in the distributionof cross sections 0 1994 American Chemical Society
Crossed Beam Rovibrational Energy Transfer from SIGlyoxal The Journal of Physical Chemistry, Vol. 98, No.29, 1994 7117
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lo 0ot Figure 1. Vibrational and rotational energy level diagrams for SItram-glyoxal. The left side shows all vibrational levels below 1200 cm-I. The two levels pumped in the inelastic scattering experiments are marked by bold lines. Arrows show the dominant vibrationally (~AIQ~ = 1) inelastic scattering channels from each of the initial levels. On the right side, the rotational and rovibrational levels reached by @KO glyoxal scattering are shown (see ref 11). The levels are denoted by the K quantum number for rotation about the u axis of glyoxal.
for the various AK processes within the rotational and the rovibrationalchannels. This distribution is one of the more subtle points to emerge from the experiments. In scattering from the lower levels, such rotational scaling is distinctive for the two types of channels (rotational vs rovibrational) and for the two initial states and for the two target gases Hz and He. It has proven to be a stringent test for the scattering calculations. We now have a chance to observe the rotational scaling from two additional initial states of quite different vibrational character. VET from the 81 level has been studied in both 300 K1”*O and low-energy supersonic jet collisions21J2 using the laser pumpdispersed fluorescence probe technique. We shall comment on how our explorations with higher collision energies and narrow Em distributions compare. We shall use a notation developed previously,ll in order to make the text easier to read. Briefly, we refer to an SIrovibrational level such as u{ = 1, K’= 0 as 5lKO. We have omitted the primes from SI state symbols except where necessary to avoid confusion. Cross sections for a specific channel such as 5lKO 5l7lK6 are given as u(5171K6), omitting the initial state. We refer to a set of cross sections such as 51KO 5’71Kn as u(5171Kn).
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Experimental Procedures and Results Full experimental details have been given el~ewhere.~*~Jl Briefly, the inelastic scattering is observed in a chamber where a pulsed skimmed beam of He carrying 5-7% glyoxal is crossed nominally at 90° by a pulsed supersonic He expansion. A 0.4cm-1 bandwidth pulsed laser pumps glyoxal at the beam intersection to the selected SIu’K’leve1 via the appropriate SlSo absorption transition. The inelasticscattering at Em = 95 meV = 770 cm-1 is observed by dispersed glyoxal SlSo fluorescence. Two spectra are acquired in the computer’s memory, both normalized to the total fluorescenceintensity on a shot-to-shot basis. One is with both beams on,and one is with the He target beam off. Subtraction of these spectra yields a so-called scattering spectrum which containsfluorescence from only those glyoxal molecules that have undergone inelastic scattering. Relative cross sectionsfor the inelastic scattering are normally extracted from the scattering spectra by computer spectral
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Frequency (cm-’) Figure 2. Collision-freedispersed fluorescence spectra (spectrometer resolution of IO-cm-’ fwhm) obtained after pumping the S1 level (top panel) and the 8’ level (bottom panel). The dashed boxes indicate the
spectral regions scanned in the inelastic scattering experiments. simulation. All necessary photophysical and spectroscopic information is available from independent sources, so that the only unknown parameters of the simulation are the relative populations of the vibrational (d) and rotational (J’K’) states produced by the inelastic scattering. With an assumed J distribution for each dK’ state (not a sensitive parameter), the relative u’K’ populations are adjusted to give an optimum simulation. The best fit populationsare the relative cross sections that we seek. As described below, we use a modification of this approach for analysis of the 5’ and 8’ scattering spectra. The collision-free dispersed fluorescence obtained with the target beam off from the two initially pumped glyoxal levels 51 and 81 at about 30 K is shown in Figure 2. The spectra have a resolution (10 cm-l) that reveals only the vibrational structure. He Glyoxal ( S I P ) Scattering. The SI-&absorption region containing the 5; band used to pump 5IKO with 0 IJ’ I10 is shown in Figure 3. The prominent rotational structure is
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7118 The Journal of Physical Chemistry, Vol. 98, No. 29, 1994
Gilbert et al. $7:. The scattering from the level 5' is thus similar to that from Oo and 72. The only detectable vibrational change is IAu71 = 1.
Several additional peaks are due to resonance fluorescence from the 72K15level. This emission arises in the following way. As one can see from Figure 3, high-K absorptions in the 7; band overlap low-Ktransitions in the 5; band. When our laser is tuned to pump it can also excite 7iK::. In our cold molecular beam, there is almost no collision-free population in the K"= 14 level, and hence thereis nocontribution from 7 W 5in thecollisionfree spectrum. However, when the target beam is turned on, some molecules in low-K" levels of the ground electronic state will be inelastically scattered into 0&14, and these molecules will be selectively pumped by our laser to 72K15. Therefore, by accident, we are picking up one particular inelastic scattering channel in the ground electronic state by laser-induced fluorescence (LIF), muchin thesame way that Gentry andco-workers23J4 used SI SOLIF to monitor SOlevel populations in their scattering experiments. Collision-free emission from the 72 level contains three strong vibronic transitions in the region covered by Figure 4 (see Figure 2 of ref 11). These are 7i, 7:, and 7:. For high-K levels, the Hiinl-London factors are small except for the rR and PP rotational transitions. Therefore, we expect to see a pair of PP and rR lines associated with each of the 7;, 7:, and 7: bands, due to this ground-state scattering 'artifact". The calculated positions of these six rotational lines are shown in Figure 4. The 5 ; and 5;7: bands constitute the most intense regions of the scattering spectrum. They are shown a t 2-cm-1-fwhm resolution in the bottom half of Figure 4 where some of the K states produced by scattering are resolved in a noisy spectrum. The poor S / N and spectral congestion make the Ksub-bandheads generally indistinct so that it is impossible to determine with precision the K state populations produced by rotational and rovibrational scattering. He + Glyoxal 8'iP Inelastic Scattering. The 8; absorption band used to prepare 8I is shown in Figure 3. As opposed to the other initial states, the pump transition 8;is forbidden and occurs with a different and less accommodating rotational contour (type A + B)25than that of the allowed bands (type C). Consequently, the 8l level is prepared with significant populations in the levels KO to 6.We designate these levels simply as 8IK". Figure 5 shows the inelastic scattering spectrum obtained a t 10-cm-I resolution from He glyoxal (81K"). While emission from many of the vibrational levels accessible in scattering would occur in the displayed region, only a few bands appear. Emission from only one of these levels, 817l, has been securely identified. The band 8i7; is immediately recognizable by analogy with similar bands from the Au7 = +1 state produced by inelastic scatteringfromthe00,72,and 5 1 initiallevels. Wealsoseeemission from 8I7l appearing in the 8:7f forbidden transition. As expected for a forbidden type A + B transition, that band has a narrower contour than the allowed bands. Three unassigned bands labeled A, B, and C occur in Figure 5 . The bandwidths are relatively narrow, all about the same as the spectrometer resolution. While such contours are typical of forbidden bands, the most intense bands from levels populated by inelastic scattering will be allowed with broad contours. Since only those bands should be visible, we do not understand why such narrow contours occur. We have considered the possibility that these structures may be due entirely to artifacts unrelated to inelastic scattering (as observed in the 5l inelastic scattering spectrum). Nominations for the sources of such artifacts are, however, not evident. On the other hand, we do not have convincing assignments assuming that they are from levels populated by vibrationally inelastic scattering. An enlarged view of these bands is given in Figure 6 that contains spectra with the H e target beam ofJ with that beam on, and the on-off scattering spectrum. Sets of spectra taken on
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vibronic bands of trans-glyoxal. The the 8; (bottom panel) SO 41 asterisksmark the K'+ K"sub-bandheads that were pumped during the inelastic scattering experiments. He + Glyoxal (5lKo) Ec,m,= 95 meV
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Frequency (cm") Figure 4. Vibrational (top) and rotational (bottom) state resolved dispersedfluorescenceinelasticscatteringspectrafor He t glyoxal (5IKO) (Em = 95 meV) with a resolution of 10 cm-I (top) and 2 cm-1 (bottom).
Thespectra weresmoothedwitha threepointslidingaverage. Themarkers show the calculated positions of vibrational band origins. The p and r bands due to inelastic scattering from the 72 level with high initial K excitation are marked (see text). The correspondingvibronic band origins are shown enclosed in brackets. The region enclosed in the dashed box is shown at a higher resolution in the bottom panel. comprised of 'R sub-bandheads. While it would appear that tuning the excitation laser to the K'= 0 K"= 1 sub-bandhead can cleanly pump 5IKO, minor absorption also occurs to another S1 state when both beams are on. +
The low-resolution (10 cm-I) inelastic scattering spectrum obtained by pumping the bandhead is shown in the top half of Figure 4. Assignments that we consider secure show that only one vibrational state is reached by inelastic scattering, namely 517l. Its presence is revealed by the three bands 5:7:, 5:7:, and
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Frequency (cm-') Figure 5. Dispersed fluorescence inelastic scattering spectra for He + glyoxal (8lP3) (Em = 95 meV) shown at 10-cm-1 (top) and 2-cm-l (bottom)resolution. Thespectrawere smoothedwith a three point sliding average. The region enclosed in the box is shown at a higher resoluton in thebottompane1,wherc theKstateresolvedinelasticscatte~ngspcctrum is displayed with the 'R K'bandheads marked. The transitions labeled A, B, and C, have not been assigned (see text). different days illustrate the reproducibility. The spectra are marked with the calculated positions of various band origins. These markers are only to aid discussion and do not infer a band assignment. Our attempts to assign the growth features A, B, and C as emission from levels populated by vibrationallyinelastic scattering have not been particularly successful. Bands B and C occur near the origins of strong bands from low SI levels, but there is no possibility of supporting such assignments by observing other bands from these levels. In scattering from all other initial levels, we can see at least two or three transitions from the proposed final scattering level. The positions of strong fluorescence bands from all levels displayed in Figure 1 may be securely calculated. Among these, only the 41 and 6,' transitions are close to feature A. While the 4; match is good, we can reject the assignment because the 4; band also expected from this level is missing. That band origin would occur at 22 921 cm-1 with about one-third of the 4; intensity, detectable at our S/N. There are no bands from 6l that can be used to confirm or reject a 6;assignment. The only other strong 61 band occurring in the region of Figure 5 is 6;7;, but it would be obscured by the 8; contour. Feature B occurs in the collision-free 81 spectrum with a maximum near the 0; band origin. Its presence there is almost certainly due to pumping of the 0; band itself by amplified spontaneous emission in the laser source. The band grows, however, whentheHe beamis tumedon. Iftheincreasedintensity was due entirely to rotational scattering from 00, a dip would occur at the band center in the scattering spectrum. The growth is, however, sufficient tooverwhelmthis dip. The region isgaining intensity from some additional source that does not broaden the contour in the manner of ordinary rovibronic scattering. One may see that feature C is close to one of the two weak bands from the initially pumped 81 level. (Both 81 bands properly subtract out in the off-on difference spectrum.) The 12: band
Frequency (cm-') Figure 6. An expanded display of the glyoxal (8IP) + He scattering spectrum at IO-cm-I resolution showing a region containing several scatteringbands. The left and right sides of the figure show scam of the same spectral regions taken on different days. All of the spectra were smoothed using a three point sliding average. The A, B, and C bands are neur the calculated 4;, O,: and 12; origins (indicatedby dashed lines). The origin of the 6: transition is also marked. The top panels show the collision-free SVLF spectrum obtained by pumping the 8l level of glyoxal with the He target beam off. The middle panels show the same region with the He target beam on. The bottom panels show the subtracted spectraon-qffthat contain signalonly from inelastically scatteredglyoxal. is the only close match for feature C. The contour of that feature is somewhat obscured by the subtracted region of the adjacent 81 emission band. One might check this possible assignment by looking for the 1217; band that should occur with about 5% of the 121 intensity. Unfortunately, that band position is obscured by 8:7,' emission. The bottom panel of Figure 5 displays the K state resolved inelastic scattering spectrum at 2 cm-1 resolution. The region shows the 8: and 8;71 vibronic bands, both of which are allowed transitions, for which type C rotational selection rules apply. The 'R bandheads due to pure rotationallyand rovibrationally inelastic scattering are assignable up to - 6 4 . In this respect the 81 scattering is similar to 00 and 72 scattering.
Discussion We shall give elsewhere11 a full comparison of the inelastic scattering characteristics from the initial states @KO, 72Ko,5lKO, and 81P. Here we consider issues specific to the 5lKO and 81Kn levels. Tbe Lowest Freqymcy Mode Dominates Vibrationally Inelastic Scattering. As shown in Figure 1, many vibrational channels are accessiblefor scattering from He glyoxal ( 9 K O and 8 ' P ) with E , = 770 cm-I (95 meV), including some particularly close to the 81 level. While the issue is not wholly resolved for 81 scattering, only the Au7 = 1 vibrational channels can be securely assigned. Evidence for the rovibrational scattering 51 5171 and 81 8171 is well developed. A search for other channels suggeststhat their cross sections are at least an order of magnitude less. The 8l search included specifically a hunt for the nearby levels 6' and 5171 with negative results. Unassigned structure does appear in the 81 scattering spectrum, but it seems unrelated to 81 scattering since the band contoursare incorrect. We discuss this issue further in the next section.
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The inelastic scattering calculations of Kroes et a1.I4predict that Au7 = 1 transitions dominate the rovibrational scattering from both initial levels 51 and 8'. While those calculations did not explore all the energetically accessible channels, none of the many that were included were competitive with the 5l 5I7l and 81 8171 channels. The low-frequency mode activity has also been observed in vibrational energy transfer from SOp-difluorobenzene.23 That molecule and glyoxal are both planar with their lowest frequency modes representingout-of-planemotion. The question then arises whether it is the low frequency or the out-of-plane character that is most responsible for the energy-transfer activity. The crude SSH-T model of polyatomic vibrational energy transfer cannot give insight to the issue of out-of-plane vs in-plane motions, but it clearly predicts larger cross sections for low-frequency modes. This prediction has been explored recently for 300 K vibrational energy transfer in SIp-difluorobenzene,2628where the predicted scaling of relative cross sections with vibrational frequencies has been compared with observations. A far more instructive exercise has come from threedimensional quantal inelastic scattering calculations for both p-difluorobenzene29and gly0xa1.I~ When the frequencies of inplane or out-of-plane fundamentals are artificially reduced to that of the lowest, the calculated cross sections always become large, within a factor of 2 or so of that for the active mode. There is no doubt that low frequency is the key ingredient for large vibrational cross sections. The Competition among Vibrational Channels. Is Scattering from 81an Exception to the Rule? The scattering spectrum after pumping 8I contains structure that cannot be securely assigned. While none has the typical rotational contours of bands from SI states populated by rovibrationalscattering, a discussion has been given in a previous section of the vibrational levels that would emerge if we are indeed seeing vibrational bands with unaccountably restricted rotational contours. Three vibrational channels would compete with the securely identified 81 8171 channel. Those channels are 8' 12l, 8I Oo, and 8l Xn where vX is some unidentified fundamental, overtone, or combination. We discuss below the implications of such assignments. As will be seen, they reinforce the proposition that the structure is an artifact unrelated to rovibrational inelastic scattering from glyoxal (81K"). If such assignments were correct, vibrational energy transfer from the initial state 8l would be unique among polyatomic molecules. Four vibrational channels would have roughly similar cross sections. At least two, the 8l Oo (band B) and 8 l - 121 (band C) channels, would involve a quantum change in a large frequency mode ( v i = 735 cm-I) that is competitive with the low-frequency AUT= +1 channel. Such successful competition with a low-frequency mode would be unique. It is, to our knowledge, not documented for any other V T, R polyatomic molecule energy transfer. This departure from conventional experience is a source of concern for the proposed fluorescence assignments for these channels. The 8' OOchannel has long been identified in thermal glyoxalglyoxal collisions.l7~30It is now known to be a consequence of E-E transfer, a process that obviouslycannot occur in our crossed beam experiments.16J8 The 81 -OOchannel has also been reported to be important for 300 Kcollisionswith raregases, stiff diatomic molecules, and other collision partners.17J0 The most recent exploration16of 300 K 8l vibrational energy transfer (for glyoxald2) found the 81 8171 channel was dominant for a widevariety of collision partners while the 81 Oo cross section was smaller by a factor of 6-20 (He) among thesegases. (The 8 l - 61 channel is the best competitor with the dominant channel, but its cross section is about a factor of 10 less). A 1987 discussion of the literature remains current.8
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Vibrational energy transfer from 8' at the very low collision energies of an expanding supersonic beam has also been studied.21.22 Under these conditions the endoergicchannel 8' 8171 is blocked, and one observes only the exoergic competition. For 10 K and 1 K collisions with He, the 8l 6' channel is dominant. The 8' Oo channel is strictly a minor competitor at 1 K but attains half the cross section of the dominant channel at 10 K. The dissociation of van der Waals complexes between glyoxal and various rare gases has also been st~died.~IJ1-33One might be tempted to use these "half-collisions" for comparisons. As is demonstrated by a comparison ofp-difluorobenzene-Ar complex di~sociation~~ with the 300 K collisional vibrational energy transfer, there may be no correspondence between favored vibrational predissociation channels and vibrational energy transfer channels. The calculated scatteringI4from 8' with 95-meV He collisions predicts that 8' scattering is ordinary rather than special and is about the same as that from 00 or 5l. The Au7 = +1 channel (81 8l7l) is the largest, exceeding all others by almost an order of magnitude. Among the investigated channels, the nearest vibrational competitor is the channel 8' 6l,whose cross section is a factor of 6 less than that for the AUT= +1 change. Emission from a level reached with such a small cross section would be at the noise level in our 8' scattering spectrum. The predicted cross sections of other vibrational channels are more than an order of magnitude less than the Au7 = +1 channel. None would be observablein our scattering spectrum. These predictionsinclude specifically the 8I Oo and 8l 12l channels. As is demonstrated elsewhere," the predictions are highly accurate for many aspects of Hz and He scattering from the initial levels Oo and 72. As far as we can tell from the data presented here, they are similarly accurate for S scattering. The predictions also match to within a factor of 3 the observed ratio ofcrosssectionsfor 8' rotationalscattering to81-8171vibrational scattering (with He). The agreement of thecalculations with all aspects of the scattering suggests to us that the bands A, B, and C are probably not connected with direct SI rovibrationally inelastic scattering. Relative Cross Sectiom for Rotationally and Rovibrationally Inelastic Scattering from the Initially Pumped Levels SIP and 8'P. We are interested in sets of relative cross sections for rotationally scattering 5IKn or 81Kn 8lKe as well as rovibrational scattering 5IKO- S171Knor8'Kn-817IKn. These channels are analogous to those diagramed in Figure 1 for OOKn scattering. The procedure for obtaining these cross sections by computer simulation of the scattering spectrum is explained in detail elsewherell where it is applied to scattering from the initial state OOKO and 72KO. Spectra for the two higher states of the present study have poorer signal-to-noise, and we have used alternative methods to learn about the cross sections. The glyoxal (SIP) He scattering spectrum displayed in Figure 4 is almost a replica of that from glyoxal ( O O K O ) He scattering. To demonstrate the similarity, we have used the experimental set of cross sections from glyoxal (@KO) scattering by He to simulate the 5lKO spectrum. The result is shown in Figure 7. By our experience, the simulation is probably as satisfactory as one can obtain for such a noisy spectrum. We conclude that the relative cross sectionsfor scatteringfrom these two initial states are the same to within our experimental error. This result is consistent with the theoretical predictions.14 To emphasize the match of cross sections, we show also in Figure 7 the simulation obtained with the different relative cross sections that occur for glyoxal (72KO) + He scattering. The simulation is now greatly degraded and, by our experience, an unacceptable match with the experimental spectrum. The S/N of the glyoxal (81Kn) + He scattering spectrum is better, and we have obtained semiquantitative cross sections for
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Crossed Beam Rovibrational Energy Transfer from SIGlyoxal The Journal of Physical Chemistry, Vol. 98, No. 29, 1994 7121
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Frequency (cm-') Figure 7. Rotational state resolved dispersed fluoresctncc inelastic scattering spectra for He + glyoxal (SI@) at Em = 95 meV. The raw data are shown as circles, and the fit is shown as a solid line. The top panel shows the fit obtained using the parameters that best fit the He glyoxal (@@) spectrum (see Table 111, ref 11). The bottom panel shows the fit obtained using the parameters that best fit the He +glyoxal (7%')) spectrum (see Table IV, ref 11). Thevibrational band origins are marked in both spectra.
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the rotationally and rovibrationally inelastic scattering by measuring the relative intensities of the 'R sub-bandheads. Such a procedure was used for our initial glyoxal (@KO) + Hz experiments7and is known to give reasonable results. For our 81Khxoss section measurements, the fluorescencequantumyields for 81 and 8I7l were assumed to be equal (see Table I1 of ref 11). The fraction of total 817l fluorescence appearing in the 8i7; band is known to be equal to the fraction of 81 fluorescence occurring in the 8 ; band (see Table I of ref 11). The relative cross sections for glyoxal (81K") He scattering are presented in Figure 8 . We have also included in that figure the cross sections for glyoxal (OOKD) He scattering. The comparison informs us that the 81 and 00 relative cross sections are the same to within our experimental error. A remarkable result is found when we now compare the cross sections for glyoxal scattering by He from four initial levels: 00, 72,51, and 8l. The scattering characteristics of the three levels @,SI, and 8l areessentiallyidentical. Thecorrespondenceextends to the competition between the rotational and rovibrational channels and, as far as we can tell, even to the distribution of cross sections for the various Mprocesses within these channels. These scattering similarities are also predicted in the theoretical calculation^.^^ In contrast, initial excitation of the active mode by pumping the 72 initial level produces distinctive scattering behavior.
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Conclusions The rovibrationally inelastic scattering from the higher vibrational levels 51 and 81 in SIglyoxal reinforces the indications of dominant activity by the lowest frequency mode that were emerging from studies of lower glyoxal levels. The scattering cross sections for Au7 = f l quantum changes in that mode, the CHO-CHO torsion v7' = 233 cm-1, exceed the cross sections for all other vibrational state changes by nearly an order of magnitude or more. This conclusion is clouded only by the uncertainty introduced by unexplained structure in the 81 scattering spectrum.
Figure 8. Rotationally resolved inelastic scattering cross sections for H e +glyoxal (8Ikn)(hollow symbols) and (0" (solid ) symbols). The cross sections are plotted against the amount of energy transferred AE from thecenter-of-mass translational motion to theinternal degrees of freedom of glyoxal. Pure rotationally inelastic scattering cross sections (Aq = 0) are shown as circles and rovibrational scattering cross sections (Aw = +1) as squares. The He + glyoxal (@KO) cross sections and error bars are from ref 11.
The state-to-state rotationally inelastic scattering from the SI glyoxal levels 00, S1,and 8l is identical as far as can be seen within the noise of the 51 and 81 experiments. The theoretical scattering predictions are in agreement. The equivalence of scattering fromtheselevels revealsa persistent separation of glyoxal motions in the collisional interaction. The rotational motion about the topaxis remains well separated from vibrational motion, including even a mode (V8) that is primarily an out-of-plane hydrogen wag (vs is in-plane). The CHO-CHO torsion, which is so remarkably harmonic in the SOstate," appears also reasonably decoupled from the motions of vs and V 8 during the collision.
Acknowledgment. Financial support from the National Science Foundation in deeply appreciated. We are also grateful for a grant from the NATO Collaborative Research Grants Program Grant SA.5-2-05 (RG.0215/89). Discussions with D. C. Clary and G.-J. Kroes have been extremely valuable. References and Notes (1) Krajnovich, D. J.; Butz, K. W.; Du, H.; Parmenter, C. S.J . Chem.
Phys. 1989, 92, 7705.
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