J. Phys. Chem. 1993,97, 1615-1621
1615
Vibrational Energy Transfer in SIpDifluorobenzene. A Comparison of Low and Room Temperature Collisions Christopher J. hrsell Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Charles S. Parmenter' Joint Institute for Laboratory Astrophysics, University of Colorado, Boulder, Colorado 80309 Received: November 25, I992
Vibrational energy transfer in SIp-difluorobenzene has been studied at low temperature within a supersonic free jet. Quantitative relative cross sections for the state-to-state vibrational energy transfer channels (Le. flow patterns) from the 8* vibrational level (evib = 346 cm-') were obtained for low energy collisions (T= 20-35 K) with He and Ar. They are qualitatively similar to analogous flow patterns obtained earlier from the 00 level for room temperature (300 K)collisions with He and Ar. Both the 300 K and the low temperature flow patterns show that the vibrational energy transfer is selective among the possible channels and that the competition among vibrational channels is distinctive for each collisional partner. The low and room te&perature flow patterns differ, however, in quantitative detail. A treatment of the standard SSH-T vibrational energy transfer model describes semiquantitatively the flow patterns at both low and room temperature and for collisions with H e and Ar. The success of a single model suggests that vibrational energy transfer in SIp-difluorobenzene is governed by the same mechanism over the entire span of collision energies associated with temperatures from -20 to 300 K.
Iabodllction
Much discussion has been given to the mechanism responsible for vibrational energy transfer (VET) at the low collisionalenergies occuring within a supersonic jet expansion. The debate has centered particularly on whether low and room temperature VET involve different mechanisms. As discussed below, several experimentalstudieswith both diatomic and polyatomic molecules have been designed to probe this issue. In this paper we offer further experimental exploration of the question. We have obtained quantitative relative cross sections for the state-to-state VET channels that occur from the g2 vibrational level (ev,b = 346 cm-I) in SIp-difluorobenzene (pDFB) for low temperature (T = 2&35 K) collisions with He and Ar. The unique appeal of pDFB is that analogous sets of cross sections, or flow patterns, are available for room temperature (300 K) VET with He and Are1 We compare the low and room temperature pDFB results by modelling, adapting first a standard VET model to describe semiquantitatively our new low temperature flow patterns for collisions with He and Ar. We then show that the same model fits the room temperature flow patterns with equal fidelity. We infer from thiscorrespondencethat the same collisional mechanism operates over the span of collision energies associated with temperatures from -20 to 300 K. One of the most productive techniques for the study of singlecollision VET in diatomics and polyatomics has been the optical pumpdispersed fluorescencetechniquee2 A molecule is optically pumped to a specificvibrational level in the first excited electronic state, and SI-SO dispersed fluorescence is then used to monitor quantitatively the VET to nearby vibrational levels. Under singlecollision conditions, this technique yields relative state-to-state cross sections or, in some experiments, absolute cross sections. Normalization of the cross sections gives quantitative VET flow patterns. The technique has been especially successful for polyatomic VET in 300 K bulbs.2 Since the initial studies on SIbenzene,j To whom correspondence should be addressed. Permanent address: Department of Chemistry, Indiana University, Bloomington, IN 47405.
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all results have shown that polyatomic VET is a highly selective process, even in the midst of complicated vibrational level structure.2 VET is governed by strong propensity rules that are seen most clearly by the severely restricted state-testate transfer channels. Among the many energetically accessible vibrational levels, VET occurs to only a few. The optical pumpdispersed fluorescence technique has also been the principal method used to learn about VET in the low temperature collisional environment achieved in a supersonic jet.4-20 Here the molecule under study is seeded in the collisional partner, generally a rare gas. The supersonic jet expansion provides an environment in which the collisional energy decreases as a function of distance from the nozzle. Temperatures of the collisional environment can be as low as 1-10 K. Efficient VET is generally observed at these low temperatures. Its persistence initially presented an enigma since conventional semiclassical theories predict13essentially no VET at these low temperatures. A new VET mechanism based on collisional resonances,a phenomenonuniquely important at low temperature, was proposed.615 Efficient VET was attributed to the lengthening of the collisional duration on account of these resonances. Much discussion has centered on whether or not collisional resonances need to be invoked to explain low temperature VET.14J5,'8-*2 Recent experimental explorations that have focussed on the size of the absolute cross sections have been particularly instructive. Studies of iodinels and naphthalene19.*0 suggest that efficient VET at low temperatures is understandable if the VET cross section is scaled by the elastic Lennard-Jones cross section as opposed to a hard sphere cross section. At low temperatures, the encounter rate is mediated by the attractive part of the intermolecular potential which leads to efficient VET. There is no need to invoke the special contributions of collisional resonances. An alternative experimental approach with polyatomics has focussed on the relative VET cross sections of thevariouschannels from a given initial state. The low temperature cross sections were early observed to display the selectivity among the possible state-to-state channels that is the hallmark of 300 K polyatomic 0 1993 American Chemical Society
1616 The Journal of Physical Chemistry, Vol. 97, No. 8, 1993
VET.15 In fact, there are no low energy polyatomic VET studies that fail to reveal this selectivity. In this respect, the low and high temperature VET look related. More recent low temperature explorations of C6D6” and of naphthalene20have produced quantitative flow patterns that have been related with some success to the general modelling of flow patterns observed for other aromatics at 300 K. The comparisons were again suggestive of similar mechanisms for both low and high temperature VET. In our present work we extend this approach to a polyatomic system that allows a much more explicit comparison of low and high temperatureflow patterns. Extensive VET data are available for SIpDFB at 300 K.192J3The 300 K He and Ar flow patterns from the Oo level provide a particularly useful benchmark. They are well replicatedby the standard Schwartz, Slawsky, HerzfeldTanczos (SSH-T) modelZ4.zsof polyatomic VET. We now obtain He and Ar VET flow patterns for SIpDFB at T = 20-35 K.The question then centers on whether the SSH-T modelling so succtssful for room temperature VET replicates the low temperature flow patterns with q u a l success. For this comparison we we a form of the SSH-T model that can be adapted to both low and room temperature experiments with all model parameters derived entirely from data that are independent of our pDFB VET experiments.
Experimental Procedures VET was studied in a supersonicjet expansion using the optical pumpdispersed fluorescence technique. pDFB (C1 a) in 3 atm of He or Ar was expanded through a nozzle of diameter D = 0.8 mm pulsed at 10Hz. The gas expanded freely into a small vacuum chamber, which had a typical background pressure of 5 X 10-5 Torr. pDFB was pumped to the 82 vibrational level in the SIstate (using the 8; band at 37212 cm-I, vac) with the frequency doubled output of a Lambda Physik excimer pumped dye laser. The laser beam was focussed by a lens to a waist diameter of about 0.5 mm and crossed the free jet at right angles. The beam centerwasplacednominallyatX/D = 2.5,wherexis thedistance from the nozzle aperture. The uncertainty in laser beam placement and the 0.5 mm laser beam waist combine to give X/D = 2.5 i 0.5. Fluorescence was collected along two directions perpendicular to both the free jet and the laser beam. Along one direction, an EM1 98l6QA photomultiplier tube monitored the total fluorescence intensity. Fluorescencealongtheother direction was imaged onto the entrance slit of a 1.7 m Czerny-Turner monochromator operating in fourth order. The dispersed fluorescencewas detected by a Hamamatsu R2079 photomultiplier tube. The photomultiplier outputs were integrated on a pulse-by-pulse basis by a home-made gated detection system which was interfaced to a personal computer. The computercontrolled the monochromator frquenc can and was used for storage and manipulation of the fluor& signals. Correction for the pulse-to-pulse variation in the laser power and beam density was made on a shot-to-shot basis by normalizing the dispersed fluorescence signal with the total fluorescence signal. ReSUltS
The SIvibrational levels that may participate in VET from the pumped 82 level (tvib = 346 cm-l) are shown in Figure 1. The actual Occurrence of VET to these levels by collisions with the He or Ar carrier gas is detected by vibrational structure in the SI-.,SOpDFB fluorescence spectrum. A dispersed fluorescence spectrum was first obtainedby pumping the 8* level in a ‘collisionfree” region far downstream (ca. X/D = 25)in order to establish spectral features due to 82 emission alone. A second spectrum was obtained by pumping 82in the ‘single-collision” region of the expansion at X/D = 2.5. Comparisons of the spectra reveal the
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00 346 2 Figure 1. All of the vibrational energy levels of SIp-difluorobcnzene (pDFB) up to about 3 5 0 ~ m - ~Thevibrational . identities are given, along 4and the change in vibrational quanta Au for with the energy gap 1 state-testate VET from the initially prepared 82 level.
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Frequency (cm-l) Figure 2. Dispersed fluorescence spectra from
