STATE-RESOLVED VIBRATIONAL-ENERGY-TRANSFER CHANNELS FROM S1 0(0) PARA-DIFLUOROBENZENE IN COLLISION WITH HE AND AR

State-to-state vibrational energy transfer (VET) from the zero-point level of S1 p-difluorobenzene (pDFB) vapor at 300 K has been studied for single collisions with He and Ar. As seen in other polyatomic VET studies, high selectivity occurs among possible channels in this 30-mode molecule. The competition among VET channels has a marked dependence on collision partner. Large absolute rate constants are measured for total VET from the 0(0) level and for the dominant state-to-state channels. The total VET rate constant is 1.6 X 10(6) Torr-1 s-1 for He and 1.8 X 10(6) Torr-1 s-1 for Ar, roughly one-fifth hard sphere for each. The transfer channels involve quantum changes in only the two lowest frequency modes, v30 = 119 cm-1 and v8 = 173 cm-1. The channel 0(0) --> 30(1) dominates VET for each gas. With Ar collisions, for which five channels have been measured, it accounts for about 90% of the transfer. With He, it is 60% of the transfer, competing with the 0(0) --> 8(1) channel that has about 30%. The absolute rate constants for the two He + pDFB channels are within a factor of 2 of values calculated by Clary using three-dimensional quantal scattering theory. A treatment of the SSH-T model accounts well for the competition between VET channels as well as for the differences between collision partners. The account is in the familiar form of propensity rules, similar to those developed earlier for VET in S1 benzene, aniline, and pyrazine.