VIBRATIONAL PREDISSOCIATIN IN THE P-FLUOROTOLUENE-ARGON COMPLEX. $CH_{3}$ VS.$CD_{3}$ INTERNAL ROTATION.

1. C.S. Parmenter and B.M. Stone, J. Chem. Phys. 84, 4710 (1986). 2. Z. Q. Zhao, Abstract for the 46th International Symposium on Molecular Spectroscopy, Columbus, OH, June 1991, p.168. 3. D. B. Moss, C. S. Parmenter, T.A Peterson, C.J. Pursell, and Z. Q. Zhao, VIIth International Symposium on Ultrafast Processes in Spectroscopy, Bayreuth German,(IOP Publishing Ltd., Bristol, UK) 1992 (in press).Author Institution: Department of Chemistry, Indiana UniversityIt is well established that internal rotation of the methyl group in p-fluorotoluene(pFT) has a profound effect on the rate of intramolecular vibrational energy redistribution (IVR), especially when compared to a system such as p-difluorbenzene (pDFB),Which lacks this extra degree of freedom. Chemical timing experiments conducted on these systems show orders of magnitude acceleration of IVR in the free rotor pFT $molecule.^{1}$ Further studies of this effect have been conducted on van der waals complexes in which the pDFB or pFT is weakly bonded to an argon atom. In these cases, the experimental evidence suggests that the internal rotation of the methyl group not only contributes substantially to IVR dynamics, but also quite heavily affects the rate of vibrational predissiociation $(VP).^{2}$ Acceleration of the dissociation is so pronounced in the pFT-Ar system that only emission from the monomer is observed, while pDFB-Ar still exhibits strong complex fluorescence. Further chemical timing investigations revealed that the isotopically substituted rotor counterpart to pFT, $pFTd_{3}$ showed no marked increase in the rate of $IVR.^{3}$ These similarities in the light and heavy rotor systems have more recently begun to be studied in the $pFTD^{3}$-Ar van der Waals complex expanded in a supersonic free jet. We are presently examining the consequences of the deuterium substitution on vibrational state selectivity and rates of VP in order to further explore and confirm the contributions of internal rotation to vibrational energy flow