We have measured rates of vibrational predissociation from 12 vibrational levels ofS1thinsp;phyphen;difluorobenzenendash;argon (pDFBndash;Ar) by timehyphen;resolved twohyphen;color multiphoton ionization. Three features of our results are particularly surprising. First, the vibrational predissociation rate does not necessarily increase with additional quanta in a given ring vibrational mode. Second, direct excitation of the presumed reaction coordinate, the van der Waals stretch, may decrease the vibrational predissociation rate. Third,pDFB mode 8 (mode 16ain the Wilson notation) shows a large vibrational frequency shift upon complexation, as in other aromatichyphen;argon complexes, but inpDFB it is at most weakly active in vibrational predissociation, in striking contrast to the other systems. These results are inconsistent with available simple theories of vibrational predissociation. We also find that the vibrational predissociation lifetime exceeds 2.7 ns even at 1634 cmminus;1excess vibrational energy, where rapid vibrational energy redistribution has been predicted. Where direct comparison is possible, the rates we measure are about 40percnt; lower than those obtained in timehyphen;integrated experiments by Parmenter and cohyphen;workers.
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