Direct measurements of vibrational predissociation of p-difluorobenzene–argon

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Abstract
We have measured rates of vibrational predissociation from 12 vibrational levels of S1 p‐difluorobenzene–argon (pDFB–Ar) by time‐resolved two‐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 16a in the Wilson notation) shows a large vibrational frequency shift upon complexation, as in other aromatic‐argon complexes, but in pDFB 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 cm−1 excess vibrational energy, where rapid vibrational energy redistribution has been predicted. Where direct comparison is possible, the rates we measure are about 40% lower than those obtained in time‐integrated experiments by Parmenter and co‐workers.