The Dissociation of Para-Difluorobenzene Van Der Waals Complexes: Spectroscopy and Theory

pDFB-Ar is one of about 15 large molecule-atom van der Waals complexes for which reports describe dissociation following excitation of specific ring levels in the excited S_1 state. Among these, the pDFB fluorescence spectroscopy proves particularly favorable. For each of numerous initial levels, it gives the vibrational predissociation (VP) lifetimes and identifies the vibrationally state-resolved VP channels including some intramolecular vibrational redistribution (IVR) channels. Even though much effort continues on theoretical modeling to understand these energy transfer phenomena, the standard models involving direct dissociation of the complex from the pumped state do not work. In contrast, simple modeling by a sequential dissociation involving, first, IVR into the van der Waals modes followed by dissociation does better. We see that IVR is often the rate limiting step. Thus, as one learns to model the IVR successfully, a major contributor to the dissociation characteristics receives proper account. Other factors also contribute, however, to the dissociation and to the resulting spectroscopy. The competitive IVR rates are calculated by a semiempirical method. The dissociation rate is then calculated as a competition between a unimolecular RRKM dissociation and a direct VP from a state with high excitation in the van der Waals modes. A formulation developed by Ewing is used for the direct VP calculation. The modeling produces an account of the branching ratios with correct prediction of the observed channels. In some cases, it even correctly predicts the branching ratios.