Some Effects of the $N_{2}$ Rotor on the Vibrational Predissociation of the p-Difluorobenzen-$N_{2}$ Complex

Author Institution: Indiana University, Bloomington, IN 47405.; MIT, Cambridge, MA 02139.We use the vibrational structure of $S_{1}-S_{0}$ fluorescence to compare one aspect of the vibrational predissociation (VP) of the van der Waals complexes pDFB-Ar and pDFB-$N_{2}$. The latter is of particular interest because the $N_{2}$ is almost a free rotor with its axis parallel to the plane of the ring. In these complexes, IVR that pumps ring vibrational energy into van der Waals modes always precedes the VP step. The IVR lifetime from the pDFB-Ar $\overline{27}^{1}$ level is more than an order magnitude longer (ca 115 nsec) than that from six other initially excited $S_{1}$ ring levels. This slow IVR is attributed to unusually poor coupling of the in plane $\nu_{27}$ H-bending motion with the van der Waals modes. Those modes are modified in pDFB-$N_{2}$, and the change is reflected in the IVR and VP characteristics. The IVR lifetime of the pDFB-$N_{2}$ complex becomes relatively short (ca 30 nsec) and an additional channel is observed. For this channel, the monomer product seems to be rotaionally hot. This kind of highly rotationally excited VP product is not common for large molecular complexes. Conjecture concerning the dynamics behind these changes will be offered