Benzonitrile and its van der Waals complexes studied in a free jet. II. Dynamics in the excited state: The effect of changing the degrees of freedom of partner molecules

Abstract

The dispersed fluorescence spectra of the van der Waals (vdW) complexes consisting of benzonitrile and various partner species were observed in a free jet following a single vibronic level (SVL) excitation. For the vdW complexes with atomic species (Kr and Ar), the fluorescence was found to come from the initially prepared state and/or from the monomer produced by vibrational predissociation. On the contrary, in the case of benzonitrile–molecule complexes (H2O, N2O, and CF3H), only the fluorescence from the relaxed vdW molecule was observed. These features are interpreted in terms of a simple general scheme of predissociation. Vibrational predissociation is considered to be a composite of the four processes: (1) radiative decay of the prepared state; (2) intracomplex vibrational energy transfer producing a relaxed vdW complex; (3) radiative decay of the relaxed vdW complex; and (4) dissociation of the relaxed vdW complex. The difference in fluorescent state between atomic and molecular vdW complexes are caused by the competition among these four processes. The relative importance among these processes is strongly dependent on the state density of the vdW modes which in turn reflects the degrees of freedom of the partner species.