Vibrational Predissociation of p-Difluorobenzene·Ar Studied by Mass-Analyzed Threshold Ionization Spectroscopy

Abstract

Mass-analyzed threshold ionization (MATI) spectroscopy has already proven to be a powerful method for the study of the fragmentation energetics and dynamics of molecular clusters in the cationic state. In this study, its application was extended to the investigation of the vibrationally induced predissociation (VP) of van der Waals (vdW) clusters in vibronically excited states (S1). To verify the feasibility of MATI spectroscopy for studies of this type, the p-difluorobenzene·Ar1 (p-DFB·Ar1) complex has been chosen as a model system. The state and mass selectivity of MATI spectroscopy promise to give useful supplementary information about the VP process, which would be difficult or even impossible to obtain by conventional methods such as fluorescence spectroscopy and time-resolved resonant two-photon ionization spectroscopy. (Butz, K. W.; et al. J. Phys. Chem. 1986, 90, 3533. Jacobson, B. A.; et al. J. Chem. Phys. 1988, 89, 5624.) In accordance with the pioneering studies of Parmenter et al., who investigated the predissociation of the vibronically excited p-DFB·Ar1 complex very extensively by means of UV fluorescence spectroscopy, (Butz et al., 1986), the MATI spectra give evidence for the strong mode selectivity of the VP process. However, from the MATI results, evidence is given that additional fragmentation channels appear, which have not been observed in the fluorescence spectra. On the basis of the fragmentation thresholds observed in the MATI spectra, we also deduced upper and lower limits for the dissociation energies of the complex in the S0, S1, and ionic ground state, which differ significantly from those determined by Parmenter et al.