VIBRATIONAL PREDISSOCIATION OF P-DIFLUOROBENZENE-CENTER-DOT-AR STUDIED BY MASS-ANALYZED THRESHOLD IONIZATION SPECTROSCOPY

Mass-analyzed threshold ionization (MATI) spectroscopy has already pro ven to be a powerful method for the study of the fragmentation energet ics and dynamics of molecular clusters in the cationic state. In this study, its application was extended to the investigation of the vibrat ionally induced predissociation (VP) of van der Waals (vdW) clusters i n vibronically excited slates (S-1). To verify the feasibility of MATI spectroscopy for studies of this type, the p-difluorobenzene Arl (p-D FB.Ar-1) 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 im possible to obtain by conventional methods such as fluorescence spectr oscopy and time-resolved resonant two-photon ionization spectroscopy. (Butz, K. W.; et al. J. Phys. Chem. 1986, 90, 3533. Jacobson, B. A.; e t al. J. Chem. Phys. 1988, 89, 5624.) In accordance with the pioneerin g studies of Parmenter et al., who investigated the predissociation of the vibronically excited p-DFB.Ar-1 complex very extensively by means of UV fluorescence spectroscopy, (Butz et al., 1986), the MATI spectr a give evidence for the strong mode selectivity of the VP process. How ever, from the MATI results, evidence is given that additional fragmen tation channels appear, which have not been observed in the fluorescen ce spectra. On the basis of the fragmentation thresholds observed in t he MATI spectra, we also deduced upper and lower limits for the dissoc iation energies of the complex in the S-0, S-1, and ionic ground state , which differ significantly from those determined by Parmenter et al.