New assignments in the UV spectroscopy of the small benzene–argonn clusters: The effects of a structure-selective vibrational predissociation

In this paper, new experimental results using UV spectroscopy of the small benzene–Ar_n clusters are presented. We have found evidence for the vibrational predissociation of the $S_1$ state of some of these species on a nanosecond time scale and we propose a new assignment for the UV spectral features of this system. This assignment is consistent with other experimental data and it accounts additionally for the previously reported spectral anomalies, in particular the ionization potential measurements of these species. The two-color R2PI spectra performed can thus be considered as nanosecond pump–probe experiments that allow us to estimate the relaxation rate of the $S_1$ state of these clusters. The vibrational relaxation is found to be strongly size and structure dependent: The two isomers [(1/1) and (2/0)] of the n=2 species exhibit different lifetimes differing by at least one order of magnitude. The size dependence of the relaxation process within the one-sided isomer series appears also paradoxical since the relaxation rates of the (3/0) and (4/0) species are found to be smaller than that of the (2/0). These properties are discussed in relationship with the symmetry of these species.