FREQUENCY-RESOLVED AND TIME-RESOLVED CLUSTER PHOTODISSOCIATION DYNAMICS IN SR+(H2O)N, SR+(NH3)N AND SR+(CH3OH)N

The study of photodissociation processes in mass-selected solvated metal ions affords a detailed look at half-collision dynamics and their relationship to structural and dynamical phenomena accompanying solvation. In photodissociation studies on the alkaline earth cation Sr+ solvated by polar solvent molecules, we have examined the nature of electronic to vibrational (E-V) energy transfer when metal-centred electronic transitions excite these systems above their dissociation thresholds. The excited electronic states of small Sr+(NH3)n and Sr+(H2O)n clusters with n = 1 or 2 show clear directional effects associated with the orientations of the excited 5p orbitals with respect to the bond axis. The interpretation of the photodissociation as an E-V energy-transfer process suggests that significant transfer of electron density from the metal centre to the solvent occurs at the intersections of the excited and ground-state potential surfaces. In larger clusters with n = 3-6, the absorption maxima shift from the visible through the infrared region of the spectrum, finally peaking near 1.5 mum for n = 6. A spectral moment analysis of the absorption cross-sections shows that [0\r2\0] for the radial distribution function of the valence electron in the ground state increases by more than a factor of 20 as n increases from 1 to 6 solvent molecules. We discuss these spectral shifts in terms of the increasing Rydberg character of the ground and excited states of the clusters, arising from the rapid solvent-dependent stabilization of ion-pair states with increasing number of solvent molecules. Our most recent experiments attempt to address the time-scale for photodissociation by using picosecond pump-probe laser techniques. Our initial results on the Sr+(NH3)2 system suggest that the dissociation is dominated by a slow process with a lifetime of 7 ns, but there is also a small contribution from a process taking place on a timescale faster than 10 ps. The extension of these measurements to a range of cluster sizes, with the goal of extracting characteristic solvent motion times in well characterized solvation environments, provides us with a probe of the solvent reorganization process accompanying electron transfer in condensed phase systems.