Frequency- and time-resolved cluster photodissociation dynamics in Sr+(H2O)n, Sr+(NH3)n and Sr+(CH3OH)n

Abstract

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 µm 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 timescale 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.