Energy dependence of the decay pathways of optically excited small gold clusters

Abstract

The pathway competition between neutral monomer and neutral dimer evaporation from optically excited odd-size gold cluster ions Au+ n, n=7–15, has been investigated as a function of cluster size and excitation energy. Gold cluster ions of these sizes are the only ones to show observable pathway competition while all other sizes exclusively evaporate either neutral monomers or neutral dimers. The investigation has been performed by photoexcitation of stored size-selected gold cluster ions with a single 10-ns laser pulse. Subsequent time-resolved observation of the delayed dissociation allows us to quantitatively determine the relative fragment yields of the respective decay channels as a function of excitation energy. Contrary to theoretical expectations, the dimer-to-monomer branching ratio of evaporated particles is found to decrease monotonously with increasing excitation energy for all cluster sizes under investigation. Possible explanations for this behaviour are discussed.