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

The pathway competition between neutral monomer and neutral dimer evaporati on 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. Gol d cluster ions of these sizes are the only ones to show observable pathway competition while all other sizes exclusively evaporate either neutral mono mers or neutral dimers. The investigation has been performed by photoexcita tion of stored size-selected gold cluster ions with a single 10-ns laser pu lse. Subsequent time-resolved observation of the delayed dissociation allow s us to quantitatively determine the relative fragment yields of the respec tive decay channels as a function of excitation energy. Contrary to theoret ical expectations, the dimer-to-monomer branching ratio of evaporated parti cles is found to decrease monotonously with increasing excitation energy fo r all cluster sizes under investigation. Possible explanations for this beh aviour are discussed.