Decay pathways of small gold clusters - The competition between monomer and dimer evaporation

The decay pathway competition between monomer and dimer evaporation of phot oexcited cluster ions Au-n(+), n = 2-27, has been investigated by photodiss ociation of size-selected gold clusters stored in a Penning trap. For n > 6 the two decay pathways are distinguished by their experimental signature i n time-resolved measurements of the dissociation. For the smaller clusters, simple fragment, spectra were used. As in the case of the other copper-gro up elements, even-numbered gold cluster ions decay exclusively by monomer e vaporation, irrespective of their size. For small odd-size gold clusters, d imer evaporation is a competitive alternative, and the smaller the odd-size d clusters, the more likely they decay by dimer evaporation. In this respec t, Au-9(+) shows an anomalous behavior, as it is less likely to evaporate d imers than its two odd-numbered neighbors, Au-7(+) and Au-11(+). This nonam er anomaly is typical for copper-group cluster ions M-9(+) (M = Cu, Ag, An) and a similar behavior is found in the anionic heptamers M-7(-). It is dis cussed in torms of the well-known electronic shell closing at n(c) = 8 atom ic valence electrons.