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

摘要

The decay pathway competition between monomer and dimer evaporation of photoexcited cluster ions Au_n~+, n = 2-27, has been investigated by photodissociation of size-selected gold clusters stored in a Penning trap. For n > 6 the two decay pathways are distinguished by their experimental signature in time-resolved measurements of the dissociation. For the smaller clusters, simple fragment spectra were used. As in the case of the other copper-group elements, even-numbered gold cluster ions decay exclusively by monomer evaporation, irrespective of their size. For small odd-size gold clusters, dimer evaporation is a competitive alternative, and the smaller the odd-sized clusters, the more likely they decay by dimer evaporation. In this respect, Au_9~+ shows an anomalous behavior, as it is less likely to evaporate dimers than its two odd-numbered neighbors, Au_7~+ and Au_(11)~+. This nonamer anomaly is typical for copper-group cluster ions M_9~+ (M = Cu, Ag, Au) and a similar behavior is found in the anionic heptamers M_7~-. It is discussed in terms of the well-known electronic shell closing at n_e = 8 atomic valence electrons.