Cluster-surface interaction studied by time-resolved two-photon photoemission

We use time-resolved two-photon photoemission to study the stability of siz e selected Ag-n(+) clusters (n = 2 - 9) deposited onto highly oriented pyro lytic graphite (HOPG) substrates at liquid nitrogen temperatures. The depos ition was carried out with variable kinetic energies of the clusters. Clust ers deposited with high kinetic energy (up to 60 eV/cluster) become fragmen ted upon impact. For low deposition energies (1-4 eV/cluster) the size depe ndence of the photoelectron spectra reveals a pronounced odd/even effect, w hich is well known for gas phase silver clusters. This indicates that the s oft deposited clusters retain their size and identity on the sample. The ph ase of the odd/even effect suggests that transient negatively charged clust er ions serve as an intermediate step in the two-photon photoemission proce ss. The lifetime of the anions rises with cluster size. This is attributed to an increasing electronic density of states for larger clusters.