Two photon photoemission of deposited silver clusters

We use time resolved two photon photoemission to study the stability of siz e selected silver clusters deposited onto highly oriented pyrolytic graphit e (HOPG) substrates. Size-selected Ag-n(+) clusters (n = 2 - 9) are deposit ed at low coverage onto HOPG surfaces at liquid nitrogen temperatures. Afte r deposition, the samples are irradiated by a series of ultrashort laser pu lse pairs. Photoelectrons created by two photon photoemission are collected in a magnetic bottle type time-of-flight photoelectron spectrometer. Their kinetic energy distribution is recorded as a function of the delay time be tween subsequent light pulses. With the exception of Ag-3 the size dependen ce of the photoelectron spectra reveals a pronounced odd/even effect, which is well known for gas phase silver clusters. This indicates that the depos ited clusters retain their size and identity on the sample. The lifetime of the photoexcitation rises with cluster size. This is attributed to an incr easing electronic density of states for larger clusters.