Cluster-Substrate Interaction on a Femtosecond Time Scale Revealed by a High-Resolution Photoemission Study of the Fermi-Level Onset

We have measured ultraviolet photoemission spectra of quasi-size-selected silver clusters at T0ex0ex=0ex0ex40K grown in the nanopits of a graphite surface. The Fermi-level onset observed shows distinct deviations from the steplike shape typical for metals. A simple model that takes into account the finite lifetime of the photohole (corresponding to a charged cluster) can explain these deviations by the cluster-substrate interaction on a femtosecond time scale, and hence provide an explanation for the differences between photoemission spectra of free and deposited clusters in general.