The lateral photoemission distribution from a defined cluster/substrate system as probed by photoemission electron microscopy

Abstract

We used photoemission electron microscopy (PEEM) to investigate the lateral distribution of the photoemission yield from a defined system of silver clusters supported by a highly oriented pyrolytic graphite (HOPG) substrate. For threshold photoemission using conventional photoemission (PE) and two-photon photoemission (2PPE) we find that distinct, well-separated emitters are responsible for the measured integral photoemission yield. Complementary characterization of the surface using STM shows that the emitter density as probed by PEEM is reduced by about three orders of magnitude in comparison to the actual cluster density. Wavelength and light polarization scans in combination with two-photon-PEEM clearly show that the origin of the 2PPE signal is related to small silver particles. Furthermore, the PEEM differentiates between inhomogeneous and homogeneous broadening effects in the 2PPE signal. This observation allows one to assign the origin of the local photoemission signal to either a distinct single silver particle or a number of coherently coupled silver particles. We conclude that the 2PPE-yield is highly selective with respect to specific properties of the supported silver particles. Our results show that in future experiments, PEEM as a highly local field probe, may be a key tool in the identification of these properties.