UV laser-induced cleaning of native-oxide-covered W photocathodes: the effect of recontamination at various pressures

The fifth harmonic (213 nm) of a picosecond Nd3+:YAG laser was used to remove small fractions of the covering oxide overlayer from polycrystalline metallic photocathodes. The evolution of the photoemitted charge (high-density electron pulses) was followed along with the surface reflectivity change, and they were interpreted in terms of the work function change of the surface. The careful choice of the laser fluence enables us to investigate the cleaning kinetics within a relatively narrow coverage range. Moreover, after recontaminating the surface at different pressures, we observe distinct kinetics as well. This latter effect is explained by the various rates of oxygen incorporation into the oxide lattice, resulting in a looser, rougher surface for high pressures and a more compact oxide for low pressures. (C) 1998 Elsevier Science S.A.