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 polycryst alline metallic photocathodes. The evolution of the photoemitted charg e (high-density electron pulses) was followed along with the surface r eflectivity change, and they were interpreted in terms of the work fun ction change of the surface. The careful choice of the laser fluence e nables us to investigate the cleaning kinetics within a relatively nar row coverage range. Moreover, after recontaminating the surface at dif ferent pressures, we observe distinct kinetics as well. This latter ef fect is explained by the various rates of oxygen incorporation into th e oxide lattice, resulting in a looser, rougher surface for high press ures and a more compact oxide for low pressures. (C) 1998 Elsevier Sci ence S.A.