Time dependent models of field-assisted photoemission

Abstract

A time-dependent model of photoemission, developed to analyze current from and laser heating of metals and dispenser photocathodes, is here applied to examine the impact of geometrical field enhancement and the contribution of tunneling to the photocurrent from a tungsten needle in terms of temporal response, quantum efficiency, and cathode performance. First, a review is given of an updated Fowler–Dubridge model of quantum efficiency including quantum mechanical effects. Second, a prolate spheroidal model of a tungsten needle is given to determine applied fields and incident angles necessary to evaluate emitted current and laser heating effects. Third, a time-dependent model of laser heating of an electron distribution is given which affects the photoemitted current and (if conditions are right) gives field and thermal components as well. Finally, the methodology is related to the experimental findings of Garcia and Brau [Nucl. Instrum. Methods Phys. Res. A 483, 273 (2002)], in which an intense laser illuminates a tungsten needle under sufficiently high fields that the photoemitted current contains field and thermal emission components and effects.