Nonlinear organic photoemitters: Their properties and possible applications in electron microscopy

Abstract

This paper describes recent measurements of the emission characteristics of laser-driven photoelectric thin films and their potential uses in microscopy. Preparation of organic film photocathodes on doped silicon and copper substrates is described. Kinetic energy spreads of photoelectrons from these emitters have been measured as a function of current density, film thickness, substrate type and laser wavelenght. It appears that film thickness and smoothness have little effect on the energy distribution, but the energy spread does increase at high current densities, probably because of space charge, and reducing the illuminating wavelenght from 337 to 266 nm causes a significant increase in energy spread. With pulsed laser illumination, the emission current can be increased by selecting a thickness which exploits an optical standing wave resonance made possible by the substrate reflectance. The current densities obtained with these cathodes are compared to values desired for applications in microscopy, and comparison is also made to other choices of photoemitters and laser illuminators. Performance limitations imposed by heating of the films and by space charge are discussed.