Monte Carlo Simulation of Discrete Space Charge Effects in Photoelectron Emission Microscopy

Interactions among discrete charges in the emission current of a photoelectron imaging system can limit its spatial resolution, significantly affecting the performance and design of emission microscopes employing high current densities. Under intense laser illumination a sample can emit photocurrent densities exceeding tens of amperes/cm2. This study describes Monte Carlo estimations of electron trajectory aberrations produced by discrete electron space charge interactions in a photoelectron emission microscope. The results also apply to photoelectron sources that might be used in other kinds of devices. A model of the emission process was used to assemble bunches of about 500 randomly positioned electrons whose individual trajectories were computed. Trajectory distortions of one member in each bunch that were due to the coulomb forces exerted by its neighbors were used to evaluate aberrations for a variety of voltages, current densities, and instrument configurations. Aberrations estimated in this way are smaller than those predicted by earlier theory or obtained in pulsed imaging experiments. Here we discuss the reasons for these differences, offer suggestions for improved instruments, and present experimental images.