Fast proton-induced electron emission from rare-gas solids and electrostatic charging effects

Abstract

We have studied electron emission from solid Ar, Kr, and Xe films induced by impact of 10--100 keV protons at normal incidence. Electron yields were measured as a function of applied anode voltage and film thickness from 200 to 7000 \AA{}. The observed electron yields are huge---hundreds of electrons per incident ion: higher, per amount of electronic energy deposited, than for any other material studied so far. We extend an electron emission model, developed for metals, to the case of insulators, and obtain electron escape depths of hundreds of nm when fitted to the dependence of electron yield on target thickness. The experiments, especially those with Ar and Kr, are not well described by the model. The reason is the presence of strong electric fields produced by charged traps in the films which, together with a low surface barrier (absence in the case of Ar) ease the extraction of electrons from the films at sufficiently high anode voltages. A hysteresis in the electron currents as a function of anode voltage is also attributed to macroscopic charging of the films. The electrostatic surface potential of the films during ion bombardment is derived by comparing the dependence of the electron emission current with anode voltage to results of computer simulations of electron trajectories near the sample.