Reflection of hydrogen atoms from metal and semiconductor targets

Abstract

Partially cesiated Si and Mo targets have been exposed to a flux of atomic hydrogen produced by thermal dissociation of hydrogen gas on a heated tungsten mesh. Hydrogen atoms in the tail of the distribution are elastically reflected from the surface, and some fraction are negatively ionized by electron transfer from the surface. The H− energy distributions and the ion yields have been measured. The backscattered H− ions have a Maxwellian energy distribution with a temperature equal to the atomic gas temperature. The ion yield depends exponentially on the inverse temperature of the gas. The highest measured yield is ∼0.01 for T=0.21 eV on Si. Monte Carlo calculations of the ionized and neutral fractions using the probability model of Rasser et al. are in good qualitative agreement with experimental results. Calculations show that most H− ions formed near the surface do not have enough kinetic energy to overcome the potential barrier due to the image charge interaction.