Dissociation and vibrational excitation ofH2molecules and wall influence on the densities in a multicusp ion source

Abstract

We have determined absolute densities of vibrationally excited hydrogen molecules with ν’ ’ up to 5 and atoms in a discharge, using resonance-enhanced multiphoton ionization. All measurements were done as a function of discharge voltage, discharge current, and gas pressure. The occupation of ν’ ’=3–5 is strongly superthermal. This effect is more pronounced at higher discharge currents. Scaling laws for the dependence of the density of various particles on the discharge parameters are presented. The analysis shows that vibrationally highly excited molecules are predominantly produced by primary electron excitation and deexcited on each wall collision. H-atom densities have also been determined by resonance-enhanced multiphoton ionization. The atoms are produced by dissociation of molecules by primary electrons, dissociation of molecules on the filaments, and collisions between positive ions and electrons and destroyed by recombination on the walls. Our analysis shows that the recombination coefficient of atoms on the walls is ≊0.1.