Investigation of H- production in plasma of dc glow discharge

Abstract

The laser photodetachment technique was applied to investigate the negative ion dynamics in the positive column of a pure hydrogen dc glow discharge at pressures P = 0.1 - 3 Torr. Upon the discharge current modulation, the negative ion H- concentration decayed with the characteristic loss time of hydrogen H(1s2S) atom concentration. The evolution of H atom concentration was investigated by the time-resolved actinometry. The axial electric field was measured by double probe technique. The analysis of the experimental data, i.e. the dependencies of H- and H concentrations as well as reduced electric field on the discharge parameters (discharge current and pressure), allowed us to determine the rate constant of H- production in the discharge, where vibrational excitation and reactions with atoms H greatly influence upon H- production and loss processes. The main processes, which contribute to H- production, are discussed in detail. The vibrational excitation of H2 is shown to increase the rate of dissociative attachment essentially not only via the excitation of the well-known lowthreshold (eth≈3 eV) resonance of H2-(X2∑u+), but also via the excitation of the high-threshold resonances of H2-(2∑g+ (eth≈6 eV), 2Πg, 2Πu (eth≈8-13 eV) etc). Detailed analysis of dissociative attachment via high-threshold resonances allowed us to describe experimental data in the whole range of E/N being investigated, whereas taking any other mechanisms into account (attachment to Rydberg states of H2 etc) failed to do that.