Formation Of Negative Hydrogen Ions In Surface And Volume Processes

The backscattering of energetic hydrogen particles, 1-1000 eV, from alkali metal surfaces provides for a relatively large yield of negative hydrogen ions. These yields are enhanced by particle reflection from surfaces consisting of partial alkali coatings over high-Z transition-metal substrates. The theoretical data supporting these observations are reviewed. The parameters leading to optimum reflection yields are summarized. In the volume of a hydrogen discharge with electron temperatures of about one electron volt, negative ions are formed by dissociative attachment to vibrationally excited molecules. The vibrational distribution is determined by e-V collisions between low energy electrons and vibrationally excited molecules, E-V singlet electron excitation processes caused by high energy (100 eV) electrons colliding with ground state molecules exciting to electronic states followed by radiative decay to higher vibrational levels, and V-T collisions between molecules resulting in transfer of vibrational excitation to translational energy. The role of these different processes as they bear on the vibrational distribution is discussed. The possibility of a volume-surface interaction leading to a high volume density of negative ions is considered.