Negative surface ionization of hydrogen and its application to plasma diagnostics

H2+ ions with energies between 50-300 eV impinging on a ThO2-surface are converted into H- ions with an efficiency between 2-7%. It is shown that the negative ions are specularly reflected from the surface with an energy of about 0.4 times the incident beam energy. It is suggested that the neutralization of the H2+ ions preceding the negative surface ionization process takes place via resonant electron capture into a repulsive state of H2. From the data presented it is concluded that a neutral particle analyzer with a low workfunction surface as the ionizing element is in principle feasible. (Auth.)