Negative surface ionization of hydrogen atoms and molecules

Abstract

The total yield of H- ions, Y(Ein). produced in backscattering of low-energy H+ and H2+ ions from polycrystalline gold, tungsten, and molybdenum converter surfaces was measured at normal incidence in the energy range Ein = 2-30 eV per nucleus. The yield per nucleus is independent of the ion mass. This indicates that the molecular ions are dissociated before colliding with the converter surface. A universal expression for Y(Ein) was developed by combining the electron tunneling theory with atomic scattering theory. This expression agrees well with measurements. The yield is completely characterized by two parameters. Eth/RE and RNη0, which can be determined experimentally: Y=o for Ein = Eth/RE, and Y approaches the maximum yield Rη0 as Ein increases. These parameters were determined from measured H- yields in ion beam backscattering experiments, as well as for backscattering of thermal distributions of hydrogen atoms. For beam experiments, the maximum yield of 0.3 per nucleus was obtained for Mo/Cs converters with 1.5 eV work function. A higher maximum yield of 0.42 was obtained from experiments on backscattering thermal distributions of H atoms. This is attributed to high extraction fields. The universal yield formula made it possible to compare the results of the two different types of experiments. © 1996 American Institute of Physics.