Electron Capture Processes on Oxidized Surfaces:H−Formation on Mg/O2and Al/

Abstract

Electron capture processes on oxygen covered surfaces were investigated on the example of H 2 formation in H 1 scattering. A strong increase in H 2 production occurs with increasing coverage from the submonolayer to oxide formation range. At low coverages these modifications are interpreted as resulting from competing local effects at oxygen chemisorption sites and nonlocal effects corresponding to work function changes. At high coverages scattering on an ionic solid is considered, with capture occurring at O d2 sites. Strong H 1 to H 2 conversion on an oxide provides an interesting method of negative ion production for injection into fusion devices. [S0031-9007(96)01643-2] PACS numbers: 79.20. Rf, 34.70.+e, 73.30.+y, 82.65.My Oxide surfaces play an important role in fields like catalysis, microelectronics, gas sensors, and in many manufacturing processes in ceramic and glass industries [1,2]. Despite their importance, the proportion of surface science work that has been devoted to date to them remains very limited as opposed to the case of metal and semiconductor surfaces. It is well known that the majority of reactions at surfaces and the formation of transient species involve charge transfer processes: the exchange of an electron or a proton [3]. The acid-base properties of an oxide catalyst are characterized by its capacity to retain an electron or a proton. The study of these transfer phenomena, which along with the surface structure play a fundamental role in determining its reactivity, is thus of great interest. On the theoretical side, efforts at a systematic investigation of the acid-base properties of a series of insulating oxides (BaO, MgO, TiO 2 , SiO 2 ) have been performed recently by Noguera et al. [4,5], who investigated the adsorption of H 1 and OH 2 and discussed charge transfer for this static case. On the experimental side, an interesting method of investigation was suggested by Souda et al. [6], who qualitatively discuss the backscattering of H 1 and He 1 in terms of the ionic-covalent nature of the surface. However, neutralization probabilities, which carry quantitative information, were not determined.A related topic, the study of oxygen adsorption on metal and semiconductor surfaces, has been the subject of numerous experimental and theoretical studies. However, here also electron transfer processes have not been studied in detail experimentally and there do not exist accurate theoretical descriptions of resonant and Auger electron transfer processes. It is common knowledge that oxygen adsorption results in an enhanced emission of secondary ions, as observed in, e.g., secondary-ion mass spectroscopy (SIMS) measurements, attributed to a decrease of electron capture. Our recent studies [7][8][9] have demonstrated that strong modifications in the rearrangement of atomic states near a surface occur in the very initial stages of oxygen uptake. These modifications were tentatively attributed to changes in such surface properties as the work function ͑f͒ and density of states.Here we investiga...

Theory of negative-ion conversion of neutral atoms in grazing scattering from alkali halide surfaces

“…6,7 The evolution of the charge state of the projectile during its interaction with ionic crystal surfaces has received attention only recently. [8][9][10][11][12][13][14][15][16] In experiments on the grazing scattering of positively charged and neutral H, O, F projectiles from alkali-halide surfaces very high fractions of negative ions were unexpectedly observed in the scattered beams. 9,13,15 Until now the concepts for electron transfer from a surface to an atom essentially descend from atom/metal surface interaction studies.…”

Formation and decomposition of hydrogen-related electron traps at hydrogenated Pd/GaAs (n-type) Schottky interfaces

“…It has been recently reported that the impact of hydrogen on oxygen-covered Ni͑100͒ surfaces produces water at temperatures between 120 and 400 K. 21 There are reports of charge transfer involving the hydrogen adsorbed on the oxide surface. [22][23][24][25] Evidence for the interaction of electrons and hydrogen comes from the observation of the formation of negatively charged hydrogen from positively charged hydrogen colliding on oxygen covered surface. 22 Theoretical studies of adsorption of hydrogen on oxide surfaces indicate charge transfer between the surface oxygen atom and the adsorbed proton.…”

“…[22][23][24][25] Evidence for the interaction of electrons and hydrogen comes from the observation of the formation of negatively charged hydrogen from positively charged hydrogen colliding on oxygen covered surface. 22 Theoretical studies of adsorption of hydrogen on oxide surfaces indicate charge transfer between the surface oxygen atom and the adsorbed proton. [23][24][25] It is believed that dissociation of hydrogen gas through transition metals results in highly active atomic hydrogen species that react selectively with oxygen atom on the supporting oxide surface.…”

Formation of negative ions in grazing scattering of neutral atoms from alkali-metal halide (100) surfaces

“…Most of these studies have focused on ion-induced electron emission [1], energy loss of the projectile [2], and charge transfer phenomena [3]. In particular, recent studies on negative-ion formation in grazing scattering of neutrals from insulator surfaces have attracted considerable attention [4][5][6][7][8][9], due to its potential application in the development of new negative-ion sources.…”

“…However, at higher velocities, this model fails to reproduce the experimental data, since electron loss, which dominates electron transfer cycles and determines the final negative-ion fraction, is not included in their model. In fact, electron loss phenomena for insulators have been observed experimentally both in surface scattering [6,15,16] and nanocapillary transmission measurements [17]. Therefore, the understanding of the destruction of negative ions on insulator surfaces is highly desired.…”