Recombination and dissociation of H2+ and H3+ ions on surfaces to form H2(v″): Negative-ion formation on low-work-function surfaces

Abstract

The recombination and dissociation of H+2 and H+3 ions incident upon metal surfaces leads to H, H2(v″), and H− products rebounding from the surface. A four-step model for H+2 -ion recombination generates H2(v″) via resonant electron capture through the b 3Σ+u and X 1Σ+g states. A molecular trajectory analysis provides final-state H2(v″) distributions for incident energies of 1, 4, 10, and 20 eV. The calculated H2:H+2 yields compare favorably with the observed yields. A similar four-step model for incident H+3 proceeds via resonant capture to form the H3(2p 2E′→2p 2A1) ground state, in turn dissociating into H+H2(v_″), with the fragment molecule rebounding to give the final H2(v″) distribution. Comparing the final populations v″≥5 for incident H+2 or H+3 shows that the H+3 ion will be more useful than H+2 for H− generation via dissociative attachment. Molecular ions incident upon low-work-function surfaces generate additional H2(v″) via resonant electron capture through excited electronic states and provide two additional sources of H− production: Direct H− production by H dissociation products rebounding from the surface and H− production through the formation of H−2 in the surface selvage that in turn dissociates into H+H−. The H−2 in the selvage is formed by resonant capture to the low-lying vibrational levels of H2(v″), and complements dissociative attachment to high-lying levels in the discharge. The H, H2(v″), and H− yields are inventoried for H+3 incident upon barium surfaces.

A vacuum-UV laser-induced fluorescence experiment for measurement of rotationally and vibrationally excited H2

“…In such environments, surfaces may influence the vibrational and rotational excitation to a large extent. On a surface, molecular hydrogen ions 6 as well as atomic hydrogen radicals 7,8 can recombine, forming rovibrationally excited H 2 . Hall et al 8 and Eenshuistra et al, 7 for example, measured vibrational overpopulations near heated filaments, where the electron density was very low.…”

Heterogeneous and homogeneous hydrogen kinetics in plasma chemistry

“…Based on the experimental diagnostics and kinetic modelling of ionizing plasma the main mechanism for H atoms production in hydrogen plasmas are believed to include the following: direct electron impact dissociation of HZ [18,25]; collisional step-wise vibrational energy redistribution until the dissociation limit [26]; dissociative attachment of electrons to vibrationally excited Hz molecules [27-291; dissociative recombination of hydrogen molecular ions [30]; mutual recombination of hydrogen positive and negative ions L j l , x ] ; dissociative neutralization of hydrogen positive molecular ions on the surfaces [33,34].…”

The Increase of Negative Hydrogen Ions Production in ECR Source by use of the Additional Low-Temperature Emitters of Electrons

“…Приведённые в [15] данные о существенной зависимости ионного тока источника от материала поверхности разрядной камеры подтверждают вывод о перспективности исследований явлений на поверхности электродов для увеличения эффективности ЭЦР источников плазмы. Перспективным каналом колебательного возбуждения молекул H 2 является взаи-модействие между водородными атомными или молекулярными ионами H + 2 и H + 3 на металлической поверхности плазменной камеры [16,17]. В результате рекомбинации на поверхности атомов газовой фазы могут образовываться колебательно возбуж-дённые молекулы водорода H 2 ( ′′ ) [18,19].…”