Excitation by and surface reflection of fast hydrogen atoms in low-pressure hydrogen discharges

Fast excited H atoms produced in collisions of fast H atoms with H₂ and observed by Doppler spectroscopy and quantitative radiometry are the dominant H{α} source in low-current, low-pressure H₂ dc discharges. For heavy-metal cathodes, backscattered fast H atoms from incident H and H⁺ atoms and H₂ and H₂⁺ molecules excite most of the H{α}. For graphite cathodes, backscattering is small and H{α} is produced by electrons and by approaching fast H atoms produced by charge transfer in H⁺-H₂ collisions and dissociation in H₂⁺-H₂ and H₂-H₂ collisions. Excitation of H{α} by ion collisions with H₂ and the cathoode is small.