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 ${\mathrm{H}}_{2}$ and observed by Doppler spectroscopy and quantitative radiometry are the dominant H\ensuremath{\alpha} source in low-current, low-pressure ${\mathrm{H}}_{2}$ dc discharges. For heavy-metal cathodes, backscattered fast H atoms from incident H and ${\mathrm{H}}^{+}$ atoms and ${\mathrm{H}}_{2}$ and ${\mathrm{H}}_{2}^{+}$ molecules excite most of the H\ensuremath{\alpha}. For graphite cathodes, backscattering is small and H\ensuremath{\alpha} is produced by electrons and by approaching fast H atoms produced by charge transfer in ${\mathrm{H}}^{+}$-${\mathrm{H}}_{2}$ collisions and dissociation in ${\mathrm{H}}_{2}^{+}$-${\mathrm{H}}_{2}$ and ${\mathrm{H}}_{2}$-${\mathrm{H}}_{2}$ collisions. Excitation of H\ensuremath{\alpha} by ion collisions with ${\mathrm{H}}_{2}$ and the cathoode is small.