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