Doppler-broadened Hα line shapes in a rf low-pressure H2 discharge

Abstract

The presented results are concerned with the shapes of the Balmer-α line emitted by a low-pressure rf 13.56-MHz discharge in hydrogen. Optical emission analysis has been done using a monochromator with a high spectral resolution (0.04 Å). Essentially, the lines present a central nearly Gaussian-shape peak with a FWHM of about 0.3 Å emerging from a wide pedestal constituted by two plateaus with widths equal to 0.9 and 1.5 Å, respectively, and by large wings whose intensity quasilinearly decreases on both sides of the central wavelength. We explain these profiles by the Doppler broadening due to fast H atoms produced by the dissociative excitation and the dissociative ionization of the H2 molecules by electron impact generally followed by the excitation of the resulting atoms towards emitting states.