Temperature measurement of atomic hydrogen produced by hot-filament dissociation in a CH4H2 mixture

Abstract The Doppler broadening of the absorption line H α at 656.5 nm has been used to measure the gas-phase temperature in a hot-filament-assisted diamond deposition reactor. This transition corresponds to an absorption from the n = 2 excited level of atomic hydrogen which is excited by application of a voltage of some tens of volts between the filament and the substrate. In spite of the very weak density of the absorbers, the H α transition can be observed by intracavity laser absorption spectroscopy. The temperature profile between the substrate and the filament is reported. A rapid decrease of about 600°C is measured in the close vicinity of the filament. Absolute concentrations of excited hydrogen are deduced from the absorption spectra and are discussed in relation to the groundstate population. The dependence of the excited atomic hydrogen concentration on the input methane concentration is also reported and discussed.