Atom and Ion Chemistry in Low Pressure Hydrogen DC Plasmas

Abstract

The chemical composition of a low-pressure hydrogen dc plasma produced in a hollow cathode discharge has been measured and modeled. The concentrations of H atoms and of H + , H 2 + and H 3 + ions were determined with a combination of optical spectroscopic and mass spectrometric techniques, over the range of pressures (p ∼0.008-0.2 mbar) investigated. The results were rationalized with the help of a zero-order kinetic model. A comparatively high fraction (∼0.1 ( 0.05) of H atoms, indicative of a relatively small wall recombination, was observed. Low ionization degrees (<10 -4 ) were obtained in all cases. In general, the ionic composition of the plasma was found to be dominated by H 3 + , except at the lowest pressures, where H 2 + was the major ion. The key physicochemical processes determining the plasma composition were identified from the comparison of experimental and model results, and are discussed in the paper.

Chemistry in glow discharges of H2/O2mixtures: diagnostics and modelling

“…In a previous study of pure H 2 plasmas in the same reactor, emission spectroscopy measurements in conjunction with a collisional radiative model [33] showed that the H 2 vibrational populations in our plasmas are concentrated in the lowest levels and can be roughly described by a vibrational temperature of ~ 3000 K. For this vibrational temperature, the population of H 2 (v≥1) is ~ 12%. Given the high threshold for electron impact dissociation of H 2 (~ 11 eV) as compared with the first vibrational quantum of H 2 (~ 0.5 eV), we do not expect a significant contribution of vibrationally excited molecules, H 2 (v), to the global electron impact dissociation of H 2 .…”

“…The DC plasma reactor used for the experiments has been described in previous works [33,34]. It consists of a stainless steel grounded hollow cathode reactor (34 cm length, 10 cm diameter) with a central anode.…”

“…The residence time of gases in the reactor is measured according to the method used in previous works [33], obtaining a value of 0.54 ± 0.10 s. Plasma currents of ~ 150 mA and voltages of 500-550 V (depending on the mixture ratio) are maintained during the experiments. An electron gun is used for plasma ignition and then turned off.…”

“…In the steady state, a small fraction of these hot atoms survives and can be detected either in the line profiles of Balmer emission lines of H atoms [81] or indirectly in the energy resolved mass spectra of H + and O + ions [81][82][83]. However, previous works carried out on this reactor for different gas mixtures at comparable pressures [33,34,[36][37][38][39] have shown that the assumption of a Maxwellian distribution is not a bad approximation for the global kinetics, and we have adopted it here for lack of better information on the actual eedf. The limitations of this approximation should be taken into account in the interpretation of the results.…”

“…Positive ions are assumed to disappear from the plasma mainly due to diffusion through the sheath to the reactor walls, where they are neutralized with a probability of 1 (see ref [34] for details on the treatment of ion diffusion). To meet the electroneutrality condition, the flux of positive ions to the walls is balanced to match the net production of positive charge in the glow [33]. Other mechanisms such as electron impact neutralization and ion-ion recombination are also considered, but their importance is orders of magnitude lower.…”