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.

The neutral and charged species present in a direct current (dc) hollow cathode, gas flow, air reactor are experimentally studied by quadrupole mass spectrometry. The degree of ionization of the plasma and the electron mean temperature with decreasing air pressure, for constant discharge current, are measured with a double Langmuir probe. The chemical composition of the plasma changes appreciably over the 3 x 10(-3) to 5 x 10(-2) mbar range investigated: at the lowest pressures studied, O2 dissociation is up to 60% and the concentration of NO is half that of N2; concerning ions, NO+ and N2+ are dominant for the whole pressure range. A kinetic model of the plasma including electrons, neutrals, and positive ions is developed to account for the experimental observations; it is consistent with energy balance and predicts that heterogeneous processes are the main source of NO and that the contribution of ions to the global chemistry of neutrals is of minor significance even for the lowest pressures.

The transients associated with the ignition and extinction of the cold plasma produced in a low-frequency, square wave modulated, hollow cathode discharge of air are characterized by time resolved mass spectrometry and emission spectroscopy. The time evolution of the concentrations of neutral products measured in the discharge (NO, N2O, N and O) is compared with the predictions of a simple kinetic model previously developed to characterize low-pressure plasmas of N2O, NO and NO2 and a good agreement is found

The distributions of ions and neutrals in low-pressure (approximately 10(-2) mbar) DC discharges of pure hydrogen and hydrogen with small admixtures (5%) of CH(4) and N(2) have been determined by mass spectrometry. Besides the mentioned plasma precursors, appreciable amounts of NH(3) and C(2)H(x) hydrocarbons, probably mostly from wall reactions, are detected in the gas phase. Primary ions, formed by electron impact in the glow region, undergo a series of charge transfer and reactive collisions that determine the ultimate ion distribution in the various plasmas. A comparison of the ion mass spectra for the different mixtures, taking into account the mass spectra of neutrals, provides interesting information on the key reactions among ions. The prevalent ion is H3+ in all cases, and the ion chemistry is dominated by protonation reactions of this ion and some of its derivatives. Besides the purely hydrogenic ions, N(2)H+, NH(4)+, and CH(5)+ are found in significant amounts. The only mixed C/N ion clearly identified is protonated acetonitrile C(2)H(4)N+. The results suggest that very little HCN is formed in the plasmas under study.

An experimental diagnosis and kinetic modeling of the ionic chemistry in low pressure DC plasmas of H(2)/Ar has been carried out. The studies were performed at pressures of 2 Pa and 0.7 Pa, in a hollow-cathode discharge reactor, using as plasma precursor a H(2)/Ar mixture with 15% Ar content. Experimental measurements include distributions of ion fluxes to the cathode, as well as electronic temperatures and densities in the plasma glow. Besides the species resulting directly from electron impact ionization (H(+), H(2)(+), Ar(+) and Ar(2+)), the ions H(3)(+) and ArH(+) were found to be formed in large amounts through protonation reactions in the glow. In spite of the not too large variation in the pressure of the two plasmas, the differences in the ion distributions are worth mentioning. In the 2 Pa discharge (but not in the 0.7 Pa), H(3)(+) was the dominant ion and ArH(+) exceeded markedly the Ar(+) signal. On the other hand, the appearance of Ar(2+) in the two plasmas points at the relevance of high energy electrons. The experimental results can be accounted for by a simple kinetic model, after including corrections for the presence of a small fraction (<3%) of high energy (>50 eV) electrons and for the attenuation of the Ar(+) ions in the sheath through asymmetric charge exchange with H(2).

The transients associated with the ignition and the extinction of the cold plasma produced in a low frequency, square-wave modulated, hollow cathode discharge of nitrogen dioxide are characterized by time resolved emission spectroscopy, mass spectrometry and electrical probes. The temporal evolution of the concentrations of neutral species created or destroyed in the NO 2 discharges are compared with the predictions of a simple kinetic model previously developed for discharges of other nitrogen oxides (N 2 O and NO). The physical conditions of pressure, gas flow rate, modulation frequency and electrical current in the NO 2 plasma were selected in order to highlight the time-dependent behaviour of some of the stable species formed in the discharge, especially the nitrogen oxide products, whose concentrations show transient maxima. The usefulness of the analysis of the transient results is emphasized as a means to evaluate the relevance of the different elementary processes and as a key to estimate the values of some of the rate constants critical to the modelling. This work is dedicated to the memory of Professor José Campos.

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