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This work is the first of two companion papers devoted to the kinetic modelling of low-pressure DC flowing discharges in N 2 -H 2 mixtures. While the present paper is mainly concerned with bulk discharge processes, the second one investigates surface processes involving dissociated N and H atoms, which are essential to understand the discharge properties. The global model combining bulk and surface processes as described in these two papers is self-contained in the sense that the sole input parameters it requires are those that can externally be chosen in experiments, namely: total gas pressure, radius and length of the discharge tube, discharge current, gas flow rate and initial gas temperature and composition (e.g., the relative hydrogen concentration X in the binary mixture (1 − X )N 2 + X H 2 at the discharge inlet). For a given set of input parameters, this model enables one to calculate the following bulk plasma properties as a function of the axial coordinate z : concentration of N 2 , H 2 , NH, NH 2 , NH 3 molecules and N, H atoms in the ground electronic state; population in the electronically excited states N 2 (A 3 + u , B 3 π g , a − u , a 1 π g , C 3 π u , a + g ), H 2 (R) (an effective high Rydberg state) and N( 2 D, 2 P); concentration of the ions N + 2 , N + 2 (B), N + 4 , H + 2 , H + 3 , HN + 2 and H − ; vibrational level populations of N 2 (X 1 + g ) and H 2 (X 1 + g ) molecules; electron density N e , mean kinetic energy 3 2 kT e , characteristic energy u k and drift velocity v d ; discharge sustaining electric field E ; average gas temperature across the tube T and wall temperature T w . The calculations are compared with data from different experiments in pure N 2 and H 2 discharges (measurements of electric field as a function of current and pressure) and in N 2 -H 2 discharges (measurements of relative changes in the electric field and the N 2 (C), N + 2 (B) concentrations as a function of the H 2 percentage). From the comparison to experiment, rate coefficients for associative ionization upon collisions between two excited N 2 molecules and deactivation of N 2 (a ) and N 2 (X, v ) by H atoms have been estimated from the model.

The probability, , of losses of N and O atoms on Pyrex walls was determined from a fit of calculated to measured concentrations [O] and [NO] in a low-pressure glow discharge in , for percentage concentrations . The kinetic model used for calculations includes a detailed description of the processes occurring in the discharge bulk and of the surface reactions of O and N atoms. It was found that and are functions of the ratio and the wall temperature, . The values of were found to increase from about to about as increases from 1% to 90% (corresponding to ). The probability was found to be independent of and to depend only on in the range . For , however, depends on , its magnitude increasing by a factor of 2 - 5 as . The kinetic model developed here for surface reactions provides closed expressions for and in terms of the rate constants and the activation energies for these reactions. It is shown that the behaviour of and is well explained by the model under the following conditions: (i) the main surface processes for the low wall temperatures involved are reversible adsorption followed by surface diffusion of the adatoms to active sites, where they may either be irreversibly adsorbed or recombine; and (ii) there exist two independent systems of active sites, with different reaction probabilities.

This work is the second of two companion papers devoted to the kinetic modelling of low-pressure DC flowing discharges in N 2 -H 2 mixtures. While the first paper was mainly concerned with bulk discharge processes, the present one investigates surface processes involving dissociated N and H atoms, which are essential to understand the discharge properties. The kinetic model for surface processes developed here takes into account: (a) physical adsorption and desorption of N and H atoms; (b) chemical adsorption and desorption of both types of atoms at vacant chemically active sites on the surface; (c) surface diffusion of physisorbed N f and H f atoms; (d) the reactions of chemisorbed N S and H S atoms with gas phase N and H atoms and with physisorbed N f and H f atoms, leading to the formation of gas phase N 2 , H 2 and chemisorbed (NH) S molecules. The latter molecules can either be desorbed or react with H, H S , H 2 to produce gas phase NH, NH 2 and NH 3 molecules. The probability of wall losses for N, H, NH, NH 2 and the rates for wall production of NH, NH 2 , NH 3 have been obtained for Pyrex glass as a function of the wall temperature and the relative concentrations of N, H, NH, NH 2 , H 2 . A number of important parameters for surface processes have been estimated from detailed analysis of experimental data. Measurements of relative changes in N, H and NH 3 concentrations in N 2 -H 2 discharges as a function of the H 2 percentage have been interpreted in terms of the model.

A theory of the positive column in electronegative gases based on fluid-type momentum equations to describe charged particle motion is presented. It is assumed that quasi-neutrality conditions prevail and the ion inertial terms are neglected. The positive ions are assumed to be created by electron collisions with neutral molecules and the negative ions to be formed by dissociative electron attachment and destroyed by detachment in reactions with neutral species. The mathematical formulation consists of a two-point boundary value problem involving two independent parameters, functions of collisional and transport data, and two eigenvalues. One of these is the central ratio of the negative ion density to the electron density, while the other is related to the ionisation-loss balance and embodies a discharge characteristic for the maintenance field. These eigenvalues and the radial density distributions of the charged species were calculated for a wide range of variation of the independent parameters. An application of the theory to a positive column in oxygen is given as an illustrative example.