Surface kinetics on N and O atoms in N-2-O-2 discharges

The probability, gamma, 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 N-2-O-2, for percentage O-2 concentrations O less than or equal to delta less than or equal to 100%. 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 gamma(N) and gamma(O) are functions of the ratio X = [O]/[N] and the wall temperature, T-w. The values of gamma(N) were found to increase from about 2 x 10(-4) to about 10(-2) as delta increases from 1% to 90% (corresponding to 10(-1) less than or equal to X less than or equal to 10(4)). The probability gamma(O) was found to be independent of delta and to depend only on T-w in the range delta = 10-90%. For delta less than or equal to 90%, however, gamma(O) depends on delta, its magnitude increasing by a factor of 2-5 as delta --> 100%. The kinetic model developed here for surface reactions provides closed expressions for gamma(N) and gamma(O) in terms of the rate constants and the activation energies for these reactions. It is shown that the behaviour of gamma(N) and gamma(O) is well explained by the model under the following conditions: (i) the main surface processes for the low wall temperatures involved (less than or equal to 400 K) 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.