Surface kinetics of N and O atoms in discharges

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Abstract
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.