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 descrip tion of the processes occurring in the discharge bulk and of the surfa ce 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) t o 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 probabil ity gamma(O) was found to be independent of delta and to depend only o n 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 fa ctor of 2-5 as delta --> 100%. The kinetic model developed here for su rface reactions provides closed expressions for gamma(N) and gamma(O) in terms of the rate constants and the activation energies for these r eactions. It is shown that the behaviour of gamma(N) and gamma(O) is w ell explained by the model under the following conditions: (i) the mai n surface processes for the low wall temperatures involved (less than or equal to 400 K) are reversible adsorption followed by surface diffu sion of the adatoms to active sites, where they may either be irrevers ibly adsorbed or recombine; and (ii) there exist two independent syste ms of active sites, with different reaction probabilities.