Measurements of the loss probability of nitrogen atoms versus temperature on various surfaces

The loss probability gamma(N) of nitrogen atoms is measured versus the temp erature on different surfaces by a method based on optical emission spectro scopy measurements. In a cylindrical quartz tube, a late Ar-N-2 post-discha rge is used. Emission of the first positive system is observed in a zone th at precedes a heated zone and in another zone that follows it. Each observe d zone is at room temperature. Indeed, in these conditions, nitrogen atoms titration by NO permits the attainment of the absolute value of the N densi ty. This method can give the dependence of the loss probability of nitrogen atoms versus the temperature. The very simple geometry used here enables t he use of foils made of any material introduced in the heated zone of the r eactor. Therefore, different wall materials can be easily studied. Calculat ion of the loss probability gamma(N) requires a 2D hydrodynamic. thermal an d kinetics modelling. It was developed and it included, diffusion to the wa lls, temperature gradients, volume and surface nitrogen atom consumptions. The first results obtained on quartz show that N atoms recombine according to an Eley-Rideal mechanism and gamma (N/quartz) = 9.3 x 10(-5) exp(-3700/R T). The loss probability of the N atoms was also determined at 300 K on iro n foils. The weak value obtained gamma(N/iron) = 6.7 x 10(-4) is probably d ue to the nitriding of the metal. (C) 1999 Elsevier Science S.A. All rights reserved.