Nitrogen dissociation in low-pressure microwave plasmas

Abstract

This work reviews a number of important aspects of the kinetic processes occurring in microwave discharge plasmas in nitrogen containing mixtures. The fundamental problems which are addressed include: (1) the determination of the non-equilibrium electron energy distribution function and its coupling to the vibrational kinetics; (2) the processes determining the vibrational distribution function in N2, N2-H2, and N2-Ar plasmas; and (3) the influence of surface kinetic processes on the whole discharge physics. The mechanisms of dissociation and ionization in N2, N2-Ar and N2-H2 are discussed in detail. The important role of plasma-wall interactions is stressed. It is shown that in N2-Ar mixtures there is an increase in the dissociation degree of N2 molecules at high Ar fractional concentrations, which can be ascribed to the contribution of fast Ar++ N2→ N+2+ Ar exchanges followed by N+2 dissociative recombination. It is further demonstrated that surface electron-ion recombination processes involving NH+2and N+2 ions are the most important sources of N(4S) gas phase atoms in N2-H2 mixtures. As the amount of H2 in the discharge increases, the channel associated with HN+2 ions becomes dominant. At nearly constant electron density there is a smooth decrease in the dissociation degree of hydrogen in N2-H2 when the H2 percentage increases up to 50%, which is due both to electron impact dissociation and H2 dissociation upon quenching of nitrogen N2(A3∑+u) and N2(a' 1∑−u) metastables.