Molecular dissociation in N2-H2 microwave discharges

A microwave N-2-H-2 discharge driven by a travelling surface wave is investigated as a source of ground state N(S-4) and H(1s) atoms. Experimental investigations have been carried out in a plasma source operating at 2.45 GHz at low-pressure conditions (p = 0.5-2 Torr). By means of optical emission spectroscopy and probe diagnostic techniques, the population densities of ground state atoms have been detected. The dissociation kinetics is discussed in the framework of a theoretical model based on a self-consistent treatment of the main discharge balances, wave electrodynamics and plasma-wall interactions. Electron-ion surface recombination processes involving HN2+ and N-2(+) ions are the most important sources of N(S-4) gas phase atoms for the conditions considered. The relative number of N(S-4) atoms in respect to the total neutral density remains approximately constant for percentages of H-2 between 10% and 50% at nearly constant electron density. The competitive interplay of two important source channels of H(1s) atoms, namely electron dissociation of H-2 and H-2 dissociation via the quenching of nitrogen N-2(a ('1) Sigma(-)(u)) and N-2 (A (3) Sigma(+)(u)) metastables, determines a smooth decrease of hydrogen dissociation when the amount of hydrogen increases up to 50% in the mixture.