AN INVESTIGATION INTO DISSOCIATIVE MECHANISMS IN NITROGENOUS GLOW-DISCHARGES BY OPTICAL-EMISSION SPECTROSCOPY

Nitrogen molecular dissociation is studied in DC diode and thermionic triode glow discharges of pure nitrogen and nitrogen-argon mixtures. Optical emission spectroscopy is used to determine the relative proportions of atomic and molecular nitrogen in the cathode sheath and plasma regions of these discharges operated at -2 kV cathode bias in the range 0.1-10 Pa. The results show that for nitrogen-argon diode discharges at 6.67 Pa total pressure, (i) dissociation rates within the sheath compare with those of pure nitrogen discharges operated at similar nitrogen pressure and the same mechanism (N-2(+)-N-2(0) dissociative charge exchange) is believed to be primarily responsible; (ii) without argon, the proportion of atomic to molecular nitrogen in the plasma is very low compared with the sheath but increases when argon is introduced, the suggested cause being reduced quenching of electron energy by nitrogen gas, leading to increased electron impact dissociation. For nitrogen-argon thermionic triode discharges at 1.33 Pa fetal pressure, the proportion of atomic to molecular nitrogen in the sheath does not significantly vary with nitrogen concentration; this is thought to arise from increased incident ion energies in the sheath, compared with the higher pressure diode case, making Ar-N-2 dissociative charge exchange collisions more prominent. The implications of these findings are discussed in relation to plasma nitriding techniques.