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. O ptical emission spectroscopy is used to determine the relative proport ions of atomic and molecular nitrogen in the cathode sheath and plasma regions of these discharges operated at -2 kV cathode bias in the ran ge 0.1-10 Pa. The results show that for nitrogen-argon diode discharge s at 6.67 Pa total pressure, (i) dissociation rates within the sheath compare with those of pure nitrogen discharges operated at similar nit rogen pressure and the same mechanism (N-2(+)-N-2(0) dissociative char ge exchange) is believed to be primarily responsible; (ii) without arg on, the proportion of atomic to molecular nitrogen in the plasma is ve ry low compared with the sheath but increases when argon is introduced , the suggested cause being reduced quenching of electron energy by ni trogen gas, leading to increased electron impact dissociation. For nit rogen-argon thermionic triode discharges at 1.33 Pa fetal pressure, th e proportion of atomic to molecular nitrogen in the sheath does not si gnificantly 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 exchang e collisions more prominent. The implications of these findings are di scussed in relation to plasma nitriding techniques.