A study of neon-nitrogen interactions in d.c. glow discharges by optical emission spectroscopy

Neon-nitrogen d.c. diode glow discharges have been investigated through the use of spatially resolved optical emission spectroscopy. This technique has enabled the discharges to be sampled from inside the cathode sheath and plasma regions. All discharges were operated at -2 kV cathode bias and 6 Pa total pressure. By evaluating spectral line intensity ratios (SLIRs) that incorporate ion species, the principal findings are as follows: first, the cathode current density at low nitrogen partial pressures is boosted by Penning ionisation of nitrogen by neon; evidence of this mechanism is provided by maxima in nitrogen ion-based SLIRs and minima in neon ion-based SLIRs occurring at low nitrogen concentrations. Second, although the optimum nitrogen partial pressure for Penning ionisation appears to be approximately 5%, the maxima in cathode current density and N₂ ⁺/N₂ ⁰ SLIRs occur at 10-15%; we speculate that a dissociative Penning ionisation mechanism is predominant at 5% but conventional (non-dissociative) Penning ionisation, with possible contributions from other mechanisms (such as electron impact ionisation of nitrogen) become significant at 10-15% nitrogen concentration.