Neon-nitrogen and argon-nitrogen thermionically supported dc triode discharges (typical of those used in plasma-assisted physical vapor deposition processes) have been investigated using optical emission spectroscopy. All discharges were operated with the cathode current density and voltage maintained at 0.06 mA/cm(2) and - 100 V. A range of nitrogen partial pressures was examined in each inert gas discharge at total pressures of 1.3 and 4.7 Pa. Spectral line intensity ratios were used to obtain information on the species within the plasma region of the discharge, from which the following effects were noted at both total pressures: (i) The N-2(+)/N-2(0) ratio was observed to increase significantly at low nitrogen partial pressures in the neon-based discharges; this is attributed to Penning ionization by neon metastables. (ii), The maximum in the N-2(+)/N-2(0) ratio and minimum in the Ne+/Ne-0 ratio occurred at similar to10% nitrogen partial pressure, indicating optimum conditions for Penning ionization. (iii) For nitrogen partial pressures of less than 10%, an increase in atomic nitrogen species was observed for both inert gas discharges; this effect is attributed to the dissociation of nitrogen molecular species (both ionized and neutral) by inert gas metastables. (C) 2003 American Vacuum Society.