Excitation of Ar lines in the cathode region of a dc discharge

Measurements of the spatial distribution of different Ar lines are reported for a flat cold cathode discharge in the j/P² range of 10^-4 to 10^-2 A cm^-2 mbar^-2, where j is the current density and P is the pressure. In this range, excitation by heavy particles and by electrons can be studied since they produce a cathode glow and a negative glow that are well separated. With increasing j/P² the heavy-particle excitation and the cathode glow increase, while the negative glow extends to a larger distance. The voltage, the pressure (P) times the lengths of the cathode glow and the cathode sheath, and the fraction of the input energy that goes into the negative glow and the cathode glow have a dependence on j/P² that is independent of pressure. We also report data on the decay of the negative glow beyond its peak, but this decay length times pressure has a dependence on j/P² that changes with pressure. The measured voltage and lengths are compared to previous data and to a recent calculation.