IONIZATION BALANCE IN THE NEGATIVE GLOW OF A HG-AR HOT-CATHODE DISCHARGE

Absorption spectroscopy is used to map the absolute density of excited Ar atoms and ground-state Hg+ ions in the negative glow of a hot cath ode Hg-Ar discharge. The experimental map of the density of excited Ar atoms is compared to, and found to agree with, a Monte Carlo simulati on of the resonance-atom density from radiation trapping. The primary production rate per unit volume of excited Ar atoms in the simulation is sharply peaked near the cathode hot spot. The experimental map of t he density of ground-state Hg+ ions is compared to, and found to agree with, a simulation based on an ambipolar-diffusion model. The product ion rate per unit volume of Hg+ in this second simulation is entirely due to Penning ionization of Hg by the excited Ar atoms. Penning ioniz ation is found to be an important, possibly the dominant, ionization m echanism in the negative glow.