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 cathode Hg-Ar discharge. The experimental map of the density of excited Ar atoms is compared to, and found to agree with, a Monte Carlo simulation 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 the density of ground-state Hg⁺ ions is compared to, and found to agree with, a simulation based on an ambipolar-diffusion model. The production rate per unit volume of Hg⁺ in this second simulation is entirely due to Penning ionization of Hg by the excited Ar atoms. Penning ionization is found to be an important, possibly the dominant, ionization mechanism in the negative glow.