Modeling of metastable argon atoms in a direct-current glow discharge

To calculate the behavior of metastable argon atoms in a direct-current glow discharge, a balance equation is constructed, taking into account all known production and loss processes of the metastable atoms. Density profiles and fluxes of the metastable atoms are computed. The relative importance of different production and loss processes determining the metastable density is calculated for the case of a molybdenum cathode in pure argon. Besides electron-impact excitation, fast-ion and atom-impact excitation are found to be the dominant production processes at the high voltages used here, while loss of the metastable atoms is caused predominantly by diffusion and also by electron quenching to the nearby resonant states. The role of metastable atoms in the total discharge is investigated. They are found to play a minor role in the ionization of argon atoms, but their part in the ionization of sputtered molybdenum cathode atoms appears to be rather important. Moreover, they seem to have a significant effect on the secondary electron emission at the cathode. The investigation also includes the influence of pressure, voltage, and current on the metastable densities and fluxes, on the relative importance of the different production and loss processes, and on the role of metastable atoms in the entire discharge.