Role of Ar2+ and Ar2+ ions in a direct current argon glow discharge:: A numerical description

A two-dimensional model has been developed for calculating the behavior of Ar2+ and Ar-2(+) ions in a direct current argon glow discharge, by the use of balance equations describing the various production and loss processes for these species, as well as their transport by diffusion and migration. These balance equations are coupled to the equations for the Ar+ ions and electrons and solved simultaneously with Poisson's equation, to obtain a self-consistent description of the charged particles behavior and the electrical characteristics in the glow discharge. Moreover, this model is combined with the other models that we have developed previously for the Ar atoms in various excited levels and the Cu atomic and ionic species, to obtain an overall description of the direct current argon glow discharge. The model is applied to typical conditions used for glow discharge mass spectrometry (pressure of 50-100 Pa, voltage of 600-1400 V, and current of 0.4-15 mA). Typical calculation results include the densities and fluxes of these ionic species, as well as the relative contributions of their production and loss processes. The Ar2+ ions are almost exclusively formed by two-electron ionization from Ar-0 atoms, and they become primarily lost by diffusion and subsequent recombination at the cell walls. The Ar-2(+) ions are mainly created by Hornbeck-Molnar and metastable-metastable associative ionization, whereas atom to molecule conversion seems to play only a minor role at the discharge conditions under study. Loss of these Ar-2(+) ions is caused primarily by diffusion and recombination at the cell walls, but dissociative recombination in the plasma plays also a significant role. We found that the ratios of Ar2+/Ar+ and Ar-2(+)/Ar+ ion densities and fluxes were in the order of 1%-10%, which is in good agreement with experimental observations. (C) 1999 American Institute of Physics. [S0021-8979(99)07920-7].