Metastable and resonance atom densities in a positive column: I. Distinctions in diffusion and radiation transport

The distinctions in the mechanisms of diffusive transport and resonance radiation transport leading to different population of the metastable and resonance levels are discussed. The analysis is based on a uniform solution of the diffusion equation and the Holstein-Biberman equation. The Green functions of the diffusion and radiation transport operators are shown to be markedly different. The Green function for the integral operator is close to the delta function and that for the diffusion operator shows a smooth logarithmic decrease. The eigenfunctions of the two operators are similar but the eigenvalues differ strongly. For this reason the density of the metastable atoms is determined by the first few modes and the density of the resonance atoms is influenced by a large number of higher modes. Diffusion carries the metastable atoms far out of the excitation zone. Resonance atoms are concentrated in the vicinity of the excitation source. The joint influence of diffusion and radiation transport is considered in the presence of intermixing channels on the basis of a constricted positive column of a discharge