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Spatial distribution of metastable and resonance atoms in a low-pressure He–Xe discharge in spot mode

A dc low-pressure discharge in a helium–xenon mixture with a hot spot on a flat oxide cathode is investigated. The zone around this cathode spot is an interesting source of excitation of metastable and resonance atoms and can be effectively used for the study of fundamental aspects, e.g. transport phenomena of these excited atoms. The method of laser atomic absorption spectroscopy is used to measure the spatial distribution of these atoms. Since excitation sources are significantly distinct from the diffusion and radiation fundamental modes there is a need for correct interpretation of the experimental results based on simultaneous solution of the diffusion equation and the equation of radiation transport. It is shown that the conventional method for the description of resonance radiation transport, which uses the effective lifetime approximation according to Holstein–Biberman, cannot reproduce the spatial distribution of excited atoms in this type of discharge. The influence of various transport mechanisms and of the collisional coupling between the two lowest excited states of xenon on the spatial distribution of these densities is analysed. It is found that the excited atoms appear on the discharge periphery mainly due to resonance radiation transport.