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The influence of excited (metastable) states of helium on the ionization and excitation processes in surface-wave sustained plasmas is studied experimentally as well as theoretically. The experimental studies are performed employing emission and absorption spectroscopy methods. The theoretical investigations are based on a collisional-radiative model coupled to self-consistent electron-energy distribution functions. The influence of the stepwise ionization on the axial distribution of the electric-field strength is discussed. Also the impact of the radial distribution of excited atoms is taken into account. Reasonable agreement between measured and calculated population densities of the 2 3 S level is found. The axial changes of the spectral line intensity ratios of a helium singlet line (4 1 S-2 1 P) to a triplet line (4 3 S-2 3 P) are found experimentally. By comparison with the theoretical model it is shown that these modifications can (to a great extent) be attributed to the changing contribution of stepwise excitation of the triplet level.
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Berndt et al. (1994) studied this question.
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