ON THE INFLUENCE OF METASTABLE ATOMS ON SURFACE-WAVE PRODUCED HELIUM PLASMAS

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.