INVESTIGATION OF AN ENHANCED GLOW STRUCTURE IN A LOW-PRESSURE RF DISCHARGE IN HELIUM

Spatially resolved plasma induced emission spectroscopy (PIE) is applied to a symmetric parallel plate discharge in helium under rf excitation at 13.56 MHz. The optical emission features are studied in the pressure range of 10-360 Pa and power densities from 6 mW/cm3 to 90 mW/cm3. At pressures above 60 Pa and power densities exceeding 50 mW/cm3 the transition from the alpha- to the gamma-mode is observed, which, in helium, is accompanied by the formation of a disk-shaped brightly luminous glow. A marked difference between the emission from levels of the triplet and singlet system of the helium atom occurs above this threshold of applied power. The spatial emission distribution of equivalent lines of the helium atom is rather different under pressure variation. Therefore the spatial distribution of the metastable levels is concluded to be substantially different between singlets and triplets. This effect is attributed to a rapid decrease of electron temperature and increase of electron density which is a characteristic of the transition between the two heating modes. Low temperature and high density of the electrons favour an overproportional growth of the associated 2 1S destruction rate, i.e., singlet to triplet conversion. A similar effect has been reported for a dc discharge by LAWLER et al. [1].