R. Flohr et al., INVESTIGATION OF AN ENHANCED GLOW STRUCTURE IN A LOW-PRESSURE RF DISCHARGE IN HELIUM, Contributions to Plasma Physics, 33(3), 1993, pp. 153-168
Spatially resolved plasma induced emission spectroscopy (PIE) is appli
ed to a symmetric parallel plate discharge in helium under rf excitati
on at 13.56 MHz. The optical emission features are studied in the pres
sure 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 h
elium, is accompanied by the formation of a disk-shaped brightly lumin
ous glow. A marked difference between the emission from levels of the
triplet and singlet system of the helium atom occurs above this thresh
old 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 conclu
ded to be substantially different between singlets and triplets. This
effect is attributed to a rapid decrease of electron temperature and i
ncrease of electron density which is a characteristic of the transitio
n between the two heating modes. Low temperature and high density of t
he electrons favour an overproportional growth of the associated 2 1S
destruction rate, i.e., singlet to triplet conversion. A similar effec
t has been reported for a dc discharge by LAWLER et al. [1].