Langmuir probe measurements were performed in connection with optical emission spectroscopy to study the a to y transition in a symmetric radiofrequency (I = 13.56 MHz) discharge between parallel plates in helium. The measurements were carried out over a wide range of pressures (30300 Pa) and discharge voltages (20-400 V). The correlation between these two diagnostic methods is investigated. The a to y transition can be seen in the sudden change both in the measured electron-energy distribution function and in the line intensities with increasing discharge voltage. The observed dramatic lowering of the electron temperature in the y mode (the secondary-electron-dominated mode) resuits in exireme differences in ine opticai emission beiween singiei ana iripiei helium lines due to spin-changing collisions of electrons and metastable atoms. It is deduced, using two different electrode materials (aluminium and stainless steel), that secondary electrons and metastable atoms play the dominant role in the formation of the two discharge modes. Furthermore, the probe measurement data are shown to be in very good agreement with those recently repotted by Godyak et a/.
The transition from the a to the y mode in a low-pressure helium RF discharge beween parallel plates is investigated by spatic-temporally resolved plasma induced emission spectroscopy. The time development of several levels of atomic helium and of the n = 4 level of ionic helium is presented over a wide range of discharge pressure (40-300 Pa) and power input (75-300 W). Single photon counting is applied to resolve the time dependence. The transition to the y or secondary electron dominated state is obselved directly in space and time dependent excitation of the helium ion level. The effect of the extreme lowering of the electron temperature resulting from the non-locai sustaining mechanism is ObseNed in the light of atomic helium lines, which show substantial differences between the singlet and the triplet systems due to spin changing collisions (singlet to triplet) coupling the metastable levels. The cross section of these collisions increases with a decrease of electron temperature below the threshold value of 0.79 eV. In addition, probe measurements in a similar, but symmetrically driven parallel plate discharge optimized for use with Langmuir probes are presented to give independent evidence to the rapid decrease of electron temperature when the transition occurs.
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 a-to the y-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 'S destructiqn rate, i.e., singlet to triplet conversion. A similar effect has been reported for a dc discharge by LAWLER et al. [I].
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