Determination of Rotational Temperatures and Free Electron Density in a Helium Microwave-Induced Plasma.
Fundamental characteristics of a low-flow, low -power helium microwave-induced plasma were examined using non-invasive spectroscopic methods. Rotational temperatures were measured using both the (o,o) origin band of OH radical at 3064 A, and the First Negative System of N_2 ^+ at 3914 A. Rotational temperatures were evaluated as a function of forward power and support gas flow rate. The data indicate that N_2^+ consistently provided twice as many emission lines as OH radical under the prevailing conditions. Further, N_2^+ derived temperatures were consistently below those obtained from hydroxyl radical. The plasma torch assembly used in this investigation is of an exclusively annular flow design. Electron number densities were examined using a variety of spectroscopic methods. A goal of this experiment was to determine if the electron concentration in this particular torch configuration was higher than the values reported for a straight, open quartz tube. The data indicate that the annular flow design affords electron densities equal to, or slightly higher than, an open tube. An examination of electron density using both hydrogen- and helium-based methodologies revealed that all line width techniques yielded erroneous information. Furthermore, the series-limit line-merging method based upon the Inglis -Teller equation was found to be unworkable for the Balmer series in this class of plasma with a prohibitively interfering spectral background. Finally, the results of this investigation indicate that the use of helium quasidegenerate emission lines possessing forbidden and allowed components may offer a very attractive and simple means of estimating free electron density.
- Pub Date:
- September 1987
- Chemistry: Analytical; Chemistry: Physical; Physics: Fluid and Plasma