Spectroscopic study of unique line broadening and inversion in low-pressure microwave generated water plasmas

摘要

It was demonstrated that low pressure (~0.2 Torr) water vapor plasmasgenerated in a 10 mm inner diameter quartz tube with an Evenson microwavecavity show at least two features which are not explained by conventionalplasma models. First, significant (> 0.25 nm) hydrogen Balmer_ line broadening,of constant width, up to 5 cm from the microwave coupler was recorded. Onlyhydrogen, and not oxygen, showed significant line broadening. This feature,observed previously in hydrogen-containing mixed gas plasmas generated withhigh voltage dc and rf discharges was explained by some researchers to resultfrom acceleration of hydrogen ions near the cathode. This explanation cannotapply to the line broadening observed in the (electrodeless) microwave plasmasgenerated in this work, particularly at distances as great as 5 cm from themicrowave coupler. Second, inversion of the line intensities of both the Lymanand Balmer series, again, at distances up to 5 cm from the coupler, wereobserved. The line inversion suggests the existence of a hitherto unknownsource of pumping of the optical power in plasmas. Finally, it is notable thatother aspects of the plasma including the OH* rotational temperature and lowelectron concentrations are quite typical of plasmas of this type.