Abstract
Low-temperature microwave-powered plasma based on hydrogen and hydrogen with noble gas mixtures are widely used as a continuous vacuum ultraviolet (VUV) source in laboratory experiments carried out to mimic the photochemistry in astrophysical environments. In this work, we present a study dedicated to optimizing such sources in terms of mono-chromaticity at Ly alpha (H(Ly alpha) line at 121.6 nm similar to 10.2 eV) and high spectral irradiance. We report the influence on the emission spectrum of a wide range of experimental conditions including gas composition (pure H-2, pure He, and H-2/He mixture), gas pressure, flow rates, and microwave power. The absolute spectral irradiance delivered by this VUV light source has been measured. With a microwave input power of 100 W, the best conditions for producing a quasi-monochromatic source are a 1% H-2/He gas mixture at a total pressure of 5 mbar and a flow rate of 2 sccm. By changing the microwave input power from 30 to 120 W, H(Ly alpha) increases by more than one order of magnitude. A comparison between the current measurements and the solar VUV spectral irradiance is reported over 115-170 nm.