Vacuum ultraviolet emission from mixed H₂:Ar/Ne/He plasmas

The emission of mixed inert/reactive gas plasmas, commonly used in a variety of industrial applications and laboratory settings, is relatively unquantified in the vacuum ultraviolet (VUV) wavelength range. VUV emission spectroscopy of mixed H₂:Ar/Ne/He inductively coupled radio frequency (RF) plasmas was conducted for pressures in the range 20-80 mTorr with 400 W input RF power and H₂ concentrations of 0% to 100%. For H₂:Ar mixtures at 80 mTorr, the Ly- α (121 nm) emission was observed to reach a maximum at 27.5% H₂: 72.5%Ar, approximately 3.3 × greater than a pure hydrogen discharge at the same pressure, and the total emission in the range 115-200 nm increased by a factor of 1.5. By increasing the pressure at which the mixtures were maintained, the maximum Ly- α emission was increased, and the H₂ concentration at which that maximum occurred decreased. Comparisons to a simple collisional model indicate a significant pressure dependence, which is consistent with observations of self-absorption of Ly- α by ground state hydrogen atoms. Experiments across the full range of H₂:Ar/Ne/He mixtures revealed that admixed gases with heavier atomic masses are more effective at increasing the Ly- α emission. A similar pressure dependence was also observed, indicating self-absorption phenomena are still highly relevant to emission spectroscopy of mixed hydrogen/inert gas plasmas. The development of high efficiency sources in the VUV wavelength range is especially useful for space applications where there is no atmospheric absorption of the VUV.