Possible Mechanism of ``Additional'' Production of H$^{-}$ in a Glow Discharge

Based on measurements of H$^{-}$ and H densities a DC glow discharge in H$_{2}$ (P=0.1-3 Torr) the rate coefficient of H$^{-}$ production as a function of $E/N$ was determined. To analyze the mechanisms of H$^{-}$ production, a simple model of H$_{2}$ vibrational excitation was developed. Estimations of vibrational level densities ($v$=3-5) obtained from VUV absorption measurements were in reasonable agreement with the calculated data. The analysis revealed that standard mechanisms of H$^{- }$production (dissociative attachment to vibrationally excited molecules H$_{2}(v)$ and molecules in Rydberg states H$_{2}$(\textit{Ry})) were not enough to explain the experimental results. In order to describe both the shape (vs $E/N)$ and the magnitude of the measured H$^{-}$ production rate coefficient, an ``additional'' source of H$^{-}$, having a strong resonant electron attachment CS in the range of $\sim $5-9 eV, should be invoked. Although H$_{2}$ has no resonances in the 5-9 eV range, water is known to strongly dissociatively attach in this range. Thus, even small amounts (0.1-1{\%}) of water vapor in the apparatus can explain the origin of the ``additional'' H$^{-}$ production. This result is corroborated by the work of Cadez \textit{et. al.} in Proc. of XXVII ICPIG, 2005. This work was supported by the RFBR (No.05-02-17649a), Scientific School - 171113.2003.2 and NATO Collaborative Linkage Grant (No.980097).