Low pressure hydrogen discharges diluted with argon explored using a global (volume averaged) model

A steady state global (volume averaged) model is used to explore the plasma parameters and the plasma chemistry of a low pressure (1--100 mTorr) high density hydrogen discharge that is diluted with argon [1]. The electron density increases, the dissociation fraction of hydrogen increases and the electron temperature decreases with increased argon dilution. We find that H$_3^+$ is the dominant positive ion up to about 30 \% argon dilution at 10 mTorr, were ArH$^+$ becomes the dominant positive ion. The reaction rates for the creation and destruction of various species are explored versus the discharge pressure. In particular we explore the role of the vibrationally excited levels of the hydrogen molecule in the creation of the negative ion H$^-$ through dissociative attachment. The role of the ArH$^+$ ion in the discharge chemistry is discussed and we find that ArH$^+$ plays a significant role in the destruction of the H$^-$ ion. Furthermore, the creation and destruction of H$_3^+$ and ArH$^+$ ions are explored. The electronegativity increases with increasing H$_2$ content and reaches a value of approximately unity in a pure H$_2$ discharge at 100 mTorr.\\[4pt] [1] A. T. Hjartarson, E. G. Thorsteinsson and J. T. Gudmundsson, Plasma Sources Sci. Technol. {\bf 19} (2010) 065008