Influence of surface materials on the volume production of negative ions in a radio-frequency driven hydrogen plasma
Negative atomic hydrogen ion (H$^{-}$) densities were measured in a pulsed low-pressure E-mode inductively-coupled radio-frequency (rf) driven plasma in hydrogen by means of laser photodetachment and a Langmuir probe. This investigation focuses on the influence of different metallic surface materials on the volume production of H$^{-}$ ions. The H$^{-}$ density was measured above a thin disc of either tungsten, stainless steel, copper, aluminium, or molybdenum placed onto the lower grounded electrode of the plasma device as a function of gas pressure and applied rf power. For copper, aluminium, and molybdenum the H$^{-}$ density was found to be quite insensitive to pressure and rf power, with values ranging between 3.6x10$^{14}$ to 5.8x10$^{14}$ m$^{-3}$. For stainless steel and tungsten, the H$^{-}$ dependency was found to be complex, apart from the case of a similar linear increase from 2.9x10$^{14}$ to 1.1x10$^{15}$ m$^{-3}$ with rf power at a pressure of 25 Pa. Two-photon absorption laser induced fluorescence was used to measure the atomic hydrogen densities and phase resolved optical emission spectroscopy was used to investigate whether the plasma dynamics were surface dependent. An explanation for the observed differences between the two sets of investigated materials is given in terms of surface reaction mechanisms for the creation of vibrationally excited hydrogen molecules.