Dissociation of hydrogen molecules on metal filaments in H^-ion sources

Abstract

Atomic hydrogen causes significant deterioration of Hion source characteristics through vibration-translation relaxation of vibrationally excited H, molecules. We propose a method for atomic hydrogen density determination based on its selective absorption by a metal (Ta in our case) covered with a monoatomic non-metal film. The temperature-dependence of the dissociation rate by the Ta filament and the contribution of dissociation on Ta filaments to the atomic density in the Hion source are measured and reported. This contribution is noticeable but not dominant when the energy injected into the discharge is iarge enough. ii is shown that the filament surface of W, Ta, Nb or W-Th behaves as a clean one with respect to H, dissociation under the actual conditions of Hion sources. According to a phenomenological theory, filaments of W, Ta and Nb dissociate H, independently of their temperature and of the gas pressure with probability about 0.2-0.3 per single impact, while a W-Th filament dissociates H, with much lower probability, which depends on pressure and temperature.

Hydrogen dissociation on high-temperature tungsten

“…The P diss data of Smith and Fite, taken at p < 0.2 mTorr and shown in Figs. 2 and 3, is slightly higher than our data, but as it is at very small p, it strongly supports a independent of p. Smith and Fite suggested that an oxygen layer might exist on their W surface at the lower T, but this is not supported by observations reported in Livshits et al [18]. Most of the difference from our data looks like an 80 K (4%) temperature shift, which is not surprising in view of the difficulty of establishing T at this level of accuracy.…”

Kinetics of reduction of a RuO2(110) film on Ru(0001) by atomic hydrogen

“…Adsorbed water species were completely absent during or after the reduction of the RuO 2 film by molecular hydrogen [12]. This difference is attributed to the high probability of adsorption of atomic H species on a clean metal surface [43] as compared with the very low sticking probability of the H 2 molecules [12].…”

“…This is both due to the removal of the oxygen atoms by forming water vapor, but also due to the change in the structure of the RuO 2 lattice. After reduction, the surface is apparently composed of side by side Ru and RuO 2 patches [41,[43][44][45], where thin metallic Ru islands are formed. Thereby the homogenous distribution of the oxide film is disrupted and the remaining oxidic species are left as patches at surface resident positions.…”

“…The presence of metallic islands or clusters trigger the recombination of atomic hydrogen species and their subsequent desorption, since the recombination coefficient of atomic hydrogen on such surfaces is approximately unity [43]. Thus, the reduction by atomic hydrogen finally becomes less effective to remove the remnant oxide.…”

Quantification of the atomic hydrogen flux as a function of filament temperature and H2 flow rate

“…27,32,[41][42][43] The reported values vary from 0.03 to 1. A study by Livshits et al 43 indicates that these inconsistencies are caused by different cleanliness of the metal surfaces, but for a clean metal surface the recombination coefficient equals 1. As the Pt sensor in our research was always degassed prior to each measurement, the value taken for c Pt as 1 is well justified.…”