Electron transfer processes in ion-surface interactions have been attracting interests in a variety of fields such as nuclear fusion energy research, surface diagnostics, chemical reaction in the interstellar mediums, etc. For hydrogen recycling in magnetically confined fusion devices, experimental studies have revealed importance of excited states in energetic neutrals of atomic hydrogen isotopes backscattered at surface of plasma facing high-Z metals like tungsten and molybdenum. However, the excited state distribution is little understood so far.
In the present work, formation of excited hydrogen atoms above surfaces of high-melting temperature metals (e.g. tungsten, molybdenum) was explained by the single electron capture by protons. Occupation probabilities of the excited atomic hydrogen levels were calculated using a one-dimensional mixed quantum-classical method for proton velocities of 0.1-1.0 a.u. The time-dependent Schroedinger equation of the electron was solved numerically using the split-operator spectral method [1], while the proton motion was described by classical trajectories. Electron-surface interaction was approximated using a semi-empirical potential proposed by Jennings et al [2]. The dielectric response of the metal surface was also taken into account at each proton-surface distance using a static linear density response function obtained by the local density functional method [3]. As the velocity becomes as high as the Fermi velocity of the metal, the probabilities of the excited levels reached to ∼10 % of the ground level. The occupation probabilities of metal conduction bands left after the single electron capture by the proton were also obtained.

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