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A new analytical potential energy surface (PES) based on new density functional theory data is constructed for the interaction of atomic hydrogen with both a clean and an H-preadsorbed β-cristobalite (001) surface. For the atomic interaction, six adsorption sites have been considered, the Si site (T1′) being the most stable one. The PES was developed as a sum of pairwise atom-atom interactions between the gas-phase hydrogen atoms and the Si and O atoms of the β-cristobalite surface. A preliminary molecular dynamics semiclassical study of the different heterogeneous processes (e.g., H2 formation via Eley-Rideal reaction, H adsorption) that occur when H collides with an H-preadsorbed β-cristobalite (001) surface was carried out. The calculations were performed for collisional energy in the range (0.06 ≤ Ekin ≤ 3.0 eV), normal incidence and a surface temperature Tsurf = 1000 K. The recombination probability reaches its maximum value of approximately 0.1 for collisional energies in the range 0.3 ≤ Ekin ≤ 0.8 eV. The H2 molecules are formed in medium-lying vibrational levels, while the energy exchanged with the surface in the recombination process is very low. [ABSTRACT FROM AUTHOR]

The Eley-Rideal recombination reaction of H chemisorbed on the four-fold site of W(001) at a surface temperature TS = 500 K was studied using the fully-dimensioned semiclassical collisional model and an accurate potential energy surface for the H-W(001) system. The recombination probabilities calculated at impact energies in the range (0.05-5) eV, the energy distributions in the final states as well as other relevant collisional data are presented and discussed. [ABSTRACT FROM AUTHOR]

Using two different silica polymorphs, we have experimentaly demonstrated the impact of the surface structure in the heterogeneous recombination of atomic oxygen. Thus, the measured recombination coefficient γ of quartz is approximately three times smaller than the γ value found for β-cristobalite at 1000 K. However, the complexity of the heterogeneous processes remains such, that Molecular Dynamics (MD) simulation became necessary to get details at the atomic level on the molecular processes underlying the experimental observations. Semi-classical MD simulations of the heterogeneous oxygen molecule formation over β-quartz are performed according to the E-R mechanism at a surface temperature of 1000 K. The interaction dynamics takes into account the energy exchange processes between the surface atoms and the O and O2 particles in the gas phase. The O2 formation probability is calculated at different collisional energies of the incoming gas oxygen atoms, together with the probabilities of the others possible reaction channels taking place at the surface. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Cross sections for elementary processes relevant in the modeling of negative ion sources are reviewed, with particular attention to the dependence on the vibrational excitation of the target molecule. Electron impact induced excitation and dissociation processes in H2 molecule and its isotopic variants are considered. Ro-vibrationally resolved cross sections for dissociation and de-excitation occurring in atom-molecule collision processes are presented. Finally the theoretical determination of hydrogen recombination probability on graphite surface is also considered. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]