TITLE: Low energy atomic and molecular collision with graphite surface

   Faculty of Physics, University of Bucharest, POB MG-11

ABSTRACT:

 

The interaction of atomic and molecular species of hydrogen with basal plane of graphite has been investigated by means of atomic cluster models of 10, 24 and 48 carbon atoms using HF-LCAO theory at the ab-initio and semi-empirical level of approximation.

The last approach was based on an original package developed for carbon clusters. Atomic migration betweenconsecutive basal planes was described by cluster models of two sheets of carbon atoms. The contribution presents the theoretical results about atomic and molecular interactions with graphite. It was found for H-bonding energy 2.6 eV, using the largest cluster model. The migration of H above the surface and between consecutive basal planes was simulated by wide calculation sessions on potential energy in each points of a net containing 450 points describing a local surface of $0.25nm^2$. A 3D interpolation approach gives the image of a hypersurface potential energy projection at a given distance to the surface. The semi-quntitative results has been indicated two significant facts related to atomic specie migration. The first is that H has the smallest displacement barrier along C-C bonds at a distance of 1.3A from the basal plane. In the case of absorbed atoms between graphite basal planes an almost free motion channel has been found being parallel with the surface. The interaction potential barrier for H atoms collision with graphite surface at the center of the carbon ring has been calculated 5.9 eV negleting surface vibration modes. The hyperfine interaction between the electron of hydrogen and the proton has been taken as a measure of the interaction betweenincident atom and local states of the target. The isotropic hyperfine constant obtained at the level of the semiempiric calculations was found 402Gs at the equilibrium position of H above a C atom at a distance of 1.3A. The corresponding value above C-C bond resulted at 426 Gs. The interaction of $H_2$ molecule with the graphite surface at low energy indicates (for the highest binding energy) an almost vertical position above a carbon atom. This result is in contradiction to some published papers but it is in agreement with some other publications. The doubts are related to the approximations involved on treating spread orbitals involved in the description of collision. They are related to delocalised electronic states which are more significant in the case oh $H_2$ than in the case of H. From the theoretical point of view the binding energy depends on the surface site but experimental data are related to a lateral average like potential. This contribution has been indicated that the dependence of the binding energy depending physisorption site and on molecular orientation, belongs to the range of -44meV to almost a repulsive potential. The rotational barrier calculation of $H_2$ bonded in the deepest equilibrium state on the graphite surface suggested a precession type movement of the molecular axis around the normal direction at very low temperature.

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