Correlated, counterpoise corrected wave function calculations on the hydrogen-coronene system were used to investigate the energy landscape and the dynamic behavior of hydrogen atoms physisorbed on graphite. The adopted MP2 correlation level, employing the aug-cc-pVDZ basis set augmented with bond functions, was selected after extensive investigation on the smaller hydrogen-benzene system. The computed physisorption energy (39.7meV) was in excellent agreement with the existing experimental value of 39.2meV for a graphite single layer (Ghio et al., 1980) and makes one confident of the computed barriers to diffusion. A simple, analytical expression of the corrugated potential energy surface fitted to the calculated energy values was then used in 3D quantum dynamical calculations of the tunnelling contribution to the diffusion coefficient. Results showed that hydrogen atoms physisorbed on graphite were highly mobile on the surface even at T = 0K. This suggested that hydrogen formation in cold, interstellar clouds could indeed occurred down to very low temperatures through recombination of hydrogen atoms previously physisorbed on the surface of dust grains.

Physisorption and Diffusion of Hydrogen Atoms on Graphite from Correlated Calculations on the H-Coronene Model System. Bonfanti, M., Martinazzo, R., Tantardini, G.F., Ponti, A.: Journal of Physical Chemistry C, 2007, 111[16], 5825-9