Quantum studies of H atom trapping on a graphite surface.

The trapping and sticking of H and D atoms on the graphite (0001) surface is examined, over the energy range of 0.1-0.9 eV. For hydrogen to chemisorb onto graphite, the bonding carbon must pucker out of the surface plane by several tenths of an angstrom. A quantum approach in which both the hydrogen and the bonding carbon atoms can move is used to model the trapping, and a potential energy surface based on density functional theory calculations is employed. It is found, for energies not too far above the 0.2 eV barrier to chemisorption that a significant fraction of the incident H or D atoms can trap. The forces on the bonding carbon are large, and it can reconstruct within 50 fs or so. After about 100 fs, most of the trapped H atoms scatter back into the gas phase, but the 5%-10% that remain can have lifetimes on the order of a picosecond or more. Calculations of the resonance eigenstates and lifetimes confirm this. An additional lattice degree of freedom is included quantum mechanically and is shown to significantly increase the amount of H that remains trapped after 1 ps. Further increasing the incident energy destabilizes the trapped state, leading to less H remaining trapped at long times. We estimate that for a full dissipative bath, the sticking probabilities should be on the order of 0.1.