A wave packet study of the dynamics of H2 formation through a collinear Eley−Rideal mechanism that explicitly handles the substrate relaxation effects is presented. The substrate used is a planar PAH which exhibits some strong similarities with a perfect graphite (0001) surface. The collision energy range investigated lies between 0.4 meV (T = 5 K) and 0.46 eV. The reaction probability is large, except at very low collision energy. Most of the available energy goes into the H2 vibration, which is much more excited than the substrate. Focusing on the surface relaxation effects, the results are compared with two widely used approximations, namely, the sudden approximation and the adiabatic approximation.

We have studied the formation of the H2 molecule on a graphite surface, when both H atoms are initially physisorbed. The graphite surface is assumed to be planar. The interaction potential is modeled to reproduce the experimental properties of H physisorption on graphite. Extending our previous work [S. Morisset, F. Aguillon, M. Sizun, and V. Sidis, J. Chem. Phys. 121, 6493 (2004)], full-dimensionality quantum calculations are presented for collision energies ranging from 4 to 50 meV. It is shown that the reaction occurs with a large cross section and produces the H2 molecule with a considerable amount of vibrational energy. The mechanism is either direct or involves the formation of an intermediate complex.