Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. II. H-physisorbed case.

Following previous investigation of collision induced (CI) processes involving hydrogen atoms chemisorbed on graphite [R. Martinazzo and G. F. Tantardini, J. Chem. Phys. 124, 124702 (2006)], the case in which the target hydrogen atom is initially physisorbed on the surface is considered here. Several adsorbate-substrate initial states of the target H atom in the physisorption well are considered, and CI processes are studied for projectile energies up to 1 eV. Results show that (i) Eley-Rideal cross sections at low collision energies may be larger than those found in the H-chemisorbed case but they rapidly decrease as the collision energy increases; (ii) product hydrogen molecules are vibrationally very excited; (iii) collision induced desorption cross sections rapidly increase, reaching saturation values greater than 10 A2; (iv) trapping of the incident atoms is found to be as efficient as the Eley-Rideal reaction at low energies and remains sizable (3-4 A2) at high energies. The latter adsorbate-induced trapping results mainly in formation of metastable hot hydrogen atoms, i.e., atoms with an excess energy channeled in the motion parallel to the surface. These atoms might contribute in explaining hydrogen formation on graphite.