Dynamical Study of the Adsorption of Hydrogen on the W(001) Surface

The interaction energy of molecular hydrogen with the W(001) surface is expressed by means of a tight-binding model potential. The binding energies of the H atom with the W(001) surface are: 3.03, 2.43 and 1.84 eV, for the four-fold, bridge and top sites, respectively. The potential is used in stochastic quasi-classical trajectories calculations to study the adsorption dynamics of H-2 on the W(001) surface. We have found that the dissociative adsorption probability P(a) decreases rapidly from 67% to 50% when the collision kinetic energy E(col) increases from 0.1 to 0.25 eV, and then it increases slowly to 55% for E(col) = 1 eV. This trend is typical of dynamical processes which proceed by means of a precursor state mechanism, where the molecule is trapped, with no activation energy, in a potential well before dissociation. When thermal beams are simulated, P(a) decreases slowly for increasing values of the beam temperature, as observed experimentally. From our calculations, it comes out that: the H-2 ro-vibrational energy is more effective than the translational one in dissociative adsorption; P(a) follows the total kinetic energy scaling, being practically independent on the angle of incidence of the H-2 beam; some corrugation effects are present on the W(001) surface.