A MONTE-CARLO QUASI-CLASSICAL TRAJECTORIES STUDY OF THE CHEMISORPTION OF HYDROGEN ON THE W(001) SURFACE

The dynamical aspects of the adsorption of H2 on the W(001) surface have been studied by performing Monte Carlo quasi-classical trajectories calculations, and using an analytic tight-binding model potential. We have found that the dissociative adsorption probability Pa undergoes a rapid decrease from 60% to 54% when increasing the collision kinetic energy Ecol from 100 to 150 meV, and after having reached the value of 50% at Ecol = 500 meV, it increases slowly to 56% for Ecol = 1 eV. This behaviour is in qualitative agreement with the results of nozzle beam experiments, which have been interpreted on the basis of parallel adsorption through a precursor path and a direct activated path. When simulating thermal beams, we have seen that Pa decreases slowly for increasing values of the beam temperature, as observed experimentally. From our calculations it emerges that: (i) when H2 is in the ground vibrational level, the translational energy is more important for adsorption than the internal one, but the reverse holds for the first-excited vibrational level; (ii) Pa is practically independent of the angle of incidence of the beam, that is, it follows the total kinetic energy scaling; (iii) a significant influence of the corrugation of the W(001) surface on Pa is evidenced; (iv) the scattering is dominated by large interconversions of the translational and internal energy components; and (v) the angular distributions of the backscattered H2 molecules are not specular to the incidence angles of the beam.