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

The dynamical aspects of the adsorption of H-2 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 P(a) undergoes a rapid decrease from 60% to 54% when increasing the collision kinetic energy E(col) from 100 to 150 meV, and after having reached the value of 50% at E(col) = 500 meV, it increases slowly to 56% for E(col) = 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 P(a) decreases slowly for increasing values of the beam temperature, as observed experimentally. From our calculations it emerges that: (i) when H-2 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) P(a) 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 P(a) 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 H-2 molecules are not specular to the incidence angles of the beam.