POLARIZATION AND CHARGE-TRANSFER DURING THE DISSOCIATION OF H-2 ON AL(110)

We present a density functional calculation within the generalized gra dient approximation of the H-2 induced dipole moment along the reactio n path for dissociation outside an Al(110) surface. The dipole moment is found to change sign during the adsorption process, being positive far from the surface and negative closer in where the antibonding H-2 level is being filled. Assuming this variation to be qualitatively sim ilar for other surfaces, we can classify the effect of an electrostati c field or an adsorbed alkali atom on the adsorption process according to whether the barrier to adsorption is in the entrance channel or th e exit channel. The electrostatic interaction model is shown to qualit atively explain why adsorbed alkalies promote the dissociation of H-2 on some metals and poison it on others.