MOLECULAR ADSORPTION ON UNRELAXED AND RELAXED IONIC STEPS - APPLICATION TO AR, CO, CO2, AND NH3 ADSORBED ON MGO(001)

The adsorption energy and the equilibrium valley of various molecules adsorbed on a step of MgO(001) substrate are determined on the basis o f interatomic potentials. The step acts as a trapping site for all mol ecules adsorbed on the lower terrace and as a barrier for molecules on the upper terrace. The height of the potential barrier increases or d ecreases when compared to the diffusion barrier on a perfect surface, depending on the electric nature of the molecule. While nonpolar admol ecules experience a potential barrier at the upper step edge, this is a characteristic of the strong ionic substrate surface field to drasti cally reduce the potential barrier for polar molecules. The relaxation and the rumpling of the step ions change the adsorption energy by abo ut 10%, resulting in an increase for a rare-gas adatom and in a decrea se for all polar species, but they have little influence on the molecu lar orientation. (C) 1996 American Institute of Physics.