THE INFLUENCE OF THE IMPACT SITE ON ORIENTATION EFFECTS IN QUANTUM NO(X (2)PI)-AG(111) SCATTERING

We report a fully quantum analysis of the impact-site dependence of or iented NO collisions with Ag(111). Our simulations are based upon the recent corrected effective-medium potential-energy surfaces of DePrist o and Alexander. The main finding is the huge influence of the impact site on both the NO rotational distributions and the steric effect. Th e global steric effect is found to vary from -8% (centre site) to +42% (atop site). Yet, calculations with the site-averaged potential surfa ces yield a residual steric effect (+8%) in good agreement with the ex perimental observations of Kleyn, Stolte and co-workers. However, it d oes not match the site average of the steric results (+19%). This sugg ests that the detailed topology of the surface should be considered in interpreting the experimental orientation effects and that models bas ed on flat surfaces may be inadequate.