Surface rainbow scattering of alkali ions from metal surfaces

Surface scattering experiments have been performed at collision energies well below 50 eV at which a number of competing processes (e.g. sputtering, penetration of the first surface layer) can be neglected. A pronounced rainbow structure is observed in the angular distribution which is compared with the results of a computer simulation especially developed for these collision energies. Changing the collision parameters, i.e. impact angle, impact energy, azimuthal orientation of the scattering plane and orientation of the surface with respect to the bulk, strongly influences the angular distributions. All these changes are well reproduced in the calculations for the same set of parameters in the model interaction potential, provided these parameters are chosen properly. It has been found to be sufficient if two-dimensional trajectory calculations are performed in order to extract information about the complete interaction potential within the surface unit mesh. For this procedure a long range order in the surface is not required. Charge exchange of the projectile ions during the collision can, in principle, distort the measured distributions. These processes may be neglected for the K + + clean W system, but are found to be non-negligible in the presence of adatoms on the W surface or for Li + scattering. Therefore the latter data, in particular the dramatic changes in the rainbow structure due to adsorbed atoms, cannot be evaluated in terms of changes in the surface corrugation as long as the influence of the neutralization of the projectiles is unknown or not taken into account in the experiment.