Azimuthal dependence of charge exchange in grazing scattering of 4 keV ions on a NaCl(100) surface:: Role of the trajectory

Our experimental measurements have revealed marked variations versus azimuthal angle of the positive and negative charge fractions produced by grazing scattering of different ions on a clean and flat NaCl(1 0 0) surface, at energies ranging from 2 to 10 keV. Depending on the electronic structure of the projectile (Na+, Ne+, F+, D+, O+), on its kinetic energy and on the scattering angle these variations are different. They can be related to the trajectories of the projectiles all along the surface. Simulations using numerical codes based either on the binary collision approximation (BCA) or on molecular dynamic (MD) calculations are used to estimate the number of 'visited' crystal sites at a given distance. It is found that the azimuthal variation of the negative charge fraction is directly related to the number of visited chlorine sites. Analysis of the trajectories show that. even under grazing incidence, some critical distances of closest approach can be reached beyond which reionisation of neutralised projectiles is possible through electron promotion mechanisms. (C) 2001 Elsevier Science B.V. All rights reserved.