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Role of the image charge and electronic corrugation on charge-fraction azimuthal variations for keV Li and Na ions interacting with Cu(110) and Ag(110) under grazing incidence.

The positive charge fraction resulting from charge exchange during the interaction of Li+ and Na+ ions on
the Ag (110) and Cu (110) metal surfaces has been measured for different energies and incident azimuthal angles
of the projectiles. Surprisingly, although metallic Cu and Ag are well described by the free-electron model, the
positive charge fraction has been observed to vary significantly according to the azimuthal angle of incidence,
for projectile energies lower than 4 keV and incidence angle below 5° with respect to the surface grazing
scattering conditions , and specifically around the low-index [001] and [110] directions. A detailed analysis of
trajectory calculations for the Li/Cu (110) system allows one to distinguish between different types of trajectories
on tops, zigzags, and in rows , the relative importance of which varies rapidly with azimuthal angle. We
obtain a good correlation between the observed azimuthal variation of the charge fraction and the mean value
of the electronic density encountered by the projectile in the vicinity of the first atomic layers. We show that
this short-range effect can be reflected on the observed variation of the charge fraction through the charge
image effect on the trajectories. This effect is shown to be very sensitive to the electronic corrugation of the
surface which is known to be low for Cu and Ag. Azimuthal experimental measurements thus appear to be a
tool to characterize such low corrugations.

View online : Phys. Rev. B 76, 045432 (2007)