Image acceleration of highly charged xenon ions in front of a metal surface

${\mathrm{Xe}}^{\mathit{q}+}$ ions with charge up to q=33 and energies 3.7q keV are scattered under a grazing angle of incidence from a clean and flat Al(111) surface. Because of the image charge interaction the ions are accelerated on the incident path towards the surface plane which results in increased effective angles of incidence for the scattered projectiles. From the angular distributions for reflected neutralized projectiles we deduce the image charge interaction energies gained by the incident ions in front of the surface. Our data are in fair agreement with a ${\mathit{q}}^{3/2}$ dependence for the image energies as predicted from a simple classical overbarrier model.