Total low-energy electron yields for the normal incidence interaction of slow, very highly charged ions ${(}^{136}$${\mathrm{Xe}}^{\mathit{q}+}$, 21≤q≤51; ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}_{}^{\mathit{q}+}{}_{}{}^{}$, 51≤q≤80) with a clean gold surface have been determined from the related measured electron emission statistics. The projectile impact energies could be reduced down to the image charge acceleration limit. The electron emission yield was found to increase proportionally with the increasing projectile charge state in all cases studied, suggesting no saturation in the ability of the Au target to provide necessary electrons within the above surface interaction time (e.g., about 280 electrons/projectile for ${\mathrm{Th}}^{79+}$ in less than ${10}^{\mathrm{-}13}$ s). Because of the relatively narrow energy distribution of the incident ions, as well as their very high charge states, the first clear measurements of the image charge acceleration from electron emission yields could be performed. Results of a quantitative study of this acceleration are in good agreement with those of a theoretical model based on classical over the barrier transitions (recently developed by J. Burgdörfer et al.).

• Received 10 May 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1943