Trajectory-Dependent Charge Exchange in Alkali Ion Scattering from a Clean Metal Surface

We report the first evidence for trajectory-dependent neutralization of hyperthermal energy alkali ions scattered from a clean metal surface. We have scattered 7.5 and 50 eV ${\mathrm{Na}}^{+}$ from Cu(001) and found that, for atoms leaving the surface with the same velocity and direction, the measured neutral fraction varies by a factor of 7 depending on the type of surface collision. The collision types are identified with scattering simulations. We suggest that coupling of the charge exchange and collision dynamics is responsible for the variations in neutralization.