  Helium spin echo experiments combined with ab initio-based Langevin molecular
dynamics simulations are used to quantify the adsorbate-substrate coupling
during the thermal diffusion of Na atoms on Cu(111). An analysis of
trajectories within the local density friction approximation allows the
contribution from electron-hole pair excitations to be separated from the total
energy dissipation. Despite the minimal electronic friction coefficient of Na
and the relatively small mass mismatch to Cu promoting efficient phononic
dissipation, about $(20\pm5)\%$ of the total energy loss is attributable to
electronic friction. The results suggest a significant role of electronic
non-adiabaticity in the rapid thermalization generally relied upon in adiabatic
diffusion theories.
