  We have measured the stopping powers and straggling of fast, highly ionized
atoms passing through thin bilayer targets made up of metals and insulators. We
were surprised to find that the energy losses as well as the straggling depend
on the ordering of the target and have small but significantly different values
on bilayer reversal. We ascribe this newly found difference in energy loss to
the surface energy loss field effect due to the differing surface wake fields
as the beam exits the target in the two cases. This finding is validated with
experiments using several different projectiles, velocities, and bilayer
targets. Both partners of the diatomic molecular ions also display similar
results. A comparison of the energy loss results with those of previous
theoretical predictions for the surface wake potential for fast ions in solids
supports the existence of a self-wake.
