Non-equilibrium energy loss for very highly charged ions in insulators

The energy loss of 144 keV Ar16+ ions in a bilayer structure of C-CaF2 has been measured. An asymmetry in the results is found depending on which layer is passed by the ion first: the energy loss is about four times larger when the CaF2 layer is traversed by the ion first. We interpret this as an indication of the existence of a nonequilibrium charge state of the Ar ions inside the solid in the case of the insulator.