Velocity dependence of the energy loss of very slow proton and deuteron beams in Cu and Ag

We present energy loss measurements of protons and deuterons down to velocities of 0.1 a.u. in very thin polycrystalline Cu and Ag foils, using the transmission technique. As in previous experiments in Au, a transition between two energy loss regimes is observed, i.e., from a stopping due to the excitation of only the free conduction band electrons at the lowest velocities, to a slowing down with the participation of both the free electrons and the loosely bound $d$ electrons. This transition causes a change of the slope in the energy loss versus ion velocity curve at a characteristic threshold velocity related to the binding energy of the $d$ electrons. As shown by model calculations these threshold velocities for Cu and Ag are shifted according to their density of states distributions.