Collective excitations, optical properties and the stopping power of materials

We compare energy loss by ions and photons via electronic excitations in metals, insulators and semiconductors using optical data from synchrotron light-source measurements. The spectra of photon-induced transverse excitations differ markedly from those of longitudinal excitations by ions: photons give up all their energy to a single electronic excitation; charged particles produce a broad spectrum of excitations shifted toward higher energies by collective effects. The widths of these collective-mode resonances in insulators and semiconductors are significantly greater than those in metals reflecting shorter collective-mode lifetimes.