Plasmon excitation in ion-solid interactions

We discuss recent advances in the area of plasmon excitations in ion-solid interactions. The basic aspects of plasmons are described, including surface modes. Kinetic plasmon excitation, a major contribution to the energy loss of fast charges in matter, should occur above a threshold ion velocity of ∼1.3 times the Fermi velocity. The unexpected observation of sub-threshold excitation is analyzed with the conclusion that the usually dominant mechanism is indirect excitation by fast secondary electrons. Slow ions with sufficiently high potential energy can excite multipole surface plasmons, as evidenced by the plasmon decay structure in the electron emission spectra