Wakes, Dynamic Screening

Abstract

Two topics on “wake” phenomena are reported. The first one concerns the density fluctuation induced in solids by swift ions. For this purpose, the knock-on collision electrons (v_e ~ 2v_p) emitted at 0° from thin carbon foils under 4.5 MeV N ⁺₂ impact were measured as a function of the orientation θ of the molecular axis of the projectile with respect to the beam axis. A marked decrease in the yield has been observed near θ ~ 25°, which is a critical angle where the density fluctuation of target valence electrons changes its feature from collective to single-particle-like. The second topic concerns a search for wake-riding electrons employing slow antiprotons as well as equivelocity protons. Electrons emitted at 0° have been measured for projectile energies from 500 keV to 750 keV. Although a deep dip is predicted at v_e ~ v_P for antiproton-gas collisions, the observed spectrum is rather smooth with indication of a bump at ~ 50 eV below the energy where the dip is anticipated. The energy and the relative intensity of the bump are found to be consistent with those predicted for electrons released from a wake-riding state.