Heavy ion channeling

Abstract

We discuss recent experiments on heavy ion channeling at high energies. Resonant-capture and loss of electrons has been studied by measurement of the charge state of ions transmitted through a thin Si crystal. The energy loss has been used to define the transverse energy of axially channeled ions and to measure the strength of the resonant processes as a function of transverse energy. We therefore first discuss briefly the average energy loss and the straggling in energy loss for channeled and random penetration. Dielectronic KLL capture (inverse KLL Auger emission) has been measured in the energy range 12–18 A MeV for Br 33+ (He like) ions and the observed dependence of the resonance strength on transverse energy is consistent with the calculated variation of the density of valence electrons seen by the ions. Coherent excitation leading to electron loss has been measured for Si 13+ (H like) ions channeled along a 〈111〉 axis at about 21.7 A MeV, where the energy in the ion rest frame of virtual photons in the 7th harmonic of the axial potential matches the energy of the transition of the bound electron from the ground state to the first excited state. A detailed theoretical discussion of this process is given. The dependence on transverse energy is very strong because the field decreases rapidly with distance from an atomic string. There is good agreement between calculations and experiment, both concerning the dependence of the strength of the resonance on transverse energy and concerning the Stark splitting, which is caused mainly by the zero'th harmonic of the transverse force from 〈111〉 atomic strings.