Crystal assisted processes in ion channeling

Abstract

Channeled ions experience a restoring force from adjacent atomic rows or planes that govern their trajectories and hence limit their interactions to those with valence or conduction electrons. Trajectory information can be gained from emergence patterns and studies of radiative electron capture (REC) can give information on the electron momentum distributions and densities in the channel. With this knowledge, one can study ion-electron collisions in dense media; in particular, dielectronic excitation and recombination. The states of the energetic channeled ions can be influenced by the crystal field and by the wake field that follows them. They can also be perturbed by the periodic electromagnetic force with a frequency depending on their velocity and the periodicity of the lattice. When the frequency of the perturbation v caused by passing along a given crystal direction with atomic spacing d at velocity v reaches a resonance, v res = ( v d )K , where K = 1, 2, 3, …, resonant coherent excitation (RCE) can occur to non-degenerate eigenstates of the ion. Ions in these excited states can then be ionized by the channel electrons or, if not ionized, can escape from the crystal and radiate. RCE has been studied in both axial and planar channeling by varying the ion velocity or, in the case of planar channeling, by varying the apparent interatomic spacing by changing the angle with respect to an axis. In both cases, information can be gained concerning the states of channeled ions and the nature of the crystal channel.