Channeling radiation: atomic and molecular physics in one and two dimensions

Energetic electrons entering a crystal in low index planar directions can be trapped into bound states of the potential made up by the atoms in the planar sheet. This potential has associated eigenstates analogous to an averaged one dimensional atom. Similarly electrons injected in an axial direction are bound into eigenstates of the two dimensional string potential characterized by two quantum numbers n,l. In some crystal directions (e.g., diamond <110>) two strings lie in close proximity and their potentials overlap so that binding to both strings occurs and two dimensional molecular levels obtained. When positrons are injected in planar directions they are bound between the atomic sheets into an almost harmonic potential. In compound crystals the potential can become more complex. Radiation arising from bound state transitions in all the above configurations has been observed (Ee- = 4-54 MeV, Ee+ = 28-54 MeV, E/sub h nu/ = 4-150 keV) in a number of materials (C, Si, Ge, LiF). Information concerning atomic potentials and valence electron densities has been obtained and is discussed. 18 references, 8 figures