Transverse cooling and heating in ion channeling

In contrast to predictions from the standard theory of ion channeling we have observed strong redistributions of initially isotropic ion beams after transmission of thin crystal foils. Depending on the experimental parameters, there can be strong enhancements, corresponding to "transverse cooling," or strong reductions, "transverse heating," of the ion flux along a crystal axis or plane. For most ions there is a transition from cooling to heating when the ion energy is decreased, which depends on the crystal direction and on the atomic numbers of the ion and of the crystal atoms. In this paper we present an overview of this newly discovered phenomenon. Redistribution of an initially isotropic flux violates basic symmetries in the theory of channeling. We have argued earlier that the observed transverse cooling or heating can be understood as a consequence of fluctuations in the charge state of the channeled ions, but a detailed explanation of the transition from cooling to heating has yet to be established. A theoretical description is the most difficult for ions with many electrons. A different type of simulation has been developed based on n-body classical trajectory Monte-Carlo procedures and the first results are discussed.