Transition from transverse cooling to heating

Recently [Phys. Rev. Lett. 83 (1999) 1759] we have reported experimental evidence for the redistribution of an isotropic ion beam after transmission through thin crystals. For not fully stripped ions we have observed either an enhancement or a reduction of the flux along crystal directions. For both phenomena a new mechanism was proposed: cooling or heating of the transverse motion by spatially correlated charge exchange processes. Further systematic measurements reveal even a transition for most ions from cooling to heating. In this paper we discuss the characteristic features of this transition for different ions transmitted through Si, Ni and Pt single crystals. A remarkably sharp dependence of this transition on the ion velocity, and ion and target atomic numbers was found. At present, no suitable model is at hand, with the formerly proposed mechanism of electron capture and loss being challenged by the new data