Our group has recently [Phys. Rev. Lett., 83 (1999) 1759] reported experimental evidence for the redistribution of an isotropic ion flux after transmission through thin crystals. For not fully stripped ions we have observed either an enhancement (cooling) or a reduction (heating) of the angular flux along crystal directions. For a possible explanation of this unpredicted effect an additional mechanism was introduced: cooling or heating of the transverse motion by spatially correlated charge exch... »