Cooling and heating of channeled ions and corresponding charge state distributions

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 exchange processes. One consequence of this mechanism could be a different charge state distribution of well-channeled versus non-channeled ions: for cooling channeled ions should exit the crystal with a higher charge state than in random directions and heating should yield the opposite result. We report here measurements that show indeed shifts of the mean charge state of channeled ions in comparison to the one of random ions. These shifts depend on the ion velocity, but there seems to be no general correlation between the shifts and the appearance of cooling or heating. (C) 2003 Elsevier B.V. All rights reserved.