Transverse cooling or heating of channeled ions by electron capture and loss

We have measured the angular distribution of energetic heavy ions after passage through a Si crystal for an incident beam with an isotropic angular distribution over angles much larger than the critical angle for channeling. Strong redistribution of the flux has been observed, in some cases an enhancement along channeling directions, and in other cases a reduction. The phenomenon is not predicted by channeling theory and cannot be reproduced by computer simulations. We propose a new mechanism: cooling or heating of the transverse motion of channeled ions due to repeated capture and loss of electrons. The cooling effect is analogous to Sisyphus cooling of very cold, trapped atoms in a strong laser field.