Abstract
Collisions between completely stripped ions and hydrogen atoms at relatively low velocities are considered where a model involving only a finite number of crossings between diabatic states should be adequate. The transition probability for many curve crossings is formulated in such a way as always to respect unitarity. Numerical results are presented for 2<or=Z<or=18 and hydrogen atoms in their ground and metastable states. A simplified analytic theory, valid for large values of Z, explains the general trend of the results.