Abstract
A propensity rule has been found for the magnetic substate distribution of excited states populated in collisions between multiply charged ions with atoms. It is observed that the m=-l substates are predominantly populated if the quantization axis is chosen to be perpendicular to the scattering plane. The result implies that it is possible to carry out reduced close-coupling calculations where only the m=-l substates for each l are included in the basis expansion. Such reduced close-coupling calculations are desirable for treating electron capture to high-n excited states. The range of applicability of the propensity rule is examined for C6+-H collisions.