On the validity of the impulse approximation for electron capture in asymmetric collisions

For electron capture in asymmetric ion-atom collisions the projectile-target charge ratio is a natural expansion parameter. Recently Macek and Taulbjerg (Phys. Rev. Lett.; 46:170 (1981)) have claimed that the anomalies of the Coulomb T matrix introduce corrections to the impulse approximation which do not depend on this ratio. It is shown here that this is due to the approximations they introduced, and that more accurate estimates lead to a dependence of their correction on the charge asymmetry. This correction may nevertheless be important even for rather asymmetric systems. (author)