Strong velocity dependence of electron capture in collisions between aligned Na*(3p) and He2+

By analyzing spectra of emitted photons, we have studied state-selective electron capture in collisions of He2+ on aligned Na*(3p) atoms that span the ''velocity-matching'' energy between projectile and target electron. We find a strong dependence of the capture cross sections on the Na*(3p) orbital alignment, i.e., aligned perpendicular or parallel to the ion beam direction. The preference changes from perpendicular to parallel at a velocity close to the classical orbital velocity of the 3p electron. Classical-trajectory Monte Carlo calculations reproduce this striking energy dependence.