Angular momentum transfer and charge cloud alignment in atomic collisions: intuitive concepts, experimental observations and semiclassical models

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Abstract
The authors discuss intuitive concepts to describe alignment and orientation effects in collision processes with, or leading to, an atomic np state. For direct excitation one can understand the atomic angular momentum transferred in terms of a rolling ball, and for excitation (de-excitation) in a molecular picture one can visualise the alignment angle of the atomic p charge cloud in terms of a transition from a body-fixed molecular picture (small internuclear distances R) to a space-fixed picture (large R). These concepts are illustrated by experimental results for e+Na* and Na++Na* collisions. Semiclassical theory is discussed for both the direct and the molecular inelastic processes, giving a theoretical foundation for these models. Detailed results are reported for the time development of the charge cloud in Na++Na* collisions as a model case, illustrating the concept of body-fixed versus space-fixed electron motion and its limitations. Further examples are the molecular process N2+Na* and the atomic process Xe+Ba* at thermal energies. In all cases long-range rotational ( Sigma - Pi ) coupling determines the charge cloud motion.