A coherence study of 2pσ-2pπ rotational coupling: Li(2²P) and He(2¹P) orientation and alignment in 1-25 keV Li⁺-He collisions

The authors have studied the alignment and orientation of the electron cloud of the Li(2 2 P) and He(2 1 P) states excited by 2p sigma -2p pi rotational coupling for impact parameters between 0.2 and 1.1 au in 1-25 keV Li + -He collisions by coherence and correlation analysis techniques. It is found that for collision energies below 5 keV the shape of the excited electron cloud is very nearly that of a p orbital, aligned perpendicular to the asymptotic internuclear axis, independent of impact parameter and of whether the electron stays on the He core or is transferred to the Li centre during the collision. These findings agree with the predictions based on an analysis of the simple diabetic MO diagram for the Li-He system. At energies above 5 keV, the shape changes significantly. Also, the alignment angle deviates from the perpendicular direction, being larger than 90 degrees for Li and smaller for He. The angular momentum perpendicular to the collision plane shows a pronounced variation with collision energy but is only weakly dependent on impact parameter. Possible origins for this behaviour are discussed.