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The mixing of state multipoles for an atomic state with unresolved fine structure in a magnetic field is investigated in its simplest case: the conversion of alignment to orientation. The combined action of the external field and the spin-orbit coupling results in a build-up of orientation and hence in the emission of circularly polarised light although the excitation process is axially symmetric and is invariant with respect to the reversal of the symmetry axis and thus the system could only show alignment initially. The case investigated is the collisional excitation of the He 33D state by He+ ions. After correction of some errors in the theoretical literature the agreement between the calculated and the observed circular polarisation of the fluorescence radiation from the collisionally excited He atoms is very good. Owing to possible objections in two previous experiments this is the first unambiguous demonstration of the alignment to orientation conversion effect.

The authors have studied the alignment and orientation of the electron cloud of the Li(22P) and He(21P) 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.

The scattering process He++He(1s2) to He++He(1s2p)1P has been investigated at low (