Orientation of Na(3p) states excited in Na-He collisions

The orientation of Na(3p) states created in 3-13 keV Na(3s)-He collisions has been studied by the polarised photon-scattered particle coincidence technique at scattering angles corresponding to the impact-parameter range 1-2 a.u. In the standard geometry, at large impact parameters the excitation process exhibits a very high degree of orientation with about 90% of the Na(3p) states having m1=-1. Strong reduction in this propensity is observed at impact parameters smaller than about 1.3 a.u., where a molecular curve crossing causes simultaneous He(n=2) excitation. In this region, also ionisation becomes important. (orig.)