Differential-excitation studies of quasi-one-electron systems. I. Na 3²P excitation in Na-He, Ne, Ar collisions

The impact-parameter dependent probabilities for Na(3s from 3p) emission in 1.5-10 keV Na-He, Ne and Ar collisions have been determined experimentally in the range from 0.4 to 3.0 au by means of scattered-particle, photon coincidence measurements. Two mechanisms are proposed to account for the observed emission. At low collision energies, the Na(3s to 3p) excitation takes place at molecular curve crossings, whereas direct excitation dominates at higher energies, around the maximum in the total emission cross section. The direct mechanism is essentially the same for all three collision systems, whereas the quasi-molecular mechanism depends strongly on the asymmetry of the collision.