Excitation of the 3p-states of Mg + and Mg ~ colliding with inert gases is treated within the coupled-channel impact parameter approximation. We have calculated the total excitation cross section and the linear polarisation of the (3p~3s)-emission as a function of the collision energy as well as the impact parameter dependence of the excitation probability and of the excitation amplitudes (expressed in terms of the ,~,Zparameters). Qualitative agreement with the experiments can be obtained when including both localised molecular orbital transitions at small internuclear distances (mechanism (i)) and the direct interaction between the Mg+'~ electrons and the inert gas atom at large internuclear distances (mechanism (ii)). We present evidence that it is inadequate at impact energies below 1 keV to compute the phase X directly from the excitation amplitudes obtained in the coupled-channel impact parameter approximation.

Results of photon-scattered atom coincidence measurements are presented for Mg-inert gas collisions. A polarisation analysis of the collision induced Mg(31p-31S) photons emitted perpendicular to the scattering plane was carried out at 1 keV collision energy as a function of the impact parameter. From these data the quantum-mechanical scattering amplitudes for the excitation of one of the two available Mg 3 s-valence electrons are derived. The results are compared with the corresponding ones for the Mg+-inert gas collisions. A good qualitative understanding of the mechanism can be obtained.