Impact parameter theory for quasi-one-electron systems: ns-np excitation, alignment and orientation in Be⁺/Mg⁺-He/Ne collisions

Excitation probabilities P(b,E) and the coherence parameters, lambda (b,E) and chi (b,E) have been calculated for 2 S to 2 P excitation of Be + (2s) and Mg + (3s) ions in collisions with ground-state He and Ne atom targets in the energy range E CM =0.5-100 keV and the impact parameter range b=0.1-5.0 au. Each ion-atom pair has been treated as a quasi-one-electron three-particle system consisting of the valence electron of the ion, the closed-shell ion core, and the closed-shell rare-gas atom. Close-coupled equations, based upon a rectilinear trajectory, have been solved for the amplitudes associated with a three-state (ns, np 0 , np 1 ) one-centre expansion of the electronic wavefunction. A Hellmann-type pseudopotential has been used for the electron-ion core interaction; for the electron-rare-gas interaction an electrostatic model was chosen. The theory predicts that high energies and/or large impact parameters lambda will tend to zero and chi to tau /2. For finite energies (impact parameters) the function lambda (b) ( lambda (E)) exhibits pronounced oscillations which correlate with rapid changes (phase jump mod Delta chi mod approximately= pi ) of the phase angle chi (b) ( chi (E)).