Differential excitation studies of quasi-one-electron systems. III. Be II 2²P excitation, alignment and orientation in Be⁺-He, Ne, Ar collisions

For pt.II see ibid., vol.12, no.15, p.2541 (1979). The impact parameter dependence of the Be II 2 2 S-2 2 P emission probability and polarisation has been studied for 1.5-15 keV Be + -He, Ne, Ar collisions by detection of photons in two directions perpendicular to the incident beam, one orthogonal and to one in the scattering plane, in coincidence with ions scattered through a selected angle theta . The studied range of reduced scattering angles tau =E theta corresponds to impact parameters b in a range of 0.3-1.7 au for helium, 0.5-2.2 au for neon and 1.2-2.3 au for argon. The emission probabilities show that at small impact parameters the excitation takes place at quasi-molecular curve crossings for the Be + -Ne, Ar systems. Direct excitation dominates for the Be + -He system and for the large impact parameter collisions of the Be + -Ne, Ar systems. In the region where the direct excitation mechanism dominates, the coherence parameters lambda (b,E) and chi (b,E) describing the relative size and phase of the quantum-mechanical excitation amplitudes for the M L =0 and M L =+or-1 magnetic sublevels have been derived for the Be + -He and Be + -Ne collisions. The lambda and chi parameters vary little with b or E for He as the target, while strong variations are found for Ne. Loss of coherence due to internal forces and cascade effects are treated in detail.