Collision-induced alignment in quasi-one-electron systems: systematical trends in resonance-line polarisation

Polarisation of the resonance-line emission measured in keV Li, Be + , Na, Mg + , K, Zn + -He, Ne, Ar collisions is analysed. For all these systems it is found that the collision-induced alignment, corrected for fine- and hyperfine-structure effects, exhibits a characteristic maximum-minimum structure as a function of the Massey parameter Delta Ea/h(cross)v. Isoelectronic systems yield almost identical results. The ratio sigma 0 / sigma 1 between the cross sections for the M L =0 and M L =+or-1 sublevels has a pronounced maximum near the maximum in the total cross section. Calculations of Nielsen and Dahler (1976) and of Courbin-Gaussorgues et al. (1979) reproduce the observed experimental trends, but the size of the variations depends critically on the choice of the interaction potential. The results are discussed in terms of the current picture of inelastic processes in collisions of quasi-one-electron systems.