Spin asymmetry in electron-impact ionization

We investigate electron-impact single ionization of He-like ions of charge state q = 0, . . . , 6 in the initial ground (1 S-1) and metastable (2( 3)S) states. Good agreement between theory and experiment is established for the total cross sections, and then the scaling behavior with q is considered as a function of the ratio u = E-i/E-I <= 10 of the incident electron energy E-i and the ionization threshold E-I. While the expected E-I(2) scaling of the cross sections is confirmed, we also find that at each scaled energy u the spin asymmetry (for the 2 S-3 state) converges rapidly to a nonzero constant with increasing q. This indicates that, despite E-I increasing with q(2), exchange effects remain undiminished on the broad scaled-energy range considered. We suggest that this physical behavior is more universal than just for the He-like ions.