Spin-dependent orientation propensities revealed in polarized-electron-polarized-photon coincidence studies.

The density matrix parametrization of collisionally excited atomic ensembles is generalized to account for a projectile spin. The elements are related to the {open_quote}{open_quote}generalized Stokes parameters{close_quote}{close_quote} of Andersen and Bartschat [J. Phys. B {bold 27}, 3189 (1994)], determined in scattered- projectile{endash}polarized-photon coincidence experiments after impact excitation by spin-polarized electrons. The well-established orientation propensity rule for unpolarized electron beam experiments is {ital not} valid for spin-resolved collisions. {copyright} {ital 1996 The American Physical Society.}