Complete experiments in electron-atom impact excitation: Present status and future prospects

An overview is given of the present understanding of excitation in electron-atom collisions, with particular emphasis on the extent to which a ''perfect scattering experiment'' in the Bederson sense has been achieved. Recent experimental and theoretical results are put into a common framework, generalizing the ideas and systematics, presented in a review paper, of the case of excitation by spin-polarized electron beams. For various levels of complexity, complete sets of coherence parameters are suggested, and their relationships to generalized Stokes parameters and generalized STU parameters are pointed out.