Some Consequences of the Compound Ion Model

Cross sections for excitation of atomic levels by electrons often vary sharply with energy near threshold, as if resonating with a state of the compound ion. Some consequences of this `compound ion model' of excitation are explored. In particular it is shown that information bearing on the validity of the model can be obtained from measurements of the angular distribution and polarization of the light emitted following excitation to radiating states. Possible applications to mercury and to helium are discussed.