Dichroism in the electron-impact ionization of excited and oriented sodium atoms

Measurements are reported for kinematically complete electron-impact ionization collisions with oriented 3 P sodium atoms excited from the ground state by right- and left-hand circularly polarized light. The measurements reveal a strong dependence of the ionization cross section on the helicity inversion of the exciting photon. This dichroic effect and the magnetic state resolved cross sections are described using the distorted-wave Born approximation and the dynamically screened Coulomb wave method. Within the context of these methods we investigate the role of different short- and long-range interactions involved in the process. For moderate values of incident energy we conclude that the shape and magnitude of the dichroism is predominantly determined by the Coulomb interactions of the outgoing electrons with the residual ion. For slow escaping electrons the dichroism is also influenced by the interelectronic correlations.