Berakdar, J; Engelns, A; Klar, H

The triply differential cross section (TDCS) for electron-impact ionization of atoms is analysed in terms of tensorial parameters representing orientation and alignment in electron - electron coincidences. Particular attention has been paid to excited alkali states because of experimental feasibility. Three dynamical models for the calculation of the TDCS have been considered. The plane-wave impulse approximation (PWIA), the first Born approximation (FBA), and the use of a correlated double continuum wavefunction as employed by Brauner et al, hereafter referred to as BBK. It is shown that the PWIA forbids any orientational effect, but allows for an alignment. The FBA generally shows an orientational dichroism, the structure of the TDCS within the FBA has been worked out for non-statistical M-state populations. Finally, final-state correlation is shown to modify drastically the FBA results even at intermediate to higher energies. Numerical calculations for H(2p) are presented for the purpose of illustration.

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