Second-order effects in large-angle coplanar symmetric (e, 2e) processes

Theoretical and experimental triple differential cross sections are compared for the ionisation process e - +He(1 1 S) to He + (1s)+2e - , at an incident electron energy of 200 eV, in a coplanar symmetric geometry. At large angles, the theoretical curve obtained by including the second Born contribution due to the initial target state differs markedly from the first Born result and exhibits a characteristic behaviour which is confirmed by the experimental data.