The effect of post-collision interaction in n3S excitation of helium by electrons

The excitation of the 4 3 S, 5 3 S, 6 3 S and 4 3 D states of helium by electron impact has been studied in the energy region around 60 eV. New experimental evidence has been obtained for the existence of an indirect excitation mechanism resulting from a post-collision interaction (PCI) between the scattered electron and the electron ejected from an intermediate autoionizing state. It is found, and made plausible, that the PCI mechanism via the excitation of autoionizing states primarily leads to the excitation of S-states.