Electron-impact excitation of the doubly excited states of helium below the N=3 He+ threshold

Measurements of the doubly excited states of helium below the [Formula Presented](N=3) threshold populated by electron impact are presented for a range of scattering angles and residual energies. The deduced values of the energies and widths for 11 of the optically forbidden states are compared with theoretical values, which shows the agreement to be good for the energies but poorer for the widths. The data verify propensity rules in Lin's [Adv. At. Mol. Phys. 22, 77 (1986)] K, T, A classification scheme for the cross sections. By comparing the [Formula Presented][Formula Presented] and [Formula Presented][Formula Presented] states at the scattering angle of 20° and the residual energy of 40 eV, we obtain a dependence of [Formula Presented] for the cross section at these conditions.