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 He+(N = 3) threshold populated by electron impact are presented for a range of scattering angles and residual energies. The deduced values of the en ergies and widths for 11 of the optically forbidden states are compare d with theoretical values, which shows the agreement to be good for th e 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 s cheme for the cross sections. By comparing the 2(s)2 S-1(e) and 3s(2 1 )S(e) stares at the scattering angle of 20 degrees and the residual en ergy of 40 eV, we obtain a dependence of n(-10.1+/-0.8) for the cross section at these conditions.