Electron impact double-excitation of helium 2l2l' autoionizing states using the (e, 2e) technique

Coplanar (e, 2e) triple differential cross sections (TDCS) measurements are reported for the helium autoionizing doubly excited states, (2s(2))(1)s, (2p(2))D-1, and (2s2p)P-1, for an incident electron energy of 250 eV and a scattering angle of -13 degrees, corresponding to a momentum transfer of 1.06 a.u. The presence of autoionization results in a clearly visible recoil peak in the TDCS structure with a shape that is strongly dependent on the orbital angular momentum L of the resonance. The resonance contributions together with the strong background of direct ionization processes lead to a complicated asymmetric structure of the resonance profile in the TDCS.