Out-of-scattering-plane $(e,2e)$ measurements and calculations are reported for the three singlet helium $2\ell 2{\ell }^{\prime }$ autoionizing levels, with 80, 100, 120, 150, and 488 eV incident-electron energies, and scattering angles ${60}^{\circ }, 50.8^{\circ }, {45}^{\circ }, 39.2^{\circ }$, and $20.5^{\circ }$, respectively. The kinematics are the same in all cases: the momentum transfer is $K=2.1$ a.u., and ejected electrons are detected in a plane that contains the momentum-transfer direction and is perpendicular to the scattering plane. The results are presented as $(e,2e)$ angular distributions energy integrated over each level. They are compared with fully nonperturbative $B$-spline $R$-matrix and hybrid second-order distorted-wave + $R$-matrix calculations.

• Received 26 January 2018

DOI:https://doi.org/10.1103/PhysRevA.97.052710