Triple Coincidence (e,γ2e) Experiment for Simultaneous Electron Impact Ionization Excitation of Helium

Simultaneous ionization and excitation of helium atoms by 500 eV electron impact is observed by a triple coincidence of an ionized slow electron, the recoiling He⁺ ion, and the radiated vacuum ultraviolet photon (λ≤30.4  nm). Kinematically complete differential cross sections are presented for the He⁺(2p)²P final ionic state, demonstrating the feasibility of a quantum mechanically complete experiment. The experimental data are compared to predictions from state-of-the-art numerical calculations. For large momentum transfers, a first-order treatment of the projectile-target interaction can reproduce the experimental angular dependence, but a second-order treatment is required to obtain consistent magnitudes.