Observation of Autoionizing Transitions in Helium Using the (e, 2e) Technique

Resonance ionization from the ($2s2p$) ${}_{}{}^1{}_{}{}^{}P$ and ($2p^2$) ${}_{}{}^1{}_{}{}^{}D$ levels of helium has been observed in an ($e, 2e$) experiment, in which the kinematics of all electrons in fully determined, at incident energies of 200 and 400 eV and a scattering angle of 10°. The results show that as the angle of emission approaches the direction $-\overrightarrow{\mathrm{Q}}$, where $\overrightarrow{\mathrm{Q}}$ is the momentum transfer to the helium atom, the resonance profile becomes more symmetric and the resonance cross section increases sharply.