Recent out-of-plane (e,2e) experiments on autoionizing levels of helium

Out-of-plane (e, 2e) measurements and calculations are reported for the helium autoionizing levels (2s²)¹S, (2p²) ¹D, (2s2p)¹P, and for direct ionization [1]. While the recoil peak almost vanishes in the angular distribution for direct ionization, it remains significant for the autoionizing levels and exhibits a characteristic shape for each orbital angular momentum L = 0, 1, 2. These findings can qualitatively be explained by an L-dependent addition to the ionization amplitude, but only a second-order model in the projectile-target interaction can quantitatively reproduce the observed magnitudes of the recoil peaks. We present the data as both angular distributions and energy spectra for the resonances. The experimental part of this work is being carried out using an existing (e,2e) apparatus modied to allow the electron gun (see figure 1) to move on the surface of a (mathematical) cone [2]. This permits the measurement of out-of-plane (e,2e) angular distributions, for a full 360, using a special geometry that allows out-of-plane conditions to be combined with the binary peak in a single measurement. Details of the apparatus will be presented.