Out-of-plane ($e,2e$) angular distributions and energy spectra of He autoionizating states

Out-of-plane $(e,2e)$ measurements and calculations are reported for the helium auto\-ionizing levels $(2s^2)^1\negthinspace S$, $(2p^2)^1\negthinspace D$, $(2s2p)^1\negthinspace P$, and for direct ionization. \footnote{B.A. deHarak, K. Bartschat, and N.L.S. Martin, {\em Phys. Rev. Lett.}, In press} While the recoil peak almost vanishes in the angular distribution for direct ionization, it remains significant for the auto\-ionizing 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.