Emission-angle-dependent post-collision interaction

We studied the autoionization electrons of two doubly excited states of helium—(2s2p${)}^1$P and (2$p^2$ ${)}^1$D—that were collisionally excited by multiply charged and relatively fast ions. The ion velocities (2–4.5 a.u.) were faster than the velocities of the emitted electrons (∼1.6 a.u.). The wings of the measured autoionization profiles were significantly different from standard Beutler-Fano profiles. These differences are largest at forward angles, smaller ion velocities, and larger ion charges. The distortion is explained with use of a classical kinematic model. After emission, the autoionization electron is accelerated if emitted in a forward direction (toward the projectile) or slowed down if emitted in a backward direction (away from the projectile). A modification of a post-collision interaction theory accounts for the interference between the bound-state electrons and the continuum electrons. Fits of the measured electron-energy spectra with use of this model show very good agreement.