Quasi-molecular electron promotion beyond the 1s$\sigma$ and 2p$\pi$ channels in slow collisions of He$^{2+}$ and He

The electron emission pattern of transfer ionization in collisions of $\mathrm{H}{\mathrm{e}}^{2+}$ with He was investigated for impact velocities between 0.53 a.u. and 0.77 a.u. (7 keV/u\char21{}15 keV/u) employing recoil-ion momentum spectroscopy. This process is known to be dominated by the promotion of the $2p\ensuremath{\pi}$ quasimolecular orbital into the continuum which results in banana-shaped areas of high electron momentum densities in the collision plane extending from the target to the projectile in velocity space. Asymmetries are explained by a coherent superposition of the $1s\ensuremath{\sigma}$ channel of quasimolecular promotion with the $2p\ensuremath{\pi}$ channel. Here we report on additional contributions from channels of higher angular momentum which emerge at the smaller impact velocities. They show up as highly structured electron emission patterns in the plane perpendicular to the direction of impact.