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

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