Angular Correlation between Autoionization Electron and Scattered Ion in Slow He⁺-He Collisions

Abstract

We have investigated the excitation of autoionization states in He⁺/He collisions at impact energies between 1 and 6 keV by energy analysis of electrons, ejected at ϑ = 180°, and by measurement of coincidences between electrons, ejected at ϑ = 135° and different azimuthal angles φ, and ions, scattered into θ = 6°. This yields complex population amplitudes for magnetic sublevels of the excited states. It turns out that mainly the He(2p²)¹D state is excited, and that the measured electron distribution is nearly rotationally symmetric around a direction that coincides with the internuclear axis of the collision partners at the distance of closest approach. If this axis is taken as the quantization axis, the m = 0 sublevel is nearly exclusively populated. This implies the following interpretation: near the distance of closest approach the electron cloud is “blown up” by the electron promotion via the 2pσ_u orbital. Since the internuclear distance is small at this instant the electron cloud does not follow the rotation of the internuclear axis but stays fixed in space. In a rotating frame this corresponds to a 2pσ-2pπ rotational coupling.