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 2pσu 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.