Charge distribution of He states, excited by electron capture in He+ (26 keV)-He collisions

Using anticrossing spectroscopy we investigated one-electron capture in He + (26 keV) – He collisions. The intensity of the triplet line λ (1s4l 3 D → 1s2p 3 P) ≈ 447.2 nm emitted by the fast atoms was measured as a function of an external electric field in the collision volume varied from −30 kV/cm to +30 kV/cm. The radiative decay of excited He atoms in an almost field-free region outside the collision volume was observed. The highly asymmetric intensity function could be reproduced theoretically using the density-matrix formalism. It indicates that the excited He atoms are in a state with a highly forward-backward asymmetric charge distribution, immediately after the charge-exchange collision.