Electric dipole moments of impact-excited He atoms

We excited helium atoms using 10--20-keV H{sub 2}{sup +} (D{sub 2}{sup +}) ion impact and investigated the intensity of the {lambda}(4{sup 3}{ital D}-2{sup 3}{ital P})=447 nm line as a function of an electric field {minus}2 kV/cm{approx lt}{ital F}{le}+2 kV/cm applied antiparallel and parallel to the ion beam. The recorded signals were strongly asymmetric with respect to the sign of the electric field. This asymmetry is explained by assuming a coherent excitation of He levels with different angular momenta (4{ital d},4{ital f} and 5{ital f},5{ital g}). These measurements demonstrate that angular momentum coherences of impact-excited states can also be observed in nonhydrogenic systems.