Electric dipole moments of impact-excited He atoms

We excited helium atoms using 10–20-keV ${\mathrm{H}}_2^{+}$ (${\mathrm{D}}_2^{+}$) ion impact and investigated the intensity of the λ($4^3$D-$2^3$P)=447 nm line as a function of an electric field -2 kV/cm≲F≤+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 (4d,4f and 5f,5g). These measurements demonstrate that angular momentum coherences of impact-excited states can also be observed in nonhydrogenic systems.