The two emission lines, Kα1α3h and Kα2α3h resulting from the two-electron transitions 1s-2 → 2s-1 2p3 2-1 and 1s-2 → 2s-1 2p1 2-1 were resolved for elemental nickel. Their measured energies agree well with calculations. Their relative intensity I(Kα1α3h)/I(Kα2α3h) ≈ 3 4 and their intensity relative to that of the Kα diagram lines is about 10-4. This is some 104 times larger than both theoretical results and the results of ion-atom collision experiments. © 1983.
Salem, S. I., Kumar, A., & Scott, B. L. (1983). Two-electron, one-photon transitions in nickel. Physics Letters A, 97(3), 100–102. https://doi.org/10.1016/0375-9601(83)90522-4