Two-electron, one-photon transitions in Cr, Fe, Co, and Cu

The two emission lines resulting from the two-electron transitions $1s^{-2\mathrm{}}\to 2s^{-1\mathrm{}}2p_{\frac{3}{2}}^{-1\mathrm{}}$, $1s^{-2\mathrm{}}\to 2s^{-1\mathrm{}}2p_{\frac{1}{2}}^{-1\mathrm{}}$ were resolved for the transition elements Cr, Fe, Co, and Cu. Their energies were determined and were found to agree well with calculations. The relative intensities of the two transitions were measured and found to be $\frac{I\left(K{\alpha }_1{\alpha }_3^h\right)}{I\left(K{\alpha }_2{\alpha }_3^h\right)}\simeq \frac{3}{4}$. For the same initial state, the intensity of the emission lines resulting from the simultaneous transitions of two electrons was found to be comparable to the emission intensity from the uncorrelated single-electron transitions. This is some 3 orders of magnitude greater than what has been previously reported, both theoretically and experimentally.