Two-electron one-photon transitions in atoms with 12 ≤ Z ≤ 80

Relativistic configuration interaction results are presented for the K alpha alpha x-rays arising from the decay of doubly ionized K shell of atoms with 12 <= Z <= 80. The x-ray energies and dipole rates are calculated in the active space approximation using multi-configuration Dirac-Fock wavefunctions with the inclusion of the Breit interaction and quantum electrodynamics corrections. The calculations include all single and double excitations from core-valence interactions. The resulting length and velocity gauge transition rates are well converged and are in good agreement with the allowed transition, while the intercombination line rate is seen to be gauge dependent. Conclusions concerning the relative importance of correlation and the Breit interaction on the intensity ratios are obtained in some detail. The sensitivity of the relative intensities of the fine structure lines to the coupling variations in both length and velocity gauges is analysed. The calculated results are compared with previous experimental and theoretical data.