Transition elements, electronic configuration integrals

Here the radial integral is defined by (26.4). The selection rules for relativistic Mfc-transitions between one-electron configurations directly follow from the non-zero conditions of submatrix element (27.1). They consist of triangular condition 7 72 and symbols (abc) at radial integrals, ensuring the even values of the perimeter of the corresponding triangle. It follows... 

The one-electron submatrix element of operator rk 1 stands for the radial integral of this quantity and the appropriate radial wave functions. Indeed, we can easily show that the right side of formula (27.7) equals zero for k = 0. Therefore, Ml-transitions occur only between levels of one and the same configuration. 

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