Accurate variational calculations are presented for the off-diagonal matrix elements of the Breit interaction between the 2P1 and 2P3 states of several members of the helium iso-electronic sequence. A comparison is made with recent Z-expansion calculations, which are accurate through terms of order α2Z3. The comparison shows that the Z-expansion method for off-diagonal mixing is potentially useful in the theory of many-electron atoms. The effect of singlet-triplet mixing on the 2P3 excitation cross sections is briefly discussed. © 1969 The American Physical Society.
Drake, Gordon W. F.. (1969). Singlet-triplet mixing in the helium sequence. Physical Review, 181 (1), 23-24.