Singlet-Triplet Mixing in Excited Helium Atoms

Experiments in collisional transfer of excitation, as well as theoretical considerations, have shown that singlet-triplet mixing occurs for nF states of helium. In this work the helium fine structure has been calculated using polarized orbitals to determine the amount of mixing and its dependence on the state (nl) of the excited electron, and on the polarization of the orbital of the inner electron. Results are presented for n=3-10 and l=2(n-1). All P states gave poor results because of a large exchange effect involving the polarization term in the wave function. However, the 3³D and 4³D levels showed improved agreement with experiment for the fine structure. The singlet-triplet mixing is essentially zero for $l=2$, is significant for $l=3$, and is virtually complete for l≥4. It is also found that the mixing decreases slightly with increasing n.