Singlet-triplet anticrossings inHe4. III. Separation and mixing of then=3−8D1andD3states

Abstract

The method of singlet-triplet anticrossing has been extended to the $n{}_{}{}^{1-3}{}_{}{}^{}D$ levels of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ with $n=3, 4, \mathrm{and} 5$. We measure both the zero-field singlet-triplet separation with a higher accuracy than optical measurements, and the antisymmetric part of the spin-orbit coupling between singlet and triplet states. The agreement with many-body perturbation calculations for singlet-triplet intervals and with the hydrogenic approximation for spin-orbit coupling is fairly good even for low-$n$ states. Values of singlet-triplet mixing in the $n=3-8\mathrm{} D$ states, derived directly from the experimental results, are significantly more accurate than previous experimental and theoretical values.