Singlet - triplet anticrossings in He. II. Then=6, 7, 8D1,3states

The method of singlet-triplet level anticrossing has been extended to the $n=6\mathrm{}{}_{}{}^{1,3}{}_{}{}^{}D$ levels of He. Measurement of this anticrossing gives a highly accurate determination of the zero-field singlet-triplet separation and a less accurate determination of the antisymmetric part of the spin-orbit coupling perturbation between the $n=6\mathrm{}{}_{}{}^{1,3}{}_{}{}^{}D$ states. The zero-field separation for $n=6\mathrm{}$, although considerably more accurate, is in essential agreement with previous optical results, confirming the accuracy of the intercombination line measurements. The singlettriplet intervals calculated by many-body perturbation theory are also found to be in very good agreement with the experimental results.