Singlet - triplet anticrossings in He. II. The n=6, 7, 8 1,3D states

The method of singlet-triplet level anticrossing has been extended to the n=6 ^1,3D 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 ^1,3D states. The zero-field separation for n=6, 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.