Forbidden singlet-triplet anticrossings in 3He : precise determination of n1D-n3D (n = 3-6) intervals

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Abstract
We have studied narrow anticrossing signals in 3He which have no equivalent in 4He because they are due to the conjonction of fine and hyperfine interactions. High precision results are obtained by the use of a high homogeneity Bitter coil driven by NMR. The experimental signals obtained are in perfect agreement with the predictions deduced from an entire diagonalization of the 3He Breit hamiltonian restricted to the (Is, nd) configuration. The values used for the radial constants are deduced from an analysis of previous experimental results obtained by various authors on 4He and 3He. These values are found to differ from the hydrogenic ones by less than 1 %. We have determined : first the singlet-triplet spin-orbit coupling constant for the 31-3D states : a(3D) = 650 ± 1 MHz and secondly the singlet-triplet separation of nD states (n = 3 to 6) with a precision of up to 5 x 10-5 in relative value. An unexpectedly large ( ∼ 10- 3), negative isotope shift is found compared to the equivalent 4He values, presumably due to a slight configuration interaction induced by a hyperfine interaction