Spin-Orbit Coupling inHe5

Experimental results on the splitting of the ground state of ${\mathrm{He}}^5$ are compared with the structures which would be expected on the basis of ($a$) Thomas relativistic spin-orbit coupling, and ($b$) the tensor spin-orbit interaction of mesotron theory. It is found that the Thomas coupling, as customarily formulated, gives an inverted doublet which is several orders of magnitude too small. The tensor force gives a result of the same order of magnitude as the experimental splitting and of the "normal" structure; these latter results can be established, however, only for a nonsingular behavior of the potential function. We discuss a modified treatment of the relativistic spin-orbit coupling, suggested by Breit, which would account for the existence of widely spaced spin-orbit multiplets.