Calculation of scalar nuclear spin-spin coupling in a noble-gas mixture

Indirect spin-spin interactions between nuclei bound in a molecule are well known in nuclear magnetic resonance, but such interactions between unbound atoms are less well studied and are often assumed to be zero. We present a precision calculation of this interaction between Xe-129 and He-3 nuclei in a gas. Relativistic, state-of-the-art electronic structure theory is used to compute the scalar coupling constant J(R) and the interatomic potential energy function V(R) as functions of the Xe-He internuclear distance R. Using virial expansion we find the nuclear spin enhancement factor kappa = -0.0105 +/- 0.0015, in excellent agreement with recent experiments. This interaction is particularly important for precision measurements using nuclear spin comagnetometers.