Calculation of the fine structure of the a 3Σ+u state of molecular helium

Using a published correlated wave function computed by Poshusta and Matsen, we have obtained a vibrationally averaged first-order spin-spin constant of -0.04089 cm^-1 for the a ³Σ⁺_u metastable state of molecular helium. This is in good agreement with the experimental value of -0.0367 cm^-1 recently reported by Lichten et al. Our results strongly suggest that second-order spin-orbit effects are small.