Fine structure and radiative lifetime of the low-lying triplet states of the helium excimer

The fine structure λ constants are determined as a function of inter-nuclear separation, r, for the low-lying triplet states, a3Σu+ and c3Σ+g, in the helium dimer. To our knowledge, this is the first reported ab initio prediction of λversusr for the c3Σ+g state. The second-order spin–orbit contributions to the λ are found to be negligible, in agreement with previous studies, leaving λ determined only the vibrationally averaged spin–spin contribution in this study. The λ constants are predicted to be negative in the binding region around re, in agreement with experiment, and show small positive maxima at large r (near the dissociation limit). The calculated λ constant for the a3Σ+u state is λ = −0.0448 cm−1, which is 22% larger (in absolute value) than the experimental value of −0.0367 cm−1. Comparison with previous ab initio calculations for the a3Σ+u state indicates more accurate internuclear distance dependence of the spin splitting constant and better convergence to the λ constant for Be(3P), which is the united atom limit. The electric dipole transition moments for the spin-forbidden transitions a3Σ+u → X1Σ+g and 13Πu → X1Σ+g are calculated by the quadratic response method, taking into account spin–orbit coupling. For the a → X transition a good agreement with previous calculations is obtained.