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Description
Relativistic, quantum electrodynamics, as well as non-adiabatic corrections and couplings, are computed for the $\text{b}\ ^3\Pi_\text{g}$ and $\text{c}\ ^3\Sigma_\text{g}^+$ electronic states of the helium dimer. The underlying Born-Oppenheimer potential energy curves are converged to 1 ppm ($1:10^6$) relative precision using a variational explicitly correlated Gaussian approach. The quantum nuclear motion is computed over the 9-(12-)dimensional $\text{b}\ ^3\Pi_\text{g}$-$\text{c}\ ^3\Sigma_\text{g}^+$ (and $\text{B}\ ^1\Pi_\text{g}$-$\text{C}\ ^1\Sigma_\text{g}^+$) electronic-spin subspace coupled by non-adiabatic and relativistic (magnetic) interactions. The electron's anomalous magnetic moment is also included; its effect is expected to be visible in high-resolution experiments. The computed rovibrational intervals and fine-structure splittings, spanning over several orders of magnitude in energy, are found to be in remarkable agreement with available high-resolution spectroscopy data. Fine-structure splittings are also predicted for the $\text{c}\ ^3\Sigma_\text{g}^+$ levels, which have not been fully resolved experimentally, yet.
J. Chem. Phys. 163, 081102 (2025); doi: 10.1063/5.0288277