High-Precision Quantum Dynamics of He$_2$ over the b $^3\Pi_\mathrm{g}$-c $^3\Sigma_\mathrm{g}^+$ Electronic Subspace by including Non-adiabatic, Relativistic and QED Corrections and Couplings

Abstract

Relativistic, quantum electrodynamics, as well as non-adiabatic corrections and couplings, are computed for the b $^3\Pi_\mathrm{g}$ and c $^3\Sigma_\mathrm{g}^+$ 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 b $^3\Pi_\mathrm{g}$-c $^3\Sigma_\mathrm{g}^+$ (and B $^1\Pi_\mathrm{g}$-C $^1\Sigma_\mathrm{g}^+$) 9-(12-)dimensional 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 rovibronic energy intervals are in excellent agreement with available high-resolution spectroscopy data, including the rovibronic b $^3\Pi_\mathrm{g}$-state fine structure. Fine-structure splittings are also predicted for the c $^3\Sigma_\mathrm{g}^+$ levels, which have not been fully resolved experimentally, yet.