With four electrons, ${\mathrm{He}}_2$ is one of very few molecules for which first-principles quantum-chemical calculations that include the treatment of nonadiabatic, relativistic, and quantum-electrodynamics corrections are possible. Precise spectroscopic measurements are needed as references to test these calculations. We report here on a spectroscopic measurement of the $c{{}^3{\mathrm{\Sigma }}_g}^{+}\leftarrow a{{}^3{\mathrm{\Sigma }}_u}^{+}$ electronic transition of ${}^4\mathrm{He}_2$ at a precision ($\mathrm{\Delta }\nu /\nu$) of $2.5\times {10}^{-10}$ and with full resolution of the rotational, spin-spin, and spin-rotational fine structures. The focus of the investigation is placed on the measurement and analysis of transitions to the rotational levels of the $c{}^3{\mathrm{\Sigma }}_g^{+}(v=4)$ state, which are located energetically above the $\mathrm{He}\left(1{}^1\mathrm{S}_0\right)+\mathrm{He}\left(2{}^3\mathrm{S}_1\right)$ dissociation limit and can decay by tunneling predissociation through a barrier in the potential of the $c$ state. The new data include a full set of the energy levels of the $a{}^3{\mathrm{\Sigma }}_u^{+}(v=0)$ and $c{}^3{\mathrm{\Sigma }}_g^{+}(v=4)$ states with rotational quantum numbers $N$ up to 9 and 10, respectively, and full sets of fine-structure intervals in these levels for comparison with the results of first-principles calculations carried out in a parallel investigation [Rácsai, Jeszenszki, Margócsy, and Mátyus, arXiv:2506.23879v1 [physics.chem-ph]. The new data were combined with data from earlier measurements of the spectrum of the $c\text{−}a$ band system of ${}^4\mathrm{He}_2$ to derive full sets of molecular constants for the $a{}^3{\mathrm{\Sigma }}_u^{+}(v=0)$ and $c{}^3{\mathrm{\Sigma }}_g^{+}(v=4)$ states with much improved precision over previous experimental results. A pronounced broadening of the linewidths of the transitions to the $c{}^3{\mathrm{\Sigma }}_g^{+}(v=4,N=10)$ fine-structure levels is attributed to tunneling predissociation through a barrier in the potential of the $c$ state and is quantitatively accounted for by calculations of the predissociation widths.

Close

PrevNext

View figure in article

• Atomic & molecular structure
• Electronic excitation & ionization
• Electronic structure of atoms & molecules
• Electronic transitions
• Fine & hyperfine structure
• Interatomic & molecular potentials
• Molecular dissociation
• Molecular spectra
• Photodissociation
• Relativistic & quantum electrodynamic effects in atoms, molecules,& ions
• Single- and few-photon ionization & excitation
• Atomic & molecular beams
• Molecules
• Optical spectroscopy
• Photoexcitation
• Photoionization