Precision Spectroscopy of the Fine Structure in the $\boldsymbol{a\;{}^3Σ_u^+(v=0)}$ and $\boldsymbol{c\;{}^3Σ_g^+(v=4)}$ States of the Helium Dimer
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2025
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| author | Wirth, V. Holdener, M. Merkt, F. |
| author_facet | Wirth, V. Holdener, M. Merkt, F. |
| contents | With four electrons, 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 $c$ $^3Σ_g^+\leftarrow a$ $^3Σ_u^+$ electronic transition of $^4$He$_2$ at a precision ($Δν/ν$) of $2.5\times 10^{-10}$ and with full resolution of the rotational, spin-spin and spin-rotational fine structures. The investigation focuses on transitions to the rotational levels of the $c$ $^3Σ_g^+(v=4)$ state, located energetically above the He$(1$ $^1S_0) + $He$(2$ $^3S_1)$ dissociation limit and decaying by tunneling predissociation through a barrier in the $c$ state. The new data include a full map of the energy levels of the $a$ $^3Σ_u^+(v=0)$ and $c$ $^3Σ_g^+(v=4)$ 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 first-principles calculations from a parallel investigation [B. Rácsai, P. Jeszenszki, A. Margócsy and E. Matyús, arXiv:2506.23879v1 (2025)]. The new data were combined with data from earlier measurements of the spectrum of the $c-a$ band system of $^4$He$_2$ to derive full sets of molecular constants for the $a$ $^3Σ_u^+(v=0)$ and $c$ $^3Σ_g^+(v=4)$ states with much improved precision over previous experimental results. A pronounced broadening of the linewidths of the transitions to the $c$ $^3Σ_g^+(v=4,N=10)$ 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2508_08060 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Precision Spectroscopy of the Fine Structure in the $\boldsymbol{a\;{}^3Σ_u^+(v=0)}$ and $\boldsymbol{c\;{}^3Σ_g^+(v=4)}$ States of the Helium Dimer Wirth, V. Holdener, M. Merkt, F. Atomic Physics Chemical Physics With four electrons, 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 $c$ $^3Σ_g^+\leftarrow a$ $^3Σ_u^+$ electronic transition of $^4$He$_2$ at a precision ($Δν/ν$) of $2.5\times 10^{-10}$ and with full resolution of the rotational, spin-spin and spin-rotational fine structures. The investigation focuses on transitions to the rotational levels of the $c$ $^3Σ_g^+(v=4)$ state, located energetically above the He$(1$ $^1S_0) + $He$(2$ $^3S_1)$ dissociation limit and decaying by tunneling predissociation through a barrier in the $c$ state. The new data include a full map of the energy levels of the $a$ $^3Σ_u^+(v=0)$ and $c$ $^3Σ_g^+(v=4)$ 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 first-principles calculations from a parallel investigation [B. Rácsai, P. Jeszenszki, A. Margócsy and E. Matyús, arXiv:2506.23879v1 (2025)]. The new data were combined with data from earlier measurements of the spectrum of the $c-a$ band system of $^4$He$_2$ to derive full sets of molecular constants for the $a$ $^3Σ_u^+(v=0)$ and $c$ $^3Σ_g^+(v=4)$ states with much improved precision over previous experimental results. A pronounced broadening of the linewidths of the transitions to the $c$ $^3Σ_g^+(v=4,N=10)$ 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. |
| title | Precision Spectroscopy of the Fine Structure in the $\boldsymbol{a\;{}^3Σ_u^+(v=0)}$ and $\boldsymbol{c\;{}^3Σ_g^+(v=4)}$ States of the Helium Dimer |
| topic | Atomic Physics Chemical Physics |
| url | https://arxiv.org/abs/2508.08060 |