氦二聚体$\boldsymbol{a\thinspace^3Σ_u^+(v=0)}$和$\boldsymbol{c\thinspace^3Σ_g^+(v=4)}$态精细结构的精密光谱学研究
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
AI总结：
针对可进行高精度第一性原理计算的氦二聚体，报道其$c\thinspace^3Σ_g^+\rightarrow a\thinspace^3Σ_u^+$电子跃迁的精密光谱测量，实现高分辨精细结构分析，推导高精度分子常数，为检验理论计算提供关键参考。
摘要：
拥有四个电子的He₂是极少数可进行包含非绝热、相对论和量子电动力学修正处理的第一性原理量子化学计算的分子之一。需要精密光谱测量作为参考来检验这些计算。本文报道了对⁴He₂的$c\thinspace^3Σ_g^+\rightarrow a\thinspace^3Σ_u^+$电子跃迁的光谱测量，其精度（Δν/ν）为$2.5×10^{-10}$，并完全分辨了转动、自旋-自旋和自旋-转动精细结构。研究聚焦于跃迁至$c\thinspace^3Σ_g^+(v=4)$态的转动能级，该能级能量高于He$(1\thinspace^1S_0)+$He$(2\thinspace^3S_1)$解离极限，且通过$c$态势垒的隧穿预解离衰变。新数据包括$a\thinspace^3Σ_u^+(v=0)$和$c\thinspace^3Σ_g^+(v=4)$态的能级全图，其转动量子数$N$分别高达9和10，以及这些能级中用于与平行研究的第一性原理计算[B. Rácsai等人，arXiv:2506.23879v1（2025）]对比的全套精细结构间隔。新数据与早期⁴He₂$c-a$带系光谱测量数据结合，推导得到$a\thinspace^3Σ_u^+(v=0)$和$c\thinspace^3Σ_g^+(v=4)$态的全套分子常数，精度较之前实验结果显著提升。跃迁至$c\thinspace^3Σ_g^+(v=4,N=10)$精细结构能级的线宽明显展宽，归因于通过$c$态势垒的隧穿预解离，且预解离宽度计算可定量解释这一现象。
英文摘要：
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.
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