Ultraprecise relative energies in the (2 0 0) vibrational band of H₂¹⁶O

The technique of Noise-Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) is employed to detect rovibrational transitions of H216O at wavelengths of 1.4 μm. This intracavity-saturation approach narrows down the typical Doppler-broadened linewidths of about 600 MHz to the sub-MHz domain. The locking of the spectroscopy laser to a frequency-comb laser and the assessment of collisional and further line broadening effects result in transition frequencies with an absolute uncertainty below 10 kHz. The lines targeted for measurement are selected by the spectroscopic-network-assisted precision spectroscopy (SNAPS) approach. The principal aim is to derive precise and accurate relative energies from a limited set of Doppler-free transitionsH216O. The 71 newly observed lines, combined with further highly accurate literature transitions, allow the determination of the relative energies for all of the 59 rovibrational states up to <i>J</i> = 6 within the (v1v2v3)=(200) vibrational parent of H216O, where <i>J</i> is the overall rotational quantum number and v1, v2, and v3 are quantum numbers associated with the symmetric stretch, bend, and antisymmetric stretch normal modes, respectively. An experimental curiosity of this study is that for strong transitions an apparent signal inversion in the Lamb-dip spectra is observed; a novelty reserved to the NICE-OHMS technique.本研究采用抗噪声腔增强光学外差分子光谱（Noise-Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy，NICE-OHMS）技术，在1.4 μm波长下探测H₂¹⁶O的振转跃迁。该腔内饱和方案可将典型的约600 MHz多普勒加宽谱线宽度压缩至亚兆赫兹量级。通过将光谱激光锁定至频率梳激光，并评估碰撞及其他谱线展宽效应，最终使得跃迁频率的绝对不确定度优于10 kHz。本研究借助光谱网络辅助精密光谱学（spectroscopic-network-assisted precision spectroscopy，SNAPS）方法筛选待测谱线，核心目标是从有限组无多普勒跃迁中提取精确且准确的相对能级能量。研究团队将71条新观测到的谱线与已发表的高精度文献谱线数据相结合，成功确定了H₂¹⁶O的(ν₁ν₂ν₃)=(200)振动母态内所有总转动量子数J≤6的59个振动态的相对能级，其中J为总转动量子数，ν₁、ν₂、ν₃分别对应对称伸缩、弯曲和反对称伸缩正则模式的量子数。本研究的一项实验新奇观测结果为：对于强跃迁，可在兰姆凹陷谱中观测到明显的信号反转现象，该特性仅为NICE-OHMS技术所独有。