Network-Based Design of Near-Infrared Lamb-Dip Experiments and the Determination of Pure Rotational Energies of H218O at kHz Accuracy

Meissa L Diouf,Wim Ubachs,Attila G Császár,Roland Tóbiás,Frank M J Cozijn,Irén Simkó,Edcel J Salumbides

doi:10.1063/5.0052744

Abstract

Taking advantage of the extreme absolute accuracy, sensitivity, and resolution of noise-immune-cavity-enhanced optical-heterodyne-molecular spectroscopy (NICE-OHMS), a variant of frequency-comb-assisted Lamb-dip saturation-spectroscopy techniques, the rotational quantum-level structure of both nuclear-spin isomers of H218O is established with an average accuracy of 2.5 kHz. Altogether, 195 carefully selected rovibrational transitions are probed. The ultrahigh sensitivity of NICE-OHMS permits the observation of lines with room-temperature absorption intensities as low as 10−27 cm molecule−1, while the superb resolution enables the detection of a doublet with a separation of only 286(17) kHz. While the NICE-OHMS experiments are performed in the near-infrared window of 7000–7350 cm−1, the lines observed allow the determination of all the pure rotational energies of H218O corresponding to J values up to 8, where J is the total rotational quantum number. Both network and quantum theory have been employed to facilitate the measurement campaign and the full exploitation of the lines resolved. For example, to minimize the experimental effort, the transitions targeted for observation were selected via the spectroscopic-network-assisted precision spectroscopy (SNAPS) scheme built upon the extended Ritz principle, the theory of spectroscopic networks, and an underlying dataset of quantum chemical origin. To ensure the overall connection of the ultraprecise rovibrational lines for both nuclear-spin isomers of H218O, the NICE-OHMS transitions are augmented with six accurate microwave lines taken from the literature. To produce absolute ortho-H218O energies, the lowest ortho energy is determined to be 23.754 904 61(19) cm−1. A reference, benchmark-quality line list of 1546 transitions, deduced from the ultrahigh-accuracy energy values determined in this study, provides calibration standards for future high-resolution spectroscopic experiments between 0–1250 and 5900–8380 cm−1.

Highlights

IntroductionDetailed analysis of the spectra of water vapor, the most important greenhouse gas in the atmosphere of the Earth, has been at the center of (ultra)high-resolution molecular spectroscopy for many decades.[1,2,3,4,5,6,7,8] During this time, ever-improving techniques have been devised to resolve spectral features in diverse environments and determine the line parameters (e.g., positions, intensities, and shapes, as well as cross sections and collisional parameters) with rapidly increasing accuracy and coverage
Detailed analysis of the spectra of water vapor, the most important greenhouse gas in the atmosphere of the Earth, has been at the center ofhigh-resolution molecular spectroscopy for many decades.[1,2,3,4,5,6,7,8] During this time, ever-improving techniques have been devised to resolve spectral features in diverse environments and determine the line parameters with rapidly increasing accuracy and coverage
While the NICE-OHMS experiments are performed in the near-infrared window of 7000–7350 cm−1, the lines observed allow the determination of all the pure rotational energies of H218O corresponding to J values up to 8, where J is the total rotational quantum number

Summary

Introduction

Detailed analysis of the spectra of water vapor, the most important greenhouse gas in the atmosphere of the Earth, has been at the center of (ultra)high-resolution molecular spectroscopy for many decades.[1,2,3,4,5,6,7,8] During this time, ever-improving techniques have been devised to resolve spectral features in diverse environments and determine the line parameters (e.g., positions, intensities, and shapes, as well as cross sections and collisional parameters) with rapidly increasing accuracy and coverage. In cases when lines of the main isotopologue are too strong, lines of less-abundant isotopologues are used to trace the chemical environment, e.g., in outer space, while in radio astronomy, H218O transitions are employed to assess isotopic ratios in the interstellar medium.[10]

Methods

Results

Conclusion