Multi-laboratory measurements of 12CH4 2v3-band line parameters: accuratedetermination of line intensities at the permille level

Kurzfassung

We present a coordinated three-laboratory study of the 2v3 R-branch of 12CH4 near 1.65 µm, aimed at determining physically accurate line intensity and line-shape parameters in a spectrally congested region of the tetradecad polyad. In Part I of the study, molecular line intensities are determined in the low-pressure Doppler broadened regime, using cavity ring-down spectroscopy (CRDS) at NIST and Fourier Transform Spectroscopy (FTS) at DLR [1]. These intensity parameters were subsequently used in Part II to constrain fitting of CRDS spectra obtained at USTC in the pressure broadened regime, from which line-shape parameters, including first-order line mixing, are extracted [2]. The analysis of all measurements employed previously determined highly accurate line positions, obtained from CRDS saturation spectroscopy [3]. This provides a common set of constraints across each independent analysis, which is critical to mitigate variable correlation and avoid coincidental effective parameter sets.

Transition intensities are shown to agree at the sub-1 permille level for isolated transitions, along with the summed intensities of individual multiplets and manifolds, consistent with Monte Carlo simulations. Individual transitions, which are subject to significant blending-induced variable correlation, differ at the sub-percent level. The accuracy limits are evaluated using Monte Carlo modeling to determine accuracy limitations, demonstrating multi-variable correlation as the limiting factor in agreement between experiments. Excellent agreement is also shown between the measured values and recent ab initio intensities [4].

In Part II of the study, spectra were obtained at pressures ranging from 10 to 50 kPa and subsequently modeled with the speed-dependent Voigt profile, including first-order line mixing and employing the intensities determined in Part I as constraints. The resulting line-shape parameters have uncertainties of less than 1%, representing a significant improvement over HITRAN 2020 [5]. The parameters obtained in Part I and II, along with scaled air-broadened parameters, are incorporated into the HITRAN 2024 methane update [6].

References:

[1] Z.D. Reed et al J. Quant. Spectrosc. and Rad. Trans. 345, 109574 (2025)

[2] R.-H. Yin et al J. Quant. Spectrosc. and Rad. Trans. 346, 109588 (2025)

[3] Votava et al., Phys. Chem. Chem. Phys. 24, 4157 (2022)

[4] M. Rey et al., Icarus 303, 114-30 (2018)

[5] I. Gordon et al. J. Quant. Spectrosc. And Rad. Trans. 277, 107949 (2022)

[6] T. Bertin et al. J. Quant. Spectrosc. And Rad. Trans. 349, 109736 (2026)