[Paper Review] A methane line list with sub-MHz accuracy in the 1250 to 1380 cm-1 range from optical frequency comb Fourier transform spectroscopy

Matthias Germann, Adrian Hjältén+7 more|arXiv (Cornell University)|Apr 13, 2022

Spectroscopy and Laser Applications54 references24 citations

TL;DR

This study presents a high-accuracy methane line list in the 1250–1380 cm⁻¹ range using optical frequency comb Fourier transform spectroscopy, achieving sub-MHz line position uncertainties (0.19–2.3 MHz). The method enables global modeling improvements with residuals reduced by a factor of 8 compared to prior absorption data and 20 compared to emission data, significantly enhancing spectral precision for atmospheric and astrophysical applications.

ABSTRACT

We use a Fourier transform spectrometer based on a difference frequency generation optical frequency comb to measure high-resolution, low-pressure, room-temperature spectra of methane in the 1250 - 1380 cm$^{-1}$ range. From these spectra, we retrieve line positions and intensities of 678 lines of two isotopologues: 157 lines from the $^{12}$CH${_4}$ ${ u}$${_4}$ fundamental band, 131 lines from the $^{13}$CH${_4}$ ${ u}$${_4}$ fundamental band, as well as 390 lines from two $^{12}$CH${_4}$ hot bands, ${ u}$${_2}$ + ${ u}$${_4}$ - ${ u}$${_2}$ and 2${ u}$${_4}$ - ${ u}$${_4}$. For another 165 lines from the $^{12}$CH${_4}$ ${ u}$${_4}$ fundamental band we retrieve line positions only. The uncertainties of the line positions range from 0.19 to 2.3 MHz, and their median value is reduced by a factor of 18 and 59 compared to the previously available data for the $^{12}$CH${_4}$ fundamental and hot bands, respectively, obtained from conventional FTIR absorption measurements. The new line positions are included in the global models of the spectrum of both methane isotopologues, and the fit residuals are reduced by a factor of 8 compared to previous absorption data, and 20 compared to emission data. The experimental line intensities have relative uncertainties in the range of 1.5 - 7.7%, similar to those in the previously available data; 235 new $^{12}$CH${_4}$ line intensities are included in the global model.

Motivation & Objective

• To obtain highly accurate line positions and intensities for methane isotopologues in the 1250–1380 cm⁻¹ range, a key atmospheric window region.
• To overcome the limitations of conventional FTIR spectroscopy, which suffers from lower precision (e.g., 1.8 MHz uncertainty) and outdated data for methane’s complex, congested spectrum.
• To improve global spectral models of methane by incorporating new, high-accuracy experimental data from a comb-based FTS system.
• To reduce fit residuals in effective Hamiltonian models by replacing or supplementing lower-accuracy data from previous studies.
• To provide a benchmark dataset with sub-MHz line position accuracy and 1.5–7.7% intensity uncertainty for applications in atmospheric science, exoplanet characterization, and molecular physics.

Proposed method

• Employed a difference frequency generation optical frequency comb (OFC) source to generate a broadband, phase-locked mid-infrared comb spanning 7.25–8 μm.
• Used a Herriot-type multi-pass absorption cell (10.436 m path length) to enhance sensitivity at low methane pressures (≤10 mTorr).
• Applied a comb-based Fourier transform spectrometer (FTS) with absolute frequency calibration via a stabilized reference laser, enabling direct traceability to the RF frequency standard.
• Implemented a two-step wavelength calibration procedure: first, line-by-line optimization of the reference laser wavelength to minimize fit residuals; second, correction for nonlinear mapping of comb modes to FTS sampling points using a local FTS scale correction.
• Calculated corrected FTS frequency spacing and reference wavelength using equations (A.11) and (A.13) to align sampling points with comb mode frequencies across the spectral range.
• Used a GPS-disciplined rubidium standard to stabilize the comb repetition rate (frep ≈ 125 MHz), ensuring long-term frequency stability and absolute accuracy.

Experimental results

Research questions

• RQ1 Can optical frequency comb-based FT spectroscopy achieve sub-MHz accuracy in line position measurements for methane in the 1250–1380 cm⁻¹ range?
• RQ2 To what extent does the new data reduce residuals in global effective Hamiltonian models of methane compared to previous absorption and emission data?
• RQ3 How do the line positions and intensities from the 12CH4 ν4 fundamental band and its hot bands compare in accuracy and precision to those from conventional FTIR measurements?
• RQ4 What is the impact of correcting for nonlinear mapping of comb modes on the FTS scale, and how does it improve spectral alignment and line fitting?
• RQ5 Can the new line list significantly improve the accuracy of methane spectral models used in atmospheric and exoplanetary remote sensing?

Key findings

• The study reports 678 lines with line position uncertainties between 0.19 and 2.3 MHz, with a median uncertainty of 430 kHz, representing a 59-fold improvement over previous emission data for hot bands and an 18-fold improvement over conventional FTIR data for the 12CH4 ν4 band.
• Line intensities were retrieved with relative uncertainties of 1.5–7.7%, comparable to previous data, and 235 new 12CH4 intensities were added to the global model.
• The new line positions reduced the fit residuals of the effective Hamiltonian model by a factor of 8 compared to previous absorption data and by a factor of 20 compared to emission data.
• The correction procedure for nonlinear FTS scale mapping successfully eliminated wavenumber-dependent offsets, enabling accurate alignment of FTS sampling points with comb mode frequencies across the spectral range.
• The global model now includes 157 lines from 12CH4 ν4, 131 from 13CH4 ν4, 390 from two 12CH4 hot bands (ν2 + ν4 – ν2 and 2ν4 – ν4), and 165 additional 12CH4 ν4 lines with positions only.
• The method achieved sub-MHz accuracy across a 130 cm⁻¹ bandwidth (1250–1380 cm⁻¹), demonstrating the feasibility of high-precision, broadband molecular spectroscopy using comb-based FTS for complex, congested spectra.