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

TL;DR

This study presents a highly precise methane line list in the 1250-1380 cm$^{-1}$ range using optical frequency comb spectroscopy, significantly improving the accuracy of spectral data for methane isotopologues.

Contribution

The paper introduces a new methane line list with sub-MHz accuracy, derived from Fourier transform spectroscopy, enhancing spectral models and reducing residuals compared to previous data.

Findings

Line position uncertainties reduced by factors of 18 and 59.

Inclusion of 235 new methane line intensities.

Residuals in spectral models decreased by factors of 8 and 20.

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${_4}$ ${\nu}$${_4}$ fundamental band, 131 lines from the $^{13}$CH${_4}$ ${\nu}$${_4}$ fundamental band, as well as 390 lines from two $^{12}$CH${_4}$ hot bands, ${\nu}$${_2}$ + ${\nu}$${_4}$ - ${\nu}$${_2}$ and 2${\nu}$${_4}$ - ${\nu}$${_4}$. For another 165 lines from the $^{12}$CH${_4}$ ${\nu}$${_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.