Atlas of Experimental and Theoretical High-temperature Methane Cross Sections from T = 295 to 1000 K in the Near-infrared

Abstract

Spectra of hot methane were recorded using a tube furnace and a high-resolution Fourier transform spectrometer. We obtained experimental absorption spectra in the 1.85-1.11 μm near-infrared region at eight temperatures ranging from 295 K up to 1000 K. We have converted these into an atlas of absorption cross sections at each temperature for the methane tetradecad, icosad and triacontad polyads, excluding some spectral intervals either strongly contaminated by water or due to baseline fringes. On the theoretical side, the spectra were simulated from the ab initio-based Reims-Tomsk TheoReTS line list for the same experimental conditions. This line list has been constructed by global variational calculations from potential energy and dipole moment surfaces followed by empirical line position corrections deduced from previously published analyses. The comparisons showed very good overall agreement between observations and theory at high spectral resolution for the tetradecad and icosad and at medium or low resolution above this range. A full set of the theoretical absorption cross sections is also included. Detailed temperature dependence of the methane absorption enables the efficient method of remotely probing the temperature of distant astronomical objects based on a comparison of relative signals in carefully selected spectral intervals. This first combined experimental and theoretical easy-to-use cross-section library in the near-infrared should be of major interest for the interpretation of current and future astronomical observations up to a resolving power of 100,000-300,000 in the range 6400-7600 cm −1 and a resolving power of 5000-10,000 in the higher wavenumber range up to 9000 cm −1 .

An Accurate, Extensive, and Practical Line List of Methane for the HITEMP Database

“…Wong et al (2019) have previously compared empirical absorption cross sections of the P 4 , P 5 and P 6 regions of CH 4 to those calculated using RNT2017 up to 1000K. The largest spectral coverage provided by Wong et al (2019) is at 600 K, and comparisons to this work and ExoMol 34to10 are shown in Fig. (10) for a convolved resolution of 2 cm −1 .…”

“…RNT2017 contains significant improvements with respect to the previous RNT2014 (Rey et al 2014a) work, for which a good general agreement with experimental spectra up to 1200 K has been observed by Hargreaves et al (2015) for the pentad (P 2 ) and octad (P 3 ) regions (2.0-3.8 µm) . RNT2017 has recently been validated against experimental observations up to 1000 K for the tetradecad (P 4 ), icosad (P 5 ) and triacontad (P 6 ) regions (1.11-1.85 µm) by Wong et al (2019) at resolutions of 0.02, 0.2 and 2.0 cm −1 . In addition, the region near 1.7 µm has also been validated to accurate (±0.002 cm −1 ) observations at 1000 K by Ghysels et al (2018) along with comparisons to MeCaSDa, HITRAN2016 and ExoMol 10to10.…”

“…For P 6 , the overall shape of the observed band is reproduced, with a slight difference in intensity. The same comparisons were made for all temperatures available from Wong et al (2019), with similar results. For all bands, this work has significantly smaller residuals than ExoMol 34to10.…”

“…The experimental measurements of Hargreaves et al (2015) and Wong et al (2019) contain spectra of CH 4 with terrestrial abundances. The room-temperature observations can be used to validate the inclusion of CH 4 isotopologues from HITRAN2016 into the hightemperature line list for this work.…”

“…High-temperature comparisons have been made to experimental observations of P 2 and P 3 from Hargreaves et al (2015) and P 4 , P 5 and P 6 from Wong et al (2019). Emission spectra for P 1 have been measured by Hargreaves et al (2012); however self-absorption effects make them inappropriate for comparisons due to unreliable line intensities.…”