Global analysis of the high temperature infrared emission spectrum of 12CH4 in the dyad (ν2/ν4) region

Abstract

We report new assignments of vibration-rotation line positions of methane ($^{12}$CH$_4$) in the so-called Dyad ($\nu_2$/$\nu_4$) region (1000 -- 1500 cm$^{-1}$), and the resulting update of the vibration-rotation effective model of methane, previously reported by Nikitin \textit{et al.} [A.V. Nikitin \textit{et al.} PCCP, \textbf{15}, (2013), 10071], up to and including the Tetradecad. High resolution (0.01 cm$^{-1}$) emission spectra of methane have been recorded up to about 1400 K using the high-enthalpy source developed at IPR associated with the Fourier transform spectrometer of the SOLEIL synchrotron facility (AILES beamline). Analysis of these spectra allowed extending rotational assignments in the well-known cold band (Dyad$-$GS) and related hot bands in the Pentad$-$Dyad system (3000 cm$^{-1}$) up to $J_{max}=30$ and 29, respectively. In addition, 8512 new transitions belonging to the Octad$-$Pentad (up to $J=28$) and Tetradecad$-$Octad (up to $J=21$) hot band systems were successfully identified. As a result, the MeCaSDa database of methane was significantly improved. The line positions assigned in this work, together with the information available in the literature, were fitted using 1096 effective parameters with a dimensionless standard deviation $\sigma = 2.09$. The root mean square deviations $d_{RMS}$ are 3.60 $\times 10^{-3}$ cm$^{-1}$ for Dyad$-$GS cold band, 4.47 $\times 10^{-3}$ cm$^{-1}$ for the Pentad$-$Dyad, 5.43 $\times 10^{-3}$ cm$^{-1}$ for the Octad$-$Pentad and 4.70 $\times 10^{-3}$ cm$^{-1}$ for the Tetradecad$-$Octad hot bands. The resulting new line list will contribute to improve opacity and radiative transfer models for hot atmospheres, such as those of hot-Jupiter type exoplanets.