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)CH4) in the so-called dyad (ν2/ν4) region (1100-1500 cm(-1)), and the resulting update of the vibration-rotation effective model of methane, previously reported by Nikitin et al. [Phys. Chem. Chem. Phys. 15, 10071 (2013)], 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 Institut de Physique de Rennes 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-ground state (GS)) and related hot bands in the pentad-dyad system (3000 cm(-1)) up to Jmax = 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 σ = 2.09. The root mean square deviations dRMS are 3.60 × 10(-3) cm(-1) for dyad-GS cold band, 4.47 ×10(-3) cm(-1) for the pentad-dyad, 5.43 × 10(-3) cm(-1) for the octad-pentad, and 4.70 × 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.

New investigation of the ν3 C–H stretching region of 12CH4 through the analysis of high temperature infrared emission spectra

“…A typical wavenumber shift of 10 −3 cm −1 was found and no calibration was applied to the experimental emission spectra. In addition, there was no inter-calibration of the measured positions in the P 1 − P 0 and P 2 − P 1 (previous work [35]) and P 2 − P 0 (this work) bands.…”

“…Our methodology is to perform successive global analyses in the main five different spectral regions (Monad, Dyad, Pentad, Octad and Tetradecad) of methane. This was recently done in the Dyad region [35] and extended to the Pentad system in the present work.…”

“…As the spectra were rather dense, the precision of measurement of line positions was significantly affected by line blending. It was determined with the help of the previous global model predictions [35] and comparisons with line positions from the literature [30,32]. We checked that the well known and strongest lines that are free of overlaps are reproduced within 0.001 cm −1 in the case of the cold band.…”

“…The emergence of new experimental techniques such as the High Enthalpy Source (HES), developed by R. Georges and coworkers, coupled to a high-resolution Fourier transform spectrometer (FTS) allows the recording of high temperature emission spectra of polyatomic molecules (C 2 H 2 and CH 4 ) [33,34,35] up to about 2000 K. Recently, a new experimental setup coupling the HES to a cw-cavity ring-down spectrometer has been developed by the same group to investigate rotationally cold hot bands of polyatomic molecules in the 1.5-1.7 µm region [36]. Hargreaves et al developed an original approach to produce emission-corrected FTIR transmittance spectra of hot gases such as methane [37] and ethane [38].…”

“…Infrared databases of methane have been recently updated with information derived from the analysis of high temperature emission spectra of the Dyad system around 1400 cm −1 [35]. Indeed, 14415 vibration-rotation transitions belonging to the cold band and to three hot bands up to the Tetradecad were assigned, allowing significant improvements of the global effective model of methane to be achieved.…”