The near-infrared (1.30–1.70 μm) absorption spectrum of methane down to 77 K

Abstract

The high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 1.30 and 1.70 µm (5860-7680 cm -1 ) using a newly developed cryogenic cell and a series of DFB diode lasers. The investigated spectral range includes part of the tetradecad and the full icosad regions for which only very partial theoretical analysis are available. The analysis of the low temperature spectrum will benefit from the reduction of the rotational congestion and from the narrowing by a factor 2 of the Doppler linewidth allowing the resolution of a number of multiplets.Moreover, the energy value and rotational assignment of the angular momentum J of the lower state of a given transition can be obtained from the temperature variation of its line intensity. This procedure is illustrated in selected spectral regions by a continuous monitoring of the spectrum during the cell cool-down to 77 K, the temperature value being calculated at each instant from the measured Doppler linewidth. A short movie showing the considerable change of a spectrum during cool-down is attached as Supplementary Material. The method applied to a 30 cm -1 section of the tetradecad spectrum around 6110 cm -1 has allowed an unambiguous determination of the J value of part of the observed transitions.

The HITRAN2012 molecular spectroscopic database

“…There are a number of ongoing and recent studies [143,144] which can further improve the near-IR parameters (4800 to 7700 cm -1 ). It is expected that an interim update of this region and a new semi-empirical list of the octad will be available within one year.…”

Line broadening coefficient calculations for methane perturbed by nitrogen

“…The vibrational structure is organized into polyads of rapidly increasing complexity when climbing the energy range up to midinfrared and visible regions [22]. As far as line positions and intensities are concerned, the most recent and complete global analysis for 12 CH 4 concerns the 0 to 4800 cm −1 region [23], while upper polyads are still under investigation and the subject of sophisticated experimental efforts [24][25][26][27][28].…”

The high sensitivity absorption spectrum of ozone (18O3 and 16O3) near 7800cm−1: Identification of the 3A2(000)–X(110) hot band superimposed to very weak vibrational bands

“…We have extensively used this so-called 'two temperatures' method to derive the empirical lower energy levels of the methane transitions [34][35][36].…”