The 1.28 μm transparency window of methane (7541–7919 cm−1): empirical line lists and temperature dependence (80 K–300 K)

Abstract

The high sensitivity absorption spectra of methane at room temperature and 80 K were recorded by CW-Cavity Ring Down Spectroscopy in the 1.28 μm transparency window (7541–7919 cm⁻¹). The empirical line parameters of 7690 and 5794 transitions were retrieved at room temperature and at 80 K, respectively. The achieved sensitivity (α_min ≈ 10⁻¹⁰ cm⁻¹) allowed detecting transitions with intensities as small as 5 × 10⁻³⁰ cm per molecule. In order to facilitate identification of the CH₃D transitions present in the CRDS spectrum of methane in “natural” isotopic abundance, the spectrum of a highly enriched CH₃D sample was recorded by differential absorption spectroscopy at room temperature and at 80 K. The CH₃D relative contribution in the considered transparency window is found to be significant only at 80 K (up to 15%) but more limited than in the 1.58 μm transparency window.

The low energy values of the transitions observed at both room temperature and 80 K were derived from the variation of their line intensities. Empirical lower states and J values have been obtained for 2821 CH₄ transitions representing 94.1 and 98.5% of the absorbance in the region at room temperature and 80 K, respectively. The good quality of these derived energy values is demonstrated by the marked propensity of the corresponding CH₄ lower state J values to be close to integers. The constructed line lists extend to higher energies the WKC (Wang–Kassi–Campargue) line lists of methane in the near infrared (1.71–1.26 μm). They allow one accounting for the temperature dependence of methane absorption between 80 K and 300 K and are of importance for the analysis of the near infrared spectrum of several planetary bodies like Titan, Uranus and Neptune. The centers of the 3ν₂ + ν₃ and 6ν₄ bands responsible of the absorption in the studied region are discussed in relation with recent theoretical calculations.