High resolution spectroscopy and the first global analysis of the Tetradecad region of methane 12CH4

Abstract

We present the first detailed analysis of the infrared spectrum of methane ¹²CH₄ in the so-called Tetradecad region from 2.1 to 1.6 μm (4760–6250 cm⁻¹). New experimental high resolution FTIR spectra at 78 K and at room temperature combined with improved theoretical modeling have allowed quantum assignments to be greatly extended in this region. A global fit of all assigned lines of ¹²CH₄ in the 0–6200 cm⁻¹ region has been performed. In the end, 3012 line positions and 1387 intensities of 45 individual subbands of the Tetradecad were modeled up to J = 14. The root mean square deviations were 0.023 cm⁻¹ for line positions and 13.86% for line intensities in the Tetradecad region itself. Although this analysis is still preliminary, it is already sufficient to characterize the stronger bands throughout the whole of the Tetradecad. The calculated integrated intensity of the polyad is 1.399 × 10⁻¹⁹ cm⁻¹/(molecule cm⁻²) at 296 K. A “definitive” theoretical modeling of this spectral region of methane requires further work, but the present success substantially improves our understanding of methane spectroscopy as needed to interpret planetary atmospheres. Lines pertaining to three-fourths of the 60 sub-vibrational bands in this polyad have been assigned.