Global analysis of the high resolution infrared spectrum of methane 12CH4 in the region from 0 to 4800 cm-1
Résumé
We report the global analysis of methane (12CH4) lines from high resolution rovibrational spectra including accurate line positions and intensities in the region 0–4800 cm−1. This covers four polyads: The Ground State Monad (rotational levels), the Dyad (940–1850 cm−1, 2 vibrational levels, 2 sublevels), the Pentad (2150–3350 cm−1, 5 vibrational levels, 9 sublevels) and the Octad (3550–4800 cm−1, 8 vibrational levels, 24 sublevels) and some of the associated hot bands (Pentad−Dyad and Octad−Dyad). New Fourier transform infrared (FTIR) spectra of the Pentad and Octad regions have been recorded with a very high resolution (better than 0.001 cm−1 instrumental bandwidth, unapodized) at 78 K using the Bruker IFS 125 HR Z¨urich prototype (ZP2001) spectrometer in combination with a long optical path collisional cooling system (S. Albert, S. Bauerecker, M. Quack and A. Steinlin,. Mol. Phys., 105, 541–558 (2007)). Existing spectra previously recorded with the FTIR spectrometer at the National Solar Observatory on Kitt Peak in Arizona were remeasured selectively to provide new intensities and positions of weaker features above 4400 cm−1. These were combined with previously reported absorption data from FTIR and laser absorption, as well as high-resolution stimulated Raman and microwave spectra. The effective Hamiltonian was expanded up to order 6 for the Ground State, order 6 for the Dyad, order 5 for the Pentad and order 5 for the Octad. A total of 16738 line positions were used in the least squares adjustment characterized by the following global root mean square deviations dRMS for line positions: 1.3 × 10−4 cm−1 for the Dyad, 6.0 × 10−4 cm−1 for the Pentad, and 3.5 × 10−3 cm−1 for the Octad. Absolute intensities were also analyzed for all the cold bands and some of the hot bands in the region under consideration and we obtained dRMS = 9.6% including 3262 experimental line intensities for the Octad. This analysis represents a large improvement over the previous one (J.-C. Hilico, O. Robert, M. Lo¨ete, S. Toumi, A. S. Pine and L. R. Brown, J. Mol. Spectrosc. 208, 1–13 (2001)) with dRMS = 0.041 cm−1 for positions and 15.6% for intensities in the Octad for a smaller data set. The new results are discussed as benchmarks in relation to accurate potential energy hypersurfaces and for atmospheric and planetary spectra.