Assignment and modelling of 12CH4 spectra in the 5550–5695, 5718–5725 and 5792–5814 cm−1 regions
Introduction
Over the last decades, an improvement of knowledge of infrared methane spectra has been a target for numerous atmospheric and astrophysical applications [1], [2], [3]. This evolution was motivated by the new challenges that appeared with the planetary missions measuring medium resolution spectra of different atmospheres. In this context, many studies have been devoted to radiative properties of the Titan atmosphere (Saturn's largest satellite) which is composed of 98.6% nitrogen and 1.4% methane at temperatures ranging between 70 K and 200 K [4], [5], [6], [7], [8], [9]. Methane band parameters are essential for accurate interpretation infrared spectral data provided by orbiting and ground-based observatories [10], [11], [12], [13], [14]. Lacking or insufficiently accurate information on spectral transitions was identified as a major issue for investigations of outer planets [15], [6], [7]. Despite recent progress [16], [17], [18], [19], [20], [21], [22], [23], the linelists [24], [25], [26], [27], [28], [29], [30] based on available analyses of laboratory experiments do not yet include all the necessary information to reproduce atmospheric methane spectra in the near infrared, particularly for relatively weak absorption features which, however, play an important role in long atmospheric optical paths [31].
Because of high tetrahedral symmetry some normal modes of methane are doubly or triply degenerate in the harmonic approximations and fall in nearby intervals. In addition to that there are accidental resonances ω1 ≅ ω3 ≅ 2ω2 ≅ 2ω4 resulting in vibrational levels grouped into so called polyads (Dyad, Pentad, Octad, etc.) [32]. The vibrational energy levels with the same value of the polyad number P = 2 V1 + V2 + 2 V3 + V4 calculated from the four vibrational quantum numbers are then in the same energy range. In this work we consider analyses of spectra belonging to the Tetradecad corresponding to P = 4. The scheme of 12CH4 vibrational states is given in Fig. 1, where the blown-up scale at the right-hand side represents sub-band centers with their symmetry types related to the present study.
The aim of the present work is to improve the line list and to extend the assignments of weak transitions of 12CH4 in the 5550–5695.25, 5718.8–5724.25, 5792.36–5814.29 cm−1 region of the Tetradecad [16]. These three small regions are the last ranges of the Tetradecad in which detailed identification of experimentally observed transitions was absent. The Tetradecad is a complex polyad containing 60 vibrational sublevels. The lower portion of the Tetradecad (mostly the 4v4 band) was first studied by Robert et al. [33], [34], [20].
A quite representative set of rovibrational assignments in the range 5550–5695.25 cm−1 was obtained in the frame of the GOSAT project from spectra recorded by Fourier Transform Spectroscopy [35], [36]. The GOSAT empirical list includes about 5200 lines and provided about 2000 rovibrational assignments in the range 5550–6240 cm−1. This linelist was included in the public database HITRAN [24]. A simultaneous fitting of assigned-to-date lines of 12CH4 in the 0–6200 cm−1 region was reported in Ref [16]. A further extended set of 13,045 calculated positions and intensities has been reported in the range 4800–5300 cm−1 in [37], in which 2725 experimental line positions have been fitted with the RMS standard deviations of 0.004 cm−1 and 1764 selected line intensities with the RMS deviation of 7.3%. Some 5718 line positions and intensities were retrieved in the 5300–5550 cm−1 region [38]. One part of WKLMC experimental line lists [18] (see also [7], [39]) at 296 K have been assigned [40] in the range 5855–6250 cm−1. This range is part of the whole WKLMC [18] lists at 296 K and 80 K measured in Grenoble in the region 5855–7912 cm−1 by Differential Absorption Spectroscopy (DAS) in the strong absorption regions (the 2v3 region of the Tetradecad [7], [39] and in the Icosad [41], [42], [43], [44]) and by high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58 µm [45], [46] and 1.28 µm transparency windows [47]. Recently 5934 line positions and intensities were retrieved in the 5695–5850 cm−1 region by Ghysels et al. also from laser spectra recorded in Grenoble [48]. Note that in the last paper several small intervals were not covered.
The GOSAT empirical list [36] includes about 2500 lines in these ranges, but only 400 of them have been assigned.
The paper is structured as follows. Experimental spectra recorded in GSMA Reims and at JPL are described in Sections 2.1 and 2.2. Section 3 is devoted to the determination of line parameters and Section 4 to spectra assignments. Section 5 gives the information on the new assigned line lists provided in the Supplementary Materials.
Section snippets
Reims FTS spectra
In the present study, a large set of FTS spectra measured at GSMA laboratory of Reims University have been used. From all these spectra, some have already been presented in former publications concerning specific studies. In the experimental series made in 2009 and in 2011, the 50 m base multipass cell from the lab was optically fitted to our high resolution step-by-step spectrometer as first described in [34] and [17]. The temperature of the cell is the room temperature. The absorption path
Line parameters in the 5550–5695.25 cm−1, 5718.8–5724.25, 5792.36–5814.29 regions
All positions and intensities were obtained from Reims and JPL spectra using the retrieval program SpectraPlot [53]. All spectral features with intensities > 1.0 × 10−25 cm−1/(molecule·cm−2) and many of the lines with intensities between 1.0 × 10−25 and 4.0 × 10−26 cm−1/(molecule·cm−2) were retrieved. We note that in the range under consideration there are stronger absorption features than in the range 5300–5550 cm−1 [38]. This made the study of weak transitions much more complicated. At the
Spectra assignment
Line-by-line assignments of crowded methane spectra is known to be a tedious and difficult task [1]. The most widespread approach for spectra analyses in past decades has been based on effective Hamiltonian (EH) and effective dipole moment (EDM) models involving empirical parameters fitted to observed lines (other notation fitted effective Hamiltonian parameters, see for example [30], [32], [61] and Refs. therein). One of the well-known problems was a rapidly increasing number of adjustable
The methane line list with assignments
The line list at 296 K including the assignments from this work is compiled and attached as Supplementary Material. A sample of this list is given in Table 5; this includes the observed positions and intensities (at 296 K) along with the quantum assignments and lower state energies and rovibrational assignments (the assignment notation format is described in Ref. [38]). Self-broadening and air-broadening coefficients obtained from Refs. [82], [83] were added to our final line list. More recent
Conclusion
The main results of the present spectrum analysis in the 5550–5695.25, 5718.8–5724.25, 5792.36–5814.29 cm−1 regions are extended assignments and improved line lists. By applying an ab initio based effective CT Hamiltonian we assigned ∼2800 new lines of 12CH4 up to J = 17. Energy levels of all sub-bands were obtained using subsequent empirical adjustment of a limited subset of EH parameters, which were statistically well-defined in the fit. The accuracy of the final empirically optimized
Acknowledgements
The research at the V.E. Zuev Institute of Atmospheric Optics was performed under contract No. 17-17-01170 with the Russian Scientific Foundation. The supports of the CNRS (France) in the frame of “Laboratoire International Associé SAMIA”, of the “Programme National de Planétologie (PNP) du CNRS, of the French ANR project e-PYTHEAS (ref: ANR-16-CE31-0005), of IDRIS / CINES computer centers of France and of the ROMEO computer center Reims-Champagne-Ardenne are acknowledged. Part of the research
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