Model, software and database for line-mixing effects in the ν3 and ν4 bands of CH4 and tests using laboratory and planetary measurements—I: N2 (and air) broadenings and the earth atmosphere (original) (raw)
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The Astrophysical Journal, 2017
Modeling atmospheres of hot exoplanets and brown dwarfs requires high-T databases that include methane as the major hydrocarbon. We report a complete theoretical line list of 12 CH 4 in the infrared range 0-13,400 cm −1 up to T max =3000 K computed via a full quantum-mechanical method from ab initio potential energy and dipole moment surfaces. Over 150 billion transitions were generated with the lower rovibrational energy cutoff 33,000 cm −1 and intensity cutoff down to 10 −33 cm/molecule to ensure convergent opacity predictions. Empirical corrections for 3.7 million of the strongest transitions permitted line position accuracies of 0.001-0.01 cm −1. Full data are partitioned into two sets. "Light lists" contain strong and medium transitions necessary for an accurate description of sharp features in absorption/emission spectra. For a fast and efficient modeling of quasi-continuum cross sections, billions of tiny lines are compressed in "super-line" libraries according to Rey et al. These combined data will be freely accessible via the TheoReTS information system (http://theorets.univ-reims.fr, http://theorets.tsu.ru), which provides a user-friendly interface for simulations of absorption coefficients, cross-sectional transmittance, and radiance. Comparisons with cold, room, and high-T experimental data show that the data reported here represent the first global theoretical methane lists suitable for high-resolution astrophysical applications.
An improved empirical line list for methane in the region of the 2ν3 band at 1.66μm
Journal of Quantitative Spectroscopy and Radiative Transfer, 2013
The absorption spectrum of methane is recorded at room temperature by differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5855-6183 cm À 1) dominated by the 2n 3 band at 1.66 mm. An empirical list of 9228 lines is constructed from a profile fitting of the very congested spectrum. The achieved noise equivalent absorption of the spectra (a min E 1 Â 10 À 7 cm À 1) allowed us to add about 4000 lines to the recent empirical list constructed in relation with the project ''Greenhouse Gases Observing Satellite'' (Nikitin A, Lyulin O, Mikhailenko S, Perevalov V, Filippov N, Grigoriev I, et al. GOSAT-2009 methane spectral list in the 5550-6236 cm À 1 range. J Quant Spectrosc Radiat Transfer 2010;111:2211-24). Lines due to the 13 CH 4 and CH 3 D isotopologues present in the methane sample in ''natural'' abundance (0.011 and 5.0 Â 10 À 4 , respectively) are systematically identified by comparison with spectra of pure CH 3 D and 13 CH 4 recorded separately. In order to derive empirical values of the lower state energy level, E emp , from the intensity ratios of the lines measured at 80 K and 296 K, the obtained 296 K line list is combined with the WKMC list of methane at 80 K (Campargue A, Leshchishina O, Wang L, Mondelain D, Kassi S, Nikitin AV. Refinements of the WKMC empirical line lists (5852-7919 cm À 1) for methane between 80 K and 296 K. J Quant Spectrosc Radiat Transfer 2012;113:1855-73). Overall 4396 lower state energy values were determined extending significantly the previous set of E emp values derived using the GOSAT list for the room temperature dataset. These improvements allow for a better accounting of the methane spectrum temperature dependence between 80 K and 300 K. A number of discrepancies between the rovibrational assignments included in the GOSAT list and our derived E emp values is discussed. The new line dataset including a large number of empirical lower state energies provides a solid basis for future theoretical modeling of the tetradecad of 12 CH 4 .
Icarus, 2006
The band model fits of Sihra [1998. Ph.D. Thesis. University of Oxford], subsequently reported by Irwin et al. [2005. Icarus 176, 255-271], to new measurements of low-temperature near-infrared self-broadened methane absorption spectra combined with earlier warmer, longer path measurements of both self-and hydrogen-broadened methane spectra measured by Strong et al. [1993. J. Quant. Spectrosc. Radiat. Transfer 50, 363-429], have been found to contain severe artefacts at wavelengths of very low methane absorption. Although spectra calculated from these new band data appear to be reliable for paths with low to medium absorption, transmissions calculated for long paths of high methane absorption, such as for Uranus, Neptune and Titan are severely compromised. The recorded laboratory transmission spectra of Sihra [1998. Ph.D. Thesis. University of Oxford] and Strong et al. [1993. J. Quant. Spectrosc. Radiat. Transfer 50, 363-429] have thus been refitted with a more robust model and new k-distribution data for both self-and hydrogen-broadened methane absorption derived. In addition, a new model of the temperature dependence of the absorption has been employed that improves the quality of the fit and should also provide more accurate extrapolations to low temperatures.
Chemical Physics, 2009
We report the global analysis of methane (12 CH 4) lines from high resolution rovibrational spectra including accurate line positions and intensities in the region 0-4800 cm À1. This covers four polyads: The Ground State Monad (rotational levels), the Dyad (940-1850 cm À1 , 2 vibrational levels, 2 sublevels), the Pentad (2150-3350 cm À1 , 5 vibrational levels, 9 sublevels) and the Octad (3550-4800 cm À1 , 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 À1 instrumental bandwidth, unapodized) at 78 K using the Bruker IFS 125 HR Zürich prototype (ZP2001) spectrometer in combination with a long optical path collisional cooling system [S. Albert, S. Bauerecker, M. Quack, A. Steinlin, Mol. Phys. 105 (2007) 541]. 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 lines above 4400 cm À1. 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 16,738 line positions were used in the least squares adjustment characterized by the following global root mean square deviations d RMS for line positions: 1.3 Â 10 À4 cm À1 for the Dyad, 6.0 Â 10 À4 cm À1 for the Pentad, and 3.5 Â 10 À3 cm À1 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 d RMS = 9.6% including 3262 experimental line intensities for the Octad. This analysis represents a large improvement over the previous one [
Spectrum of hot methane in astronomical objects using a comprehensive computed line list
Proceedings of the National Academy of Sciences of the United States of America, 2014
Hot methane spectra are important in environments ranging from flames to the atmospheres of cool stars and exoplanets. A new spectroscopic line list, 10to10, for (12)CH4 containing almost 10 billion transitions is presented. This comprehensive line list covers a broad spectroscopic range and is applicable for temperatures up to 1,500 K. Previous methane data are incomplete, leading to underestimated opacities at short wavelengths and elevated temperatures. Use of 10to10 in models of the bright T4.5 brown dwarf 2MASS 0559-14 leads to significantly better agreement with observations and in studies of the hot Jupiter exoplanet HD 189733b leads to up to a 20-fold increase in methane abundance. It is demonstrated that proper inclusion of the huge increase in hot transitions which are important at elevated temperatures is crucial for accurate characterizations of atmospheres of brown dwarfs and exoplanets, especially when observed in the near-infrared.
Journal of Geophysical Research, 1993
Near-infrared 10-cm-1 resolution spectra of methane obtained at various temperatures, pressures, and abundances are fit to a quasi-random narrow-band model. Exponential-sum absorption coefi%ients for three temperatures (112 K, 188 K, and 295 K), and 20 pressures from 0.0001 to 5.6 bars, applicable to the cold environments of the major planets, are then derived from the band model for the 230 wavelengths measured from 1.6 to 2.5/zm. Root-mean-square deviations between the laboratory and the exponential-sum synthetic transmissions are reported for the best fitting 50 wavelengths. Deviations relevant to broadband, 1% spectral resolution observations such as planned to be acquired by Galileo/Near Infrared Mapping Spectrometer (NIMS) and Cassini/Visual and Infrared Mapping Spectrometer are also presented. The validity of exponential-sum coefiScients derived from broadband (10 cm-1) transmission data is demonstrated via direct comparison with line-by-line calculations. The complete atlas of coefiScients is available from the Planetary Data System-Planetary Atmospheres Discipline Node. eta!., 1991]. In addition, both the Galileo and Cassini missions plan on comprehensive near-IR monitoring of Jupiter, Saturn, and Titan during their extensive orbital tours beginning in late 1995. The strength of methane gas absorption spans 7 orders of magnitude in the 1.6-to 2.5-/xm region, thus enabling various spectral features to probe a range of atmospheric pressures extending from millibars to bars. Yet very little has been done to exploit the information contained in this spectral Copyright 1993 by the American Geophysical Union. Paper number 92JE02808. 0148-0227/93/92JE-02808505.00 region. This is due to both (1) the lack of quantitative information on the behavior of near-infrared methane gas absorptions over the range of pressures and temperatures extant in the outer planets, and (2) the lack of a reliable method for assessing the relevant radiative transfer processes in vertically inhomogeneous scattering atmospheres. A decade ago, Benner and Fink [1980] demonstrated that pressure-independent broadband (10-,• resolution) methane absorption coefficients can be used with Beer's law to accurately determine planetary reflectivities at wavelengths less than 1 _•zm. However, they demonstrated as well that beyond-•0.9/zm, more sophisticated band models including pressure dependencies are, at the least, required. Yet such band models are inapplicable to vertically inhomogeneous planetary atmospheres wherein temperatures and pressures vary significantly with altitude, and wherein significant aerosol scattering is present. These difficulties may, in theory, be surmounted via a detailed line-by-line analysis over subwavenumber spectral intervals. However, apart from the computational expense, the extremely complicated nature of the methane spectrum has prevented the assignment of all but the strongest near-IR lines. Recently, Giver et ai. [1990] addressed problem 1 above, with their acquisition of laboratory transmission data over the relevant range of temperatures and pressures. Recent progress has occurred in problem 2 as well. In particular, Goody and Yung [1989], Goody et ai. [1989], and Lacis and Oinas [1991] have demonstrated the efficacy of a new technique, the correlated-k method, for handling molecular absorptions at broad spectral resolutions in vertically inho
A far wing lineshape for H2 broadened CH4 infrared transitions
Journal of Quantitative …, 2002
Remote sensing investigations of the deep atmosphere of Jupiter, through spectral atmospheric windows, strongly depends on the assumed line shape for the far wings of collisionally broadened infrared transitions. It was shown in [1] that the large uncertainty on the line profile for CH ...
The high-resolution far-infrared spectrum of methane at the SOLEIL synchrotron
Journal of Quantitative Spectroscopy and Radiative Transfer, 2010
As a tetrahedral molecule, methane has no permanent dipole moment. Its spectrum, however, displays faint absorption lines in the THz region, due to centrifugal distorsion effects. This is important for planetary applications since this region is used to measure methane concentration in some planetary atmospheres, in particular on Titan. Up to now, all measurements relied either on some old low resolution infrared absorption spectra, or on high resolution Stark measurements for low J values only. Even if these results have been reexamined recently [E. H. Wishnow, G. S. Orton, I. Ozier and H. P. Gush, J. Quant. Spectrosc. Radiat. Transfer 103, 102-117 (2007)], it seemed highly desirable to obtain much more precise laboratory data.
Precise methane absorption measurements in the 1.64 μm spectral region for the MERLIN mission
Journal of geophysical research. Atmospheres : JGR, 2016
In this article we describe a high-precision laboratory measurement targeting the R(6) manifold of the 2ν 3 band of (12)CH4. Accurate physical models of this absorption spectrum will be required by the Franco-German, Methane Remote Sensing LIDAR (MERLIN) space mission for retrievals of atmospheric methane. The analysis uses the Hartmann-Tran profile for modeling line shape and also includes line-mixing effects. To this end, six high-resolution and high signal-to-noise absorption spectra of air-broadened methane were recorded using a frequency-stabilized cavity ring-down spectroscopy apparatus. Sample conditions corresponded to room temperature and spanned total sample pressures of 40 hPa - 1013 hPa with methane molar fractions between 1 μmol mol(-1) and 12 μmol mol(-1). All spectroscopic model parameters were simultaneously adjusted in a multispectrum nonlinear least-squares fit to the six measured spectra. Comparison of the fitted model to the measured spectra reveals the ability t...