Line List For Methane Research Articles

ExoMol line lists – LVII. High accuracy ro-vibrational line list for methane (CH4)

ABSTRACT The MM ro-vibrational line list for methane (12CH4) is presented; MM covers wavelengths λ > 0.83 μm (wavenumbers up to 12 000 cm−1) and contains over 50 billion transitions between 9155 208 states with total angular momentum J ≤ 60. MM was generated through solution of the nuclear motion Schrödinger equation using variational program trove for an empirically derived potential energy surface (PES) and a new high-level ab initio dipole moment surface. The PES was constructed by fitting the ro-vibrational energies of CH4 to a set of highly accurate, experimentally derived energies. Molecular states are classified using the Td(M) symmetry group and are fully assigned with rotation and vibration quantum numbers. The MM line list is adapted to high-resolution applications by replacing the calculated ro-vibrational energies with the experimentally derived values where available, namely for 23 208 states with J ≤ 27 below 9986 cm−1. Doing so leads to over 1000 000 experimentally derived CH4 lines compared to approximately $330\, 000$ lines of 12CH4 in the HITRAN data base. The MM line list is shown to be more complete than the recent HITEMP methane line list. Methane spectra computed using MM across a broad range of temperatures and wavenumbers show excellent agreement with experiment. The MM line list supersedes the previous ExoMol methane line lists 10to10 and 30to10 both in terms of accuracy and coverage. Together with the pre-computed ExoMolOP molecular atmospheric opacity tables, it is now the recommended CH4 data set in the ExoMol data base (www.exomol.com).

A methane line list with sub-MHz accuracy in the 1250 to 1380 cm−1 range from optical frequency comb Fourier transform spectroscopy

We use a Fourier transform spectrometer based on a difference frequency generation optical frequency comb to measure high-resolution, low-pressure, room-temperature spectra of methane in the 1250 – 1380-cm−1 range. From these spectra, we retrieve line positions and intensities of 678 lines of two isotopologues: 157 lines from the 12CH4 ν4 fundamental band, 131 lines from the 13CH4 ν4 fundamental band, as well as 390 lines from two 12CH4 hot bands, ν2 + ν4 – ν2 and 2ν4 – ν4. For another 165 lines from the 12CH4 ν4 fundamental band we retrieve line positions only. The uncertainties of the line positions range from 0.19 to 2.3 MHz, and their median value is reduced by a factor of 18 and 59 compared to the previously available data for the 12CH4 fundamental and hot bands, respectively, obtained from conventional FTIR absorption measurements. The new line positions are included in the global models of the spectrum of both methane isotopologues, and the fit residuals are reduced by a factor of 8 compared to previous absorption data, and 20 compared to emission data. The experimental line intensities have relative uncertainties in the range of 1.5 – 7.7%, similar to those in the previously available data; 235 new 12CH4 line intensities are included in the global model.

Terahertz Rotational Spectroscopy of Greenhouse Gases Using Long Interaction Path-Lengths

Even if on-board mm-wave/THz heterodyne receivers have been developed to measure greenhouse gases (GHGs) atmospheric profiles, rotational spectroscopy rests under-exploited for their monitoring unlike IR rovibrational spectroscopy. The present study deals with the ability of THz spectroscopy using long interaction path-lengths for GHG laboratory investigations. High-resolution THz signatures of non-polar greenhouse molecules may be observed by probing very weak centrifugal distortion induced rotational transitions. To illustrate, new measurements on CH4 and CF4 have been carried out. For CH4, pure rotational transitions, recorded by cw-THz photomixing up to 2.6 THz in a White type cell adjusted to 20 m, have allowed to update the methane line list of atmospheric databases. Concerning CF4, Fabry-Perot THz absorption spectroscopy with a km effective pathlength was required to detect line intensities lower than 10−27 cm−1/(moleccm−2). Contrary to previous synchrotron-based FT-FIR measurements, the tetrahedral splitting of CF4 THz lines is fully resolved. Finally, quantitative measurements of N2O and O3 gas traces have been performed in an atmospheric simulation chamber using a submm-wave amplified multiplier chain coupled to a Chernin type multi-pass cell on a 200 m path-length. The THz monitoring of these two polar GHGs at tropospheric and stratospheric concentrations may be now considered.

An Accurate, Extensive, and Practical Line List of Methane for the HITEMP Database

A methane line list for the HITEMP spectroscopic database, covering 0-13,400 cm$^{-1}$ ($>$746 nm), is presented. To create this compilation, ab initio line lists of $^{12}$CH$_{4}$ from Rey et al. (2017) ApJ, 847, 105 (provided at separate temperatures in the TheoReTS information system), are now combined with HITRAN2016 methane data to produce a single line list suitable for high-temperature line-by-line calculations up to 2000 K. An effective-temperature interpolation model was created in order to represent continuum-like features at any temperature of interest. This model is advantageous to previously-used approaches that employ so-called ``super-lines'', which are suitable only at a given temperature and require separate line lists for different temperatures. The resultant HITEMP line list contains $\sim$32 million lines and is significantly more flexible than alternative line lists of methane, while accuracy required for astrophysical or combustion applications is retained. Comparisons against experimental observations of methane absorption at high temperatures have been used to demonstrate the accuracy of the new work. The line list includes both strong lines and quasi-continuum features and is provided in the common user-friendly HITRAN/HITEMP format, making it the most practical methane line list for radiative transfer modeling at high-temperature conditions.

Measurement of the spectral line positions in the 2v3 R(6) manifold of methane

A spectral line list for methane in the 6075.3–6078 cm−1 region with the intensity cutoff of 1 × 10−26 cm/molecule at 296 K is presented. We have accurately measured 96 line positions by using cavity ring-down spectroscopy with temperature-controlled cavity. The accuracy of most line positions exceeds that of previous studies. The improvement is due mainly to the accurate line positions obtained from recording high-resolution saturated absorption (Lamb-dip) spectra.

A hybrid line list for CH4 and hot methane continuum.

Molecular line lists (a catalogue of transition frequencies and line strengths) are important for modelling absorption and emission processes in atmospheres of different astronomical objects, such as cool stars and exoplanets. In order to be applicable for high temperatures, line lists for molecules like methane must contain billions of transitions, which makes their direct (line-by-line usage) application in radiative transfer calculations impracticable. Here we suggest a new, hybrid line list format to mitigate this problem, based on the idea of temperature-dependent absorption continuum. The line list is partitioned into a large set of relatively weak lines and a small set of important, stronger lines. The weaker lines are then used either to construct a temperature-dependent (but pressure-independent) set of intensity cross sections or are blended into a greatly reduced set of 'super-lines'. The strong lines are kept in the form of temperature-independent Einstein A coefficients. A line list for methane (CH4) is constructed as a combination of 17 million strong absorption lines relative to the reference absorption spectra and a background methane continuum in two temperature-dependent forms of cross sections and super-lines. This approach significantly eases the use of large high temperature line lists as the computationally expensive calculation of pressure- dependent profiles (e.g. Voigt) only need to be performed for a relatively small number of lines. Both the line list and cross sections were generated using a new 34 billion methane line list (known as 34to10), which extends the 10to10 line list to higher temperatures (up to 2000 K). The new hybrid scheme can be applied to any large line lists containing billions of transitions. We recommend using super-lines generated on a high resolution grid based on a resolving power of R = 1,000,000 to model the molecular continuum as a more flexible alternative to the temperature-dependent cross sections.

Spectral lines of methane measured up to 2.6 THz at sub-MHz accuracy with a CW-THz photomixing spectrometer: Line positions of rotational transitions induced by centrifugal distortion

Several Doppler-limited rotational transitions of methane induced by centrifugal distortion have been measured with an unprecedented frequency accuracy using a THz photomixing synthesizer based on a frequency comb. Compared to previous synchrotron based FT-Far-IR measurements of Boudon et al. (Ref. [1]), the accuracy of the line frequency measurements is improved by one order of magnitude; this yields a corresponding increase of two orders of magnitude to the weighting of these transitions in the global fit. The rotational transitions in the ν4←ν4 hot band are measured for the first time by the broad spectral coverage of the photomixing CW-THz spectrometer providing access up to R(5) transitions at 2.6THz. The new global fit including the present lines has been used to update the methane line list of the HITRAN database. Some small, but significant variations of the parameter values are observed and are accompanied by a reduction of the 1-σ uncertainties on the rotational (B0) and centrifugal distortion (D0) constants.

An empirical line list for methane near 1 µm (9028–10,435 cm−1)

The highly congested absorption spectrum of methane near 1µm was recorded by Fourier Transform Spectroscopy at room temperature. The detectivity threshold of the recordings corresponds to an intensity cut off on the order of 2×10−26cm/molecule. An empirical line list is constructed for the 9028–10,435cm−1 spectral region. It consists of (i) 7212 lines in the 9520–10,435cm−1 interval corresponding to the tetracontad and (ii) 574 lines in the 9028–9200cm−1 interval corresponding to the high energy region of the triacontad, dominated by the 3ν3 band near 9046cm−1. The first line list fills an important gap in the HITRAN database.The obtained data set constitutes a solid basis for future rovibrational assignments on the basis of advanced theoretical methods.

An empirical line list for methane in the 1.25 μm transparency window

The high resolution absorption spectrum of methane has been recorded at room temperature by high sensitivity Cavity Ring Down Spectroscopy between 7908.5 and 8345cm−1. The investigated spectral region corresponds to the high energy part of the 1.25μm transparency window where experimental measurements were missing. The extremely congested spectrum is mainly due to the superposition of weak bands of the triacontad in strong interaction. An empirical list of more than 12700 absorption features is built from a multiline fit of the spectrum.

Spectrum of hot methane in astronomical objects using a comprehensive computed line list.

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.

An improved empirical line list for methane in the region of the 2ν3 band at 1.66 μm

The absorption spectrum of methane is recorded at room temperature by differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5855–6183cm−1) dominated by the 2ν3 band at 1.66μm. 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 (αmin≈1×10−7cm−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−1range. J Quant Spectrosc Radiat Transfer 2010;111:2211–24). Lines due to the 13CH4 and CH3D isotopologues present in the methane sample in “natural” abundance (0.011 and 5.0×10−4, respectively) are systematically identified by comparison with spectra of pure CH3D and 13CH4 recorded separately. In order to derive empirical values of the lower state energy level, Eemp, from the intensity ratios of the lines measured at 80K and 296K, the obtained 296K line list is combined with the WKMC list of methane at 80K (Campargue A, Leshchishina O, Wang L, Mondelain D, Kassi S, Nikitin AV. Refinements of the WKMC empirical line lists (5852–7919 cm−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 Eemp 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 80K and 300K. A number of discrepancies between the rovibrational assignments included in the GOSAT list and our derived Eemp 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 12CH4.

Refinements of the WKMC empirical line lists (5852–7919 cm−1) for methane between 80 K and 296 K

During the last 4 years, empirical line lists for methane at room temperature and at 80K were constructed from spectra recorded by (i) differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5852−6195cm−1) and in the icosad (6717–7589cm−1) and (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58μm and 1.28μm transparency windows (6165–6750cm−1 and 7541–7919cm−1, respectively). We have recently constructed the global line lists for methane in “natural” isotopic abundance, covering the spectral region from 5854 to 7919cm−1 (Campargue A, Wang L, Kassi S, Mondelain D, Bézard B, Lellouch E, et al., An empirical line list for methane in the 1.26–1.71μm region for planetary investigations (T=80–300K). Application to Titan, Icarus 219 (2012) 110–128). These WKMC (Wang, Kassi, Mondelain, Campargue) empirical lists include about 43,000 and 46,420 lines at 80±3K and 296±3K, respectively. The “two temperature method” provided lower state energy values, Eemp, for about 24,000 transitions allowing us to account satisfactorily for the temperature dependence of the methane absorption over the considered region. The obtained lists have been already successfully applied in a large range of temperature conditions existing on Titan, Uranus, Pluto, Saturn and Jupiter.In the present contribution, we provide some improvements to our lists by using literature data to extend the set of lower state energy values and by correcting the distortion of the high Eemp values (J>10) due to the temperature gradient existing in the cryogenic cell used for the recordings. The proposed refinements are found to have an overall limited impact but they may be significant in some spectral intervals below 6500cm−1.The new version of our lists at 80K and 296K is provided as Supplementary Material: the WKMC@80K+ and WKMC@296K lists are adapted for planetary and atmospheric applications, respectively. The WKMC@80K+ list is made applicable over a wider range of temperatures and shows satisfactory extrapolation capabilities up to room temperature. It was obtained by transferring to the 80K list the 27,580 single lines present only in the 296K list, with corresponding lower state energy values chosen to make them below the detectivity limit at 80K.In the discussion, the different line lists and databases available for methane in the near infrared are compared and some suggestions are given.

An empirical line list for methane in the 1.26–1.71 μm region for planetary investigations (T = 80–300 K). Application to Titan

Insufficient knowledge of the near infrared spectrum of methane is an important limitation for the analysis of the spectra of Titan and of the outer planetary atmospheres in general. The work reported here is the result of a long-term project aiming to provide astronomers with a line by line list for precise calculations of the methane absorption in the near infrared region. We thus present here our best to date empirical line list between 5854 and 7919cm−1 (1.71–1.26μm) and apply it to Titan, demonstrating its capability to significantly improve planetary spectral analysis.In recent contributions, we have obtained empirical line lists at room temperature and at 80K (suitable for Titan conditions) from spectra recorded by (i) Differential Absorption Spectroscopy (DAS) in the high energy part of the tetradecad (5854–6195cm−1) and in the icosad (6717–7589cm−1) (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58μm and 1.28μm transparency windows (6165–6750cm−1 and 7541–7919cm−1, respectively). In this work, we construct the global line lists for methane in “natural” isotopic abundance, covering the entire spectral region from 5854 to 7919cm−1. These WKMC (for Wang, Kassi, Mondelain, Campargue) empirical lists include 42,988 and 46,320 lines at 80±3K and 296±3K, respectively and are assembled here with some important improvements:(i)New spectroscopic parameters for the 5854–6148cm−1 region at 80K which increase significantly the number of observations and lower state energy determinations.(ii)Transitions of 13CH4 and CH3D were systematically identified by comparison with DAS spectra of highly enriched 13CH4 and CH3D, recorded at the same temperatures.(iii)In the 1.58μm transparency window where CH3D lines contribute importantly to the methane spectrum at 80K, the set of CH3D lower state energy values was completed by using recent DAS results for pure CH3D.The “two temperature method” provided lower state energy values for about 24,000 transitions from the ratios of their line intensities at 80K and 296K. The clear propensity of the derived low J values of 12CH4 and 13CH4 to be integer illustrates the quality of the lower state energy values. The obtained data sets allow us to account for most of the temperature dependence of the absorption over the considered region.To illustrate the interest of the WKMC line lists for planetary applications, we perform simulations of Titan spectra at different resolutions taken from the ground with instruments such as the FTS at the CFHT and CRIRES at the VLT or by the DISR instrument on board the Huygens probe. The agreement between the simulations and the observations clearly demonstrates an important improvement with respect to previous works.

The near-IR spectrum of Titan modeled with an improved methane line list

We have obtained spatially resolved spectra of Titan in the near-infrared J, H and K bands at a resolving power of ∼5000 using the near-infrared integral field spectrometer (NIFS) on the Gemini North 8 m telescope. Using recent data from the Cassini/Huygens mission on the atmospheric composition and surface and aerosol properties, we develop a multiple-scattering radiative transfer model for the Titan atmosphere. The Titan spectrum at these wavelengths is dominated by absorption due to methane with a series of strong absorption band systems separated by window regions where the surface of Titan can be seen. We use a line-by-line approach to derive the methane absorption coefficients. The methane spectrum is only accurately represented in standard line lists down to ∼2.1 μm. However, by making use of recent laboratory data and modeling of the methane spectrum we are able to construct a new line list that can be used down to 1.3 μm. The new line list allows us to generate spectra that are a good match to the observations at all wavelengths longer than 1.3 μm and allow us to model regions, such as the 1.55 μm window that could not be studied usefully with previous line lists such as HITRAN 2008. We point out the importance of the far-wing line shape of strong methane lines in determining the shape of the methane windows. Line shapes with Lorentzian, and sub-Lorentzian regions are needed to match the shape of the windows, but different shape parameters are needed for the 1.55 μm and 2 μm windows. After the methane lines are modeled our observations are sensitive to additional absorptions, and we use the data in the 1.55 μm region to determine a D/ H ratio of 1.77 ± 0.20 × 10 −4, and a CO mixing ratio of 50 ± 11 ppmv. In the 2 μm window we detect absorption features that can be identified with the ν 5 + 3 ν 6 and 2 ν 3 + 2 ν 6 bands of CH 3D.

The 1.58 μm transparency window of methane (6165–6750 cm −1): Empirical line list and temperature dependence between 80 and 296 K

The empirical line parameters of over 12,000 methane transitions have been obtained at 80 K in the 1.58 μm transparency window (6165–6750 cm −1) which is of importance for planetary applications. This line list (WKC-80K) was constructed from high sensitivity spectra of normal abundance methane recorded by CW-Cavity Ring Down Spectroscopy at low temperature. The minimum intensity reported is on the order of 5×10 −30 cm/molecule. High resolution Fourier transform spectra have also been recorded using enriched CH 3D samples at 90–120 K in order to facilitate identification of monodeuterated methane features in the methane line list at 80 K. The CH 3D relative contribution in the considered region is observed to be much larger at 80 K than at room temperature. In particular, CH 3D is found dominant in a narrow spectral window near 6300 cm −1 corresponding to the highest transparency region. Using a similar line list constructed at room temperature (Campargue A, Wang L, Liu AW, Hu SM and Kassi S. Empirical line parameters of methane in the 1.63–1.48 μm transparency window by high sensitivity Cavity Ring Down Spectroscopy. Chem Phys 2010;373:203–10.), the low energy values of the transitions observed both at 80 K and at room temperature were derived from the variation of their line intensities. Empirical lower states and J-values have been obtained for 5671 CH 4 and 1572 CH 3D transitions representing the most part of the absorbance in the region. The good quality of these derived energy values is demonstrated by the marked propensity of the corresponding CH 4 lower state J values to be close to integers. The WKC line lists at 80 K and room temperature provided as Supplementary Material allow one accounting for the temperature dependence of methane absorption between these two temperatures. The importance of the 80 K line list for the study of Titan and other methane containing planetary atmospheres is underlined and further improvements are proposed. The resulting information will advance the theoretical modeling of the methane spectrum in the 1.58 μm transparency window.

Hot methane spectra for astrophysical applications

We have recorded laboratory high resolution Fourier transform spectra of methane (CH 4) emission at 800, 1000 and 1273 K with an effective hot path length of 51 cm . The spectra span 2000– 5000 cm −1 (resolution 0.020 cm −1 , nominal pressure 16.3 Torr ) and 5000– 6400 cm −1 (resolution 0.040 cm −1 , nominal pressure 79 Torr) for each temperature. We have determined line positions and intensities for 15 434, 22 163 and 25 727 emission lines in the 800, 1000 and 1273 K spectra, respectively. Positions have been calibrated to a 1 σ accuracy of better than 0.001 cm −1 in the 2000– 5000 cm −1 spectra and 0.003 cm −1 in the 5000– 6400 cm −1 spectra based on the 2001 methane line list update in the HITRAN database. We have calculated absolute line intensity calibration factors for these lines based on HITRAN intensities scaled to the appropriate temperature. In addition, since some methane lines in our spectra appeared in absorption, we have directly used intensities calculated from HITRAN for 757, 839 and 1094 additional lines in the 800, 1000 and 1273 K spectra, respectively. We expect our new hot methane line lists to be useful for direct comparison and simulation of the spectral energy distribution from brown dwarfs and hot extrasolar giant planets, and may also be used for comparison with line lists generated by both ab initio quantum chemists and more traditional theoretical spectroscopists. Furthermore, the method we have developed for obtaining high temperature line lists can be applied to other molecules in the HITRAN database.