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

doi:10.1016/j.jqsrt.2012.12.004

Elsevier

Journal of Quantitative Spectroscopy and Radiative Transfer

Volume 118, March 2013, Pages 49-59

Journal of Quantitative Spectroscopy and Radiative Transfer

https://doi.org/10.1016/j.jqsrt.201212004 Get rights and content

Abstract

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⁻¹) dominated by the 2ν₃ band at 1.66 μm. 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 (α_min×ばつ10⁻⁷ cm⁻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⁻¹ range. 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 ×ばつ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, 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⁻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 ¹²CH₄.

Highlights

► New absorption spectra of methane at 296 K in the 2ν₃ region have been recorded. ► An empirical list of 9228 lines at 296 K is constructed including about 4000 new lines. ► The 2-temperature method is applied to this list combined to the WKMC list at 80 K. ► 4396 empirical lower state energy values are determined. ► The obtained results improve significantly the GOSAT list in the region.

Introduction

We have recently constructed empirical lists at 80±3 K and 296±3 K for methane in "natural" isotopic abundance in the 5852–7919 cm⁻¹ region for applications in planetary and Earth atmospheric science, respectively [1], [2]. The positions and intensities included in the WKMC (Wang, Kassi, Mondelain, Campargue) lists were obtained from spectra recorded at 80 K and room temperature by (i) differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5852–6195 cm⁻¹) [1], [3], [4] and in the icosad (6717–7589 cm⁻¹) [5], [6], [7], [8] and (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58 μm [9], [10], [11], [12], [13] and 1.28 μm [14] transparency windows (6165–6750 cm⁻¹ and 7541–7919 cm⁻¹, respectively). The WKMC lists in Refs. [1], [2] include about 43,000 and 46,420 lines at 80 K and 296 K, respectively. The "two temperature (2T) method" [15], [16] which consists in deriving empirical values of the lower state energy level, E_emp, from the ratio of the line intensities measured at 80 K and 296 K, has provided E_emp values for about 24,000 transitions in common in the two datasets. This set of E_emp values allowed us to satisfactorily account for the temperature dependence of the methane absorption between 80 K and 300 K. The obtained lists have already been successfully applied to a large range of temperature conditions existing on Titan [2], [17], [18], Pluto [19], Uranus [20], Saturn [21] and Jupiter [21].

The 5852.5–6165.7 cm⁻¹ interval of the tetradecad at 296 K is the only region for which we did not construct a new list at 296 K because the GOSAT list of methane constructed in relation with the Greenhouse gases Observing SATellite (GOSAT) project was available [22] (see Fig. 1). The GOSAT list was obtained from FTS measurements in the 5550–6236 cm⁻¹ region. In the 5852.5–6165.7 cm⁻¹ interval in common with the WKMC list, the GOSAT list provides 5110 lines at 296 K while our DAS list at 80 K includes 7797 lines. Three thousand eighty three E_emp values were derived by applying the 2T-method to the GOSAT intensities at 296 K and our DAS intensities at 80 K [1], leaving without known temperature dependence about 2000 (40%) and 4700 (60%) line intensities at 296 K and 80 K, respectively. The lines left without empirical value of their lower state energy are those measured only at 80 K or at 296 K. Even if these "single lines" are weak or super weak (they represent only a few percent of the total absorption), it is desirable to determine their lower state energy levels.

Fig. 1 shows an overview of the GOSAT list from Ref. [22]. The weakest lines have an intensity on the order of ×ばつ10⁻²⁶ cm/molecule but a large number of line intensity values are obviously rounded values such as ×ばつ10⁻²⁵, ×ばつ10^–25 up to ×ばつ10^–24 cm/molecule (see the corresponding aligned dots on Fig. 1) indicating that they are estimate values with large uncertainties. These inaccurate intensities have a direct impact on the retrieval quality of the lower state energy computed by the 2T-method.

In order to improve our set of E_emp values both in quality and quantity, we decided to construct a higher sensitivity room temperature empirical line list from DAS recordings at 296 K. The gain in sensitivity is expected to allow for new E_emp determinations by providing counterparts at 296 K to the single lines of the 80 K dataset.

After a brief description of the experiment details (Section 2), the positions and intensities at room temperature will be derived and compared to GOSAT list in Section 3. Then in Section 4, lines due to the CH₃D and ¹³CH₄ isotopologues present in "natural" abundance in the methane sample will be systematically identified. The set of E_emp values will be derived by the 2T-method in Section 5 and compared, to the results previously obtained [1], [22].

Section snippets

Experiment

The high sensitivity absorption spectrum of methane was recorded at room temperature by differential absorption spectroscopy between 5855 and 6183 cm⁻¹ (1.71–1.62 μm). The experimental setup has been described in details in Ref. [16]. Briefly, a series of 14 DFB fibered diode lasers (20 mW butterfly mounted, by NEL) is used as light sources allowing for a continuous coverage of the 5855–6183 cm⁻¹ range. Each DFB laser diode is tuned over about 30 cm⁻¹ by a slow temperature scan from −10 to 60 °C

Line intensity retrieval

The line centers and intensities were determined using an interactive least squares multi-line fitting program assuming a Voigt profile (http://sourceforge.net/projects/fityk/ version v 1.1.1). Note that the DFB line width (10 MHz at most) is much smaller than the Doppler broadening [350 MHz HWHM] and therefore neglected. The width of the Gaussian component was fixed to the theoretical value of the Doppler width of ¹²CH₄. Considering the low pressure of the recordings, the pressure broadening is

Isotopologue identification

Contrary to the methane transparency windows where the contribution of the minor isotopologues (¹³CH₄ and CH₃D) may largely exceed their relative abundance [10], [11], [16], in the considered region of strong absorption, the ¹³CH₄ and CH₃D contributions scale roughly according to their respective relative abundances.

The ¹³CH₄ transitions of our methane list were marked one by one by comparing with the ¹³CH₄ line list attached to Ref. [27]. This list was constructed from spectra of highly

Determination of the lower state energy

The intensity of an absorption line follows the temperature dependence of the population of the lower state energy given by the Boltzmann law. The Boltzmann factor and then the E_emp lower state energy of a given transition can then be obtained from the ratio of its line intensity at 80 K and 296 K [15], [16]:

\frac{S_{ν_{0}} (T_{1})}{S_{ν_{0}} (T_{0})} = \frac{Z (T_{0})}{Z (T_{1})} \exp [- E_{e m p} (\frac{1}{k T_{1}} - \frac{1}{k T_{0}})]

where

S_{v_{0}}

and Z are the line intensities and partition functions, respectively, and T₀=296 K and T₁=80 K in our case. Following previous studies by

Comparison to the GOSAT assignments

Considering the number of interacting vibrational states (14 levels, 60 sublevels), the theoretical modeling of the tetradecad region of methane in the frame of the effective operators approach is a considerable task. This project is in progress but the theoretical approach is not yet able to reproduce the experimental spectrum. Nevertheless, Nikitin et al. [22] could attach a high number of rovibrational assignments to the lines of the GOSAT empirical list. In our region (5855–6183 cm⁻¹), 2279

Conclusion

The positions and strengths at 296 K of 9228 transitions were obtained from the analysis of the spectrum of methane recorded by direct absorption spectroscopy between 5855 and 6183 cm⁻¹. On the basis of line position coincidences, 4396 transitions were found in common with the DAS list at 80 K (list WKMC@80K from Ref. [1]) allowing to determine the corresponding lower state energies by using the 2T-method. Compared to the empirical list constructed in the frame of the GOSAT project [22], the

Acknowledgments

This work is part of the ANR project "CH4@Titan" (ref: BLAN08-2_321467). The support of the Groupement de Recherche International SAMIA (Spectroscopie d'Absorption des Molécules d'Intérêt Atmosphérique) between CNRS (France), RFBR (Russia) and CAS (China) is acknowledged.

References (30)

A. Campargue et al.
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)
A. Campargue et al.
An empirical line list for methane in the 1.26–1.71 μm region for planetary investigations (T=80–300 K). Application to Titan
Icarus
(2012)
B. Gao et al.
Empirical low energy values for methane transitions in the 5852–6181 cm⁻¹ region by absorption spectroscopy at 81 K
J Mol Spectrosc
(2009)
L. Wang et al.
Temperature dependence of the absorption spectrum of CH₄ in the region of the 2ν₃ band at 1.66 μm by absorption spectroscopy at 81 K
J Quant Spectrosc Radiat Transfer
(2010)
E. Sciamma-O'Brien et al.
Experimental low energy values of CH4 transitions near 1.33 μm by absorption spectroscopy at 81 K
J Quant Spectrosc Radiat Transfer
(2009)
A. Campargue et al.
Temperature dependence of the absorption spectrum of CH₄ by high resolution spectroscopy at 81 K: (II) The Icosad region (1.49–1.30 μm)
J Quant Spectrosc Radiat Transfer
(2010)
L. Wang et al.
The absorption spectrum of methane at 80 and 294 K in the icosad (6717–7589 cm⁻¹): Improved empirical line lists, isotopologue identification and temperature dependence
J Quant Spectrosc Radiat Transfer
(2011)
A.W. Liu et al.
High sensitivity CW-cavity ring down spectroscopy of CH₄ in the 1.55 μm transparency window
Chem Phys Lett
(2007)
L. Wang et al.
High sensitivity absorption spectroscopy of methane at 80 K in the 1.58 μm transparency window: temperature dependence and importance of the CH₃D contribution
J Mol Spectrosc
(2010)
A. Campargue et al.
Empirical line parameters of methane in the 1.63–1.48 μm transparency window by high sensitivity Cavity Ring Down Spectroscopy
Chem Phys
(2010)

A.V. Nikitin et al.

First assignment of the 5ν₄ and ν₂+4ν₄ band systems of ¹²CH₄ in the 6287–6550 cm⁻¹ region

J Quant Spectrosc Radiat Transfer

(2011)

L. Wang et al.

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

J Quant Spectrosc Radiat Transfer

(2011)

Y. Lu et al.

The CH₃D absorption spectrum in the 1.58 μm transparency window of methane: empirical line lists and temperature dependence between 80 K and 296 K

J Quant Spectrosc Radiat Transfer

(2011)

J. Bailey et al.

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

Icarus

(2011)

L.A. Sromovsky et al.

Comparison of line-by-line and band models of near-IR methane absorption applied to outer planet atmospheres

Icarus

(2012)

Cited by (32)

The 2015 edition of the GEISA spectroscopic database
2016, Journal of Molecular Spectroscopy
The GEISA database (Gestion et Etude des Informations Spectroscopiques Atmosphériques: Management and Study of Atmospheric Spectroscopic Information) has been developed and maintained by the ARA/ABC(t) group at LMD since 1974. GEISA is constantly evolving, taking into account the best available spectroscopic data. This paper presents the 2015 release of GEISA (GEISA-2015), which updates the last edition of 2011 and celebrates the 40th anniversary of the database. Significant updates and additions have been implemented in the three following independent databases of GEISA.
The "line parameters database" contains 52 molecular species (118 isotopologues) and transitions in the spectral range from 10⁻⁶ to 35,877.031 cm⁻¹, representing 5,067,351 entries, against 3,794,297 in GEISA-2011. Among the previously existing molecules, 20 molecular species have been updated. A new molecule (SO₃) has been added. HDO, isotopologue of H₂O, is now identified as an independent molecular species. Seven new isotopologues have been added to the GEISA-2015 database.
The "cross section sub-database" has been enriched by the addition of 43 new molecular species in its infrared part, 4 molecules (ethane, propane, acetone, acetonitrile) are also updated; they represent 3% of the update. A new section is added, in the near-infrared spectral region, involving 7 molecular species: CH₃CN, CH₃I, CH₃O₂, H₂CO, HO₂, HONO, NH₃.
The "microphysical and optical properties of atmospheric aerosols sub-database" has been updated for the first time since 2003. It contains more than 40 species originating from NCAR and 20 from the ARIA archive of Oxford University.
As for the previous versions, this new release of GEISA and associated management software facilities are implemented and freely accessible on the AERIS/ESPRI atmospheric chemistry data center website.
Methane line parameters in the HITRAN2012 database
2013, Journal of Quantitative Spectroscopy and Radiative Transfer
Citation Excerpt :
Empirical values of the lower state energy level, Eemp, of the transitions that appeared in both the 80 K and 296 K lists were derived by the so-called "two temperature (2T) method" i.e. from the ratio of the line intensities measured at 80 K and 296 K [51,52]. The "final" version of the WKLMC empirical lists for methane at 80 K and 296 K were recently released [32] by merging a previous version of the list [36] with new results obtained by DAS in the 2ν3 region (5852–6183 cm−1) [37]. Except for very accurate 12CH4 frequencies from Zolot et al. [53] inserted for 112 positions of the 2ν3 multiplets, the HITRAN2012 list in the 5855.15–7918.94 cm−1 region matches the positions and intensities in the WKLMC list at 296 K attached to Ref. [32].
The compilation of methane molecular line parameters was updated to include new global analyses and measurements for ¹²CH₄, ¹³CH₄ and ¹²CH₃D. Over 70% of the methane parameters in HITRAN2008 were replaced; existing parameters retained were the microwave lines and the Dyad of ¹³CH₄ near 7 μm and ν₆ of ¹³CH₃D near 8.7 μm, ¹²CH₃D (7–4076 cm⁻¹), hot bands of ¹²CH₄ (1887–3370 cm⁻¹) and normal sample CH₄ (4800–5550 cm⁻¹ and 8000–9200 cm⁻¹). With a minimum intensity at 296 K in units of cm⁻¹/(molecule cm⁻²) set to 10^–37 for the far-IR and 10^–29 for the mid- and near-IR, the methane database increased from 290,091 lines in HITRAN2008 to 468,013 lines, and three-fourths of these involved the main isotopologue. For ¹²CH₄ and ¹³CH₄, bands from the ground state were revised up to 4800 cm⁻¹. For the first time, ¹³CH₄ and ¹²CH₃D line parameters near 2.3 μm were included. Above 5550 cm⁻¹, the new compilation was based on empirical measurements. Prior laboratory results were replaced with extensive new measurements using FTIR (5550–5852 cm⁻¹), differential absorption spectroscopy (DAS) and Cavity Ring Down Spectroscopy (CRDS) (5852–7912 cm⁻¹). Ground state J values for nearly half of the measured lines in this range were obtained, either by confirming quantum assignments of analyses or by using spectra at 80 and 296 K. Finally, over 11,000 measured positions, intensities and empirical lower state energies (obtained using cold CH₄) were also added for the first time between 10,923 and 11,502 cm⁻¹. Available pressure broadening measurements from HITRAN2008 were transferred into the new compilation, but 99% of the lines were given crudely-estimated coefficients. New measured intensities and broadening coefficients were included for far-IR transitions, and high accuracy line positions were inserted for the stronger P, Q and R branch transitions of ν₃ at 3.3 μm and 2ν₃ at 1.66 μm.
New dipole moment surfaces of methane
2013, Chemical Physics Letters
New dipole moment surfaces (DMS) of methane are constructed using extended ab initio CCSD(T) calculations at 19 882 nuclear configurations. The DMS analytical representation is determined through an expansion in symmetry adapted products of internal nonlinear coordinates involving 967 parameters up to the 6th order. Integrated intensities of seven lower polyads up to J = 30 for ¹²CH₄ and ¹³CH₄ are in a good agreement with the HITRAN 2008 database, and with other available experimental data.
The WKLMC empirical line lists (5852-7919 cm-1) for methane between 80 K and 296 K: "Final" lists for atmospheric and planetary applications
2013, Journal of Molecular Spectroscopy
The "final" version of the WKLMC (Wang, Kassi, Leshchishina, Mondelain, Campargue) empirical lists for methane at 80 K and 296 K is provided for the 5852–7919 cm⁻¹ region. It was obtained by combining the WKMC list [A. Campargue, O. Leshchishina, L. Wang, D. Mondelain, S. Kassi, A.V. Nikitin, J. Quant. Spectrosc. Radiat. Transfer 113 (2012) 1855–1873] with new results obtained by differential absorption spectroscopy (DAS) in the 2ν₃ region (5852–6183 cm⁻¹) [A. Campargue, O. Leshchishina, D. Mondelain, S. Kassi and A. Coustenis, J. Quant. Spectrosc. Radiat. Transfer 118 (2013) 49–59]. Two empirical lists for methane at 296 K and 80 K in "natural" abundance are provided for atmospheric and planetary applications, respectively. They include the isotopologue identification (¹²CH₄, ¹³CH₄ and CH₃D) and the lower state energy level, E_emp, derived from the ratio of the line intensities measured at 296 K and 80 K. The 296 K list includes 55 262 absorption lines. The 80 K list is made applicable over a wider range of temperatures by including 30 282 lines observed only at 296 K, with corresponding lower state energy values chosen to make them below the detectivity limit at 80 K. Overall, the 80 K list includes 73 225 absorption lines. For the two temperatures, we provide the list in HITRAN format together with a file allowing for a line by line traceability of the listed spectroscopic parameters. The extrapolation capabilities of the 296 K list towards high temperature (∼1200 K) are discussed.
Spectrum of hot methane in astronomical objects using a comprehensive computed line list
2014, Proceedings of the National Academy of Sciences of the United States of America
ExoMol line lists-IV. The rotation-vibration spectrum of methane up to 1500 K
2014, Monthly Notices of the Royal Astronomical Society

View all citing articles on Scopus

View full text

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

Abstract

Highlights

Introduction

Section snippets

Experiment

Line intensity retrieval

Isotopologue identification

Determination of the lower state energy

Comparison to the GOSAT assignments

Conclusion

Acknowledgments

J Quant Spectrosc Radiat Transfer

Icarus

J Mol Spectrosc

J Quant Spectrosc Radiat Transfer

J Quant Spectrosc Radiat Transfer

J Quant Spectrosc Radiat Transfer

J Quant Spectrosc Radiat Transfer

Chem Phys Lett

J Mol Spectrosc

Chem Phys

J Quant Spectrosc Radiat Transfer

J Quant Spectrosc Radiat Transfer

J Quant Spectrosc Radiat Transfer

Icarus

Icarus

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