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The 13CH4 absorption spectrum at 80 K: Assignment and modeling of the lower part of the Tetradecad in the 4970–5470 cm−1 spectral range
Accepted Manuscript
The 13 CH4 absorption spectrum at 80 K: Assignment and modeling of the lower part of the Tetradecad in the 4970–5470 cm−1 spectral range Evgeniya Starikova , Keeyoon Sung , Andrei V. Nikitin , Michael Rey , Arlan W. Mantz , Mary Ann H. Smith PII: DOI: Reference:
S0022-4073(17)30653-2 10.1016/j.jqsrt.2017.11.022 JQSRT 5913
To appear in:
Journal of Quantitative Spectroscopy & Radiative Transfer
Received date: Revised date: Accepted date:
22 August 2017 23 November 2017 23 November 2017
Please cite this article as: Evgeniya Starikova , Keeyoon Sung , Andrei V. Nikitin , Michael Rey , Arlan W. Mantz , Mary Ann H. Smith , The 13 CH4 absorption spectrum at 80 K: Assignment and modeling of the lower part of the Tetradecad in the 4970–5470 cm−1 spectral range, Journal of Quantitative Spectroscopy & Radiative Transfer (2017), doi: 10.1016/j.jqsrt.2017.11.022
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Highlights: Obtained a high resolution 13CH4 spectrum at 80 K in the 4970-6200 cm-1 region. Assigned 1387 line positions and modeled the lower part of Tetradecad to the rms deviation being 0.002 cm-1. Fit 737 measured line intensities to the rms deviation of 10%. Compiled experimental and theoretical line lists at 80 K with assignments.
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The 13CH4 absorption spectrum at 80 K: Assignment and modeling of the lower part of the Tetradecad in the 4970–5470 cm-1 spectral range Evgeniya Starikova1,2*, Keeyoon Sung3, Andrei V. Nikitin1,2, Michael Rey4, Arlan W. Mantz5, Mary Ann H. Smith6 1
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Laboratory of Theoretical Spectroscopy of IAO SB RAN, av. 1, AkademicianZuev square, 634021 Tomsk, Russia 2 Laboratory of Quantum Mechanics of Molecules and Radiative Processes, Tomsk State University, 36 Lenin Avenue, 634050 Tomsk, Russia 3 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 4 GSMA, UMR CNRS 7331, Université de Reims Champagne Ardenne, Moulin de la Housse, BP 1039 51687 Reims Cedex 2, France 5 Department of Physics, Astronomy and Geophysics, Connecticut College, New London, CT, USA 6 Science Directorate, NASA Langley Research Center, Hampton, VA 23681, USA
* Corresponding author: [email protected] Abstract
A 13C-enriched spectrum of methane (13CH4) was recorded at Doppler-limited resolution (0.0044 cm-1) at 80 K covering the entire Tetradecad region in the 4970-6200 cm-1. In this paper we 13
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present the spectral analysis of the lower part of the
CH4 Tetradecad in the 4970-5470 cm-1
region. Starting with a non-empirical effective Hamiltonian derived by high-order Contact
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Transformations (CT) from the ab initio potential energy surface (PES), the effective Hamiltonian parameters have been determined for this region by extending the previous DAS
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(Differential Absorption Spectroscopy) spectral analysis for the upper part of the Tetradecad in the 5853-6200 cm-1 region. In total, 1387 rovibrational transitions were assigned belonging to five cold bands of the Tetradecad up to Jmax=11. Their positions were fitted with an rms
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deviation of 1.610-3 cm-1. Measured line intensities for 737 transitions were modeled using the effective dipole transition moments approach with an rms deviation of about 10%. Finally, the
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observed transitions were incorporated to fit simultaneously the
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CH4 Hamiltonian parameters
for the {Ground state / Dyad / Pentad / Octad / Tetradecad} system and the dipole moment parameters for the {Ground state - Tetradecad} system. Keywords: methane; 13CH4; absorption spectroscopy; spectra analyses; effective Hamiltonian; lower Tetradecad; variational calculations; HITRAN Running Head: 13CH4 absorption near 1.9 μm
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Number of pages: 17 Number of Figures: 4 Number of Tables: 4 Number supplemental files: 2
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1. Introduction
The study of methane absorption has attracted a continuously increasing interest both in laboratory high-resolution spectroscopy and in the quantum-mechanical global modeling for the remote sensing of planetary atmospheres [1, 2] and astrophysical applications [3-6]. Methane is the second strongest anthropogenic greenhouse gas following CO2 [7]. Accurate values of line
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positions and intensities are necessary to retrieve the methane concentration and the distribution profile by spectroscopic methods.
Measurements and analyses of microwave and infrared bands have been reported for 12
CH4 [8-16],
13
CH4 [17-23] and CH3D [24-28] (and references therein). Many of them were
either reported to the public domain including HITRAN and GEISA databases [29-34] or
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accessible via the VAMDC European web portal [35, 36]. However, further improvement of line parameters is still needed for an accurate radiative transfer modeling, particularly in the Octad
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and Tetradecad regions [37]. Measurements of methane isotopologues in the Earth’s atmosphere [38, 39] could provide powerful constraints in the identification of its specific sources and sinks. Several authors [40-46] have developed a geophysical model of CH4 mole fraction and the C/12C isotope ratio in methane to reconstruct the global history of CH4 emissions to the
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atmosphere. However, most of spectral analyses have focused on its principal isotopologue, 12
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CH4, leaving the spectroscopic data available for
13
C isotopic methane analysis much sparser.
A comprehensive review of the laboratory studies of methane until 2013 by Brown et al. [8]
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reported that the HITRAN 2012 database [32] contained much less experimental data for 13CH4 line-by-line spectroscopic parameters by factor of 4 for positions and by factor of 16 for intensities compared to that for 12CH4. Since then, a series of recent publications on 13CH4 have become available [19-23]. Ab initio predictions of isotopic methane spectra including vibrationrotation line positions and intensities have been performed by Rey et al. [47-49] using a variational method described in [50, 51], but experimental measurements and analyses are yet missing or incomplete in several spectral ranges of the overtone and combination bands, 3
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including the Tetradecad region of this study. Note that some vibrational levels of 12CH4, CH3D, CHD3, and 13CH4 isotopologues have been also calculated in Ref. [52] using an empirically fitted PES. The infrared spectra of 12CH4 and 13CH4 can be grouped in polyads [53] of closely lying bands because of the approximate relation between vibrational frequencies, i.e., ω1 ≈ ω3 ≈ 2ω2 ≈
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2ω4. Each polyad is defined by the integer polyad number P = 2(v1+v3)+v2+v4, where the (v1,v2,v3,v4) are principal vibrational normal mode quantum numbers. P = 0 corresponds to the ground vibrational state, and the polyads P=1, 2, 3, 4, 5, … correspond to the Dyad, Pentad, Octad, Tetradecad, Icosad, and so on in the increasing wavenumber intervals. Previously line-byline measurements and analyses of
13
CH4 have been reported by Champion et al. [54] in the
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Dyad range, by Jouvard et al. [55] in the Pentad range, and by Niderer et al. [17,18] and Brown et al. [20] in the Octad region. Starikova et al. [22] have assigned the 13CH4 line list constructed in Grenoble by laser direct absorption spectroscopy (DAS) at 80 K and 296 K in the upper part of the Tetradecad (5853-6201 cm-1). The corresponding line parameters have been included in the HITRAN 2016 release [34]. The analyses of the Tetradecad for the principal isotopologue 12
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CH4 were recently improved by Nikitin et al. [11, 16], but the spectral data of
13
CH4 for the
Tetradecad region are not yet complete. The present work is dedicated to the extension of the
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measurements and rovibrational assignments of the high-resolution spectrum of
13
CH4 in the
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lower part of Tetradecad in the 4970–5470 cm region.
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2. Experimental details
To this end, we recorded a
13
C-enriched spectrum at 79.8 K covering the entire
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Tetradecad region in the 4970-6200 cm-1 using the Bruker IFS-125HR Fourier transform spectrometer with a custom-built cryogenic multipass Herriott cell at the Jet Propulsion
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Laboratory. The Herriott cell is vacuum-coupled to the FT-IR spectrometer and has excellent temperature control. The total optical path length of the cell is 20.941(6) m. Other details of the configuration and performance of the cell can be found in [56].
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Table 1. Instrumental configuration and experimental conditions
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Interferometer (Bruker IFS-125HR) IR source Tungsten lamp (50 W) Beam splitter CaF2 Resolution 0.0044 cm-1 (unapodized) Aperture (diameter) 1.15 mm Focal length 418 mm (provided by the Bruker Optics) Optical filter range 4640 – 6200 cm-1 Detector InSb (LN2 cooled) FTS pressure < 0.010 hPa 13 Sample gas CH4 (13C, 99%) Cell pressure 5.112(1) Torr Cell Temperature 79.9(1) K Path length 20.941(6)m Cell window CaF2 (wedged) Vacuum box window CaF2 (wedged) $ More information can be found in [55].
As summarized in Table 1, the experimental setup consisted of a Tungsten lamp, a CaF2 beam splitter, and a LN2 cooled InSb detector. The band pass of the filter was 4650 – 6200 cm-1, broad
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enough to capture the entire region of the Tetradecad bands in one single spectrum, securing the best consistency for the transitions measured. The S/N ratio better than 500 was achieved by
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coadding multiple scans. A background empty-cell spectrum was also obtained under the same optics conditions, which was used to generate a transmittance spectrum. The cell temperature was monitored using a silicon-diode sensor throughout the data-taking period and the fluctuation
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or drift of the cell temperatures was kept less than ±0.1 K during the entire scanning. The 13CH4 spectrum was frequency-calibrated to the accuracy of 0.0006 cm-1 using a single frequency
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calibration factor derived from 42 features of water in the ν2+ν3 band [32] observed in the background spectrum. Since the water features were arising from residual water in the evacuated
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FTS chamber, not from any sample impurity in the cell, these residual water features were effectively cancelled out in the rationed (i.e. transmittance) spectrum, whose overview is presented in Fig. 1.
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Fig. 1. An overview of the 13CH4 spectrum recorded at 79.8 K, which captures all the cold bands in the Tetradecad region from 5000 to 6200 cm-1. In this work, we have focused on the lower Tetradecad region from 4970-5470 cm-1.
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3. Rovibrational assignment and modeling
As in our previous works on methane studies [11, 16, 22], the theoretical analysis was
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carried out using the effective Hamiltonian (EH) approach with the polyad vibration extrapolation scheme [53,57]. In general, the procedure of the assignment is based on preliminary calculations using potential energy surface (PES) and dipole moment surfaces
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(DMS) reported by Nikitin, Rey and Tyuterev [58, 59], hereafter referred to as NRT surfaces. The experimental data were initially analyzed using a non-empirical EH for the methane polyads
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constructed from the same NRT PES via high order Contact Transformation (CT) procedure [60, 61]. This algorithm permits us to obtain the high-accuracy resonance coupling parameters
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from the potential surface in order to eliminate the ambiguity of the purely empirical models [6264]. The detailed descriptions of the CT calculations with application to methane polyads and their comparison with pure ab initio calculations and experimental data for
12
CH4 have been
reported by Tyuterev et al. [61]. In this work, we apply the same approach for the experimental data reduction additionally using the assignments determined from the previous analysis of the 13
CH4 laser absorption spectrum in the 5853-6200 cm-1 region [19, 22], corresponding to the
upper part of the Tetradecad. This has greatly simplified the modeling of the 13CH4 spectrum in 6
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the region under investigation. In total, 1387 transitions out of 1521 observed lines were rovibrationally assigned in the 4970-5470 cm-1 region, most of which belong to the 4ν4 and ν2+3ν4 band systems, as illustrated in Fig. 2. For the modeling of the observed line positions and intensities we used the MIRS software [65, 66] designed for the calculations and fitting of the energy levels, line positions and
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intensities of polyatomic molecules in the representation of the irreducible tensor operators (ITO). The MIRS program provides both the direct spectra calculation and the fit of the experimental data by the least squares method. The use of the polyad extrapolation scheme to fit the Tetradecad data implies the inclusion of all the terms and the corresponding constants obtained in the previous analyses of all the lower polyads involving the Dyad (900–1700 cm-1),
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Pentad (2000–3200 cm-1) and Octad (3500–4800 cm-1). Throughout the spectrum identification and data modeling the visual control of assignment is necessary. For spectra visualization and graphical assignment of the high-resolution spectrum we used the publically available graphical
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tool, SpectraPlot [67].
Fig. 2. An overview of the assigned transitions in the 13CH4 spectrum recorded at 80 K between 4970 and 5470 cm-1. Different colors are used to indicate different band systems contributing to the spectrum.
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4. Data fit statistics The detailed statistics for our assigned transitions are summarized in Table 2, described in two separate parts. In the first one (A) we summarized the results of line positions and intensity fits of two band systems, located in the analyzed ranges: These are 4ν4 band system with seven sub-bands with the sub-band centers in the range 5093-5210 cm-1 region and the ν2+3ν4 band
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system with also seven sub-bands whose centres are located in the 5348-5440 cm-1 range. The second part (B) shadowed in grey corresponds to the fit results of the bands located above 5470 cm-1, in which only a few P-branch transitions of the ν1+2ν4, ν3+2ν4, and 2ν2+2ν4 band systems were assigned in the lower part Tetradecad region (see Fig. 2).
Only the lower part of the Tetradecad was modeled in the present work. At this stage, the
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6-th order Effective Hamiltonian contains 4195 parameters including 72 parameters of Dyad, 382 parameters of Pentad, 1202 parameters of Octad and 2539 parameters of Tetradecad. In total, 650 effective Hamiltonian parameters were adjusted to fit 12144 observed transitions covering Dyad, Pentad, Octad [17, 18, 20, 68] in addition to the lower part of Tetradecad range that was assigned in this work. Among them, 58, 210 and 322 parameters specific to the Dyad,
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Pentad and Octad, respectively, were fitted. Only 60 parameters specific for the lower part of the Tetradecad were adjusted in the present analysis; the parameters corresponding to the upper
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Tetradecad bands were fixed to the values as determined from the DAS spectrum analysis of the previous work [22] in the 5850-6200 cm-1 region. The interaction parameters of the lower part of
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the Tetradecad with other bands were not adjusted instead held fixed to their initial values predicted from the PES using MOL_CT code [61, 69]. The set of obtained parameters permits us to constrain the positions of 1387 observed transitions of the
13
CH4 Tetradecad to a maximum
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root mean square (rms) deviation of 1.6 ×10-3 cm-1. The fit residuals for the line positions are shown on the upper panel of Fig 3.
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To derive the parameters of the effective dipole transition moment operator the intensities
of 737 selected transitions of the observed bands were included in the fit. We were able to reproduce this set of selected line intensities with an rms deviation of 10%. Detailed statistics of the intensity fit for the two frequency ranges are given in Table 2, and the fit residuals for line intensities in the range 4970-5470 cm-1 are given in the lower panel of Fig. 3.
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The detailed description of Effective Hamiltonian construction will be discussed when the complete description of the 13CH4 Tetradecad in the full spectral range (4800-6200 cm-1) will
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be available in a future work.
Fig. 3. Fit residuals for line position (upper panel) and intensities (lower panel) in the 13CH4 lower part of the Tetadecad region. Different colors are used to indicate the different band systems contributing to the spectrum.
Band
Upper sublevel
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Table 2. An overview of the rovibrational assignments of the 13CH4 transitions obtained at 80 K between 4970 and 5470 cm-1 region and their corresponding line position and intensity fit statistics. Sub-band centre (cm-1)
Positions
Intensities
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Nb. rms Nb. rms Jmin, Jmax data (10-3 cm-1) data (%) -1 (A) analysis of bands located in the spectral range under investigation: 4970-5470 cm A1 1a 5093.2 20 3,7 1.3 7 9.7 F2 1b 5114.375 160 0,9 1.5 104 10.7 E 1c 5137.79 73 1,8 2.0 39 7.9 F2 2b 5180.681 102 0,7 2.0 61 10.0 4ν4 E 2c 5198.22 53 1,8 1.5 26 8.1 F1 5199.937 107 1,7 1.6 58 10.3 A1 2a 5209.6 16 5,8 2.2 8 14.8 F2 1d 5348.561 110 1,8 1.5 44 10.6 F1 1e 5367.605 109 1,8 1.9 58 10.3 E 5401.99 70 1,9 1.2 41 10.0 F2 2d 5406.464 109 0,9 1.3 67 9.8 ν2+3ν4 F1 2e 5414.24 95 1,9 1.1 56 9.4 F2 3d 5421.630 135 1,9 1.3 81 10.8 F1 3e 5439.50 98 1,7 1.4 50 11.8 (B) preliminary analysis of bands having center located in upper wavenumber range > 5470 cm-1 A1 5476.0 8 4,7 1.4 F2 5503.7 14 2,6 1.6 1 21.6 ν1+2ν4 E 5515.2 13 4,8 2.1 F2 1f 5563.4 22 7,9 1.2 5 8.1 A1 5582.4 12 5,8 1.3 8 5.8 ν3+2ν4 F2 2f 5590.0 14 6,8 1.5 4 13.3 F1 1g 5590.4 18 5,8 1.2 10 8.3
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2ν2+2ν4
E F1 2g F2 3f F1 F2
5591.4 5600.5 5601.9 5640.3 5653.3
5 10 6 3 5 1387
7,7 6,9 6,7 7,7 7,8
0.8 1.3 2.1 0.9 1.3 1.6
1 3 3 1 1 737
21.0 11.7 14.3 19.2 13.6 10.3
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Total Notes: The vibrational sublevels are enumerated here for each band for a given symmetry type. The ranking numbers within the Tetradecad are the following: a N=1 and N=2 for A1 1 and A1 2; b N=1 and N=2 for F2 1 and F2 2; c N=1 and N=2 for E 1 and E 2; d N=3, N=4 and N=5 for F2 1, F2 2 and F2 3; e N=2, N=3, and N=4 for F1 1, F1 2 and F1 3; f N=7, N=8 and N=9 for F2 1, F2 2 and F2 3; g N=5 and N=6 for F1 1 and F1 2.
Note that high-sensitivity laser measurements of 13CH4 have been reported in the Icosad range by Campargue et al. [23], but their line-by-line assignments are not yet available because of the complexity of the corresponding overlapping band patterns. The extension of the theory to these
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higher wavenumber ranges will be the aim of a further work.
5. Line lists
Using the determined parameters of the effective Hamiltonian and of the effective dipole transition moment we have compiled a list of 2993 transitions from the lower part of the
13
CH4
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Tetradecad in the 4970-5470 cm-1 region. The intensity cut off was fixed to 1×10-27 cm/molecule with Jmax = 11. A synthetic spectrum of the bands simulated from this list shows a good
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agreement with the experimental spectrum, as illustrated in Fig. 4. In the Electronic Supplements we provide two sets of the line lists. One set is for the experimental line list that includes our assignments for 1387 lines, where the line positions and intensities included in the corresponding
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fits are flagged by the ‘+’ sign. The second set of the line list contains all the calculated transitions with the cut-offs mentioned above. The corresponding examples of the file formats
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are given in Tables 3 and 4, respectively.
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Fig. 4. Comparison between the experimental spectrum of 13CH4 at 80 K (lower panel) with simulation based on the effective Hamiltonian/Dipole moment models (upper panel) in the 4970-5470 cm-1 region of the 13CH4 Tetradecad.
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Table 3. Sample of Electronic Supplement data for the observed 13CH4 transitions at 80 K along with assignments in the 4970–5470 cm-1 region.
4971.7648 4972.2288 4973.9976 4974.8067 4975.8727 4976.0377 4976.1915 4976.5440 4994.6437 4994.9357
+ + + + + + + + + +
S, Intensities at 80 K cm/molec 2.808E-25 + 1.456E-25 2.869E-25 + 4.053E-25 + 1.387E-25 1.002E-25 1.369E-25 1.841E-25 + 3.352E-25 + 1.485E-24 +
Rotational Assignments Low. state Up. State 0 7 F1 2 4 6 F2 1 0 7 E 1 4 6 E 1 0 7 F2 2 4 6 F1 1 0 7 A2 1 4 6 A1 1 0 7 E 1 4 6 E 2 0 8 E 1 4 7 E 3 0 8 F1 1 4 7 F2 4 0 8 A1 1 4 7 A2 2 0 7 F2 1 4 6 F1 3 0 6 A1 1 4 5 A2 1
Vibrational assignments Low. state Up. state 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2
E lower cm-1 293.192 293.184 293.178 293.168 293.184 376.753 376.751 376.748 293.140 219.955
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cm-1
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Notes: in this table, the columns are: 1. o: measured line positions. 2. S (80 K): measured line intensities in cm-1/(molecule cm-2) at 80 K 3. Lower state rovibrational assignment are given by the vibrational polyad number P, the rotational quantum number J, the rovibrational symmetry type C (Td irreducible representation) and the rovibrational ranking index N. 4. Upper state rovibrational assignments in the same format 5. Lower vibrational band assignments in terms of the principal vibrational quanta (v1,v2,v3,v4) and vibrational symmetry type CV (Td irreducible representation) 6. Upper vibrational band assignments in the same format 7. E lower: lower state energies [in cm-1]
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4971.7624 4972.2250 4973.7192 4973.9958 4974.8040 4975.8720 4976.0407 4976.1938 4976.5428 4982.2584
S, Intensity at 80 K cm/molec 2.502E-25 1.036E-25 4.076E-26 2.589E-25 4.101E-25 8.537E-26 6.092E-26 8.892E-26 1.439E-25 6.636E-27
Rotational assignment
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Table 4. Sample of Electronic Supplement data for the calculated line list of the 13CH4 methane transitions at 80 K for the 4970–5470 cm-1 region.
Low. state 0 7 F1 2 0 7 E 1 0 7 F1 2 0 7 F2 2 0 7 A2 1 0 7 E 1 0 8 E 1 0 8 F1 1 0 8 A1 1 0 8 F2 1
4 4 4 4 4 4 4 4 4 4
Up. state 6 F2 1 6 E 1 6 F2 2 6 F1 1 6 A1 1 6 E 2 7 E 3 7 F2 4 7 A2 2 7 F1 5
V V 57 V 60 V 60 V 57 V 57 V 60 V 57 V 57 V 57 V 58
Vibrational assignment Low. state 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1
Up. state 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 A1
E lower cm-1 293.192 293.184 293.192 293.178 293.168 293.184 376.753 376.751 376.748 376.803
Notes: in this table, the columns are: 1. o: calculated line positions. 2. S (80 K): calculated line intensities in cm-1/(molecule cm-2) at 80 K 3. Lower state rovibrational assignments are given by the vibrational polyad number P, the rotational quantum number J, the rovibrational symmetry type C (Td irreducible representation) and the rovibrational ranking index N. 4. Upper state rovibrational assignments in the same format
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5. 6. 7. 8.
V n is the number of vibrational sub band in the MIRS [65,66] labeling Lower vibrational band assignments in terms of the principal vibrational quanta (v1,v2,v3,v4) and vibrational symmetry type CV (Td irreducible representation) Upper vibrational band assignments in the same format E lower: lower state energies [in cm-1]
6. Conclusion The main goal of this study was the assignment of high-resolution FTS spectrum of 13C-
lower part of the
13
CH4 Tetradecad. The new analysis of the
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enriched methane in the range of 4970-5470 cm-1 at 80 K. This spectral range corresponds to the 13
CH4 Tetradecad presented here
would contribute to improving the methane database in this region.
The analysis in this work was performed using the effective Hamiltonian and the effective dipole transition moment expressed in terms of irreducible tensor operators for a full
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account of tetrahedral symmetry [57, 65, 66], from which we assigned 1387 rovibrational transitions belonging to five rovibrational band systems of 13CH4: 4ν4, ν2+3ν4, ν1+2ν4, ν3+2ν4 and 2ν2+2ν4 involving 14 sub-bands and modeled the line positions with an rms standard deviation of 0.0016 cm-1. The set of 737 selected line intensities was fitted with an rms of 10%. This work resulted in a calculated line list that contains 2993 transitions up to J = 11. Further studies on the
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other regions of the Tetradecad will be continued in a subsequent work. The results will be used for empirical corrections of the theoretical line lists of methane isotopologues distributed via the
applications [71].
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Acknowledgments
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TheoReTs information system [70] and for the modeling of methane absorption for planetary
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This study was performed in the frame of the LIA SAMIA between CNRS (France) and RFBR (Russia) №16-53-16022. The support from Mendeleev funding program of Tomsk State University, from French National Planetology (PNP) programm and from French ANR ePYTHEAS project are acknowledged. Part of research described in this paper is performed at the Jet Propulsion Laboratory, California Institute of Technology, Connecticut College, and NASA Langley Research Center under contracts and cooperative agreements with the National Aeronautics and Space Administration.
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The 13 CH4 absorption spectrum at 80 K: Assignment and modeling of the lower part of the Tetradecad in the 4970–5470 cm−1 spectral range Evgeniya Starikova , Keeyoon Sung , Andrei V. Nikitin , Michael Rey , Arlan W. Mantz , Mary Ann H. Smith PII: DOI: Reference:
S0022-4073(17)30653-2 10.1016/j.jqsrt.2017.11.022 JQSRT 5913
To appear in:
Journal of Quantitative Spectroscopy & Radiative Transfer
Received date: Revised date: Accepted date:
22 August 2017 23 November 2017 23 November 2017
Please cite this article as: Evgeniya Starikova , Keeyoon Sung , Andrei V. Nikitin , Michael Rey , Arlan W. Mantz , Mary Ann H. Smith , The 13 CH4 absorption spectrum at 80 K: Assignment and modeling of the lower part of the Tetradecad in the 4970–5470 cm−1 spectral range, Journal of Quantitative Spectroscopy & Radiative Transfer (2017), doi: 10.1016/j.jqsrt.2017.11.022
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Highlights: Obtained a high resolution 13CH4 spectrum at 80 K in the 4970-6200 cm-1 region. Assigned 1387 line positions and modeled the lower part of Tetradecad to the rms deviation being 0.002 cm-1. Fit 737 measured line intensities to the rms deviation of 10%. Compiled experimental and theoretical line lists at 80 K with assignments.
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The 13CH4 absorption spectrum at 80 K: Assignment and modeling of the lower part of the Tetradecad in the 4970–5470 cm-1 spectral range Evgeniya Starikova1,2*, Keeyoon Sung3, Andrei V. Nikitin1,2, Michael Rey4, Arlan W. Mantz5, Mary Ann H. Smith6 1
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Laboratory of Theoretical Spectroscopy of IAO SB RAN, av. 1, AkademicianZuev square, 634021 Tomsk, Russia 2 Laboratory of Quantum Mechanics of Molecules and Radiative Processes, Tomsk State University, 36 Lenin Avenue, 634050 Tomsk, Russia 3 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 4 GSMA, UMR CNRS 7331, Université de Reims Champagne Ardenne, Moulin de la Housse, BP 1039 51687 Reims Cedex 2, France 5 Department of Physics, Astronomy and Geophysics, Connecticut College, New London, CT, USA 6 Science Directorate, NASA Langley Research Center, Hampton, VA 23681, USA
* Corresponding author: [email protected] Abstract
A 13C-enriched spectrum of methane (13CH4) was recorded at Doppler-limited resolution (0.0044 cm-1) at 80 K covering the entire Tetradecad region in the 4970-6200 cm-1. In this paper we 13
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present the spectral analysis of the lower part of the
CH4 Tetradecad in the 4970-5470 cm-1
region. Starting with a non-empirical effective Hamiltonian derived by high-order Contact
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Transformations (CT) from the ab initio potential energy surface (PES), the effective Hamiltonian parameters have been determined for this region by extending the previous DAS
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(Differential Absorption Spectroscopy) spectral analysis for the upper part of the Tetradecad in the 5853-6200 cm-1 region. In total, 1387 rovibrational transitions were assigned belonging to five cold bands of the Tetradecad up to Jmax=11. Their positions were fitted with an rms
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deviation of 1.610-3 cm-1. Measured line intensities for 737 transitions were modeled using the effective dipole transition moments approach with an rms deviation of about 10%. Finally, the
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observed transitions were incorporated to fit simultaneously the
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CH4 Hamiltonian parameters
for the {Ground state / Dyad / Pentad / Octad / Tetradecad} system and the dipole moment parameters for the {Ground state - Tetradecad} system. Keywords: methane; 13CH4; absorption spectroscopy; spectra analyses; effective Hamiltonian; lower Tetradecad; variational calculations; HITRAN Running Head: 13CH4 absorption near 1.9 μm
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Number of pages: 17 Number of Figures: 4 Number of Tables: 4 Number supplemental files: 2
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1. Introduction
The study of methane absorption has attracted a continuously increasing interest both in laboratory high-resolution spectroscopy and in the quantum-mechanical global modeling for the remote sensing of planetary atmospheres [1, 2] and astrophysical applications [3-6]. Methane is the second strongest anthropogenic greenhouse gas following CO2 [7]. Accurate values of line
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positions and intensities are necessary to retrieve the methane concentration and the distribution profile by spectroscopic methods.
Measurements and analyses of microwave and infrared bands have been reported for 12
CH4 [8-16],
13
CH4 [17-23] and CH3D [24-28] (and references therein). Many of them were
either reported to the public domain including HITRAN and GEISA databases [29-34] or
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accessible via the VAMDC European web portal [35, 36]. However, further improvement of line parameters is still needed for an accurate radiative transfer modeling, particularly in the Octad
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and Tetradecad regions [37]. Measurements of methane isotopologues in the Earth’s atmosphere [38, 39] could provide powerful constraints in the identification of its specific sources and sinks. Several authors [40-46] have developed a geophysical model of CH4 mole fraction and the C/12C isotope ratio in methane to reconstruct the global history of CH4 emissions to the
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atmosphere. However, most of spectral analyses have focused on its principal isotopologue, 12
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CH4, leaving the spectroscopic data available for
13
C isotopic methane analysis much sparser.
A comprehensive review of the laboratory studies of methane until 2013 by Brown et al. [8]
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reported that the HITRAN 2012 database [32] contained much less experimental data for 13CH4 line-by-line spectroscopic parameters by factor of 4 for positions and by factor of 16 for intensities compared to that for 12CH4. Since then, a series of recent publications on 13CH4 have become available [19-23]. Ab initio predictions of isotopic methane spectra including vibrationrotation line positions and intensities have been performed by Rey et al. [47-49] using a variational method described in [50, 51], but experimental measurements and analyses are yet missing or incomplete in several spectral ranges of the overtone and combination bands, 3
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including the Tetradecad region of this study. Note that some vibrational levels of 12CH4, CH3D, CHD3, and 13CH4 isotopologues have been also calculated in Ref. [52] using an empirically fitted PES. The infrared spectra of 12CH4 and 13CH4 can be grouped in polyads [53] of closely lying bands because of the approximate relation between vibrational frequencies, i.e., ω1 ≈ ω3 ≈ 2ω2 ≈
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2ω4. Each polyad is defined by the integer polyad number P = 2(v1+v3)+v2+v4, where the (v1,v2,v3,v4) are principal vibrational normal mode quantum numbers. P = 0 corresponds to the ground vibrational state, and the polyads P=1, 2, 3, 4, 5, … correspond to the Dyad, Pentad, Octad, Tetradecad, Icosad, and so on in the increasing wavenumber intervals. Previously line-byline measurements and analyses of
13
CH4 have been reported by Champion et al. [54] in the
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Dyad range, by Jouvard et al. [55] in the Pentad range, and by Niderer et al. [17,18] and Brown et al. [20] in the Octad region. Starikova et al. [22] have assigned the 13CH4 line list constructed in Grenoble by laser direct absorption spectroscopy (DAS) at 80 K and 296 K in the upper part of the Tetradecad (5853-6201 cm-1). The corresponding line parameters have been included in the HITRAN 2016 release [34]. The analyses of the Tetradecad for the principal isotopologue 12
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CH4 were recently improved by Nikitin et al. [11, 16], but the spectral data of
13
CH4 for the
Tetradecad region are not yet complete. The present work is dedicated to the extension of the
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measurements and rovibrational assignments of the high-resolution spectrum of
13
CH4 in the
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lower part of Tetradecad in the 4970–5470 cm region.
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2. Experimental details
To this end, we recorded a
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C-enriched spectrum at 79.8 K covering the entire
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Tetradecad region in the 4970-6200 cm-1 using the Bruker IFS-125HR Fourier transform spectrometer with a custom-built cryogenic multipass Herriott cell at the Jet Propulsion
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Laboratory. The Herriott cell is vacuum-coupled to the FT-IR spectrometer and has excellent temperature control. The total optical path length of the cell is 20.941(6) m. Other details of the configuration and performance of the cell can be found in [56].
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Table 1. Instrumental configuration and experimental conditions
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Interferometer (Bruker IFS-125HR) IR source Tungsten lamp (50 W) Beam splitter CaF2 Resolution 0.0044 cm-1 (unapodized) Aperture (diameter) 1.15 mm Focal length 418 mm (provided by the Bruker Optics) Optical filter range 4640 – 6200 cm-1 Detector InSb (LN2 cooled) FTS pressure < 0.010 hPa 13 Sample gas CH4 (13C, 99%) Cell pressure 5.112(1) Torr Cell Temperature 79.9(1) K Path length 20.941(6)m Cell window CaF2 (wedged) Vacuum box window CaF2 (wedged) $ More information can be found in [55].
As summarized in Table 1, the experimental setup consisted of a Tungsten lamp, a CaF2 beam splitter, and a LN2 cooled InSb detector. The band pass of the filter was 4650 – 6200 cm-1, broad
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enough to capture the entire region of the Tetradecad bands in one single spectrum, securing the best consistency for the transitions measured. The S/N ratio better than 500 was achieved by
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coadding multiple scans. A background empty-cell spectrum was also obtained under the same optics conditions, which was used to generate a transmittance spectrum. The cell temperature was monitored using a silicon-diode sensor throughout the data-taking period and the fluctuation
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or drift of the cell temperatures was kept less than ±0.1 K during the entire scanning. The 13CH4 spectrum was frequency-calibrated to the accuracy of 0.0006 cm-1 using a single frequency
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calibration factor derived from 42 features of water in the ν2+ν3 band [32] observed in the background spectrum. Since the water features were arising from residual water in the evacuated
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FTS chamber, not from any sample impurity in the cell, these residual water features were effectively cancelled out in the rationed (i.e. transmittance) spectrum, whose overview is presented in Fig. 1.
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Fig. 1. An overview of the 13CH4 spectrum recorded at 79.8 K, which captures all the cold bands in the Tetradecad region from 5000 to 6200 cm-1. In this work, we have focused on the lower Tetradecad region from 4970-5470 cm-1.
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3. Rovibrational assignment and modeling
As in our previous works on methane studies [11, 16, 22], the theoretical analysis was
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carried out using the effective Hamiltonian (EH) approach with the polyad vibration extrapolation scheme [53,57]. In general, the procedure of the assignment is based on preliminary calculations using potential energy surface (PES) and dipole moment surfaces
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(DMS) reported by Nikitin, Rey and Tyuterev [58, 59], hereafter referred to as NRT surfaces. The experimental data were initially analyzed using a non-empirical EH for the methane polyads
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constructed from the same NRT PES via high order Contact Transformation (CT) procedure [60, 61]. This algorithm permits us to obtain the high-accuracy resonance coupling parameters
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from the potential surface in order to eliminate the ambiguity of the purely empirical models [6264]. The detailed descriptions of the CT calculations with application to methane polyads and their comparison with pure ab initio calculations and experimental data for
12
CH4 have been
reported by Tyuterev et al. [61]. In this work, we apply the same approach for the experimental data reduction additionally using the assignments determined from the previous analysis of the 13
CH4 laser absorption spectrum in the 5853-6200 cm-1 region [19, 22], corresponding to the
upper part of the Tetradecad. This has greatly simplified the modeling of the 13CH4 spectrum in 6
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the region under investigation. In total, 1387 transitions out of 1521 observed lines were rovibrationally assigned in the 4970-5470 cm-1 region, most of which belong to the 4ν4 and ν2+3ν4 band systems, as illustrated in Fig. 2. For the modeling of the observed line positions and intensities we used the MIRS software [65, 66] designed for the calculations and fitting of the energy levels, line positions and
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intensities of polyatomic molecules in the representation of the irreducible tensor operators (ITO). The MIRS program provides both the direct spectra calculation and the fit of the experimental data by the least squares method. The use of the polyad extrapolation scheme to fit the Tetradecad data implies the inclusion of all the terms and the corresponding constants obtained in the previous analyses of all the lower polyads involving the Dyad (900–1700 cm-1),
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Pentad (2000–3200 cm-1) and Octad (3500–4800 cm-1). Throughout the spectrum identification and data modeling the visual control of assignment is necessary. For spectra visualization and graphical assignment of the high-resolution spectrum we used the publically available graphical
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tool, SpectraPlot [67].
Fig. 2. An overview of the assigned transitions in the 13CH4 spectrum recorded at 80 K between 4970 and 5470 cm-1. Different colors are used to indicate different band systems contributing to the spectrum.
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4. Data fit statistics The detailed statistics for our assigned transitions are summarized in Table 2, described in two separate parts. In the first one (A) we summarized the results of line positions and intensity fits of two band systems, located in the analyzed ranges: These are 4ν4 band system with seven sub-bands with the sub-band centers in the range 5093-5210 cm-1 region and the ν2+3ν4 band
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system with also seven sub-bands whose centres are located in the 5348-5440 cm-1 range. The second part (B) shadowed in grey corresponds to the fit results of the bands located above 5470 cm-1, in which only a few P-branch transitions of the ν1+2ν4, ν3+2ν4, and 2ν2+2ν4 band systems were assigned in the lower part Tetradecad region (see Fig. 2).
Only the lower part of the Tetradecad was modeled in the present work. At this stage, the
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6-th order Effective Hamiltonian contains 4195 parameters including 72 parameters of Dyad, 382 parameters of Pentad, 1202 parameters of Octad and 2539 parameters of Tetradecad. In total, 650 effective Hamiltonian parameters were adjusted to fit 12144 observed transitions covering Dyad, Pentad, Octad [17, 18, 20, 68] in addition to the lower part of Tetradecad range that was assigned in this work. Among them, 58, 210 and 322 parameters specific to the Dyad,
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Pentad and Octad, respectively, were fitted. Only 60 parameters specific for the lower part of the Tetradecad were adjusted in the present analysis; the parameters corresponding to the upper
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Tetradecad bands were fixed to the values as determined from the DAS spectrum analysis of the previous work [22] in the 5850-6200 cm-1 region. The interaction parameters of the lower part of
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the Tetradecad with other bands were not adjusted instead held fixed to their initial values predicted from the PES using MOL_CT code [61, 69]. The set of obtained parameters permits us to constrain the positions of 1387 observed transitions of the
13
CH4 Tetradecad to a maximum
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root mean square (rms) deviation of 1.6 ×10-3 cm-1. The fit residuals for the line positions are shown on the upper panel of Fig 3.
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To derive the parameters of the effective dipole transition moment operator the intensities
of 737 selected transitions of the observed bands were included in the fit. We were able to reproduce this set of selected line intensities with an rms deviation of 10%. Detailed statistics of the intensity fit for the two frequency ranges are given in Table 2, and the fit residuals for line intensities in the range 4970-5470 cm-1 are given in the lower panel of Fig. 3.
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The detailed description of Effective Hamiltonian construction will be discussed when the complete description of the 13CH4 Tetradecad in the full spectral range (4800-6200 cm-1) will
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be available in a future work.
Fig. 3. Fit residuals for line position (upper panel) and intensities (lower panel) in the 13CH4 lower part of the Tetadecad region. Different colors are used to indicate the different band systems contributing to the spectrum.
Band
Upper sublevel
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Table 2. An overview of the rovibrational assignments of the 13CH4 transitions obtained at 80 K between 4970 and 5470 cm-1 region and their corresponding line position and intensity fit statistics. Sub-band centre (cm-1)
Positions
Intensities
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Nb. rms Nb. rms Jmin, Jmax data (10-3 cm-1) data (%) -1 (A) analysis of bands located in the spectral range under investigation: 4970-5470 cm A1 1a 5093.2 20 3,7 1.3 7 9.7 F2 1b 5114.375 160 0,9 1.5 104 10.7 E 1c 5137.79 73 1,8 2.0 39 7.9 F2 2b 5180.681 102 0,7 2.0 61 10.0 4ν4 E 2c 5198.22 53 1,8 1.5 26 8.1 F1 5199.937 107 1,7 1.6 58 10.3 A1 2a 5209.6 16 5,8 2.2 8 14.8 F2 1d 5348.561 110 1,8 1.5 44 10.6 F1 1e 5367.605 109 1,8 1.9 58 10.3 E 5401.99 70 1,9 1.2 41 10.0 F2 2d 5406.464 109 0,9 1.3 67 9.8 ν2+3ν4 F1 2e 5414.24 95 1,9 1.1 56 9.4 F2 3d 5421.630 135 1,9 1.3 81 10.8 F1 3e 5439.50 98 1,7 1.4 50 11.8 (B) preliminary analysis of bands having center located in upper wavenumber range > 5470 cm-1 A1 5476.0 8 4,7 1.4 F2 5503.7 14 2,6 1.6 1 21.6 ν1+2ν4 E 5515.2 13 4,8 2.1 F2 1f 5563.4 22 7,9 1.2 5 8.1 A1 5582.4 12 5,8 1.3 8 5.8 ν3+2ν4 F2 2f 5590.0 14 6,8 1.5 4 13.3 F1 1g 5590.4 18 5,8 1.2 10 8.3
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2ν2+2ν4
E F1 2g F2 3f F1 F2
5591.4 5600.5 5601.9 5640.3 5653.3
5 10 6 3 5 1387
7,7 6,9 6,7 7,7 7,8
0.8 1.3 2.1 0.9 1.3 1.6
1 3 3 1 1 737
21.0 11.7 14.3 19.2 13.6 10.3
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Total Notes: The vibrational sublevels are enumerated here for each band for a given symmetry type. The ranking numbers within the Tetradecad are the following: a N=1 and N=2 for A1 1 and A1 2; b N=1 and N=2 for F2 1 and F2 2; c N=1 and N=2 for E 1 and E 2; d N=3, N=4 and N=5 for F2 1, F2 2 and F2 3; e N=2, N=3, and N=4 for F1 1, F1 2 and F1 3; f N=7, N=8 and N=9 for F2 1, F2 2 and F2 3; g N=5 and N=6 for F1 1 and F1 2.
Note that high-sensitivity laser measurements of 13CH4 have been reported in the Icosad range by Campargue et al. [23], but their line-by-line assignments are not yet available because of the complexity of the corresponding overlapping band patterns. The extension of the theory to these
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higher wavenumber ranges will be the aim of a further work.
5. Line lists
Using the determined parameters of the effective Hamiltonian and of the effective dipole transition moment we have compiled a list of 2993 transitions from the lower part of the
13
CH4
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Tetradecad in the 4970-5470 cm-1 region. The intensity cut off was fixed to 1×10-27 cm/molecule with Jmax = 11. A synthetic spectrum of the bands simulated from this list shows a good
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agreement with the experimental spectrum, as illustrated in Fig. 4. In the Electronic Supplements we provide two sets of the line lists. One set is for the experimental line list that includes our assignments for 1387 lines, where the line positions and intensities included in the corresponding
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fits are flagged by the ‘+’ sign. The second set of the line list contains all the calculated transitions with the cut-offs mentioned above. The corresponding examples of the file formats
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are given in Tables 3 and 4, respectively.
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Fig. 4. Comparison between the experimental spectrum of 13CH4 at 80 K (lower panel) with simulation based on the effective Hamiltonian/Dipole moment models (upper panel) in the 4970-5470 cm-1 region of the 13CH4 Tetradecad.
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Table 3. Sample of Electronic Supplement data for the observed 13CH4 transitions at 80 K along with assignments in the 4970–5470 cm-1 region.
4971.7648 4972.2288 4973.9976 4974.8067 4975.8727 4976.0377 4976.1915 4976.5440 4994.6437 4994.9357
+ + + + + + + + + +
S, Intensities at 80 K cm/molec 2.808E-25 + 1.456E-25 2.869E-25 + 4.053E-25 + 1.387E-25 1.002E-25 1.369E-25 1.841E-25 + 3.352E-25 + 1.485E-24 +
Rotational Assignments Low. state Up. State 0 7 F1 2 4 6 F2 1 0 7 E 1 4 6 E 1 0 7 F2 2 4 6 F1 1 0 7 A2 1 4 6 A1 1 0 7 E 1 4 6 E 2 0 8 E 1 4 7 E 3 0 8 F1 1 4 7 F2 4 0 8 A1 1 4 7 A2 2 0 7 F2 1 4 6 F1 3 0 6 A1 1 4 5 A2 1
Vibrational assignments Low. state Up. state 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2 0000 A1 0004 F2
E lower cm-1 293.192 293.184 293.178 293.168 293.184 376.753 376.751 376.748 293.140 219.955
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cm-1
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Notes: in this table, the columns are: 1. o: measured line positions. 2. S (80 K): measured line intensities in cm-1/(molecule cm-2) at 80 K 3. Lower state rovibrational assignment are given by the vibrational polyad number P, the rotational quantum number J, the rovibrational symmetry type C (Td irreducible representation) and the rovibrational ranking index N. 4. Upper state rovibrational assignments in the same format 5. Lower vibrational band assignments in terms of the principal vibrational quanta (v1,v2,v3,v4) and vibrational symmetry type CV (Td irreducible representation) 6. Upper vibrational band assignments in the same format 7. E lower: lower state energies [in cm-1]
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4971.7624 4972.2250 4973.7192 4973.9958 4974.8040 4975.8720 4976.0407 4976.1938 4976.5428 4982.2584
S, Intensity at 80 K cm/molec 2.502E-25 1.036E-25 4.076E-26 2.589E-25 4.101E-25 8.537E-26 6.092E-26 8.892E-26 1.439E-25 6.636E-27
Rotational assignment
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cm-1
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Table 4. Sample of Electronic Supplement data for the calculated line list of the 13CH4 methane transitions at 80 K for the 4970–5470 cm-1 region.
Low. state 0 7 F1 2 0 7 E 1 0 7 F1 2 0 7 F2 2 0 7 A2 1 0 7 E 1 0 8 E 1 0 8 F1 1 0 8 A1 1 0 8 F2 1
4 4 4 4 4 4 4 4 4 4
Up. state 6 F2 1 6 E 1 6 F2 2 6 F1 1 6 A1 1 6 E 2 7 E 3 7 F2 4 7 A2 2 7 F1 5
V V 57 V 60 V 60 V 57 V 57 V 60 V 57 V 57 V 57 V 58
Vibrational assignment Low. state 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1 0000 A1
Up. state 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 F2 0004 A1
E lower cm-1 293.192 293.184 293.192 293.178 293.168 293.184 376.753 376.751 376.748 376.803
Notes: in this table, the columns are: 1. o: calculated line positions. 2. S (80 K): calculated line intensities in cm-1/(molecule cm-2) at 80 K 3. Lower state rovibrational assignments are given by the vibrational polyad number P, the rotational quantum number J, the rovibrational symmetry type C (Td irreducible representation) and the rovibrational ranking index N. 4. Upper state rovibrational assignments in the same format
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5. 6. 7. 8.
V n is the number of vibrational sub band in the MIRS [65,66] labeling Lower vibrational band assignments in terms of the principal vibrational quanta (v1,v2,v3,v4) and vibrational symmetry type CV (Td irreducible representation) Upper vibrational band assignments in the same format E lower: lower state energies [in cm-1]
6. Conclusion The main goal of this study was the assignment of high-resolution FTS spectrum of 13C-
lower part of the
13
CH4 Tetradecad. The new analysis of the
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enriched methane in the range of 4970-5470 cm-1 at 80 K. This spectral range corresponds to the 13
CH4 Tetradecad presented here
would contribute to improving the methane database in this region.
The analysis in this work was performed using the effective Hamiltonian and the effective dipole transition moment expressed in terms of irreducible tensor operators for a full
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account of tetrahedral symmetry [57, 65, 66], from which we assigned 1387 rovibrational transitions belonging to five rovibrational band systems of 13CH4: 4ν4, ν2+3ν4, ν1+2ν4, ν3+2ν4 and 2ν2+2ν4 involving 14 sub-bands and modeled the line positions with an rms standard deviation of 0.0016 cm-1. The set of 737 selected line intensities was fitted with an rms of 10%. This work resulted in a calculated line list that contains 2993 transitions up to J = 11. Further studies on the
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other regions of the Tetradecad will be continued in a subsequent work. The results will be used for empirical corrections of the theoretical line lists of methane isotopologues distributed via the
applications [71].
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Acknowledgments
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TheoReTs information system [70] and for the modeling of methane absorption for planetary
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This study was performed in the frame of the LIA SAMIA between CNRS (France) and RFBR (Russia) №16-53-16022. The support from Mendeleev funding program of Tomsk State University, from French National Planetology (PNP) programm and from French ANR ePYTHEAS project are acknowledged. Part of research described in this paper is performed at the Jet Propulsion Laboratory, California Institute of Technology, Connecticut College, and NASA Langley Research Center under contracts and cooperative agreements with the National Aeronautics and Space Administration.
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