the absorption spectrum of methane at 80 and 294 k in the icosad (6717–7589 cm−1): improved...
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Journal of Quantitative Spectroscopy &Radiative Transfer
Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–57
0022-40
doi:10.1
n Corr
E-m
journal homepage: www.elsevier.com/locate/jqsrt
The absorption spectrum of methane at 80 and 294 K in the icosad(6717–7589 cm�1): Improved empirical line lists, isotopologueidentification and temperature dependence
Le Wang, Didier Mondelain, Samir Kassi, Alain Campargue n
Universite Grenoble 1/CNRS, UMR5588 LIPhy, Grenoble F-38041, France
a r t i c l e i n f o
Article history:
Received 4 July 2011
Received in revised form
30 August 2011
Accepted 12 September 2011Available online 16 September 2011
Keywords:
Methane
CH4
CH3D13CH4
HITRAN
Titan
73/$ - see front matter & 2011 Elsevier Ltd. A
016/j.jqsrt.2011.09.003
esponding author.
ail address: [email protected]
a b s t r a c t
Using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers, new high
resolution spectra of methane have been recorded at 80 K and room temperature
by differential absorption spectroscopy (DAS) between 6717 and 7589 cm�1
(1.49–1.32 mm). The investigated spectral region corresponds to the very congested
icosad, which is not tractable by theory. Empirical lists of 19,940 and 24,001 lines were
constructed from the 80 K and room temperature spectrum, respectively. The room
temperature list adds about 8500 features to the empirical list of 15,375 lines at 296 K
adopted in the HITRAN database from the original work of L. Brown (Brown, L. Empirical
line parameters of methane from 1.1 to 2.1 mm. JQSRT 2005;96:251–70). A number of
relatively strong CH4 lines located near strong water lines were found missing in the
HITRAN line list. The improved sensitivity allowed adding more than 7000 lines to our
previous list of about 12,000 transitions at 80 K (Campargue A, Wang L, Kassi S, Masat
M, Votava O. Temperature dependence of the absorption spectrum of CH4 by high
resolution spectroscopy at 81 K: (II) The Icosad region (1.49–1.30 mm). JQSRT
2010;111:1141–51). In order to facilitate identification of the transitions of the
different methane isotopologues present in ‘‘natural’’ isotopic abundance, spectra of
highly enriched CH3D and 13CH4 samples were recorded with the same experimental
setup, both at room temperature and at 80 K.
From the variation of the line strengths between 80 K and 294 K, the low energy
values of about 12,000 transitions were determined. They allow accounting for the
temperature dependence of 84 and 93% of the methane absorbance in the region, at
room temperature and 80 K, respectively. As a result, we provide as supplementary
material two complete line lists at 80 K and 294 K, including CH3D and 13CH4
identification and lower state energy values.
& 2011 Elsevier Ltd. All rights reserved.
1. Introduction
The absorption spectrum of methane shows a succes-sion of strong absorption regions separated by intervals ofweak opacity, namely the methane transparency windows.
ll rights reserved.
(A. Campargue).
The overall appearance of the spectrum reflects the struc-ture of the CH4 vibrational levels, which are organized inpolyads of states in interaction. Due to approximate rela-tions between stretching (n1, n3) and bending (n2, n4)vibrational frequencies, n1ffin3ffi2n2ffi2n4, a polyad isdefined by the set of nearby levels with the same polyadnumber, P¼2(v1þv3)þv2þv4.
We are involved in a vast project [1–15] devoted to thedetailed characterization of the near infrared absorption
Fig. 1. Overview of the spectrum of methane in the icosad region (6400–
7800 cm�1) at room temperature and 80 K. Upper panel: The HITRAN
line list [16,17] at 296 K is superimposed to the DAS line list at 294 K (in
red). Lower panel: Line lists constructed from DAS spectra at 80 K (black
and red circles correspond to Ref. [7] and this work, respectively). The
line lists in the surrounding transparency windows (in gray) were
obtained in Refs. [12,13] by CRDS at 80 K. (For interpretation of the
references to color in this figure legend, the reader is referred to the web
version of this article.)
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–5748
spectrum of methane in the 5850–7920 cm�1 region(1.71–1.26 mm) for atmospheric and planetary applica-tions. The whole region includes the high energy part ofthe tetradecad (P¼4), the full icosad (P¼5) and the 1.58and 1.28 mm transparency windows of importance forastronomical applications. In the present work, we revisitthe icosad which spans the spectral range 6400–7600 cm�1 and contains 20 vibrational levels and 134sub-levels. Due to this high density of states in interac-tion, the spectrum is highly congested (20–30 transitionsper cm�1 at room temperature) and no regular rotationalprogressions can be identified even for the lowest rota-tional states, with the exception of part of the intensen2þ2n3 band at 7510 cm�1. Standard iterative techniquesof spectral analysis cannot be applied. In consequence, theicosad remains at present mostly unassigned, which is animportant problem for planetary applications as the lineintensities cannot be extrapolated to the different tem-peratures existing in the studied objects.
In such situation, the most efficient approach todetermine the Boltzmann factor, which rules the tem-perature dependence of the line strengths is to derive thelower state energy from the variation of the intensitiesmeasured at two temperatures. We have systematicallyapplied this ‘‘two temperature method’’ to spectrarecorded at room temperature (RT) and 80 K. In the 1.58and 1.28 mm transparency windows, the RT and 80 Kspectra were recorded by high sensitivity CW-Cavity RingDown Spectroscopy (CRDS) [1,5,9,12,13]. In the tetrade-cad [6] and icosad [7] regions corresponding to strongabsorption regions, we combined RT line intensitiesavailable in the literature to our intensity values at 80 Kretrieved from spectra recorded by Differential Absorp-tion Spectroscopy (DAS).
In our preceding study of the icosad region [7], the RTdata were taken from the HITRAN database [16]. TheHITRAN line list reproduces the empirical list constructedby L. Brown at room temperature (29674 K) by highresolution Fourier Transform Spectroscopy (FTS) [17]. Theoverview of the HITRAN (296 K) and DAS2010 (80 K) linelists are presented in Fig.1.
In Ref. [7] covering the 6717–7351 cm�1 region, thepositions and strengths of 9389 transitions were obtainedat 80 K while the HITRAN line list includes 12,250 transi-tions at 296 K in the same region. The application of thetwo temperature method provided lower state energyvalues of a total of 4646 transitions representing 68.4%and 79.4% of the total absorbance in the region, at 296 and80 K, respectively. The obtained results indicated that amajor limitation of the procedure was due to the insuffi-cient sensitivity of the HITRAN line list. The intensitycutoff of the HITRAN line list [16,17] in the region variesfrom 4�10�26 to 4�10�25 cm/molecule while the mini-mum value of the line intensities at 81 K was in the orderof 10�26 cm/molecule [7] (see Fig.1). The main purpose ofthe present work is to construct a more complete line listat room temperature by lowering the detectivity thresh-old from new DAS recordings in order to be able to derivea higher number of lower state energy values. For thispurpose, RT spectra were recorded with a new version ofour DAS setup. It turned out that this new setup allowed
not only a gain in sensitivity compared to the HITRAN linelist but a gain in sensitivity of one order of magnitudecompared to the detectivity limit of our preceding DASrecordings at 80 K [7]. We then decided to construct newDAS line lists both at room temperature and at 80 K.
The highly congested spectrum in the icosad includeslines due to different isotopologues present in naturalabundance in the methane sample (12CH4, 13CH4 andCH3D). Neither the HITRAN line list nor our DAS2010 list[7] at 80 K provides the isotopologue identification of thetransitions. In the present work, highly enriched DASspectra of 13CH4 and CH3D have been recorded with thesame experimental setup and for the same temperatureconditions (80 and 294 K) in order to discriminate the13CH4 and CH3D lines in the methane line lists.
The rest of this report is organized as follows. Theexperimental setup and line list construction aredescribed in Section 2. The identification of the isotopo-logue and the determination of the low energy values arepresented in Sections 3 and 4, respectively. In Section 5,we illustrate the achievements of our lists by a compar-ison with the HITRAN line list and literature data.
2. Experiment and construction of the line lists
The same experimental setup was used to record thespectra of methane in ‘‘natural’’ isotopic abundance, high
Fig. 2. Methane spectrum near 7414 cm�1. Upper panel: HITRAN line list
(296 K) [16,17]. Lower panel: DAS spectrum recorded at 294 K
(P¼10.0 Torr). The lines marked by star are newly observed.
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–57 49
purity CH3D (ISOTEC, min 98 at% D) and high purity 13CH4
(ISOTEC, min 99 at% 13C) by differential absorption spec-troscopy. This DAS spectrometer has been described indetails in Ref. [13]. It is an improved version of the oneused in our previous studies of the tetradecad and icosadregions [2–4,7,8]. The spectra were recorded between6717 and 7589 cm�1 (1.49–1.32 mm) at 80 K and at roomtemperature (RT¼294 K). This spectral interval was con-tinuously covered thanks to a series of thirty DFB fibereddiode lasers. The main part of the laser power is sent intoa 1.42 m long cryogenic absorption cell [2,5] where thegas is cooled down to a temperature of 8072 K withliquid nitrogen [2,5,9]. This temperature is referred to the‘‘liquid nitrogen temperature’’ (LNT) in the following. Anabsorption path length of 284 cm is achieved with oneround trip in this cell. For each laser diode, the recordedtransmission spectrum is the ratio of a transmittedspectrum over a reference spectrum acquired simulta-neously over the whole laser tuning range (�35 cm�1).This tuning range is obtained from the superimposition ofa fast periodic current ramp and a slow temperature ramp[2,13]. Each final spectrum contains about 105 spectralpoints separated by a 10 MHz interval, well below theDoppler width of methane lines at 80 K (about 150 MHzHWHM). The noise equivalent absorption (NEA) is in theorder of aminE5�10�8 cm�1 both at RT and 80 K. Duringthe recordings, the pressure was measured by a capacitancegage (MKS Baratron type 626B, 10 Torr range, 0.25% accu-racy). The same pressure values were used for the differentisotopologues: 10 Torr at RT and 1.0 and 6.0 Torr at 80 K.
A first wavenumber calibration of each spectrum isobtained from a wavelength meter (621A, Bristol Instru-ments). To increase the wavenumber accuracy an addi-tional absolute calibration is done by statisticallymatching the recorded line positions with the accuratepositions of reference lines. For ‘‘natural’’ methane at bothRT and 80 K, we adopted as reference the HITRAN positionvalues. For 13CH4, a rough calibration was obtained usinga peak list constructed from FTS spectra of highlyenriched 13CH4 available on Kitt Peak archive [18]. Thiscalibration of the 13CH4 spectra was then refined using asreference the 13CH4 lines identified in our DAS line list ofmethane. This procedure has the advantage to provide aconsistent calibration of the 13CH4 and methane spectra.The calibration of the CH3D spectra uses line positionsfrom a room temperature FTS spectrum of CH3D recordedat USTC-Hefei (P¼20 Torr, l¼105 m, unapodized resolu-tion of 0.015 cm�1) [9]. This FTS spectrum was calibratedagainst line positions of H2O (present as an impurity)from the HITRAN database [16].
For each complete laser diode spectrum, the rms valuesof the difference between the DAS and reference linepositions are generally between 5�10�4 and1�10�3 cm�1. The average uncertainty of the absolutevalues of the centers of the well isolated lines is thenbelieved to be less than 1�10�3 cm�1, for the threestudied isotopologues.
The considerable change of the appearance of thespectra induced by cooling down to 80 K has been illu-strated in Refs. [2,4,7] for the whole icosad region as wellas in small spectral sections. Fig. 2 shows the spectral
congestion of the RT spectrum together with theimproved sensitivity compared to the HITRAN line list.
The line strength Sv0(cm/molecule) of a rovibrational
transition centered at n0, was obtained from the inte-grated line absorbance,
RlineavUldv (in cm�1/molecule),
expressed as
Zlineavldv¼
Zline
lnI0ðvÞ
IðvÞ
� �dv¼ Sv0
ðTÞNl ð1Þ
where ðI0ðvÞ=IðvÞÞ is the ratio of the incident intensity tothe transmitted intensity, l is the absorption path lengthin cm, n is the wavenumber in cm�1, a(n) is the absorp-tion coefficient in cm�1, and N is the molecular concen-tration in molecule/cm3 obtained from the measuredpressure value: P¼NkBT.
The pressure self broadening at 80 K of the vibrationalbands of methane has been measured for the n3 and 2n3
bands (0.20 cm�1/atm [19]). It leads to a value of2.6�10�3 cm�1 (HWHM) at 10.0 Torr. This value beingabout half that of the Doppler broadening (HWHM5.7�10�3 cm�1), a Voigt function of the wavenumberwas adopted as line profile. An interactive multi-linefitting program was used to reproduce the spectrum[20]. The local baseline (assumed to be a quadraticfunction of the wavenumber) and the three parametersof each Voigt profile (line center, integrated absorbance,HWHM of the Lorentzian component) were fitted. TheHWHM of the Gaussian component was fixed to itstheoretical value for 12CH4. In the case of blended linesor lines with low signal to noise ratios, the LorentzianHWHM was constrained to the average value obtainedfrom nearby isolated lines. Fig. 3 shows an example ofcomparison between the measured and fitted spectra. Therms of the residuals is on the order of 2�10�5, whichcorresponds to absorption coefficients on the order of7�10�8 cm�1, not far from the typical noise equivalentabsorption (aminE5�10�8 cm�1). This sensitivity leadsto minimum values on the order of 1�10�27 and1�10�26 cm/molecule for the line intensities at 80 Kand RT, respectively.
Fig. 3. An example of simulation of the CH4 spectrum at room tem-
perature (same spectral interval than Fig. 2). From top to bottom:
(a) Experimental spectrum at 294 K (P¼10.0 Torr).
(b) Simulated spectrum resulting from the multi-line fitting procedure.
(c) Residuals between the simulated and experimental spectra.
Fig. 4. Identification of the 13CH4 lines in the spectrum of ‘‘natural’’
methane at 80 K by comparison of the methane and 13CH4 spectra near
7493 cm�1. The two spectra were recorded at 80 K with a pressure of 6.0
and 1.0 Torr, respectively. The chosen interval shows the Q branch of the
n2þ2n3 band of 13CH4.
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–5750
The complete line lists at RT and 80 K were obtainedby gathering the lists corresponding to the different DFBlaser diodes, each extending over about 35 cm�1.
While the presence of lines from impurity can be ruledout in the 80 K spectrum, impurity lines due to H2O, CO2,HF, CO etc. may be present in the RT line list. On the basisof the corresponding HITRAN line lists [16], we found thatH2O lines were present in our RT spectrum. Indeed, thelow energy part of the investigated region corresponds toa very strong absorption system of H2O with line inten-sities as high as 10�20 cm/molecule. The relative concen-tration of water in our sample was estimated to be about2�10�4, which is sufficient to lead to the observation ofabout one hundred lines, which were discarded from ourRT list. Water was found to contribute importantly toabout one hundred additional blended lines. In thesecases, the water contribution to the line strength wasestimated from nearby H2O lines and subtracted.
The final dataset consists of 24,001 and 19,940 lines atRT and 80 K, respectively. The resulting density of lines(27.5 and 22.9/cm�1) reflects the considerable congestionof the spectrum and gives an idea of the tremendouseffort required in the line fitting process. For comparison,in the same spectral interval, the HITRAN line list at 296 Kprovides 15,375 transitions while our previous DAS linelist at 80 K includes 12,485 features [7]. The overviewcomparison presented in Fig. 1 shows that, compared toprevious data, the gain in sensitivity ranges between oneand two orders of magnitude.
3. Identification of the 13CH4 and CH3D lines
In the transparency windows, it is well known that thecontribution of CH3D to the total absorption may largelyexceed its relative abundance [7,9,15] but in the strongabsorption regions dominated by vibrational bands invol-ving strong CH excitation, the 13CH4 and CH3D contribu-tions are expected to roughly scale according to theirrelative abundances. Let us underline that the CH3D/CH4
abundance ratio in our sample was measured to be
5.0�10�4 using the 3n2 band of CH3D near 6400 cm�1
[10]. This value differs significantly from the relativeabundance adopted in HITRAN (6.157�10�4). The 18%difference is the usual value of CH3D depletion in ‘‘naturalgas’’ on Earth [21]. This value is confirmed using our veryrecent intensity measurements of highly enriched CH3Dat 80 K near 1.58 mm: we obtain an average value of4.975(96)�10�4 for the ratios of more than 3000 lineintensities measured in ‘‘natural’’ methane or for pureCH3D [15]. The 13CH4 relative abundance is believed to bevery close to the HITRAN value (1.1103�10�2) as the 13Cfractionation is always very limited.
Considering that the intensity values span over four orfive orders of magnitude, 13CH4 and CH3D lines areundoubtedly present in our methane line lists over mostof the studied region. As we did not construct DAS linelists for the minor isotopologues, the 13CH4 and CH3Dlines were marked one by one by visual comparison of themethane line lists at RT and 80 K to the corresponding13CH4 and CH3D spectra. Both positions and relativeintensities were used as criteria. As an example, Fig. 4shows a comparison of the methane and 13CH4 spectra at80 K in a spectral section around 7493 cm�1, whichincludes the Q branch of the n2þ2n3 band of 13CH4. Inthe region displayed in Fig. 4, the identification of the13CH4 lines in the methane spectrum is easy but it is notthe case in the much more congested RT spectrum where13CH4 lines are very often blended with 12CH4 lines.Table 1 summarizes the amount of 13CH4 and CH3Dtransitions identified at RT and 80 K, in terms of numbersof lines and corresponding intensities. For instance, in the80 K list, 2650 lines were attributed to 13CH4 and 828additional features were found to involve both 13CH4 and12CH4 contributions. Fig. 5 shows an overview of the RTand 80 K line lists where 13CH4 and CH3D lines have beenhighlighted. The overall aspect of the 13CH4 contributionnicely follows the 12CH4 envelope with intensities valuesdivided by a factor of about 100 corresponding to the 1.1%relative abundance of 13CH4. This is a consequence of the
Table 1Statistics of the DAS measurements of methane at 80 and 294 K in the 6717–7589 cm�1 region and comparison to HITRAN [16] and Ref. [7].
Total 12CH413CH4 Blend 12CH4/13CH4 CH3D Blend 12CH4/CH3D
Intensities (10�20 cm/molecule)80 K
This work 4.375 4.331 (99.00%) 3.334E-2 (0.765%) 9.624E-3 (0.22%) 4.083E-4 (0.0093%) 3.442E-4 (0.0079%)
Ref. [7] a 4.478 4.478
Room Temperature
This work 4.005 3.962 (98.91%) 1.258E-2 (0.31%) 3.10E-2 (0.77%) 0 0
HITRAN 3.898 3.898
Number of lines80 K
This work 19,944 16,024 2650 828 323 115
Ref. [7] a 12,485
Room Temperature
This work 24,001 21,990 764 1247 0 0
HITRAN 15,375 15,375
a A larger region (6717–7655 cm�1) was studied in Ref. [7], the given values correspond to the 6717–7589 cm�1 region, presently studied.
Fig. 5. 13CH4 and CH3D lines in the icosad lists of ‘‘natural’’ methane at
294 K (upper panel) and 80 K (lower panel) between 6717 and
7589 cm�1. The blue and orange circles highlight the 13CH4 and CH3D
lines while the full black dots correspond to 12CH4 transitions.
(For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–57 51
limited amplitude (some tens of wavenumbers [21]) ofthe vibrational shifts induced by the isotopic substitutionof the carbon atom. For instance, the Q branch of then2þ2n3 band of 13CH4 is clearly apparent near 7493 cm�1,with an isotopic shift on the order of 17 cm�1 comparedto the main isotopologue. According to Table 1, the sum ofthe 13CH4 intensities (including blended lines) represents0.98 and 1.09% of the total absorbance at 80 K and RT,
respectively which is consistent with the 1.1% value of the13CH4 abundance. Note that we tried to identify 13CH4
lines in the HITRAN line list and found that they arepractically absent due to a lack of sensitivity of theoriginal data or to the choice of the intensity cutoff toinclude the lines in the HITRAN database.
CH3D lines could be safely identified only in the 80 KDAS line list. The failure to identify CH3D lines in the roomtemperature line list can be explained by the reduceddynamics on the intensity scale at RT (104 compared to105 at 80 K), the extreme congestion of the spectrum dueto the larger Doppler broadening at RT and the weaknessof the CH3D lines resulting from the 5�10�4 relativeabundance of this isotopologue.
4. Determination of the lower state energy
The lower state energy values of the CH4 transitionswere calculated using the above mentioned ‘‘two tem-perature’’ method [2,3]. The lower state energy, E, of atransition is obtained from the ratio of the line intensitiesat 294 and 80 K:
lnSv0ðT1ÞZðT1Þ
Sv0ðT0ÞZðT0Þ
� �¼�E
1
kT0�
1
kT1
� �ð2Þ
where T0¼294 K and T1¼80 K, Sv0and Z(T) are the
corresponding line intensities and partition function,respectively. The values of the partition function at 294and 80 K given in HITRAN for 12CH4 [16] leads toðZð294KÞ=Zð80KÞÞ ¼ 6:98023. This ratio is practically iden-tical for 13CH4 and CH3D and the two temperature methodcan be applied identically to the three isotopologues.
Considering the congestion of the spectrum, matchingthe lines corresponding to the same transition in the RTand LNT line lists is a crucial step of the procedure. Weadopted a strict criterion in order to insure that a RT lineand a LNT line correspond to the same transition: thedifference, d, of their centers must differ by less than0.002 cm�1. This value takes into account the uncertain-ties on the two line center determinations and possibleerror on the wavenumber calibration. It corresponds to
Fig. 7. Examples of empirical lower J assignments derived for transitions
near 7187 cm�1. The pressure of the spectra at 294 and 80 K were 10.0
and 6.0 Torr, respectively. Note the reduced Doppler broadening of the
line profiles at 80 K.
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–5752
the one-fifth and one-tenth of the Doppler width (FWHM)at LNT and RT, respectively.
10,255 pairs of CH4 lines were found to fulfill thiscriterion allowing for the derivation of the correspondinglower state energies (Eq. (2)). The associated transitions,which are highlighted in the overview plots presented inFig. 6, represent 77.6 and 88.8% of the total absorbance atRT and LNT, respectively. The corresponding empirical J
values were computed from Jemp ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1=4ÞþðE=B0Þ
p�ð1=2Þ
where B0¼5.214 cm�1 is the 12CH4 and 13CH4 ground staterotational constant. As an example, the empirical J valuesare given on the RT and 80 K spectra displayed in Fig. 7.
In Fig. 8, the empirical J values are plotted versus theline centers. The observed propensity of the obtained J
values to be close to integers reflects the quality of theresults. Our direct comparison of the 80 K and RT spectrashowed that sometimes the do0.002 cm�1 criterion onthe line positions is too strict for strongly blended or veryweak lines. Additional pairs of lines with d slightlyexceeding 0.002 cm�1 correspond to the same transitionsand should be associated. We relaxed the coincidencecriterion up to 0.003 cm�1, which allowed us to derive1689 additional lower state energy values representing6.6% and 4.3% of the total absorbance at RT and 80 K,respectively. The derived low energy values of the lineswith 0.002odo0.003 cm�1 are expected to be less
Fig. 6. Overview of the icosad line lists of methane between 6717 and
7589 cm�1, at 294 K (upper panel) and 80 K (lower panel). The green
circles highlight the 10255 associated transitions for which it was
possible to derive the lower energy values (do0.002 cm�1). (For
interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
Fig. 8. Empirical lower J values versus the line positions in the icosad of
methane (6717–7589 cm�1). The empirical J values were obtained from
the strengths of the methane transitions at 80 and 294 K by associating
lines with centers differing by less than 0.002 cm�1.
accurate, they are marked (n) in the line lists providedas Supplementary material. It is worth mentioning that avery small fraction of coinciding positions correspond toblended lines assigned to different isotopologues (mainly12CH4 and 13CH4). Nevertheless, we chose to leave theobtained low energy value in our line list as it may help toaccount for the temperature dependence of the consid-ered absorption feature in an effective way. Overall theseweak lines represent about 1.6 and 0.6% of the totalabsorbance at RT and 80 K, respectively.
The complete lists of lines measured at 80 and 294 Kare provided as two separate files. They include theisotopologue identification, the lower state energy andempirical J values for the pairs of associated transitions.Table 2 gives a sample of the RT line list.
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–57 53
5. Discussion
5.1. Comparison to the HITRAN line list
In our region, the HITRAN database reproduces theempirical line list constructed by L. Brown from FTSspectra recorded at Kitt Peak [17]. In our spectral interval,
Table 2Wavenumbers and line strengths of the methane transitions measured at 29
temperature line list of 24,001 transitions provided as Supplementary materia
T¼294 K T¼80 K
ISO Center (cm�1) Intensity (cm/mol) ISO
12C/13C 7015.0369 1.418E-25 13C
7015.0909 1.271E-23
7015.1058 1.139E-24
7015.1582 5.865E-25
7015.1885 9.624E-25
7015.2301 5.682E-24
7015.2905 5.908E-25
7015.3469 1.897E-25
7015.4016 3.915E-24
7015.4241 1.396E-25
7015.4562 1.098E-24
7015.4826 1.928E-25
7015.5493 6.618E-25
7015.5609 1.374E-2412C/13C 7015.5845 8.331E-2612C/13C 7015.6149 1.092E-25 12C/13C
7015.6962 4.822E-25
7015.7122 1.393E-24
7015.7558 2.814E-24
7015.8060 9.587E-26
7015.8256 2.781E-2413C 7015.8765 1.837E-25
7015.8928 9.342E-25
7016.0445 2.930E-24
7016.0782 2.254E-25
7016.1425 6.145E-25
7016.1748 3.102E-25
7016.2318 1.263E-23
7016.2598 4.782E-24
7016.2732 5.324E-25
7016.3614 5.455E-2512C/13C 7016.3838 1.451E-25 13C
7016.4085 1.211E-24
7016.4784 5.814E-26
7016.4945 2.831E-25
7016.5190 6.201E-26 CH3D
7016.5702 1.119E-24 12C/13C12C/13C 7016.6026 2.137E-25
7016.6389 6.087E-26
7016.6596 5.919E-25
7016.6740 1.092E-24
7016.7251 8.276E-25
7016.7473 3.582E-24
7016.7794 1.382E-24 12C/13C
7016.8163 1.908E-24
7016.8392 1.910E-25
7016.9086 8.364E-26
7016.9394 2.123E-24
7016.9712 6.741E-25
7017.0041 1.771E-25
7017.0289 4.507E-24
7017.0547 6.044E-25
Notes: The low energy values, E, were obtained for the transitions whose
(do0.002 cm�1). The empirical J values of CH4 transitions are given in the las
centers at 80 and 294 K differ by 0.002odo0.003 cm�1. 12C/13C marks the lin
Brown’s line list is based on two spectra recorded at296.3 K with the following pressure and path length:6.3 Torr, 433.0 m and 26.3 Torr, 25.0 m. L. Brown gavean overall absolute accuracy of 10% for the intensities butthis value is probably conservative and corresponds to theworst cases of highly blended lines [17]. The sum of our24,001 DAS intensities at RT is only 3% larger than the
4 K by DAS near 7016 cm�1. This Table is a small section of the room
l for the entire 6717–7589 cm�1 region.
E (cm�1) Jemp
Center (cm�1) Intensity (cm/mol)
7015.0354 8.160E-27 366.53 7.88
7015.0906 1.050E-23 163.02 5.10
7015.1049 1.020E-24 156.86 4.99
7015.1581 4.309E-25 171.98 5.25
7015.1889 3.463E-25 226.51 6.09
7015.2298 2.046E-24 226.45 6.09
7015.2891 9.158E-26 290.84 6.97
7015.3474 9.114E-26 204.43 5.77
7015.4015 1.467E-24 223.41 6.05
7015.4232 1.209E-25 159.41 5.04
7015.4584 1.370E-24 131.52n 4.53n
7015.4842 1.582E-25 163.54 5.11
7015.5495 3.094E-26 382.40 8.06
7015.5604 6.115E-26 386.16 8.10
7015.6151 9.660E-27 333.68 7.50
7015.7141 6.471E-25 207.00 5.81
7015.8253 2.483E-24 157.09 5.00
7016.0442 2.569E-24 158.47 5.02
7016.1424 3.162E-26 375.07 7.98
7016.1744 4.689E-26 292.76 6.99
7016.2339 1.288E-25 498.71n 9.27n
7016.2603 3.713E-24 167.76 5.18
7016.2745 3.047E-25 191.06 5.56
7016.3606 1.002E-24 101.98 3.94
7016.3822 1.625E-26 315.67 7.28
7016.4083 2.192E-24 103.10 3.96
7016.4755 2.122E-26 225.42n 6.08n
7016.4918 2.743E-25 150.84n 4.89n
7016.5174 9.577E-27 291.12
7016.5729 4.191E-26 399.34n 8.24n
7016.6022 9.470E-27 386.49 8.10
7016.6382 3.396E-27 368.90 7.91
7016.6583 7.829E-27 478.85 9.07
7016.6728 5.497E-26 376.75 7.99
7016.7250 2.870E-25 229.33 6.13
7016.7471 1.465E-24 216.72 5.95
7016.7785 7.629E-26 369.71 7.91
7016.8161 2.811E-26 470.61 8.99
7016.8379 8.243E-27 388.51 8.13
7016.9387 9.786E-26 383.48 8.07
7016.9722 2.004E-25 241.09 6.30
7017.0285 8.024E-24 104.37 3.99
7017.0576 1.777E-24 66.05n 3.09n
center coincides with that of a line observed in the 294 K spectrum
t column. The ‘‘*’’ symbol in the last column marks the lines whose line
es with 12CH4 and 13CH4 contributions.
Fig. 9. Comparison of the low resolution simulations of the spectrum of
methane (P¼1.0 Torr) at room temperature. Upper panel: Spectra
obtained from the HITRAN line list (black) and DAS line list at 294 K
(green). Lower panel: Relative difference (DAS�HITRAN)/HITRAN in %.
(For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
Fig. 10. Comparison of the methane spectrum simulated from the
HITRAN line list at room temperature to the DAS spectrum at 294 K.
Upper panel: Spectra obtained from the HITRAN line list (black dots) and
DAS line list at 294 K (green solid line). A number of relatively strong
CH4 lines are missing in the HITRAN list as a consequence of their close
vicinity to very strong water lines. One residual H2O line is marked in
the DAS spectrum. Lower panel: DAS spectrum of methane at 80 K
(P¼6.00 Torr) used to confirm that the missing lines are not due to
impurities. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–5754
sum of the 15,375 intensities included in the HITRANdatabase (see Table 1). This difference cannot be simplyinterpreted as due to the 8900 very weak lines, whichwere newly measured by DAS because the uncertainty onthe intensity of the strongest lines may have more impacton the overall absorbance than the myriad of new lines. Inorder to get an overview comparison of our line list at294 K to the HITRAN list, we have simulated a ‘‘lowresolution’’ absorption spectrum of methane by affectinga normalized Gaussian profile (HWHM¼5.0 cm�1) toeach line of the two lists (note that strictly speakingthe broadening of the lines is not equivalent to theconvolution of the transmittance by a low resolutionapparatus function). The obtained simulations are com-pared in Fig. 9, which includes the relative difference,(DAS�HITRAN)/HITRAN. The overall agreement is satis-factory (rms of the residuals of 5.6%). Nevertheless, asignificant deviation is observed between 6830 and7050 cm�1, the HITRAN absorbance being on averagelarger by 8%. In contrast, the DAS absorbance is signifi-cantly larger around 7200 cm�1 corresponding to a stron-ger absorption region. We have performed a systematicline by line comparison and found that a few relativelystrong lines (intensity values larger than 5�10�24 cm/molecule) are missing in the HITRAN line list below7400 cm�1. Such situation is illustrated in Fig. 10, whichshows the 80 K spectrum in order to confirm that themissing lines are not due to impurities. We found that themissing lines are located in the vicinity of strong waterlines and concluded that the FTS spectra used by L. Brownto construct her line list probably contained more waterlines than ours, which prevented the retrieval of a fewmethane lines. In fact, this potential issue was explicitlymentioned by L. Brown in her original work [17]. Let usunderline that these cases are scarce and that the overallagreement between the two line lists is significantlybetter than the (probably overestimated) 10% uncertaintyattached to the FTS intensities.
5.2. Temperature dependence of the absorption spectrum in
the icosad region
In order to estimate the impact of the new observa-tions both at RT and 80 K, the obtained J values have beenplotted as a function of the line intensities and comparedto our previous results of Ref. [7] (Fig. 11). The increasedsensitivity of the present DAS recordings allowed dou-bling the number of J determinations in the region (6063[7] compared to 11,944 for do0.003 cm�1). Fig. 11 showsthat these new determinations correspond mostly to highJ transitions, which are very weak at 80 K. These new highJ determinations will help to better account for thetemperature dependence of the spectrum around roomtemperature.
The transitions with J determination include practi-cally all the strong or medium lines (see Fig. 6) but thenumber of J derivation represents only half of the totalnumber of lines measured at RT and 80 K. In order toestimate the fraction of intensities with known tempera-ture, we compare in Fig. 12 the low resolution simulationsat RT and 80 K to simulations limited to the lines withdetermined lower state J values. This figure shows thatmost of the temperature dependence of the absorptionhas been determined over the whole region, the fractionof absorption with unknown temperature dependencebeing limited to a few % at LNT.
The quality of the retrieved J values can be estimatedfrom the histograms of the number of lines and corre-sponding sum of intensities (Fig. 13). The contrastbetween integer and non integer J values is morepronounced for the intensities than for the number oflines as the uncertainty on the intensities values (andthen on the J values) is larger for the weakest lines andthen, on average, non integer J values correspond to
Fig. 11. Empirical J values of the CH4 transitions observed between 6717
and 7589 cm�1 versus the line intensities at 80 K (upper panel) and
294 K (lower panel). The orange dots correspond to the results obtained
in Ref. [7] by associating HITRAN intensity values at 296 K to previous
DAS intensity values at 80 K. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of
this article.)
Fig. 12. Fraction of the methane absorption at 294 K (upper panel) and
80 K (lower panel) for which the temperature dependence has been
derived in the 6750–7580 cm�1 region. The red and blue solid lines
correspond to a low resolution simulation (HWHM¼5.0 cm�1) of the
methane spectrum at 294 and 80 K, respectively (P¼1 Torr) while the
areas filled in gray correspond to a similar simulation limited to the lines
with lower state energy derived by the ‘‘two temperature’’ method. (For
interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–57 55
smaller intensities. While 71.4% of the J determinations(do0.002 cm�1) leads to J values falling within a 70.25interval around integer values, these ‘‘integers’’ J valuescorrespond to 82.5% and 80.9% of the total absorbance at80 K and RT, respectively. The comparison with theresults of Ref. [7] included in Fig. 13 shows that the gainconcerns not only the amount of derived J values but alsoits quality as illustrated by the better contrast betweeninteger and non integer J values.
The histograms show that the integer-non integeralternation degrades for the low and high J values(J¼0,1 and J49). These distortions have different origins,which can be analyzed considering the theoretical valuesof the ratios of the line strengths at 80 K and 296 K, listedin Table 3.
The high J value distortion has been identified as dueto the small sections of warm gas lying between the endsof the cold jacket and the windows of the cryogenic cell[12]. Compared to a cryogenic cell at a uniform tempera-ture of 80 K, this warmer section of gas leads to anoverestimation of the LNT intensities of the high J transi-tions and then to an underestimated empirical value of J.The temperature gradient has a negligible effect for J
values smaller than 9 but, above J¼10, the decrease of thestrength by cooling is so considerable (a factor of 243 forJ¼10) that, the 1% section of warm gas starts to have alarger contribution to the measured intensity than the99% section at 80 K. Contrary to the high energy region of
the tetradecad [12], in the icosad region, this effect hasnevertheless a very limited impact because few J410lines are detected (see Fig.8).
The lack of contrast of the histograms for J¼0,1 has adifferent origin. It is probably due to the fact that thecontrast of the intensity ratios of two successive J valuesis minimum for J¼0,1 (see Table 3). The J¼0 and 1 ratiosdiffer by only 10%, which may be insufficient to discrimi-nate these two J values due to larger uncertainties on thestrength values in case of highly blended transitions. Inorder to determine the low J values, which are of impor-tance for theoretical modeling, the two temperaturemethod has been recently applied in part of the icosadregion, to jet spectra at 25 K combined with our 80 K bulkmeasurements [8]. The contrast for the J¼0,1 levels beingmore favorable for this couple of temperature values, thelow J values (Jo4) could be reliably determined for 59transitions and the centers of nine sub bands in the 7070–7350 cm�1 region [8] were estimated.
5.3. Previous rovibrational assignments
A global theoretical interpretation of the 12CH4 tetradecadis in progress in the frame of the effective Hamiltonianapproach but a global analysis of the icosad seems out ofreach considering the number of effective parameters to be
Fig. 13. Histograms of the empirical lower J values obtained for methane transitions in the 6717–7589 cm�1 region. The left and right hand panels show
the results obtained in this work and in Ref. [7], respectively. Upper panel: Count of the obtained J values with a step interval of 0.5. Lower panels:
Corresponding sum of line intensities at RT or 80 K for each interval. Note that the ‘‘integer’’ J values correspond to 82.5 and 80.9% of the total absorbance
at 80 K and RT, respectively.
Table 3Calculated ratios of the intensities at 80 and 296 K of methane (12CH4 and 13CH4) versus the rotational quantum number of the lower state.
J 0 1 2 3 4 5 6 7 8 9 10 11 12
S80/S296 6.99 6.10 4.66 3.10 1.81 9.19�10�1 4.08�10�1 1.58�10�1 5.37�10�2 1.59�10�2 4.12�10�3 9.30�10�4 1.84�10�4
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–5756
determined. At present, rovibrational assignments of icosadtransitions are limited to a few bands:
(i)
The n2þ2n3 band centered at 7510.33 cm�1 forms asequence of transitions, which is strongly dominatingthe high energy part of the icosad. From the periodicappearance of the rotational structure, lower J valueswere determined for 77 transitions as given in theHITRAN database [16]. A complete rovibrationalassignment for 21 of these transitions were reportedin Ref. [22] from FTS spectra at RT combined withCRDS spectra in a supersonic slit expansion.(ii)
A similar sequence of transitions shifted to higherenergies by about 60 cm�1 has been evidenced andassigned to a F2 sub band of 3n2þn3 in our recent CRDSstudy of the 1.28 mm transparency window [13].(iii)
In her original paper [17], L. Brown obtained the Jassignment of a sequence of 13 transitions of the R
branch of the n2þ4n4 (or n1þ3n4) band near6765 cm�1 by comparison of their intensities withthose of the n3 band. Our previous study of the icosadconfirmed 11 of these J values and allowed to extendthe J assignments to 28 transitions of the P, Q, and R
branches [7].
(iv) The most significant achievement for the icosad isthe recent modeling of the 5n4 band system on the
basis of our CRDS spectra at 80 K [11]. These veryweak bands are responsible of most of the absorptionin the 1.58 mm transparency window. Being the low-est energy bands of the icosad, they are less affectedby the many rovibrational interactions perturbingthe other icosad bands and could be consideredmostly ‘‘separately’’ [11].
We hope that the constructed line lists in particularthe 80 K dataset including the empirical low J values andthe isotopologue identification will help for further theo-retical interpretation of the icosad. An original approach[21] consist in considering simultaneously the 13CH4 and12CH4 spectra (the 2650 transitions included in our 80 Klist may constitute a good starting point). The comparisonof the 13CH4 and 12CH4 spectra may give key insight forthe rovibrational assignments for both isotopologues asthe rotational structure of the 13CH4 and 12CH4 bands aremostly identical, each 13CH4 band being simply shifted bya value depending on the upper vibrational state [21].
6. Conclusion
New high sensitivity spectra have been recorded at80 K and 294 K in the icosad region (6717–7589 cm�1) for‘‘natural’’ methane and highly enriched 13CH4 and CH3D.
L. Wang et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 47–57 57
The methane line lists constructed at 80 and 294 Kimprove significantly the line lists available in the litera-ture both in terms of completeness and accuracy. For thefirst time, the 13CH4 and CH3D lines contributing to themethane spectrum in the icosad could be identified.Compared to our previous study using the HITRAN inten-sity values at room temperature, the increased sensitivityof the present recordings has allowed duplicating thenumber of determinations of lower state energy values(11,944 instead of 6063 [7] for do0.003 cm�1). It is nowpossible to account for most of the temperature depen-dence of the absorption of methane in a wide range oftemperature conditions, for example, those relevant toconditions existing in the atmosphere of the giant outerplanets and of Saturn’s satellite, Titan.
The present work represents a major step to completethe WKMC (Wang, Kassi, Mondelain, Campargue) empiri-cal line lists for ‘‘natural’’ methane at 80 and 296 Kbetween 5850 and 7920 cm�1. The two transparencywindows surrounding the presently studied icosad havebeen recently characterized by applying the two tem-perature method to CRDS spectra at 80 and 294 K (seeFig. 1). The CH3D contribution to the absorption has beenfound dominant in the 1.58 mm transparency window. Onthe basis of the new DAS recordings for 13CH4 and CH3D,the identification of the CH3D transitions will be com-pleted and that of 13CH4 lines will be performed beforereleasing the global line list.
Acknowledgments
This work is part of the ANR project ‘‘CH4@Titan’’ (ref:BLAN08-2_321467). The support of the Groupement deRecherche International SAMIA between CNRS (France),RFBR (Russia), and CAS (China) is acknowledged.
Appendix A. Supplementary material
Supplementary data associated with this article can befound in the online version at doi:10.1016/j.jqsrt.2011.09.003.
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