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The 13CH4 absorption spectrum in the Icosad range (6600–7692 cm−1) at 80 K and 296 K: Empirical line lists and temperature dependence
Journal of Molecular Spectroscopy xxx (2016) xxx–xxx
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The 13CH4 absorption spectrum in the Icosad range (6600–7692 cm1) at 80 K and 296 K: Empirical line lists and temperature dependence A. Campargue a,b,⇑, S. Béguier a,b, Y. Zbiri a,b, D. Mondelain a,b, S. Kassi a,b, E.V. Karlovets c, A.V. Nikitin c,d, M. Rey e, E.N. Starikova c,d, Vl.G. Tyuterev e a
Univ. Grenoble Alpes, LIPhy, F-38000 Grenoble, France CNRS, LIPhy, F-38000 Grenoble, France Laboratory of Quantum Mechanics of Molecules and Radiative Processes, Tomsk State University, 36 Lenin Avenue, 634050 Tomsk, Russia d Laboratory of Theoretical Spectroscopy of IAO SB RAN, av. 1, Akademician Zuev square, 634021 Tomsk, Russia e GSMA, UMR CNRS 7331, Université de Reims Champagne Ardenne, Moulin de la Housse, BP 1039, 51687 Reims Cedex 2, France b c
a r t i c l e
i n f o
Article history: Received 30 November 2015 In revised form 6 January 2016 Accepted 14 January 2016 Available online xxxx Keywords: Methane 13 CH4 Isotope Absorption spectroscopy HITRAN
a b s t r a c t The 13CH4 absorption spectrum has been recorded at 296 K and 80 K in the Icosad range between 6600 and 7700 cm1. The achieved noise equivalent absorption of the spectra recorded by differential absorption spectroscopy (DAS) is about amin 1.5 107 cm1. Two empirical line lists were constructed including 17,792 and 24,139 lines at 80 K and 296 K, respectively. For comparison, the HITRAN database provides only 1040 13CH4 lines in the region determined from methane spectra with natural isotopic abundance. Empirical values of the lower state energy level, Eemp, were systematically derived from the intensity ratios of the lines measured at 80 K and 296 K. Overall 10,792 Eemp values were determined providing accurate temperature dependence for most of the 13CH4 absorption in the region (93% and 82% at 80 K and 296 K, respectively). The quality of the derived empirical values of the lower state rotational quantum number, Jemp, is illustrated by their clear propensity to be close to an integer. A good agreement is achieved between our small Jemp values, with previous accurate determinations obtained by applying the 2T method to jet and 80 K spectra. The line lists at 296 K and 80 K which are provided as Supplementary material will be used for future rovibrational assignments based on accurate variational calculations. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction In the recent years, we have constructed the WKLMC empirical lists (Wang, Kassi, Leshchishina, Mondelain, Campargue) at 80 K and 296 K for methane in ‘‘natural” isotopic abundance in the 5852–7919 cm1 region, for applications in planetary and Earth atmospheric science, respectively [1,2]. The positions and intensities were obtained from spectra recorded at 80 K and room temperature by differential absorption spectroscopy (DAS) in the strong absorption regions (the 2m3 region of the Tetradecad [2–4] and the Icosad [6]) and by high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58 lm [7–9] and 1.28 lm [10] transparency windows. (Let us recall that, as a result of the high Tetrahedral Td symmetry of the CH4 molecule, the vibrational levels exhibit a polyad structure characterized by the polyad number ⇑ Corresponding author at: Univ. Grenoble Alpes, LIPhy, F-38000 Grenoble, France. E-mail address: [email protected] (A. Campargue).
P = 2(v1 + v3) + v2 + v4 where vi are the normal mode vibrational quantum numbers. The Tetradecad and the Icosad corresponds to the P = 4 and 5 polyad, respectively). In the WKLMC lists, lines of CH3D and 13CH4 present in ‘‘natural” isotopic abundance in the methane sample (0.05% and 1.1%, respectively) were systematically identified by visual comparison with DAS spectra of the corresponding highly enriched species recorded separately. The line list provided by the HITRAN database [11] for methane in our region reproduces mostly the WKLMC list at 296 K [12]. This applies to the main isotopologue, 12CH4, and also to the 13CH4 isotopologue. In particular, as no empirical line list was available for pure 13CH4 above 6150 cm1, the HITRAN 13 CH4 data were extracted from the WKLMC normal sample list [12]. Consequently, like the WKLMC list, the HITRAN list of 13CH4 is not complete above 6150 cm1, many lines of 13CH4 present in natural methane being obscured by stronger 12CH4 absorption lines. This is the reason why we have undertaken a systematic DAS study of 13CH4 using methane samples highly enriched in 13C. In
http://dx.doi.org/10.1016/j.jms.2016.01.005 0022-2852/Ó 2016 Elsevier Inc. All rights reserved.
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A. Campargue et al. / Journal of Molecular Spectroscopy xxx (2016) xxx–xxx
a first contribution [13], we revisited the high energy part of the 13 CH4 Tetradecad (5850–6200 cm1) previously investigated by Fourier Transform Spectroscopy [14]. Our DAS spectra at 80 K and 296 K allowed extending the amount of observations and to enlarge the set of determinations of lower state energy levels. The present contribution is a very similar study devoted to the wide and highly congested Icosad region (6600–7700 cm1). In the following sections, we provide the experiment details (Section 2) and describe the construction of the room and cold temperature line lists (Section 3). Then we apply the 2temperature method (Section 4) to derive the lower state energy of the transitions and their corresponding empirical J values. In Section 5, the obtained results are discussed in relation to literature data.
2. Experiment The 13CH4 absorption spectrum was recorded by differential absorption spectroscopy at room temperature and at 80 K. The reader is referred to Refs. [15,16] for a detailed description of the DAS spectrometer and of the cryogenic cell cooled at liquid nitrogen temperature. The room temperature recordings were performed in the 6599.8–7691.9 cm1 range using a series of 38 fiber-connected Distributed Feed-Back (DFB) diode lasers. The 80 K recordings cover the 6600.0–7657.3 cm1 range because the highest energy DFB diode was not used as a consequence of the weakness of the 80 K absorption above 7657 cm1. Fig. 1 shows an overview of the recordings with alternate colour for successive DFB diode lasers. For each diode laser, the recorded transmission spectrum is the ratio of a transmitted spectrum over a reference spectrum acquired simultaneously over the whole laser tuning range (about 30 cm1). This tuning range is obtained from the superimposition of a fast
periodic current ramp and a slow temperature ramp from 10 to 60 °C [14], within 12 min. Using a set of two fibered beam splitters (90/10 and 50/50), the laser power was distributed between the absorption cell, a homemade etalon and a wavelength meter (Bristol Instruments 621A). Each final spectrum contains about 105 spectral points separated by a 10 MHz interval, well below the Doppler width of methane lines at 80 K (about 160 MHz HWHM). The 142 cm long absorption cell was used in a round trip configuration leading to absorption path length of 284 cm. The methane sample (from Sigma Aldrich) has a stated chemical purity higher than 99% and 13C enrichment higher than 99%. In fact, the strongest 12CH4 lines of the region could not be detected in our spectra, indicating that the 13C enrichment is significantly better than 99%. The pressure was continuously measured by a capacitance gauge (MKS Baratron type 626B, 10 Torr range, 0.25% accuracy). The room temperature and 80 K spectra were recorded at a pressure 10.0 and 6.0 Torr, respectively. Additional 80 K spectra were recorded at 1.0 Torr in the 7452–7560 cm1 interval in order to measure the strongest lines of the m2 + 2m3 band. The measured external temperature of the cell varied from 295.0 K to 296.6 K during the recordings at room temperature. In Ref. [13], the temperature in the cryogenic cell was determined to be 79.5 ± 1 K from the Doppler broadening of the absorption profiles of 400 lines recorded at 1.0 Torr. This value agrees with our previously temperature determinations using the same method [17] or from the rotational distribution of the line intensities [14]. The wavenumber scale was obtained by combination of the wavelength meter readings and the etalon fringes. To increase the wavenumber accuracy an additional absolute calibration was performed by a statistical matching of the recorded spectrum with reference line positions. We used as reference positions those obtained from a room temperature spectrum of 13CH4 recorded at USTC-Hefei by Fourier Transform Spectroscopy (P = 4 Torr, l = 15 m) [13]. This FTS spectrum was itself calibrated against line positions of H2O (present as an impurity) from the HITRAN database [11]. We estimate to 1.5 103 the uncertainty on the reported line positions. The DAS spectra displayed in Fig. 2 shows the considerable change of the spectrum by cooling down to 80 K. The chosen interval near 6725 cm1 corresponds to the low energy range of the Icosad where are located many high J transitions which mostly
αmin≈1.5x10-7 cm-1
Fig. 1. Overview comparison of the spectrum of 13CH4 recorded by DAS at 80 K (lower panel) and 296 K (upper panel) in the Icosad (6600–7692 cm1). The pressure was 6.0 and 10.0 Torr, respectively. The red and black colors correspond to successive DFB diode lasers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Comparison of the spectra of 13CH4 recorded by DAS at 80 K (upper panel) and 296 K (lower panel) near 6725 cm1. The pressure was 6.0 and 10.0 Torr, respectively. The insert illustrates the noise level corresponding to a noise equivalent absorption amin 1.5 107 cm1.
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vanish at 80 K. The noise equivalent absorbance evaluated as the rms deviation of the baseline fluctuation is on the order of 4.5 105 corresponding to a detectivity threshold of amin 1.5 107 cm1. 3. Line intensity retrieval Line centers and intensities were determined using a home made interactive least squares multi-line fitting program written in LabVIEW and C++. The DFB line width (2 MHz jitter) is much smaller than the Doppler broadening and therefore neglected. The profile was assumed to be of Voigt type with the width of the Gaussian component fixed to the theoretical value of the Doppler width of 13CH4. The Voigt profile was computed using the CPF subroutine proposed by Humlicek [18]. The integrated line absorbance, Lorentzian width and the corresponding local baseline (assumed to be a linear function of the wavenumber) were obtained from the multi-line fit. Fig. 3 shows an example of comparison between the measured and fitted spectra. The fitting process was particularly laborious; in particular the construction of the 296 K list was made difficult by the systematic line overlappings. As a rule, we preferred to limit the number of components required to reproduce a blended absorption feature and constrained the separation between two nearby lines to be larger than 6 103 and 8 103 cm1 at 80 K and 296 K, respectively. The line intensity, Sv 0 (cm/molecule), of a rovibrational transition centered at m0 was derived from the integrated line absorbance, Iv 0 (cm1):
Z Im0 ðTÞ ¼
line
am ldv ¼
Z ln line
I0 ðmÞ dv ¼ Sm0 ðTÞNl IðmÞ
Fig. 4. Overview comparison of the empirical line lists of 13CH4 between 6600 and 7692 cm1 at 80 K (upper panel) and 296 K (lower panel). The HITRAN line list of 13 CH4 is displayed on the lower panel. Line intensities correspond to the pure 13CH4 species.
ð1Þ
I0 ðmÞ is the ratio of the incident intensity to the transmitted IðmÞ intensity, l is the absorption pathlength in cm, m is the wavenumber in cm1, a(m) is the absorption coefficient in cm1, N is the molecular concentration in molecule/cm3 obtained from the measured pressure value: P = NkT. where
The final lists consist of 24,139 and 17,792 lines at 296 K and 80 K, respectively (see Fig. 4). The corresponding average density of lines is thus about 23 and 17 lines per cm1 at 296 K and 80 K, respectively. Minimum intensity values are on the order of 5 1027 cm/molecule, slightly lower at 80 K as a result of a higher gas density for a given pressure value (factor of 4) and of the narrowing of the Doppler line profile (factor of 2). Note that the strongest lines of 12CH4, water and carbon dioxide were not found in our room temperature spectra. We thus believe that the attached line lists are mostly free of impurity lines. 4. Determination of the lower state energy The intensity of an absorption line follows the temperature dependence of the population of the lower state given by the Boltzmann law. The 2T method consists in deriving the lower state energy, Eemp, using the ratio of the line intensities measured at two temperatures [17]:
Sm0 ðT 1 Þ ZðT 0 Þ 1 1 ¼ exp Eemp kT 1 kT 0 Sm0 ðT 0 Þ ZðT 1 Þ
Fig. 3. An example of simulation of the 13CH4 spectrum recorded by DAS at 296 K. Upper panel: Experimental spectrum (P = 10.0 Torr), Middle panel: Simulated spectrum resulting from the line fitting procedure (a Voigt profile was affected to each line). The sticks correspond to the obtained peak list Lower panel: Residuals between the simulated and experimental spectra.
ð2Þ
where Sv 0 and Z are the intensity and partition function, respectively (T0 = 296 K and T1 = 80 K in our case). The corresponding values of the partition function were taken from the HITRAN database Zð296 KÞ ¼ 7:0534. Note that the rigid approximafor 13CH4 [11]: Zð80 KÞ 13 tion is valid for CH4 and Eemp = B0J(J + 1) where B0 = 5.214 cm1 is the 13CH4 ground state rotational constant. Considering the large value of B0, most of the molecules are distributed among ground state rotational levels with J 6 12 (E 6 810 cm1). Hot band transitions are negligible at 80 K and are relatively weak at 296 K as the first vibrational levels have energy on the order of 1300 cm1. In other words, methane lines have no more than thirteen ways to
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A. Campargue et al. / Journal of Molecular Spectroscopy xxx (2016) xxx–xxx
evolve with temperature [2] depending on the J quantum numbers of the lower state. The distinct temperature dependence allows discriminating the different lower state J values from the ratio of a line Sv 0 ð80 KÞ intensity measured at 80 and 296 K. For instance, the Sv 0 ð296 KÞ intensity ratio is close to 1 for J = 5 and decreases from 7.0 for J = 0 to 4.1 103 for J = 10 [2]. The association of the lines corresponding to the same transition in the 296 K and 80 K lists was performed automatically using the coincidence of their line centers as the only criterion. We fixed to 0.003 cm1 the maximum value of the difference, d, of the 296 K and 80 K line centers, according to their estimated uncertainties. It corresponds to only one third of the Doppler width (FWHM) at 80 K. 10,792 pairs of lines were found to fulfil the d < 0.003 cm1 criterion allowing for the derivation of the corresponding lower state energies. (Note that, as the line position difference between two successive lines was constrained to be larger than 6 103 and 8 103 cm1 at 80 and 296 K, respectively, a given line cannot be associated to two lines at the other temperature). Fig. 5 shows the overview of the sets of associated and ‘‘single” lines (left- and right-hand panels, respectively). The sum of the intensities of the lines with Eemp values represents 93% and 82% of the total at 80 K and 296 K, respectively but corresponds to only 60.7% and 44.7% of the total number of lines, respectively. As the bending vibrational levels have a relative population on the order of 103 at 296 K, well below the dynamic range of our intensity values, the ‘‘single‘‘ lines at 296 K include a number of hot band transitions which completely vanish at 80 K. The lower state empirical Jemp values were computed from qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E 12. As an example, the derived empirical J values J emp ¼ 14 þ Bemp 0 have been indicated on the spectra displayed in Fig. 6. The propensity of the obtained J values to be close to integers is illustrated in Fig. 7 which presents an overview of the retrieved J values as a function of the line intensities at 80 K and 296 K. The histograms presented in Fig. 8 in terms of number of lines and of the intensity that they bring at 296 K and 80 K show that the contrast between integer and non-integer Jemp values is more pronounced for the intensities than for the number of lines. Indeed,
Fig. 6. Example of empirical lower J values derived for transitions near 6986 cm1. The pressure of the spectra at 296 K and 80 K was 10.0 and 6.0 Torr, respectively.
non-integer Jemp values correspond in general to weak lines with larger uncertainty on the intensity values. The larger error affecting the weakest lines is reflected on the Jemp values included in Fig. 6 and in Figs. 7 and 8 for J > 9 Table 1 summarizes the statistics of our two lists and includes a comparison to the HITRAN line list [11]. The complete lists at 296 K and 80 K are provided as separate Supplementary Materials. Each list includes the position and intensity of the coincident lines used to derive the Eemp and Jemp values, when available. A sample of the 296 K list is reproduced in Table 2. Note that neglecting the
Fig. 5. Overview comparison of the 13CH4 Icosad lines with and without lower state energy value derived by the 2T method (right and left panels, respectively). The corresponding amount of lines and the relative fraction of their summed intensity are given on each panel.
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A. Campargue et al. / Journal of Molecular Spectroscopy xxx (2016) xxx–xxx Table 1 Statistics for the empirical line lists of the
CH4 Icosad (6600–7692 cm1).
13
T
Sum of intensities (1020 cm/molecule)
Number of lines
Number of Eemp
This work HITRAN2012 [11]
296 K
4.00 1.18
24,139 1040
10,792 523
This work
80 K
4.29
17,792
10,792
Table 2 Wavenumbers and intensities of the absorption lines of 13CH4 recorded by DAS at 296 K and 80 K near 7193 cm1. Intensities are given for pure 13CH4. The empirical values of the lower state energy levels, Eemp and corresponding Jemp values were obtained by the 2T method for the transitions whose centers at 296 K and at 80 K coincide within 0.003 cm1. This table is a small section of the list of 24,139 lines at 296 K attached as Supplementary Material. A similar list is also provided for T = 80 K.
Fig. 7. Empirical J values of the 13CH4 transitions derived between 6600 and 7692 cm1 versus the line intensities at 296 K (upper panel) and 80 K (lower panel).
T = 296 K
T = 80 K
Line center Line intensity (cm1) (cm/molecule)
Line center Line intensity (cm1) (cm/molecule)
Eemp Jemp (cm1)
7192.7544 7192.8215 7192.8645 7192.8932 7192.9441 7192.9987 7193.0387 7193.0667 7193.1339 7193.1541 7193.1775 7193.2077 7193.2431 7193.2988 7193.3172 7193.3404 7193.4093 7193.4656 7193.5334 7193.5649 7193.6170 7193.6655 7193.7347 7193.7805 7193.8331
7192.7565 7192.8228 7192.8663 7192.8947
6.72E25 4.29E26 6.03E25 2.68E25
221.70 383.40 374.50 314.14
7193.0002 7193.0391 7193.0696
1.54E25 3.92E24 3.12E25
383.46 8.07 168.19 5.19 375.90 7.98
7193.1541
2.40E25
369.80 7.92
7193.2078
3.38E26
390.73 8.15
7193.3000 7193.3162 7193.3431 7193.4110 7193.4655
4.85E23 4.22E24 1.48E23 8.08E24 1.20E23
108.58 185.69 168.31 223.11 143.74
7193.5658
1.54E25
312.48 7.24
7193.6675 7193.7365 7193.7819 7193.8350
2.77E26 2.41E23 3.48E24 4.16E25
294.23 106.87 147.63 301.11
1.75E24 9.32E25 1.17E23 2.35E24 1.32E24 3.35E24 5.05E24 6.14E24 1.62E24 4.36E24 5.88E25 8.08E25 1.69E24 2.86E23 6.85E24 1.91E23 2.14E23 1.12E23 2.81E25 1.32E24 7.40E26 1.87E25 1.39E23 3.42E24 3.06E24
6.02 8.07 7.97 7.26
4.08 5.47 5.19 6.04 4.76
7.01 4.04 4.83 7.10
centrifugal distortion terms results in relatively small error of about 1 cm1 in Eemp for J 10. Major sources of non-integer Jemp values are due to intensity uncertainties and possible mismatch of blended lines. 5. Discussion
Fig. 8. Histograms of the empirical lower Jemp values obtained for 13CH4 in the 6600–7692 cm1 region. Upper panel: Count of the obtained Jemp values with a step interval of 0.1. Medium and lower panels: Corresponding sum of line intensities at 296 K and 80 K, respectively.
As mentioned in the introduction, previous 13CH4 spectroscopic data in the region are scarce and the 13CH4 HITRAN line list [11,12] is very incomplete (see Fig. 4) as it was obtained from the WKLMC line list of natural methane [1]. The present data obtained with pure 13CH4 enlarge considerably the observations (about 1040 lines in the HITRAN database compared to more than 24,000 lines presently measured at 296 K) and improve their accuracy, in particular for line intensities. The total HITRAN intensity of the 13 CH4 lines represents less than one third of the intensity sum measured in this work at 296 K (Table 1). For about half of the 13 CH4 lines of the region, the WKLMC@296 K provided Eemp values which are reproduced in HITRAN. For the remaining lines missing Eemp, an artificial value of 333.33 cm1 was added in the HITRAN list for the lower state energy. This value should be considered as a label indicating that the lower state energy is unknown. Let us note that no complete rovibrational assignments are provided in HITRAN, even for the m2 + 2m3 band near 7494 cm1 for which J values are unambiguous.
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In a recent contribution, Ulenikov et al. [19] reported the FTS high resolution spectrum of 12CH4 and 13CH4 from 1000 to 12,000 cm1 recorded at ETH Zurich at 80 K and 298 K. A multireflection collisional cooling cell with absorption path length between 5 and 15 m was used for the low temperature recordings. A number of vibrational term values were determined from the corresponding P(1) transitions reaching the pure vibrational levels (J = 0). Unfortunately no line list was constructed and only the P(1) line center values were provided by the authors (Tables IV and V of Ref. [19]). Partial vibrational assignment was reported for a number of bands up to 11,300 cm1 on the basis of an effective Hamiltonian model. In the case of the Icosad of the 13CH4 isotopologue, only the P(1) line of the m2 + 2m3 band was reported. In the case of the Icosad of the main isotopologue, 12CH4, part of the derived vibrational terms was not confirmed by our recent full rovibrational assignments based on high accuracy variational calculations [20]. Of particular relevance for the present study is the work by Votava et al. [21] who reported the Icosad absorption spectrum of 13CH4 in a supersonic jet providing a 32 K rotational temperature. A total of 339 lines, including absolute intensities, were measured in the center of the Icosad between 7200 and 7354 cm1 [21]. In order to apply the 2T method, the jet data was combined with our DAS data at 80 K and an 80 K DAS line list was constructed for the 7173–7367 cm1 interval [21]. Note that this 80 K line list is incorporated mostly unchanged in the presently reported global list. All the transitions detected in jet conditions were also measured by DAS at 80 K, allowing for the derivation of their lower state empirical energy. As a consequence of the efficient rotational cooling, the 339 derived Jemp values are all smaller than 4. Six R(0) transitions could be unambiguously identified. As discussed in Refs. [5,22], the combination of 80 K line intensities to jet intensity data instead to room temperature intensities provides more accurate determination of the low Eemp values. For instance, while the J = 0 and J = 1 lower state J values can hardly be discriminated from their intensity ratios at 80 and 296 K (7.0 and 6.1 [2], respectively), they are well identified from the jet/80 K ratio of their intensities. This is mostly due to the total absence of line overlapping in the jet spectrum which makes the line intensity retrieval more accurate. We have systematically compared the Jemp values presently obtained from 80 K/296 K intensities to those derived in Ref. [21] from jet/80 K intensities. According to the histogram presented Fig. 9, 69% of the rounded Jemp values agree with those of Ref. [21] which are expected to be more accurate; 88% correspond to
|Jemp J| < 1. The future rovibrational assignments of the spectrum using theoretical calculations (see below) will probably confirm the quality of the Jemp values derived in Ref. [21].
6. Concluding remarks In this work, we have constructed the first complete empirical line lists for 13CH4 at 80 K and 296 K in the very congested Icosad region. This significantly extends scarce spectroscopic information provided by the HITRAN list in this range. The obtained experimental data could be important for determination of 13C/12C isotopic ratio from planetary spectra: a non-negligible contribution of 13 CH4 in the methane opacity of Titan atmosphere has been detected in lower spectral ranges as discussed in [23,24]. On the other hand our measurements will be valuable for extending theoretical studies of the isotopic effects [25] in highly excited vibrational states of methane. Accurate first-principles calculations of rotationally resolved spectra of five-atomics were still a remote goal a few years ago, but have recently become feasible. Despite a considerable spectral congestion (about 20 lines/cm1), the methane Icosad spectrum for the major 12CH4 isotopologue has been rovibrationally assigned using accurate ab initio potential energy surfaces (PES) [26] and dipole moment surfaces (DMS) [27] and variational methods [28,29] developed by Reims and Tomsk teams. A mostly complete identification of the 12CH4 Icosad bands (involving more than 10,000 lines of 108 sub-bands) in the 6280–7800 cm1 range has been performed [20] for the first time on the basis of ab initio intensity predictions [30]. The further work on the assignment of the experimental 13CH4 lines reported in the present paper will benefit from ab initio calculations for the isotopic shifts in vibrational band centers due to the 12 C ? 13C substitution. These shifts appear to be quite irregular ranging between 1 and 40 cm1 depending on vibrational quantum numbers. Previous studies have shown that efficient variational methods are able to predict these shifts very accurately [25], with error bars of the order of 0.01 cm1, at least for the lower polyads. We plan extending this approach to the theoretical interpretation of our new data. Previously determined [20] empirical vibrational energies of the 12CH4 Icosad and isotopic shifts predicted by theory for each band allow for unambiguous one-to-one correspondence and for transfer of most of the 12CH4 assignments to the 13CH4 line list. In turn, the 13CH4 data could provide a further check for the 12 CH4 assignments. Isotopic shift approach has been recently validated in the 2m3 range (5850–6200 cm1) of the 13CH4 Tetradecad [31] where the 13CH4 DAS spectrum at 80 K could be modelled using the effective Hamiltonian derived from the PES [26] by the contact transformation method [32]. The rovibrational assignment of the present 80 K and 296 K line lists will help calibrating 13CH4 transitions in theoretical predictions [33] of methane spectra for higher temperature conditions.
Acknowledgments The support of the LIA SAMIA between CNRS (France) and RFBR (Russia) as well as of Tomsk State University D. Mendeleev funding program is acknowledged. This work was performed in the frame of the Labex OSUG@2020 (ANR10 LABX56).
Appendix A. Supplementary material Fig. 9. Histogram of the differences between the lower state Jemp values determined by the 2T method applied to the 80 K/296 K DAS intensities (this work) and to the 32 K jet and 80 K DAS intensities (Ref. [21]).
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jms.2016.01.005.
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References [1] A. Campargue, O. Leshchishina, L. Wang, D. Mondelain, S. Kassi, J. Mol. Spectrosc. 291 (2013) 16–22. [2] A. Campargue, L. Wang, D. Mondelain, S. Kassi, B. Bézard, E. Lellouch, M. Hirtzig, A. Coustenis, C. de Bergh, P. Drossart, Icarus 219 (2012) 110–128. [3] A. Campargue, O. Leshchishina, L. Wang, D. Mondelain, S. Kassi, A.V. Nikitin, J. Quant. Spectrosc. Radiat. Trans. 113 (2012) 1855–1873. [4] A. Campargue, O. Leshchishina, D. Mondelain, S. Kassi, A. Coustenis, J. Quant. Spectrosc. Radiat. Trans. 118 (2013) 49–59. [5] O. Votava, M. Mašát, P. Pracna, S. Kassi, A. Campargue, Phys. Chem. Chem. Phys. 12 (2010) 3145–3155. [6] L. Wang, D. Mondelain, S. Kassi, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 113 (2012) 47–57. [7] A. Campargue, L. Wang, A.W. Liu, S.M. Hu, S. Kassi, Chem. Phys. 373 (2010) 203–210. [8] A.V. Nikitin, X. Thomas, L. Régalia, L. Daumont, P. Von der Heyden, Vl.G. Tyuterev, L. Wang, S. Kassi, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 112 (2011) 28–40. [9] L. Wang, S. Kassi, A. Liu, S. Hu, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 112 (2011) 937–951. [10] D. Mondelain, S. Kassi, L. Wang, A. Campargue, Phys. Chem. Chem. Phys. 17 (2011) 7985–7996. [11] L.S. Rothman, I.E. Gordon, Y. Babikov, A. Barbe, D. Chris Benner, P.F. Bernath, M. Birk, L. Bizzocchi, V. Boudon, L.R. Brown, A. Campargue, K. Chance, L. Coudert, V.M. Devi, B.J. Drouin, A. Fayt, J.-M. Flaud, R.R. Gamache, J. Harrison, J.-M. Hartmann, C. Hill, J.T. Hodges, D. Jacquemart, A. Jolly, J. Lamouroux, R.J. LeRoy, G. Lia, D. Longo, C.J. Mackie, S.T. Massie, S. Mikhailenko, H.S.P. Müller, O.V. Naumenko, A.V. Nikitin, J. Orphal, V. Perevalov, A. Perrin, E.R. Polovtseva, C. Richard, M.A.H. Smith, E. Starikova, K. Sung, S. Tashkun, J. Tennyson, G.C. Toon, Vl.G. Tyuterev, J. Vander Auwera, G. Wagner, J. Quant. Spectrosc. Radiat. Trans. 130 (2013) 4. [12] L.R. Brown, K. Sung, D.C. Benner, V.M. Devi, V. Boudon, T. Gabard, et al., J. Quant. Spectrosc. Radiat. Trans. 130 (2013) 201–219. [13] A. Campargue, J. Lopez Segovia, S. Béguier, S. Kassi, D. Mondelain, J. Quant. Spectrosc. Radiat. Trans. 152 (2015) 140–148.
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[14] O.M. Lyulin, S. Kassi, K. Sung, L.R. Brown, A. Campargue, J. Mol. Spectrosc. 261 (2010) 91–100. [15] Y. Lu, D. Mondelain, S. Kassi, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 112 (2011) 2683–2697. [16] Bo. Gao, S. Kassi, A. Campargue, J. Mol. Spectrosc. 253 (2009) 55–63. [17] S. Kassi, B. Gao, D. Romanini, A. Campargue, Phys. Chem. Chem. Phys. 10 (2008) 4410–4419. [18] J. Humlicek, J. Quant. Spectrosc. Radiat. Trans. 21 (1979) 309–313. [19] O.N. Ulenikov, E.S. Bekhtereva, S. Albert, S. Bauerecker, H.M. Niederer, M. Quack, J. Chem. Phys. 141 (2014) 234302. [20] M. Rey, A.V. Nikitin, A. Campargue, S. Kassi, D. Mondelain, V.G. Tyuterev, Phys. Chem. Chem. Phys. 18 (2016) 176–189. [21] O. Votava, M. Mašát, P. Pracna, D. Mondelain, S. Kassi, A.W. Liu, S.M. Hu, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 149 (2014) 64–71. [22] M. Mašát, P. Pracna, D. Mondelain, S. Kassi, A. Campargue, O. Votava, J. Mol. Spectrosc. 291 (2013) 9–15. [23] C. De Bergh, R. Courtin, B. Bézard, A. Coustenis, E. Lellouch, M. Hirtzig, et al., Plan. Space Sci. 61 (2012) 85–98. [24] M. Hirtzig, B. Bézard, E. Lellouch, A. Coustenis, C. De Bergh, P. Drossart, et al., Icarus 226 (2013) 470–486. [25] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, J. Mol. Spectrosc. 291 (2013) 85–97. [26] A.V. Nikitin, M. Rey, Vl.G. Tyuterev, Chem. Phys. Lett. 501 (2011) 179–186. [27] A.V. Nikitin, M. Rey, Vl.G. Tyuterev, Chem. Phys. Lett. 565 (2013) 5–11. [28] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, J. Chem. Phys. 136 (2012) 244106. [29] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, Phys. Chem. Chem. Phys. 15 (2013) 10049– 10061. [30] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, J. Quant. Spectrosc. Radiat. Trans. 164 (2015) 207–220. [31] E. Starikova, A.V. Nikitin, M. Rey, S.A. Tashkun, D. Mondelain, S. Kassi, A. Campargue, Vl.G. Tyuterev, J. Quant. Spectrosc. Radiat. Trans., http://dx.doi. org/10.1016/j.jqsrt.2015.12.023. [32] Vl.G. Tyuterev, S.A. Tashkun, M. Rey, R. Kochanov, A. Nikitin, T. Delahaye, J. Phys. Chem. A 117 (2013) 13779–13805. [33] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, Astrophys. J. 789 (2014) 1–10.
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The 13CH4 absorption spectrum in the Icosad range (6600–7692 cm1) at 80 K and 296 K: Empirical line lists and temperature dependence A. Campargue a,b,⇑, S. Béguier a,b, Y. Zbiri a,b, D. Mondelain a,b, S. Kassi a,b, E.V. Karlovets c, A.V. Nikitin c,d, M. Rey e, E.N. Starikova c,d, Vl.G. Tyuterev e a
Univ. Grenoble Alpes, LIPhy, F-38000 Grenoble, France CNRS, LIPhy, F-38000 Grenoble, France Laboratory of Quantum Mechanics of Molecules and Radiative Processes, Tomsk State University, 36 Lenin Avenue, 634050 Tomsk, Russia d Laboratory of Theoretical Spectroscopy of IAO SB RAN, av. 1, Akademician Zuev square, 634021 Tomsk, Russia e GSMA, UMR CNRS 7331, Université de Reims Champagne Ardenne, Moulin de la Housse, BP 1039, 51687 Reims Cedex 2, France b c
a r t i c l e
i n f o
Article history: Received 30 November 2015 In revised form 6 January 2016 Accepted 14 January 2016 Available online xxxx Keywords: Methane 13 CH4 Isotope Absorption spectroscopy HITRAN
a b s t r a c t The 13CH4 absorption spectrum has been recorded at 296 K and 80 K in the Icosad range between 6600 and 7700 cm1. The achieved noise equivalent absorption of the spectra recorded by differential absorption spectroscopy (DAS) is about amin 1.5 107 cm1. Two empirical line lists were constructed including 17,792 and 24,139 lines at 80 K and 296 K, respectively. For comparison, the HITRAN database provides only 1040 13CH4 lines in the region determined from methane spectra with natural isotopic abundance. Empirical values of the lower state energy level, Eemp, were systematically derived from the intensity ratios of the lines measured at 80 K and 296 K. Overall 10,792 Eemp values were determined providing accurate temperature dependence for most of the 13CH4 absorption in the region (93% and 82% at 80 K and 296 K, respectively). The quality of the derived empirical values of the lower state rotational quantum number, Jemp, is illustrated by their clear propensity to be close to an integer. A good agreement is achieved between our small Jemp values, with previous accurate determinations obtained by applying the 2T method to jet and 80 K spectra. The line lists at 296 K and 80 K which are provided as Supplementary material will be used for future rovibrational assignments based on accurate variational calculations. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction In the recent years, we have constructed the WKLMC empirical lists (Wang, Kassi, Leshchishina, Mondelain, Campargue) at 80 K and 296 K for methane in ‘‘natural” isotopic abundance in the 5852–7919 cm1 region, for applications in planetary and Earth atmospheric science, respectively [1,2]. The positions and intensities were obtained from spectra recorded at 80 K and room temperature by differential absorption spectroscopy (DAS) in the strong absorption regions (the 2m3 region of the Tetradecad [2–4] and the Icosad [6]) and by high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58 lm [7–9] and 1.28 lm [10] transparency windows. (Let us recall that, as a result of the high Tetrahedral Td symmetry of the CH4 molecule, the vibrational levels exhibit a polyad structure characterized by the polyad number ⇑ Corresponding author at: Univ. Grenoble Alpes, LIPhy, F-38000 Grenoble, France. E-mail address: [email protected] (A. Campargue).
P = 2(v1 + v3) + v2 + v4 where vi are the normal mode vibrational quantum numbers. The Tetradecad and the Icosad corresponds to the P = 4 and 5 polyad, respectively). In the WKLMC lists, lines of CH3D and 13CH4 present in ‘‘natural” isotopic abundance in the methane sample (0.05% and 1.1%, respectively) were systematically identified by visual comparison with DAS spectra of the corresponding highly enriched species recorded separately. The line list provided by the HITRAN database [11] for methane in our region reproduces mostly the WKLMC list at 296 K [12]. This applies to the main isotopologue, 12CH4, and also to the 13CH4 isotopologue. In particular, as no empirical line list was available for pure 13CH4 above 6150 cm1, the HITRAN 13 CH4 data were extracted from the WKLMC normal sample list [12]. Consequently, like the WKLMC list, the HITRAN list of 13CH4 is not complete above 6150 cm1, many lines of 13CH4 present in natural methane being obscured by stronger 12CH4 absorption lines. This is the reason why we have undertaken a systematic DAS study of 13CH4 using methane samples highly enriched in 13C. In
http://dx.doi.org/10.1016/j.jms.2016.01.005 0022-2852/Ó 2016 Elsevier Inc. All rights reserved.
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A. Campargue et al. / Journal of Molecular Spectroscopy xxx (2016) xxx–xxx
a first contribution [13], we revisited the high energy part of the 13 CH4 Tetradecad (5850–6200 cm1) previously investigated by Fourier Transform Spectroscopy [14]. Our DAS spectra at 80 K and 296 K allowed extending the amount of observations and to enlarge the set of determinations of lower state energy levels. The present contribution is a very similar study devoted to the wide and highly congested Icosad region (6600–7700 cm1). In the following sections, we provide the experiment details (Section 2) and describe the construction of the room and cold temperature line lists (Section 3). Then we apply the 2temperature method (Section 4) to derive the lower state energy of the transitions and their corresponding empirical J values. In Section 5, the obtained results are discussed in relation to literature data.
2. Experiment The 13CH4 absorption spectrum was recorded by differential absorption spectroscopy at room temperature and at 80 K. The reader is referred to Refs. [15,16] for a detailed description of the DAS spectrometer and of the cryogenic cell cooled at liquid nitrogen temperature. The room temperature recordings were performed in the 6599.8–7691.9 cm1 range using a series of 38 fiber-connected Distributed Feed-Back (DFB) diode lasers. The 80 K recordings cover the 6600.0–7657.3 cm1 range because the highest energy DFB diode was not used as a consequence of the weakness of the 80 K absorption above 7657 cm1. Fig. 1 shows an overview of the recordings with alternate colour for successive DFB diode lasers. For each diode laser, the recorded transmission spectrum is the ratio of a transmitted spectrum over a reference spectrum acquired simultaneously over the whole laser tuning range (about 30 cm1). This tuning range is obtained from the superimposition of a fast
periodic current ramp and a slow temperature ramp from 10 to 60 °C [14], within 12 min. Using a set of two fibered beam splitters (90/10 and 50/50), the laser power was distributed between the absorption cell, a homemade etalon and a wavelength meter (Bristol Instruments 621A). Each final spectrum contains about 105 spectral points separated by a 10 MHz interval, well below the Doppler width of methane lines at 80 K (about 160 MHz HWHM). The 142 cm long absorption cell was used in a round trip configuration leading to absorption path length of 284 cm. The methane sample (from Sigma Aldrich) has a stated chemical purity higher than 99% and 13C enrichment higher than 99%. In fact, the strongest 12CH4 lines of the region could not be detected in our spectra, indicating that the 13C enrichment is significantly better than 99%. The pressure was continuously measured by a capacitance gauge (MKS Baratron type 626B, 10 Torr range, 0.25% accuracy). The room temperature and 80 K spectra were recorded at a pressure 10.0 and 6.0 Torr, respectively. Additional 80 K spectra were recorded at 1.0 Torr in the 7452–7560 cm1 interval in order to measure the strongest lines of the m2 + 2m3 band. The measured external temperature of the cell varied from 295.0 K to 296.6 K during the recordings at room temperature. In Ref. [13], the temperature in the cryogenic cell was determined to be 79.5 ± 1 K from the Doppler broadening of the absorption profiles of 400 lines recorded at 1.0 Torr. This value agrees with our previously temperature determinations using the same method [17] or from the rotational distribution of the line intensities [14]. The wavenumber scale was obtained by combination of the wavelength meter readings and the etalon fringes. To increase the wavenumber accuracy an additional absolute calibration was performed by a statistical matching of the recorded spectrum with reference line positions. We used as reference positions those obtained from a room temperature spectrum of 13CH4 recorded at USTC-Hefei by Fourier Transform Spectroscopy (P = 4 Torr, l = 15 m) [13]. This FTS spectrum was itself calibrated against line positions of H2O (present as an impurity) from the HITRAN database [11]. We estimate to 1.5 103 the uncertainty on the reported line positions. The DAS spectra displayed in Fig. 2 shows the considerable change of the spectrum by cooling down to 80 K. The chosen interval near 6725 cm1 corresponds to the low energy range of the Icosad where are located many high J transitions which mostly
αmin≈1.5x10-7 cm-1
Fig. 1. Overview comparison of the spectrum of 13CH4 recorded by DAS at 80 K (lower panel) and 296 K (upper panel) in the Icosad (6600–7692 cm1). The pressure was 6.0 and 10.0 Torr, respectively. The red and black colors correspond to successive DFB diode lasers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Comparison of the spectra of 13CH4 recorded by DAS at 80 K (upper panel) and 296 K (lower panel) near 6725 cm1. The pressure was 6.0 and 10.0 Torr, respectively. The insert illustrates the noise level corresponding to a noise equivalent absorption amin 1.5 107 cm1.
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vanish at 80 K. The noise equivalent absorbance evaluated as the rms deviation of the baseline fluctuation is on the order of 4.5 105 corresponding to a detectivity threshold of amin 1.5 107 cm1. 3. Line intensity retrieval Line centers and intensities were determined using a home made interactive least squares multi-line fitting program written in LabVIEW and C++. The DFB line width (2 MHz jitter) is much smaller than the Doppler broadening and therefore neglected. The profile was assumed to be of Voigt type with the width of the Gaussian component fixed to the theoretical value of the Doppler width of 13CH4. The Voigt profile was computed using the CPF subroutine proposed by Humlicek [18]. The integrated line absorbance, Lorentzian width and the corresponding local baseline (assumed to be a linear function of the wavenumber) were obtained from the multi-line fit. Fig. 3 shows an example of comparison between the measured and fitted spectra. The fitting process was particularly laborious; in particular the construction of the 296 K list was made difficult by the systematic line overlappings. As a rule, we preferred to limit the number of components required to reproduce a blended absorption feature and constrained the separation between two nearby lines to be larger than 6 103 and 8 103 cm1 at 80 K and 296 K, respectively. The line intensity, Sv 0 (cm/molecule), of a rovibrational transition centered at m0 was derived from the integrated line absorbance, Iv 0 (cm1):
Z Im0 ðTÞ ¼
line
am ldv ¼
Z ln line
I0 ðmÞ dv ¼ Sm0 ðTÞNl IðmÞ
Fig. 4. Overview comparison of the empirical line lists of 13CH4 between 6600 and 7692 cm1 at 80 K (upper panel) and 296 K (lower panel). The HITRAN line list of 13 CH4 is displayed on the lower panel. Line intensities correspond to the pure 13CH4 species.
ð1Þ
I0 ðmÞ is the ratio of the incident intensity to the transmitted IðmÞ intensity, l is the absorption pathlength in cm, m is the wavenumber in cm1, a(m) is the absorption coefficient in cm1, N is the molecular concentration in molecule/cm3 obtained from the measured pressure value: P = NkT. where
The final lists consist of 24,139 and 17,792 lines at 296 K and 80 K, respectively (see Fig. 4). The corresponding average density of lines is thus about 23 and 17 lines per cm1 at 296 K and 80 K, respectively. Minimum intensity values are on the order of 5 1027 cm/molecule, slightly lower at 80 K as a result of a higher gas density for a given pressure value (factor of 4) and of the narrowing of the Doppler line profile (factor of 2). Note that the strongest lines of 12CH4, water and carbon dioxide were not found in our room temperature spectra. We thus believe that the attached line lists are mostly free of impurity lines. 4. Determination of the lower state energy The intensity of an absorption line follows the temperature dependence of the population of the lower state given by the Boltzmann law. The 2T method consists in deriving the lower state energy, Eemp, using the ratio of the line intensities measured at two temperatures [17]:
Sm0 ðT 1 Þ ZðT 0 Þ 1 1 ¼ exp Eemp kT 1 kT 0 Sm0 ðT 0 Þ ZðT 1 Þ
Fig. 3. An example of simulation of the 13CH4 spectrum recorded by DAS at 296 K. Upper panel: Experimental spectrum (P = 10.0 Torr), Middle panel: Simulated spectrum resulting from the line fitting procedure (a Voigt profile was affected to each line). The sticks correspond to the obtained peak list Lower panel: Residuals between the simulated and experimental spectra.
ð2Þ
where Sv 0 and Z are the intensity and partition function, respectively (T0 = 296 K and T1 = 80 K in our case). The corresponding values of the partition function were taken from the HITRAN database Zð296 KÞ ¼ 7:0534. Note that the rigid approximafor 13CH4 [11]: Zð80 KÞ 13 tion is valid for CH4 and Eemp = B0J(J + 1) where B0 = 5.214 cm1 is the 13CH4 ground state rotational constant. Considering the large value of B0, most of the molecules are distributed among ground state rotational levels with J 6 12 (E 6 810 cm1). Hot band transitions are negligible at 80 K and are relatively weak at 296 K as the first vibrational levels have energy on the order of 1300 cm1. In other words, methane lines have no more than thirteen ways to
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A. Campargue et al. / Journal of Molecular Spectroscopy xxx (2016) xxx–xxx
evolve with temperature [2] depending on the J quantum numbers of the lower state. The distinct temperature dependence allows discriminating the different lower state J values from the ratio of a line Sv 0 ð80 KÞ intensity measured at 80 and 296 K. For instance, the Sv 0 ð296 KÞ intensity ratio is close to 1 for J = 5 and decreases from 7.0 for J = 0 to 4.1 103 for J = 10 [2]. The association of the lines corresponding to the same transition in the 296 K and 80 K lists was performed automatically using the coincidence of their line centers as the only criterion. We fixed to 0.003 cm1 the maximum value of the difference, d, of the 296 K and 80 K line centers, according to their estimated uncertainties. It corresponds to only one third of the Doppler width (FWHM) at 80 K. 10,792 pairs of lines were found to fulfil the d < 0.003 cm1 criterion allowing for the derivation of the corresponding lower state energies. (Note that, as the line position difference between two successive lines was constrained to be larger than 6 103 and 8 103 cm1 at 80 and 296 K, respectively, a given line cannot be associated to two lines at the other temperature). Fig. 5 shows the overview of the sets of associated and ‘‘single” lines (left- and right-hand panels, respectively). The sum of the intensities of the lines with Eemp values represents 93% and 82% of the total at 80 K and 296 K, respectively but corresponds to only 60.7% and 44.7% of the total number of lines, respectively. As the bending vibrational levels have a relative population on the order of 103 at 296 K, well below the dynamic range of our intensity values, the ‘‘single‘‘ lines at 296 K include a number of hot band transitions which completely vanish at 80 K. The lower state empirical Jemp values were computed from qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E 12. As an example, the derived empirical J values J emp ¼ 14 þ Bemp 0 have been indicated on the spectra displayed in Fig. 6. The propensity of the obtained J values to be close to integers is illustrated in Fig. 7 which presents an overview of the retrieved J values as a function of the line intensities at 80 K and 296 K. The histograms presented in Fig. 8 in terms of number of lines and of the intensity that they bring at 296 K and 80 K show that the contrast between integer and non-integer Jemp values is more pronounced for the intensities than for the number of lines. Indeed,
Fig. 6. Example of empirical lower J values derived for transitions near 6986 cm1. The pressure of the spectra at 296 K and 80 K was 10.0 and 6.0 Torr, respectively.
non-integer Jemp values correspond in general to weak lines with larger uncertainty on the intensity values. The larger error affecting the weakest lines is reflected on the Jemp values included in Fig. 6 and in Figs. 7 and 8 for J > 9 Table 1 summarizes the statistics of our two lists and includes a comparison to the HITRAN line list [11]. The complete lists at 296 K and 80 K are provided as separate Supplementary Materials. Each list includes the position and intensity of the coincident lines used to derive the Eemp and Jemp values, when available. A sample of the 296 K list is reproduced in Table 2. Note that neglecting the
Fig. 5. Overview comparison of the 13CH4 Icosad lines with and without lower state energy value derived by the 2T method (right and left panels, respectively). The corresponding amount of lines and the relative fraction of their summed intensity are given on each panel.
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A. Campargue et al. / Journal of Molecular Spectroscopy xxx (2016) xxx–xxx Table 1 Statistics for the empirical line lists of the
CH4 Icosad (6600–7692 cm1).
13
T
Sum of intensities (1020 cm/molecule)
Number of lines
Number of Eemp
This work HITRAN2012 [11]
296 K
4.00 1.18
24,139 1040
10,792 523
This work
80 K
4.29
17,792
10,792
Table 2 Wavenumbers and intensities of the absorption lines of 13CH4 recorded by DAS at 296 K and 80 K near 7193 cm1. Intensities are given for pure 13CH4. The empirical values of the lower state energy levels, Eemp and corresponding Jemp values were obtained by the 2T method for the transitions whose centers at 296 K and at 80 K coincide within 0.003 cm1. This table is a small section of the list of 24,139 lines at 296 K attached as Supplementary Material. A similar list is also provided for T = 80 K.
Fig. 7. Empirical J values of the 13CH4 transitions derived between 6600 and 7692 cm1 versus the line intensities at 296 K (upper panel) and 80 K (lower panel).
T = 296 K
T = 80 K
Line center Line intensity (cm1) (cm/molecule)
Line center Line intensity (cm1) (cm/molecule)
Eemp Jemp (cm1)
7192.7544 7192.8215 7192.8645 7192.8932 7192.9441 7192.9987 7193.0387 7193.0667 7193.1339 7193.1541 7193.1775 7193.2077 7193.2431 7193.2988 7193.3172 7193.3404 7193.4093 7193.4656 7193.5334 7193.5649 7193.6170 7193.6655 7193.7347 7193.7805 7193.8331
7192.7565 7192.8228 7192.8663 7192.8947
6.72E25 4.29E26 6.03E25 2.68E25
221.70 383.40 374.50 314.14
7193.0002 7193.0391 7193.0696
1.54E25 3.92E24 3.12E25
383.46 8.07 168.19 5.19 375.90 7.98
7193.1541
2.40E25
369.80 7.92
7193.2078
3.38E26
390.73 8.15
7193.3000 7193.3162 7193.3431 7193.4110 7193.4655
4.85E23 4.22E24 1.48E23 8.08E24 1.20E23
108.58 185.69 168.31 223.11 143.74
7193.5658
1.54E25
312.48 7.24
7193.6675 7193.7365 7193.7819 7193.8350
2.77E26 2.41E23 3.48E24 4.16E25
294.23 106.87 147.63 301.11
1.75E24 9.32E25 1.17E23 2.35E24 1.32E24 3.35E24 5.05E24 6.14E24 1.62E24 4.36E24 5.88E25 8.08E25 1.69E24 2.86E23 6.85E24 1.91E23 2.14E23 1.12E23 2.81E25 1.32E24 7.40E26 1.87E25 1.39E23 3.42E24 3.06E24
6.02 8.07 7.97 7.26
4.08 5.47 5.19 6.04 4.76
7.01 4.04 4.83 7.10
centrifugal distortion terms results in relatively small error of about 1 cm1 in Eemp for J 10. Major sources of non-integer Jemp values are due to intensity uncertainties and possible mismatch of blended lines. 5. Discussion
Fig. 8. Histograms of the empirical lower Jemp values obtained for 13CH4 in the 6600–7692 cm1 region. Upper panel: Count of the obtained Jemp values with a step interval of 0.1. Medium and lower panels: Corresponding sum of line intensities at 296 K and 80 K, respectively.
As mentioned in the introduction, previous 13CH4 spectroscopic data in the region are scarce and the 13CH4 HITRAN line list [11,12] is very incomplete (see Fig. 4) as it was obtained from the WKLMC line list of natural methane [1]. The present data obtained with pure 13CH4 enlarge considerably the observations (about 1040 lines in the HITRAN database compared to more than 24,000 lines presently measured at 296 K) and improve their accuracy, in particular for line intensities. The total HITRAN intensity of the 13 CH4 lines represents less than one third of the intensity sum measured in this work at 296 K (Table 1). For about half of the 13 CH4 lines of the region, the WKLMC@296 K provided Eemp values which are reproduced in HITRAN. For the remaining lines missing Eemp, an artificial value of 333.33 cm1 was added in the HITRAN list for the lower state energy. This value should be considered as a label indicating that the lower state energy is unknown. Let us note that no complete rovibrational assignments are provided in HITRAN, even for the m2 + 2m3 band near 7494 cm1 for which J values are unambiguous.
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In a recent contribution, Ulenikov et al. [19] reported the FTS high resolution spectrum of 12CH4 and 13CH4 from 1000 to 12,000 cm1 recorded at ETH Zurich at 80 K and 298 K. A multireflection collisional cooling cell with absorption path length between 5 and 15 m was used for the low temperature recordings. A number of vibrational term values were determined from the corresponding P(1) transitions reaching the pure vibrational levels (J = 0). Unfortunately no line list was constructed and only the P(1) line center values were provided by the authors (Tables IV and V of Ref. [19]). Partial vibrational assignment was reported for a number of bands up to 11,300 cm1 on the basis of an effective Hamiltonian model. In the case of the Icosad of the 13CH4 isotopologue, only the P(1) line of the m2 + 2m3 band was reported. In the case of the Icosad of the main isotopologue, 12CH4, part of the derived vibrational terms was not confirmed by our recent full rovibrational assignments based on high accuracy variational calculations [20]. Of particular relevance for the present study is the work by Votava et al. [21] who reported the Icosad absorption spectrum of 13CH4 in a supersonic jet providing a 32 K rotational temperature. A total of 339 lines, including absolute intensities, were measured in the center of the Icosad between 7200 and 7354 cm1 [21]. In order to apply the 2T method, the jet data was combined with our DAS data at 80 K and an 80 K DAS line list was constructed for the 7173–7367 cm1 interval [21]. Note that this 80 K line list is incorporated mostly unchanged in the presently reported global list. All the transitions detected in jet conditions were also measured by DAS at 80 K, allowing for the derivation of their lower state empirical energy. As a consequence of the efficient rotational cooling, the 339 derived Jemp values are all smaller than 4. Six R(0) transitions could be unambiguously identified. As discussed in Refs. [5,22], the combination of 80 K line intensities to jet intensity data instead to room temperature intensities provides more accurate determination of the low Eemp values. For instance, while the J = 0 and J = 1 lower state J values can hardly be discriminated from their intensity ratios at 80 and 296 K (7.0 and 6.1 [2], respectively), they are well identified from the jet/80 K ratio of their intensities. This is mostly due to the total absence of line overlapping in the jet spectrum which makes the line intensity retrieval more accurate. We have systematically compared the Jemp values presently obtained from 80 K/296 K intensities to those derived in Ref. [21] from jet/80 K intensities. According to the histogram presented Fig. 9, 69% of the rounded Jemp values agree with those of Ref. [21] which are expected to be more accurate; 88% correspond to
|Jemp J| < 1. The future rovibrational assignments of the spectrum using theoretical calculations (see below) will probably confirm the quality of the Jemp values derived in Ref. [21].
6. Concluding remarks In this work, we have constructed the first complete empirical line lists for 13CH4 at 80 K and 296 K in the very congested Icosad region. This significantly extends scarce spectroscopic information provided by the HITRAN list in this range. The obtained experimental data could be important for determination of 13C/12C isotopic ratio from planetary spectra: a non-negligible contribution of 13 CH4 in the methane opacity of Titan atmosphere has been detected in lower spectral ranges as discussed in [23,24]. On the other hand our measurements will be valuable for extending theoretical studies of the isotopic effects [25] in highly excited vibrational states of methane. Accurate first-principles calculations of rotationally resolved spectra of five-atomics were still a remote goal a few years ago, but have recently become feasible. Despite a considerable spectral congestion (about 20 lines/cm1), the methane Icosad spectrum for the major 12CH4 isotopologue has been rovibrationally assigned using accurate ab initio potential energy surfaces (PES) [26] and dipole moment surfaces (DMS) [27] and variational methods [28,29] developed by Reims and Tomsk teams. A mostly complete identification of the 12CH4 Icosad bands (involving more than 10,000 lines of 108 sub-bands) in the 6280–7800 cm1 range has been performed [20] for the first time on the basis of ab initio intensity predictions [30]. The further work on the assignment of the experimental 13CH4 lines reported in the present paper will benefit from ab initio calculations for the isotopic shifts in vibrational band centers due to the 12 C ? 13C substitution. These shifts appear to be quite irregular ranging between 1 and 40 cm1 depending on vibrational quantum numbers. Previous studies have shown that efficient variational methods are able to predict these shifts very accurately [25], with error bars of the order of 0.01 cm1, at least for the lower polyads. We plan extending this approach to the theoretical interpretation of our new data. Previously determined [20] empirical vibrational energies of the 12CH4 Icosad and isotopic shifts predicted by theory for each band allow for unambiguous one-to-one correspondence and for transfer of most of the 12CH4 assignments to the 13CH4 line list. In turn, the 13CH4 data could provide a further check for the 12 CH4 assignments. Isotopic shift approach has been recently validated in the 2m3 range (5850–6200 cm1) of the 13CH4 Tetradecad [31] where the 13CH4 DAS spectrum at 80 K could be modelled using the effective Hamiltonian derived from the PES [26] by the contact transformation method [32]. The rovibrational assignment of the present 80 K and 296 K line lists will help calibrating 13CH4 transitions in theoretical predictions [33] of methane spectra for higher temperature conditions.
Acknowledgments The support of the LIA SAMIA between CNRS (France) and RFBR (Russia) as well as of Tomsk State University D. Mendeleev funding program is acknowledged. This work was performed in the frame of the Labex OSUG@2020 (ANR10 LABX56).
Appendix A. Supplementary material Fig. 9. Histogram of the differences between the lower state Jemp values determined by the 2T method applied to the 80 K/296 K DAS intensities (this work) and to the 32 K jet and 80 K DAS intensities (Ref. [21]).
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jms.2016.01.005.
Please cite this article in press as: A. Campargue et al., J. Mol. Spectrosc. (2016), http://dx.doi.org/10.1016/j.jms.2016.01.005
A. Campargue et al. / Journal of Molecular Spectroscopy xxx (2016) xxx–xxx
References [1] A. Campargue, O. Leshchishina, L. Wang, D. Mondelain, S. Kassi, J. Mol. Spectrosc. 291 (2013) 16–22. [2] A. Campargue, L. Wang, D. Mondelain, S. Kassi, B. Bézard, E. Lellouch, M. Hirtzig, A. Coustenis, C. de Bergh, P. Drossart, Icarus 219 (2012) 110–128. [3] A. Campargue, O. Leshchishina, L. Wang, D. Mondelain, S. Kassi, A.V. Nikitin, J. Quant. Spectrosc. Radiat. Trans. 113 (2012) 1855–1873. [4] A. Campargue, O. Leshchishina, D. Mondelain, S. Kassi, A. Coustenis, J. Quant. Spectrosc. Radiat. Trans. 118 (2013) 49–59. [5] O. Votava, M. Mašát, P. Pracna, S. Kassi, A. Campargue, Phys. Chem. Chem. Phys. 12 (2010) 3145–3155. [6] L. Wang, D. Mondelain, S. Kassi, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 113 (2012) 47–57. [7] A. Campargue, L. Wang, A.W. Liu, S.M. Hu, S. Kassi, Chem. Phys. 373 (2010) 203–210. [8] A.V. Nikitin, X. Thomas, L. Régalia, L. Daumont, P. Von der Heyden, Vl.G. Tyuterev, L. Wang, S. Kassi, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 112 (2011) 28–40. [9] L. Wang, S. Kassi, A. Liu, S. Hu, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 112 (2011) 937–951. [10] D. Mondelain, S. Kassi, L. Wang, A. Campargue, Phys. Chem. Chem. Phys. 17 (2011) 7985–7996. [11] L.S. Rothman, I.E. Gordon, Y. Babikov, A. Barbe, D. Chris Benner, P.F. Bernath, M. Birk, L. Bizzocchi, V. Boudon, L.R. Brown, A. Campargue, K. Chance, L. Coudert, V.M. Devi, B.J. Drouin, A. Fayt, J.-M. Flaud, R.R. Gamache, J. Harrison, J.-M. Hartmann, C. Hill, J.T. Hodges, D. Jacquemart, A. Jolly, J. Lamouroux, R.J. LeRoy, G. Lia, D. Longo, C.J. Mackie, S.T. Massie, S. Mikhailenko, H.S.P. Müller, O.V. Naumenko, A.V. Nikitin, J. Orphal, V. Perevalov, A. Perrin, E.R. Polovtseva, C. Richard, M.A.H. Smith, E. Starikova, K. Sung, S. Tashkun, J. Tennyson, G.C. Toon, Vl.G. Tyuterev, J. Vander Auwera, G. Wagner, J. Quant. Spectrosc. Radiat. Trans. 130 (2013) 4. [12] L.R. Brown, K. Sung, D.C. Benner, V.M. Devi, V. Boudon, T. Gabard, et al., J. Quant. Spectrosc. Radiat. Trans. 130 (2013) 201–219. [13] A. Campargue, J. Lopez Segovia, S. Béguier, S. Kassi, D. Mondelain, J. Quant. Spectrosc. Radiat. Trans. 152 (2015) 140–148.
7
[14] O.M. Lyulin, S. Kassi, K. Sung, L.R. Brown, A. Campargue, J. Mol. Spectrosc. 261 (2010) 91–100. [15] Y. Lu, D. Mondelain, S. Kassi, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 112 (2011) 2683–2697. [16] Bo. Gao, S. Kassi, A. Campargue, J. Mol. Spectrosc. 253 (2009) 55–63. [17] S. Kassi, B. Gao, D. Romanini, A. Campargue, Phys. Chem. Chem. Phys. 10 (2008) 4410–4419. [18] J. Humlicek, J. Quant. Spectrosc. Radiat. Trans. 21 (1979) 309–313. [19] O.N. Ulenikov, E.S. Bekhtereva, S. Albert, S. Bauerecker, H.M. Niederer, M. Quack, J. Chem. Phys. 141 (2014) 234302. [20] M. Rey, A.V. Nikitin, A. Campargue, S. Kassi, D. Mondelain, V.G. Tyuterev, Phys. Chem. Chem. Phys. 18 (2016) 176–189. [21] O. Votava, M. Mašát, P. Pracna, D. Mondelain, S. Kassi, A.W. Liu, S.M. Hu, A. Campargue, J. Quant. Spectrosc. Radiat. Trans. 149 (2014) 64–71. [22] M. Mašát, P. Pracna, D. Mondelain, S. Kassi, A. Campargue, O. Votava, J. Mol. Spectrosc. 291 (2013) 9–15. [23] C. De Bergh, R. Courtin, B. Bézard, A. Coustenis, E. Lellouch, M. Hirtzig, et al., Plan. Space Sci. 61 (2012) 85–98. [24] M. Hirtzig, B. Bézard, E. Lellouch, A. Coustenis, C. De Bergh, P. Drossart, et al., Icarus 226 (2013) 470–486. [25] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, J. Mol. Spectrosc. 291 (2013) 85–97. [26] A.V. Nikitin, M. Rey, Vl.G. Tyuterev, Chem. Phys. Lett. 501 (2011) 179–186. [27] A.V. Nikitin, M. Rey, Vl.G. Tyuterev, Chem. Phys. Lett. 565 (2013) 5–11. [28] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, J. Chem. Phys. 136 (2012) 244106. [29] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, Phys. Chem. Chem. Phys. 15 (2013) 10049– 10061. [30] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, J. Quant. Spectrosc. Radiat. Trans. 164 (2015) 207–220. [31] E. Starikova, A.V. Nikitin, M. Rey, S.A. Tashkun, D. Mondelain, S. Kassi, A. Campargue, Vl.G. Tyuterev, J. Quant. Spectrosc. Radiat. Trans., http://dx.doi. org/10.1016/j.jqsrt.2015.12.023. [32] Vl.G. Tyuterev, S.A. Tashkun, M. Rey, R. Kochanov, A. Nikitin, T. Delahaye, J. Phys. Chem. A 117 (2013) 13779–13805. [33] M. Rey, A.V. Nikitin, Vl.G. Tyuterev, Astrophys. J. 789 (2014) 1–10.
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