Temperature dependence of rotational relaxation of methane in the 2ν3 vibrational state by self- and nitrogen-collisions and comparison with line broadening measurements

Sung

et al. 2010

a b s t r a c tThe high resolution absorption spectra of 13 CH 4 were recorded at 81 K by differential absorption spectroscopy using a cryogenic cell and a series of distributed feed back (DFB) diode lasers and at room temperature by Fourier transform spectroscopy. The investigated spectral region corresponds to the high energy part of the 13 CH 4 tetradecad dominated by the 2m 3 overtone near 5988 cm À1 . Empirical line lists were constructed containing, respectively, 1629 13 CH 4 transitions detected at 81 K (5852-6124 cm À1 ) and 3481 features (including 85 lines of 12 CH 4 ) measured at room temperature (5850-6150 cm À1 ); the smallest measured intensities are about 3 Â 10 À26 and 4 Â 10 À25 cm/molecule at 81 and 296 K, respectively. The lower state energy values were derived for 1196 13 CH 4 transitions from the variation of the line intensities between 81 and 296 K. These transitions represent 99.2% and 84.6% of the total absorbance in the region, at 81 and 296 K, respectively. Over 400 additional weak features were measured at 81 K and could not be matched to lines observed at room temperature. The quality of the resulting empirical low energy values is demonstrated by the excellent agreement with the already-assigned transitions and the clear propensity of the empirical low J values to be close to integers. The two line lists at 81 and at 296 K provided as Supplementary material will enable future theoretical analyses of the upper 13 CH 4 tetradecad.

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Section: Low Temperature Spectrasupporting

confidence: 86%

Determination of the low energy values of 13CH4 transitions in the 2ν3 region near 1.66μm from absorption spectra at 296 and 81K

Lyulin

Sung

et al. 2010

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“…A few additional recordings were performed with lower pressure values in order to avoid saturation of the strongest lines. The self broadening coefficient of methane at low temperature has recently been reported for the 2m 3 band [9]. Its value, which is expected to be mostly independent of the vibrational band, is on the order of 0.21 cm À1 /atm (half width at half medium) at 80 K i.e.…”

Section: Line Intensity Retrievalmentioning

confidence: 95%

Empirical low energy values for methane transitions in the 5852–6181cm−1 region by absorption spectroscopy at 81K

Gao

Campargue

2009

a b s t r a c tThe high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 1.62 and 1.71 lm (5852-6181 cm À1 ) using a newly developed cryogenic cell and a series of distributed feedback (DFB) laser diodes. The minimum value of the measured line intensities is on the order of 3 Â 10 À26 cm/molecule The investigated spectral range corresponds to the high energy part of the tetradecad dominated by the 2m 3 band for which a theoretical treatment is not yet available. The positions and strengths at 81 K of 2187 transitions were obtained from the spectrum analysis. From the values of the line strength at liquid nitrogen and room temperatures, the low energy values of 845 transitions could be determined. The obtained results are discussed in relation with the previous work of Margolis and compared to the line list provided by the HITRAN database. IntroductionMethane is a strong window effect gas whose increasing concentration participates to the global warming of the Earth atmosphere. Methane absorption has an even stronger impact on the radiative budget of the atmospheres of the giant outer planets and of Saturn's satellite Titan. The knowledge of the methane absorption spectrum in conditions similar to those existing on these planets is then a prerequisite for the modeling of their structure and climate. Despite important experimental and theoretical efforts [1], the highly congested absorption spectrum of methane above 6000 cm À1 is still resisting to a satisfactory interpretation. Empirical line-by-line spectroscopic parameters as included in HITRAN [2] or GEISA [3] databases are far from fulfilling the needs as for most of the transitions, rovibrational assignments or at least the energy of the transitions lower levels are not provided. Consequently, line intensities cannot be computed at different temperatures which make the available line lists of limited use in planetology.An alternative to the theoretical interpretation is an empirical determination of the lower state energy from the temperature dependence of the line intensities. Two spectra recorded at different temperatures are sufficient to deduce the low energy level of a transition from the ratio of the line strengths values. This method has been applied to methane in a few previous investigations. For instance, Pierre et al.[4] used two spectra recorded at 295 and 149 K by Fourier Transform Spectroscopy (FTS) to determine lower state quantum numbers in the region of the 3m 3 band near 9000 cm À1 , Tsukamoto et al. [5] could assign 215 transitions with respect to J 00 among the 269 transitions of the 3m 1 + m 3 that they recorded at 77 K. Of particular relevance for this study is the investigation by Margolis of the 5500-6150 cm À1 spectral region by FTS at room [6] and reduced [7] temperatures. Using spectra recorded at 180-220 K with a 0.8 m long cryogenic cell, he could derive the lower energy value, E 00 of 1600 transitions. Except for the theoretically assigned lines ...

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“…A Voigt function of the wavenumber was adopted for the line profile as the pressure self broadening has a significant contribution in particular at LNT: at 79 K and 10 Torr, the self broadening (HWHM 2.6 Â 10 À3 cm À1 [24]) is about half the Doppler broadening (HWHM 5.0 Â 10 À3 cm À1 ). The local baseline (assumed to be a cubic function of the wavenumber) and the three parameters of each Voigt profile (line center, integrated absorption coefficient, HWHM of the Lorentzian component) were fitted.…”

Section: Construction Of the Line Listsmentioning

confidence: 99%

High sensitivity absorption spectroscopy of methane at 80K in the 1.58μm transparency window: Temperature dependence and importance of the CH3D contribution

Wang

Liu

et al. 2010