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

Abstract

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 ...

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

“…The distance between the high reflective mirrors and the cryostat ends is about 0.5 cm. As mentioned above, this cryogenic cell with the external cylinders fitted by a pair of flanges housing anti-reflection coated windows, is the one which was used to study the LNT absorption spectrum by differential absorption spectroscopy in the surrounding strong absorption regions [4][5][6][7].…”

“…The two high absorbing regions surrounding the methane transparency window at 1.58 lm were characterized by differential absorption spectroscopy (DAS) at 81 K using a specifically designed cryogenic cell and a series of several tens Distributed Feed Back (DFB) diode lasers. This experimental approach provided a sufficient sensitivity (a min $ 10 À6 cm À1 ) in the high energy part of the tetradecad (5850-6180 cm À1 ) [4,5] and in the icosad region (6700-7700 cm À1 ) [6,7]. In the 1.58 lm transparency window corresponding to the 6180-6700 cm À1 gap, a higher 0022-2852/$ -see front matter Ó 2010 Elsevier Inc. All rights reserved.…”

“…Such laboratory measurements are nevertheless highly suitable because the transparency windows are used to analyze the deeper atmosphere of the giant outer planets and Titan and to infer the surface reflectivity with direct implications on its composition. For instance, Titan's atmosphere is roughly equivalent to a 20 km cell at T = 85 K, containing N 2 at a pressure of 1.5 bar and approximately 75 mbar of CH 4 . In absence of accurate laboratory measurements, empirical bands models are generally used by astronomers but they display important deficiencies in the transparency windows [1,2].…”

Pluto's lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations

“…The three components were shifted according to their individual Doppler shifts, and then convolved to the instrumental resolution of 60 000, determined by fitting the width of the telluric lines (and corresponding to an effective source size of 0.33 ). For modelling the Pluto spectrum, we used a recent CH 4 line list (Gao et al 2009), based on laboratory measurements (positions and intensities) at 81 K and including lower energy levels for 845 lines, determined by comparison with the intensities at 296 K collected in the HITRAN database. Although the temperature of laboratory data is similar to Pluto's, we used only lines for which energy levels were available, in order to avoid dubious extrapolation towards lower temperatures.…”

Line broadening coefficient calculations for methane perturbed by nitrogen

“…The vibrational structure is organized into polyads of rapidly increasing complexity when climbing the energy range up to midinfrared and visible regions [22]. As far as line positions and intensities are concerned, the most recent and complete global analysis for 12 CH 4 concerns the 0 to 4800 cm −1 region [23], while upper polyads are still under investigation and the subject of sophisticated experimental efforts [24][25][26][27][28].…”