Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81K: (II) The icosad region (1.49–1.30μm)

Abstract

Temperature dependence of the absorption spectrum of CH4 by high resolution spectroscopy at 81 K: (II) The icosad region (1.49-1.30 µm). AbstractThe high resolution absorption spectrum of methane has been recorded at liquid nitrogen temperature by direct absorption spectroscopy between 6717 and 7351 cm -1 (1.49 -1.36 m) using a cryogenic cell and a series of Distributed Feed Back (DFB) diode lasers.The investigated spectral region corresponds to the very congested low energy part of the icosad for which the HITRAN database provides neither rovibrational assignments nor the lower state energies. The positions and strengths at 81 K of 9389 transitions were obtained from the spectrum analysis. The minimum value of the measured line intensities (at 81 K) is on the order of 10 -26 cm/molecule. From the variation of the line strength between 81 K and 296 K, the low energy values of a total of 4646 transitions were determined. They represent 79.4 % and 68.4% of the total absorbance in the region at 81 and 296 K, respectively, and include 28 transitions assigned to the  2 +4 4 band near 6765 cm -1 . The reliability of the method based on the association of lines with coinciding centers in the 81K and 296 K spectra is discussed. The results of the present analysis have been combined with previously analyzed high energy part of the icosad dominated by the  2 +2 3 band near 7510 cm -1 . The line list for the whole icosad (6717-7655 cm -1 ) consists of 12865 transitions at 81 K.

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

“…The CW-CRDS spectrum of methane displayed as background (light gray) in the upper panel shows that, at room temperature, the CH 3 D transitions are completely obscured by much stronger CH 4 transitions. [3][4][5][6][7]. This ''two temperature method" proved to be robust and reliable to account for the temperature dependence of the methane absorption.…”

“…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].…”

“…[5,7] for the high energy part of the tetradecad and the icosad, respectively, together with the present CW-CRDS line list in the transparency window. Fig.…”

“…[7]. Indeed a strict criterion is chosen to insure that a RT line and a LNT line correspond to the same transition: the difference, d, of their line centers must differ by less than 0.002 cm À1 .…”

“…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.…”