The high-resolution far-infrared spectrum of methane at the SOLEIL synchrotron

Abstract

As a tetrahedral molecule, methane has no permanent dipole moment. Its spectrum, however, displays faint absorption lines in the THz region, due to centrifugal distorsion effects. This is important for planetary applications since this region is used to measure methane concentration in some planetary atmospheres, in particular on Titan. Up to now, all measurements relied either on some old low resolution infrared absorption spectra, or on high resolution Stark measurements for low J values only. Even if these results have been reexamined recently [E. H. Wishnow, G. S. Orton, I. Ozier and H. P. Gush, J. Quant. Spectrosc. Radiat. Transfer 103, 102-117 (2007)], it seemed highly desirable to obtain much more precise laboratory data. The high-intensity synchrotron radiation, combined with a 151.75 ± 0.1 m optical path in a White cell and a Bruker IFS 125 HR FTIR spectrometer at the AILES beamline of SOLEIL, enabled us to record this very weak spectrum at high resolution for the first time. Spectra were obtained in the 50-500 cm −1 wavenumber range at 296 K and 9.91, 20, 50 and 100 mbar with a resolution of 0.00074, 0.00134, 0.0034 and 0.0067 cm −1 (FWHM of the sinc function), 6 This paper is dedicated to Dr. Larry Rothman on the occasion of his 70th birthday.

Sub-millimetre spectroscopy of Saturn’s trace gases fromHerschel/SPIRE

“…Methane: CH 4 at 41.9 cm −1 was used to test our line-byline technique for estimating the abundances of trace gases in Saturn's atmosphere. Using two different sources for the rotational line intensities, SPIRE produced 1σ estimates of the mole fraction of (4.3 ± 0.3) × 10 −3 using the compilation of Brown et al (2003) with the dipole measurements of Wishnow et al (2007); and a mole fraction of (4.8 ± 0.3) × 10 −3 with the compilation of Boudon et al (2010). These results are broadly consistent with the results from Cassini far-infrared observations (Flasar et al 2005;Fletcher et al 2009b) and with the enrichment of carbon in Saturn compared to Jupiter.…”

“…These two SPIRE measurements are broadly consistent (to within one standard deviation) with the results from Cassini/CIRS -(4.7 ± 0.2) × 10 −3 (Fletcher et al 2009b) and (4.5 ± 0.9) × 10 −3 (Flasar et al 2005), which both used the compilation of Brown et al (2003) with the scaling factor from Wishnow et al (2007). Converting the two estimates into a C/H ratio for Saturn's bulk composition, we obtain carbon enrichments (relative to the solar chemical composition of Grevesse et al 2007) of 9.9 ± 0.7 and 11.1 ± 0.7 for the Wishnow et al (2007) and Boudon et al (2010) line intensities, respectively, consistent with the hypothesis that Saturn contains more carbon than Jupiter due to the increasing core-to-envelope ratio as we move away from the Sun (e.g., Owen & Encrenaz 2006).…”

“…Experiments with a Van Vleck Weisskopf lineshape for both PH 3 and NH 3 showed negligible differences at the resolution of the SPIRE observations. Although our preliminary analysis uses spectroscopic line data for methane from the work of Brown et al (2003) with a multiplicative factor of 1.154 for the line intensities (based on the results of Wishnow et al 2007, which is also present in the latest HITRAN 2008 compilation), we also use a more recent line list from Boudon et al (2010) in Sect. 4.1 to show potential uncertainties in the CH 4 mole fraction.…”

“…As described in Sect. 3, we estimate the mole fraction of CH 4 using two sources of line intensities: (i) the compilation of Brown et al (2003) with a multiplicative factor of 1.154 based on the new dipole measurements of Wishnow et al (2007); and (ii) the compilation of Boudon et al (2010), where the line intensities are weaker by a factor 2.2 × 10 −8 Improvement over Weisstein & Serabyn (1996) HCP N 3.3 × 10 −9 1.0 × 10 −5 Improvement over Weisstein & Serabyn (1996) CS N 8.9 × 10 −11 2.5 × 10 −7 First upper limit, 21.2 cm −1 SO 2 N 2 .5 × 10 −10 1.7 × 10 −7 First upper limit H 2 S N 3 .6 × 10 −10 2.3 × 10 −7 Improvement over Weisstein & Serabyn (1996) CH 3 Cl N 2.5 × 10 −10 1.2 × 10 −7 First upper limit CH 3 OH N 3.0 × 10 −10 1.1 × 10 −7 First upper limit H 2 CO N 6.5 × 10 −11 5.6 × 10 −8 First upper limit, 21 cm −1…”

High-resolution infrared spectroscopy with synchrotron sources

“…This is a model molecule for understanding protein folding, and analysis of the spectra is currently underway, starting with assignment and fitting of a band of the less abundant cis isomer around 580 cm À1 . Boudon and co-workers [48] have recorded the pure rotational spectrum of methane in the 70-200 cm À1 range using an absorption path of 150 m at room temperature. These weak ''forbidden" transitions arise from the small dipole moment induced by centrifugal distortion.…”