High temperature infrared spectroscopy

Jupiter - Artist’s impression of a hot Jupiter transiting in front of its host star. Credit: ESA, NASA and Frédéric Pont (Geneva University Observatory)

The modeling of the atmosphere of brown dwarves, hot Jupiters or circumstellar envelopes depends upon radiative transfer codes which require a good knowledge of the opacity of the constituents at relevant high temperatures (1000 to 2500 K). Accurate laboratory spectroscopic data remain insufficient in this temperature domain.

In our laboratory, we have developed an approach in which a high temperature source is combined with a Fourier Transform spectrometer. This instrument is employed to generate infrared emission spectra of small hydrocarbons in thermodynamic equilibrium, from which absorption cross sections are extracted.

The second method is based on adiabatic expansion in a vacuum chamber of a hot gas initially heated to 2000 K (hypersonic free jet) and probed by ultrasensitive Cavity Ring Down Spectroscopy. The heating populates the excited vibrational states, which have a major contribution to the spectrum at high temperature (hot bands), and which do not relax significantly during the expansion. However, the hypersonic expansion relaxes very efficiently the rotational states which lead to a strong simplification of the spectrum and facilitates its analysis. This technique is called out-of-equilibrium spectroscopy.

In particular, one of the objectives is to produce spectroscopic data of small hydrocarbons such as methane CH4, acetylene C2H2 or ethylene C2H4, which can strongly affect the thermal structure of these objects.

Beyond astrophysics, these studies concern research fields of combustion and hypersonics (shockwave of atmospheric re-entry, trail of ballistic objects).

 

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Spectre expérimental enregistré à l’équilibre thermodynamique à 1355 K (panneau supérieur), comparé à un spectre synthétique obtenu à l’aide d’un modèle global effectif (CQP, ULB) qui intègre nos données. L’agrandissement (panneau de droite) montre un très bon accord contrairement aux spectres synthétiques de la base HITRAN dont l’utilisation n’est pas encore adaptée au domaine des hautes températures.

Selected Publications

  • Emission spectroscopy from optically thick laboratory acetylene samples at high temperature, Audrey Moudens; Robert Georges; Abdessamad Benidar; Badr Amyay; M. Herman; Andre Fayt; Bertrand Plez, Journal of Quantitative Spectroscopy and Radiative Transfer, 2011, 112, pp. 540-549, DOI
  • Vibration-rotation pattern in acetylene (II): Introduction to Coriolis coupling in the global model and analysis of emission spectra of hot acetylene around 3 μm, Amyay B, Robert S, Herman M, Fayt A, Raghavendra B, Moudens A, Thiévin J, Rowe B, Georges R, J. Chem. Phys. 131, 114301 (2009)
  • High temperature emission spectroscopy of methane, Thiévin J, Georges R, Carles S, Benidar A, Rowe BR, Champion J-P, J Quant Spectrosc Rad Trans, 109, 2027-2036 (2008)