M. LOUVIOT1, N. SUAS-DAVID2, S. KASSI3, M. REY4, V. BOUDON1, R. GEORGES2 1 Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS/Université de Bourgogne, 9 Avenue Alain Savary, BP 47870, F-21078 Dijon Cedex, France, 2 Institut de Physique de Rennes, UMR 6251, Campus de Beaulieu, Bât 11C, Université de Rennes 1/CNRS, F-35042 Rennes Cedex, France 3 Laboratoire Interdisciplinaire de Physique, Univ. Grenoble 1/CNRS, LIPhy UMR 5588, Grenoble F-38041, France 4 Groupe de Spectrométrie Moléculaire et Atmosphérique, UMR CNRS 6089, Université de Reims, U.F.R. Sciences Exactes et Naturelles, B.P.1039, 51687 Reims Cedex 2, France ! Here, we highlight the presence of two rotationally cold gas into the jet since Boltzmann plots show two signiﬁcant slopes. It demonstrates that the jet is composed of two regions: the isentropic core or the silence zone and the boundary layers surrounding the core due to the residual gas in the chamber at the thermodynamic equilibrium. This second contribution is thus rotationally hotter than the core. 1. Experimental setup 2. Aims of the setup and experimental results Introduction 4. Analysis of the CH4 spectrum Conclusion and perspectives 0 1000 2000 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000 P0: Ground State 1 P1: Dyad 2 P2: Pentad 9 P3: Octad 24 P4: Tetradecad 60 P5: Icosad 134 P6: Triacontad 280 P7: Tetracontad 538 P8: Pentacontakaipentad 996 P9: Heptacontad 1746 Simulated spectrum × 100 Huygens DISR/ULIS ï Altitude 30 km Saturn at 727 nm = 13755 cmï1 Titan, CH4 window 939 nm = 10650 cmï1 Number of vibrational sublevels Wavenumber / cmï1 This work: analysis of hot CRD spectra of the Tetradecad region of methane. ! The study of speciﬁc astronomical objects like brown dwarfs and giant exoplanets is currently relevant in the astrophysics ﬁeld. ! Despite the difﬁculty to fully exploit the numerous spectroscopic data recorded, the presence of hot methane in the atmosphere of these bodies is well known. ! This encouraged us to use an original experimental setup developed at Institut de Physique de Rennes to record absorption spectra of a hypersonic gas expansion – initially heated at high temperature – with the high sensitive cavity ring down spectrometer (CRDS) developed by the Laboratoire LIPhy de Grenoble. ! The analysis was performed at Laboratoire Interdisciplinaire Carnot de Bourgogne in cooperation with of the Groupe de Spectroscopie Moléculaire et Atmosphérique in Reims. The polyads of methane Ar Ar Towards pumping Hollow porous graphite rod High-pressure reservoir (~1000 Torr) Low-pressure chamber (~0.09 Torr) High reﬂectivity mirrors (>99.99%) Copper electrode Non porous graphite pierced electrode (2 mm hole) Argon protecting ﬂow Hypersonic gas expansion Diode laser probe beam ! Two mixtures have been used: ﬁrst, a mixture of argon and carbon monoxide and second, a mixture of argon and methane. Mixtures were contained at high pressure in a reservoir (~ 1000 Torr) and heated at very high temperature (~ 2000 K), thanks to the high enthalpy source developed at IPR [1]. ! The High Enthalpy Source, developed in Rennes [1] was connected to a low pressure chamber (~ 0.09 Torr). The gas jet was performed through a 2 mm circular hole. ! The CRD spectrometer, developed by the LAME group from the LIPhy laboratory in Grenoble, has been placed perpendicularly to the axis of the jet to record the absorption spectra. ! High-resolution spectra of CO and CH4 have thus been investigated in the [5920-6030] cm-1 infrared spectral range. Five laser diodes have been used to cover this region (~ 30 cm-1 per laser diode). [1] J. Thiévin, R. Georges, S. Carles, A. Benidar, B. Rowe and J.-P. Champion, High- temperature emission spectroscopy of methane, J. Quant. Spectrosc. Radiat. Transfer, 109, 2027 —2036 (2008). ! The principle of this setup is the production of hypersonic gas expansions. This has been possible thanks to a high pressure reservoir connected to a low pressure chamb