V. BOUDON, Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS--Université de Bourgogne, 9 Av. A. Savary, BP 47870, F-21078 Dijon Cedex, France; L. MANCERON, Laboratoire de Dynamique, Interactions et Réactivité, CNRS UMR 7075, 4 Place Jussieu, F-75252 Paris Cedex, France; F. KWABIA-TCHANA, Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583, Université Paris-Est Créteil et Université Paris-Diderot, 61 Avenue du Général de Gaulle, 94010 Créteil Cedex, France; P. ROY, Ligne AILES -- Synchrotron SOLEIL, L'Orme des Merisiers, F-91192 Gif-sur-Yvette, France.

Sulfur hexafluoride (SF6) is a greenhouse gas of anthropogenic origin, whose strong infrared absorption in the nu3 S--F stretching region near 948 cm-1 induces a global warming potential 23900 times bigger than CO2. This heavy species features many hot bands at room temperature (at which the ground state population is only 30 %), especially those originating from the v6=1 state. Unfortunately, the nu6 band itself (near 347 cm-1) being, in first approximation, both infrared and Raman inactive, no reliable information could be obtained about it up to now. A long time ago, some authors suggested\footnoteW. B. Person, B. J. Krohn, J. Mol. Spectrosc.\/ 98, 229--257 (1983), C. Chappados, G. Birnbaum, J. Mol. Spectrosc.\/ 105, 206--214 (1984). that this band may be slightly activated through Coriolis interaction and may appear as a very faint band, with an integrated intensity about 2 millionths of that of nu3. Using a new cryogenic multipass cell with 93 m optical path length and regulated at 165\pm 2 K temperature, we recorded a spectrum of the nu6 far-infrared region thanks to the performances of the AILES Beamline at the SOLEIL french synchrotron facility. Low temperature was used to avoid the presence of the 2 nu6- nu6 hot band and to reduce the neighboring, stronger nu4- nu2 difference band. We are thus able to confirm that the small feature in this region, previously viewed at low-resolution is indeed nu6. We present its fully resolved spectrum. It appears to be activated thanks to unidentified faint interactions resulting in the presence of a first-order dipole moment term that induces unusual selection rules. This spectrum was analyzed thanks to the XTDS software package, leading to accurate molecular spectroscopic parameters that should be useful to model the hot bands of SF6.