Molecular Physics, 2014 Vol. 112, No. 18, 2504–2514, http://dx.doi.org/10.1080/00268976.2014.909059 INVITED ARTICLE High-resolution spectroscopy of difference and combination bands of SF 6 to elucidate the ν 3 + ν 1 - ν 1 and ν 3 + ν 2 - ν 2 hot band structures in the ν 3 region M. Faye a , A. Le Ven a , V. Boudon a , ∗ , L. Manceron b , c , d , P. Asselin b , c , P. Soulard b , c , F. Kwabia Tchana e and P. Roy d a Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS–Universit´ e de Bourgogne, Dijon Cedex, France; b Sorbonne Universit´ es, UPMC Univ. Paris 06, UMR 8233, MONARIS, Paris, France; c CNRS, UMR 8233, MONARIS, Paris, France; d Ligne AILES- Synchrotron SOLEIL, L’Orme des Merisiers, Gif-sur-Yvette Cedex, France; e Laboratoire Interuniversitaire des Syst` emes Atmosph´ eriques, UMR CNRS 7583, Universit´ e Paris-Est Cr´ eteil et Universit´ e Paris-Diderot, Cr´ eteil Cedex, France (Received 11 February 2014; accepted 21 March 2014) The strong infrared absorption in the ν 3 S–F stretching region of sulphur hexafluoride (SF 6 ) near 948 cm −1 makes it a powerful greenhouse gas. Although its present concentration in the atmosphere is very low, it is increasing rapidly, due to industrial pollution. The ground state population of this heavy species is only 32% at room temperature and thus many hot bands are present. Consequently, a reliable remote-sensing spectroscopic detection and monitoring of this species require an accurate modelling of these hot bands. We used two experimental set-ups at the SOLEIL French synchrotron facility to record some difference and combination bands of SF 6 : (1) a new cryogenic multiple pass cell with 93 m optical path length and regulated at 163 ± 2 K temperature and (2) the Jet-AILES supersonic expansion set-up. With this, we could obtain high-resolution absorption spectra of the ν 3 − ν 1 , ν 3 − ν 2 , ν 1 + ν 3 and ν 2 + ν 3 bands at low temperature. These spectra could be assigned and analysed, thanks to the SPVIEW and XTDS computer programs developed in Dijon. We performed two global fits of effective Hamiltonian parameters. The first one is a global fit of the ground state, ν 2 , ν 3 , ν 3 − ν 2 , ν 2 + ν 3 ,2ν 3 and 2ν 3 − ν 3 rovibrational parameters, using the present spectra and previous infrared, Raman and two-photon absorption data. This allows a consistent refinement of the effective Hamiltonian parameters for all the implied vibrational levels and a new simulation of the 2ν 3 + ν 2 − ν 2 hot band. The second global fit involves the present ν 3 − ν 1 and ν 1 + ν 3 lines, together with previous ν 1 Raman data, in order to obtain refined ν 1 parameters and also ν 1 + ν 3 parameters in a consistent way. This allows to simulate the ν 3 + ν 1 − ν 1 hot band. Keywords: sulphur hexafluoride; greenhouse gas; infrared absorption; supersonic expansion; tensorial formalism; low temperature 1. Introduction Sulphur hexafluoride (SF 6 ) is a strong greenhouse gas of anthropogenic origin with a very long lifetime (ca. 3200 years [1,2]), whose concentration is rapidly increasing in the atmosphere [3–5]. As such, it is included in the Kyoto protocol as a species whose emissions should be monitored and reduced [6–8]. It is measured using Earth-observing satellites [9,10], and this gas is also used as a tracer for atmospheric gas transport [11–13]. The HITRAN (HIgh-resolution TRANsmission molec- ular absorption database) [14,15] and GEISA (Gestion et Etude des Informations Spectroscopiques Atmosph´ eriques) [16] public spectroscopic databases used for atmospheric chemistry measurements, however, only contain either cross-sections or line lists for the strongly absorbing ν 3 fundamental band of SF 6 , around 948 cm −1 , resulting from high-precision studies [17]. This is, however, largely insuf- ficient to account for the whole atmospheric absorption (and greenhouse capability) of SF 6 , since numerous hot bands overlap the cold band spectrum and form a complicated ∗ Corresponding author. Email: Vincent.Boudon@u-bourgogne.fr pattern, due to the existence of low-lying vibrational states (at room temperature, only 32% of the SF 6 molecules lie in the ground vibrational state). At present, these hot bands are spectroscopically poorly characterised. In order to contribute to this question, several years ago, we have undertaken a systematic study of all low-lying and combination vibrational levels that may be involved in the hot bands of the ν 3 region [18,19], using both infrared (IR) absorption [20–23] and stimulated Raman spectroscopy [24–27] at high resolution. Analyses and line list gener- ation are performed, thanks to the HTDS (Highly-spherical Top Data System) [28] module of the XTDS (eXtended Top Data System) software [29], based on the tensorial formalism developed in the Dijon group for such highly symmetrical molecules [30–32]. In this work, we continue this investigation by consid- ering two difference bands, namely ν 3 − ν 1 and ν 3 − ν 2 , lying in the far IR (FIR) region. Already reported by other authors [33], these bands can provide very accurate data, and thus parameters, for the ν 1 and ν 2 fundamentals (ν 3 being already well known [17]). As a matter of fact, we C  2014 Taylor & Francis