Author Institution: Laboratoire de Physico-Chimie de l'Atmosphere, EA 4493, Universite du Littoral Cote d'Opale, 59140 Dunkerque, France; Institut des Sciences Moleculaires d'Orsay, CNRS, UMR 8214, Universite Paris XI, bat. 210, 91405 Orsay Cedex, France; SOLEIL Synchrotron, AILES beamline, L'orme des Merisiers, Saint-Aubin, 91192 Gif-Sur-Yvette, FranceSulfur monoxide SO (X3Σ−) is a well-known interstellar radical identified in a wide variety of astrophysical environments 184, L59 (1973)} which is particularly abundant in star forming regions, \textit{Astrophys. J.} 315, 621 (1987)}. Due to its high reactivity and its role in chemical reactions involving O and S atoms, SO is also a reaction intermediate in combustion processes and chemistry of the Earth atmosphere, \textit{J. Mol. Spectrosc.} 124, 379 (1987)}. %The pure rotational spectrum of SO has been extensively studied in the laboratory from sub-millimeter to THz wavelength, \textit{J. Mol. Speectrosc.} 182, 85 (1997)}. However no far infrared (FIR) broadband investigation on this radical has been reported up to date, and several rotational transitions belonging to the spectral windows of the HIFI instrument (on board of the Herschel satellite) require accurate laboratory measurements. %\vspace{1em} We have recorded pure rotational transitions of SO in the THz spectral range using synchrotron-based Fourier-Transform (FT) FIR and continous wave (CW) THz techniques. A FT-FIR spectrum of SO has been recorded at the AILES beamline of SOLEIL synchrotron in the spectral range 44--93 \wn~using a resolution of 0.001~\wn~allowing an accuracy on line position of 0.00007~\wn~(∼ 2 MHz). A multipass absorption discharge cell aligned to an absorption path length of 24 m has been used, \textit{Rev. Sci. Instrum.} 82, 113106 (2011)}. A continuous electrical discharge (1~A / 980~V) in a flowing mixture of H2S, He, H2 and air (respectively at pressure of 0.01, 1.15, 0.14 and 0.06~mbar) was used to produce SO. On this spectrum, 102 transitions of SO have been identified with N=31 to 65. Among the observed lines, 99 are detected for the first time (22 new transitions belong to the HIFI spectral windows). Due to our limited instrumental resolution, transitions involving N ranging from 31 to 43 show unresolved fine structure triplets. Recently, in order to observe all fine structure components in the HIFI spectral windows, we have recorded a high resolution CW-THz spectrum of SO, \textit{J. Mol. Struct.} 1006, 13 (2011)}. At the time of the writing, this spectrum was under analysis
