International Journal of Infrared and Millimeter Waves, Vol. 12, No. 9, 1991 NEW FAR INFRARED MEASUREMENTS OF THE ROTATIONAL SPECTRUM OF CYANOACETYLENE HC3N W. Chen, R. Bocquet, G. Wlodarczak, and D. Boucher Laboratoire de Spectroscopie Hertzienne (UA C.N.R.S.249) Bat. P5, Universitd de Lille I 59655 Villeneuve d'Ascq, France Received June 12, 1991 Abstract Several rotational lines of cyanoacetylene in the ground state and the first excited state v7=1 have been measured between 580 and 1100 GHz with a new FIR laser sidebands spectrometer. A global analysis which takes into account the previous measurements on this molecule is performed. Key words : FIR laser sidebands, spectroscopy, cyanoacetylene. Introduction Cyanoacetylene HC3 N has been the subject of numerous studies since 1950. The first microwave measurements of the J=3-2 line have been published by Westenberg and Wilson (1) for the 987 0195~9271/91/0900-0987506.50/0 9 1991 Plenum Publishing Corporatio~