Spectroscopy of key molecular species in the Far-Infrared

In this paper we briefly analyze the type of molecular transitions and the contribution of polyatomic molecules to the Far Infrared spectrum of interstellar and circumstellar clouds. For that aim, we present our recent work on molecular first Far-IR detections. All the results presented here were taken in the direction of Sgr B2, close to the Galactic Center. Our study is based oil Fabry-Perot observations carried out with the Infrared Space Observatory (ISO) Long Wavelength Spectrometer LWS. In addition, we report. our last Far IR and millimeter observations of water vapour. This incorporates several thermal ortho/para H2O transitions and ground-based observations of the 3(13)-2(20) maser transition at 183.31 GHz. Our first Far-IR identifications include molecules such as NH, NH2, C-3, H3O+. They mainly represent the whole typos of molecular transitions likely to occur in molecular clouds Far IR spectra. In addition to traze the dense gas with never observed before molecular transitions, this work represents a preparatory study for the spectroscopy programs that could be done with FIRST in the Far-IR.