PRISMAS : PRobing InterStellar Molecules with Absorption line Studies

We will carry out a comprehensive spectroscopic study of key molecularline carriers, probing interstellar hydrides and carbon chains. Our investigation will entail highresolution HIFI spectroscopyof some 25 molecules towards 8 sources, and full spectral scans withPACS. The target hydrides contain the elements H, D, C, N, O, F andCl. We will take advantage of the strong dust emission from massive star forming regions to detect multiple absorption components from foreground clouds of diverse properties that are known to intersect the selected sightlines, along with emission and absorption intrinsic to thebackground sources. Our investigation will provide a wealthof new information about interstellar hydrides addressing key puzzles posed by previous observations from the groundsince the 1940.s, and recently with ISO, SWAS, and ODIN and leavingan important Herschel legacy to astrochemistry and ISM science. Wewill address the role of high temperature chemical reactions in theformation of interstellar molecules, and the question of how suchreactions might be driven. We will also investigate the role of grainsurface reactions in interstellar chemistry, and the growth of carbonmolecules, bridging the gap between molecules and aggregates, asunique spectroscopic signatures of carbon chains and rings,are accessible in the FIR.Many of the lines that are detectable with Herschel in the localUniverse become accessible to groundbased observatories forredshifted sources. Our programme will provide an uniquebenchmark for the studies of molecular gas at high redshift with ALMA. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]