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The Herschel-guaranteed time key programme PRobing InterStellar Molecules with Absorption line Studies (PRISMAS)(1) is providing a survey of the interstellar hydrides containing the elements C, O, N, F and Cl. As the building blocks of interstellar molecules, hydrides provide key information on their formation pathways. They can also be used as tracers of important physical and chemical properties of the interstellar gas that are difficult to measure otherwise. This paper presents an analysis of two sight-lines investigated by the PRISMAS project, towards the star-forming regions W49N and W51. By combining the information extracted from the detected spectral lines, we present an analysis of the physical properties of the diffuse interstellar gas, including the electron abundance, the fraction of gas in molecular form, and constraints on the cosmic ray ionization rate and the gas density.

With a 3.5 m diameter telescope passively cooled to approximately 80 K, and a science payload comprising two direct detection cameras/medium resolution imaging spectrometers (PACS and SPIRE) and a very high spectral resolution heterodyne spectrometer (HIFI), the Herschel Space Observatory is providing extraordinary observational opportunities in the 55-670 μm spectral range. HIFI has opened for the first time to high-resolution spectroscopy the submillimetre band that includes the fundamental rotational transitions of interstellar hydrides, the basic building blocks of astrochemistry. We discuss a recent HIFI discovery of metastable rotational transitions of the hydronium ion (protonated water, H(3)O(+)), with rotational level energies up to 1200 K above the ground state, in absorption towards Sagittarius B2(N) in the Galactic centre. Hydronium is an important molecular ion in the oxygen chemical network. Earlier HIFI observations have indicated a general deficiency of H(3)O(+) in the diffuse gas in the Galactic disc. The presence of hot H(3)O(+) towards Sagittarius B2(N) thus appears to be related to the unique physical conditions in the central molecular zone, manifested, for example, by the widespread presence of abundant H(3)(+). One intriguing theory for the high rotational temperature characterizing the population of the H(3)O(+) metastable levels may be formation pumping in molecular gas irradiated by X-rays emitted by the Galactic centre black hole. Alternatively, the pervasive presence of enhanced turbulence in the central molecular zone may give rise to shocks in the lower-density medium that is exposed to energetic radiation.