Collisional data of high quality is a prerequisite for a comprehensive analysis of interstellar emission spectra. Concerning small hydrides, recent collisional data have been computed for CH, CH+, NH and OH. In all these studies, however, the helium atom was employed as a substitute for H2, for simplicity. It is now well known that significant differences exist between He and H2, even when this latter is in its ground para rotational state (j=0). Collisions with He atoms thus provide guidance at the order-of-magnitude level or better but they can certainly not reproduce H2 (or H) to within 30% accuracy, which is necessary to guarantee high confidence in radiative transfer calculations and to provide a robust determination of molecular column densities. Furthermore, all these studies were performed within the rigid rotor approximation. While this approximation is valid in the case of He atoms, it is highly questionable in the case of H or H2 because the reactive channels are expected to play a role, even at low temperatures. The two main objectives of the present project are i) to tackle the collisional excitation of reactive interstellar species, namely hydride radicals and ions, and to compare theory with experiment and ii) to assess the impact of the collisional rate coefficients on coupled radiative transfer/chemical models and to compare predictions with observations. It should be emphasized that until recently, analogue studies have been essentially restricted to non-reactive systems. The present project therefore represents a major step forward both in the understanding of molecular energy transfer and in the analysis of interstellar spectra.

A second objective of the project is to investigate the nuclear-spin chemistry of the multiply-hydrogenated species (H2O, NH3, etc.) which drives the ortho-to-para ratios that are now accurately measured with, in particular, the Herschel telescope. Spin-state chemistry is expected to provide new molecular line diagnostics of great astrochemical relevance.

A third objective is to provide collisional data for the isotopologues of the most abundant hydrides, in particular the deuterated species (e.g. NH2D). This data is of crucial importance to determine robust (D/H) ratios in the interstellar medium.

The present project is composed of three main tasks. The first task (Theory) concerns the computation of collisional and reactive cross sections and rate coefficients for small interstellar hydrides. The second task (Experiment) is devoted to laboratory measurements of rate coefficients and comparisons with theoretical data. The third and last task (Astrophysical Modeling) focuses on the use of the data in radiative transfer and chemical models. Each task is in turn composed of a couple of sub-tasks which contribute to and are related to the three main tasks as outlined below.

From the strong multi-disciplinary character of the proposal, we expect a broad impact of the results, both in chemical physics and astrochemistry. One of the important actions of the project will be dissemination of the results through the two communities via publications, press releases, database, code repositories and web activities.
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