8230 Phys. Chem. Chem. Phys., 2011, 13, 8230–8235 This journal is c the Owner Societies 2011 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 8230–8235 Rotational quenching of monodeuterated water by hydrogen molecules Laurent Wiesenfeld, a Yohann Scribano b and Alexandre Faure* a Received 18th November 2010, Accepted 15th February 2011 DOI: 10.1039/c0cp02591g Cross sections and rate coefficients for low lying rotational transitions in HDO induced by para and ortho-H 2 collisions are presented for the first time. Calculations have been performed at the close-coupling and coupled-states levels with the deuterated variant of the H 2 O–H 2 interaction potential of Valiron et al. [J. Chem. Phys., 2008, 129, 134306]. Rate coefficients are presented for temperatures between 5 and 100 K and are compared to the corresponding rates for H 2 O and D 2 O. Significant differences caused by the isotopic substitution, in particular the C 2v symmetry breaking, are observed. Finally, our rates are found to be significantly larger (by up to three orders of magnitude at 50 K) than the corresponding HDO–He rates and should lead to a thorough re-estimation of the abundance of interstellar HDO. 1. Introduction Water is one of the key molecules in the physical and chemical evolution of star and planet-forming regions. It is the most abundant component in the icy mantles of interstellar grains and, in the warm regions surrounding young stars (T B 100 K), it plays a crucial role in the cooling of the gas. 1 Owing to strong atmospheric telluric absorption, however, H 16 2 O low- energy lines are almost invisible from the ground and can only be observed under special conditions, e.g. maser emission. 2 Rare water isotopologues like HDO suffer much less telluric absorption and can be observed at millimetre wavelength, e.g. the recent high resolution image of HDO towards a solar- type protostar. 3 Furthermore, observing the deuterated forms of water can help to understand the origin of water in the interstellar medium and its possible link with the D/H ratios observed in comets and in the oceans on the Earth. Investigation of the abundance of water and its isotopologues in space is one of the main target of the HIFI heterodyne instrument aboard the Herschel Space Observatory, which was launched in may 2009. Recent Herschel detections towards star forming regions include HDO (Comito et al. 4 ) and D 2 O (Vastel et al. 5 ) as well as the first (tentative) identification of HD 18 O (Bergin et al. 6 ). In order to interpret the observed spectra in terms of local physical conditions and relative abundances, radiative transfer modeling is necessary which, in turn, requires the knowledge of rates for collisional (de)excitation. At low density, the population of molecular levels is indeed determined by a competition between the radiative and collisional processes. The molecular populations are therefore generally far from local thermodynamical equilibrium (LTE). 7 In star forming regions, the main colliding partner is H 2 . In contrast to H 2 O and D 2 O for which collisional rates with H 2 have been recently computed, 8–10 the only current collisional rates for HDO are those of Green for helium atoms 11 and those of Faure et al. for electron-impact excitation. 12 Here we report calculations of cross sections and rates for the rotational (de)excitation of HDO by H 2 molecules, with a focus on the lowest four rotational levels. Our main objective is to assess the importance and magnitude of the isotopic substitution on the water excitation and also to compare the HDO–H 2 rates with the corresponding HDO–He rates currently employed in astronomical models. The paper is organized as follows: the section below describes the HDO–H 2 potential energy surface. Inelastic scattering calculations are presented and discussed in section 3. The HDO rates are briefly compared to those of H 2 O and D 2 O in section 4. Conclusions are drawn in section 5. 2. Potential energy surface Recently, a nine dimensional (9D) potential energy surface (PES) was developed by Valiron et al. 13,14 for the H 2 O–H 2 system using high accuracy ab initio methods. It was obtained by combining standard CCSD(T) calculations with explicitly correlated CCSD(T)-R12 calculations, fully described in the original papers. This PES is independent of nuclear masses and can be employed for any water isotopologues. Its high accuracy has been spectacularly confirmed very recently by differential cross sections measurements, 15 pressure broadening 16 and molecular beam experiments, 17 and bound state calculations. 18,19 Valiron et al. were able to establish that employing state-averaged geometries is a reliable approxima- tion for including zero-point vibrational effects within a a UJF-Grenoble 1/CNRS-INSU, Institut de Plane ´tologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-38041, France. E-mail: afaure@obs.ujf-grenoble.fr b Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 5209, CNRS - Universite ´ de Bourgogne, 9, av. Alain Savary, B.P. 47870, F-21078 Dijon Cedex, France PCCP Dynamic Article Links www.rsc.org/pccp PAPER