The potential energy surface of H(2)O-H(2) is of great importance for quantum chemistry as a test case for H(2)O-molecule interactions. It is also required for a detailed understanding of important astrophysical processes, namely, the collisional excitation of water, including the pumping of water masers and the formation of molecular hydrogen on icy interstellar dust grains. We have calculated the interaction for H(2)O-H(2) by performing both rigid-rotor (five-dimensional) and non-rigid-rotor (nine-dimensional) calculations using the coupled-cluster theory at the level of singles and doubles with perturbative corrections for triple excitations [CCSD(T)] with moderately large but thoroughly selected basis set. The resulting surface was further calibrated using high precision explicitly correlated CCSD(T)-R12 calculations on a subset of the rigid-rotor intermolecular geometries. The vibrationally averaged potential is presented in some details and is compared with the most recent rigid-rotor calculations. We explain, in particular, as to why vibrationally averaged rigid-rotor geometries are a better choice than equilibrium geometries. Our fit of the vibrationally averaged surface provides for the first time an accuracy of approximately 3 cm(-1) in the van der Waals minimum region of the interaction. The overall accuracy of the nine-dimensional surface and fit is lower but remains of the order of 3%-4% of the anisotropy in the domain spanned by the vibrational functions.

The hydrogen and water molecules are ubiquitous in the Universe. Their mutual collisions drive water masers and other line emission in various astronomical environments, notably molecular clouds and star forming regions. We report here a full nine-dimensional interaction potential for H 2 O−H 2 calibrated using high-accuracy, explicitly correlated wavefunctions. All degrees of freedom are included using a systematic procedure transferable to other small molecules of astrophysical or atmospherical relevance. As a first application, we present rate constants for the vibrational relaxation of the v 2 bending mode of H 2 O obtained from quasi-classical trajectory calculations in the temperature range 500−4000 K. Our high temperature (T ≥ 1500 K) results are found compatible with the single experimental value at 295 K. Our rates are also significantly larger than those currently used in the astrophysical literature and will lead to a thorough re-interpretation of vibrationally excited water emission spectra from space.

Aims. Using a newly determined 5D potential energy surface for H 2 -H 2 O we provide an extended and revised set of rate coefficients for de-excitation of the lowest 10 para-and 10 ortho-rotational levels of H 2 O by collisions with para-( j = 0) and ortho-H 2 ( j = 1), for kinetic temperatures from 5 K to 20 K. Methods. Our close coupling scattering calculations involve a slightly improved set of coupled channels with respect to previous calculations. In addition, we discuss the influence of several features of this new 5D interaction on the rotational excitation cross sections. Results. The new interaction potential leads to significantly different rate coefficients for collisions with para-H 2 ( j = 0). In particular the de-excitation rate coefficient for the 1 10 to 1 01 transition is increased by up to 300% at 5 K. At 20 K this increase is 75%. Rate coefficients for collisions with ortho-H 2 ( j = 1) are modified to a lesser extent, by up to 40%. The influence of the new potential on collisions with both para-( j = 0) and ortho-H 2 ( j = 1) is expected to become less pronounced at higher temperatures.

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Context. Ammonia and its deuterated isotopologues probe physical conditions in dense molecular cloud cores. The time-dependence of deuterium fractionation and the relative abundances of different nuclear spin modifications are supposed to provide means of determining the evolutionary stages of these objects. Aims. We aim to test the current understanding of spin-state chemistry of deuterated species by determining the abundances and spin ratios of NH 2 D, NHD 2 , and ND 3 in a quiescent, dense cloud. Methods. Spectral lines of NH 3 , NH 2 D, NHD 2 , ND 3 , and N 2 D + were observed towards a dense, starless core in Ophiuchus with the APEX, GBT, and IRAM 30-m telescopes. The observations were interpreted using a gas-grain chemistry model combined with radiative transfer calculations. The chemistry model distinguishes between the different nuclear spin states of light hydrogen molecules, ammonia, and their deuterated forms. Different desorption schemes can be considered. Results. High deuterium fractionation ratios with NH 2 D/NH 3 ∼ 0.4, NHD 2 /NH 2 D ∼ 0.2, and ND 3 /NHD 2 ∼ 0.06 are found in the core. The observed ortho/para ratios of NH 2 D and NHD 2 are close to the corresponding nuclear spin statistical weights. The chemistry model can approximately reproduce the observed abundances, but predicts uniformly too low ortho/para-NH 2 D, and too large ortho/para-NHD 2 ratios. The longevity of N 2 H + and NH 3 in dense gas, which is prerequisite to their strong deuteration, can be attributed to the chemical inertia of N 2 on grain surfaces. Conclusions. The discrepancies between the chemistry model and the observations are likely to be caused by the fact that the model assumes complete scrambling in principal gas-phase deuteration reactions of ammonia, which means that all the nuclei are mixed in reactive collisions. If, instead, these reactions occur through proton hop/hydrogen abstraction processes, statistical spin ratios are to be expected. The present results suggest that while the deuteration of ammonia changes with physical conditions and time, the nuclear spin ratios of ammonia isotopologues do not probe the evolutionary stage of a cloud.

We report extensive theoretical calculations on the rotation-inversion excitation of interstellar ammonia (NH 3 ) due to collisions with atomic and molecular hydrogen (both para-and ortho-H 2 ). Close-coupling calculations are performed for total energies in the range 1-2000 cm −1 and rotational cross sections are obtained for all transitions among the lowest 17 and 34 rotation-inversion levels of ortho-and para-NH 3 , respectively. Rate coefficients are deduced for kinetic temperatures up to 200 K. Propensity rules for the three colliding partners are discussed and we also compare the new results to previous calculations for the spherically symmetrical He and para-H 2 projectiles. Significant differences are found between the different sets of calculations. Finally, we test the impact of the new rate coefficients on the calibration of the ammonia thermometer. We find that the calibration curve is only weakly sensitive to the colliding partner and we confirm that the ammonia thermometer is robust.

We present theoretical studies on the scattering resonances in rotationally inelastic collisions of NH 3 and ND 3 molecules with H 2 molecules. We use the quantum close-coupling method to compute state-to-state integral and differential cross sections for the NH 3 /ND 3-H 2 system for collision energies between 5 and 70 cm −1 , using a previously reported potential energy surface [Maret et al., Mon. Not. R. Astron. Soc. 399, 425 (2009)]. We identify the resonances as shape or Feshbach resonances. To analyze these, we use an adiabatic bender model, as well as examination at the scattering wave functions and lifetimes. The strength and width of the resonance peaks suggest that they could be observed in a crossed molecular beam experiment involving a Stark-decelerated NH 3 beam. C 2015 AIP Publishing LLC. [http://dx.

We study the gravitational collapse of prestellar sources and the associated evolution of their chemical composition. We use the University of Grenoble Alpes Astrochemical Network (UGAN), which includes reactions involving the different nuclearspin states of H 2 , H + 3 , and of the hydrides of carbon, nitrogen, oxygen, and sulfur, for reactions involving up to seven protons. In addition, species-to-species rate coefficients are provided for the ortho/para interconversion of the H + 3 + H 2 system and isotopic variants. The composition of the medium is followed from an initial steady state through the early phase of isothermal gravitational collapse. Both the freeze-out of the molecules on to grains and the coagulation of the grains were incorporated in the model. The predicted abundances and column densities of the spin isomers of ammonia and its deuterated forms are compared with those measured recently towards the prestellar cores H-MM1, L16293E, and Barnard B1. We find that gas-phase processes alone account satisfactorily for the observations, without recourse to grain-surface reactions. In particular, our model reproduces both the isotopologue abundance ratios and the ortho:para ratios of NH 2 D and NHD 2 within observational uncertainties. More accurate observations are necessary to distinguish between full scrambling processesas assumed in our gas-phase network-and direct nucleus-or atom-exchange reactions.