François Lique

Astronomical identification of CN-, the smallest observed molecular anion

We present the first astronomical detection of a diatomic negative ion, the cyanide anion CN-, as well as quantum mechanical calculations of the excitation of this anion through collisions with para-H2. CN- is identified through the observation of the J = 2-1 and J = 3-2 rotational transitions in the C-star envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles indicate that CN-, like the large anion C6H-, is formed in the outer regions of the envelope. Chemical and excitation model calculations suggest that this species forms from the reaction of large carbon anions with N atoms, rather than from the radiative attachment of an electron to CN, as is the case for large molecular anions. The unexpectedly large abundance derived for CN-, 0.25 % relative to CN, makes likely its detection in other astronomical sources. A parallel search for the small anion C2H- remains so far unconclusive, despite the previous tentative identification of the J = 1-0 rotational transition. The abundance of C2H- in IRC +10216 is found to be vanishingly small, < 0.0014 % relative to C2H.

Breakdown of the Born-Oppenheimer approximation in the F+ o-D2 -> DF + D reaction.

The reaction of F with H2 and its isotopomers is the paradigm for an exothermic triatomic abstraction reaction. In a crossed-beam scattering experiment, we determined relative integral and differential cross sections for reaction of the ground F(2P3/2) and excited F*(2P1/2) spin-orbit states with D2 for collision energies of 0.25 to 1.2 kilocalorie/mole. At the lowest collision energy, F* is ∼1.6 times more reactive than F, although reaction of F* is forbidden within the Born-Oppenheimer (BO) approximation. As the collision energy increases, the BO-allowed reaction rapidly dominates. We found excellent agreement between multistate, quantum reactive scattering calculations and both the measured energy dependence of the F*/F reactivity ratio and the differential cross sections. This agreement confirms the fundamental understanding of the factors controlling electronic nonadiabaticity in abstraction reactions.

BASECOL2012: A collisional database repository and web service within the Virtual Atomic and Molecular Data Centre (VAMDC)

The BASECOL2012 database is a repository of collisional data and a web service within the Virtual Atomic and Molecular Data Centre (VAMDC, http://www.vamdc.eu). It contains rate coefficients for the collisional excitation of rotational, ro-vibrational, vibrational, fine, and hyperfine levels of molecules by atoms, molecules, and electrons, as well as fine-structure excitation of some atoms that are relevant to interstellar and circumstellar astrophysical applications. Submissions of new published collisional rate coefficients sets are welcome, and they will be critically evaluated before inclusion in the database. In addition, BASECOL2012 provides spectroscopic data queried dynamically from various spectroscopic databases using the VAMDC technology. These spectroscopic data are conveniently matched to the in-house collisional excitation rate coefficients using the SPECTCOL sofware package (http:// vamdc.eu/software), and the combined sets of data can be downloaded from the BASECOL2012 website. As a partner of the VAMDC, BASECOL2012 is accessible from the general VAMDC portal (http://portal.vamdc.eu) and from user tools such as SPECTCOL.

Observation of partial wave resonances in low-energy O2-H2 inelastic collisions.

Quantum Collision Course Our experience of a world apparently governed by classical physics is a consequence of the fact that quantum mechanical effects average out in size regimes much larger than nanometers. Even at the molecular level, the quantized nature of rotational energy distributions is often obscured by averaging effects. Chefdeville et al. (p. 1094; see the Perspective by Casavecchia and Alexander) have observed a striking manifestation of quantized rotation in the scattering trajectories of colliding H2 and O2 molecular beams. The experimentally resolved partial wave resonances show essentially complete agreement with theoretical calculations and deviate starkly from classical collision paradigms. Purely quantum mechanical effects are manifested in an experiment probing crossed molecular beams. [Also see Perspective by Casavecchia and Alexander] Partial wave resonances predicted to occur in bimolecular collision processes have proven challenging to observe experimentally. Here, we report crossed-beam experiments and quantum-scattering calculations on inelastic collisions between ground-state O2 and H2 molecules that provide state-to-state cross sections for rotational excitation of O2 (rotational state N = 1, j = 0) to O2 (N = 1, j = 1) in the vicinity of the thermodynamic threshold at 3.96 centimeter−1. The close agreement between experimental and theoretical results confirms the classically forbidden character of this collision-induced transition, which occurs exclusively in a purely quantum mechanical regime via shape and Feshbach resonances arising from partial waves with total angular momentum (J) = 2 to 4.

Benchmarks for the generation of interaction potentials for scattering calculations: applications to rotationally inelastic collisions of C4 (X3Σ−g) with He

We present an application of recently developed, explicitly correlated, partially spin-restricted coupled-cluster RCCSD(T)-F12x (x = A/B) methods [G. Knizia, T. B. Adler, and H.-J. Werner, J. Chem. Phys., 2009, 130, 054104] for the generation of multi-dimensional potential energy surfaces (PESs) for scattering calculations. We test the method on the O(2)-He van der Waals model system by a comparison with standard orbital-based coupled-cluster techniques, employing correlation-consistent atomic basis sets (aug-cc-pVXZ, X = T, Q, 5, 6) and a complete basis set. From this comparison, it is obvious that the RCCSD(T)-F12/aug-cc-pVTZ approach is accurate enough for the description of short and long-range interactions with low computational effort. We apply this new method in studies of the interaction of the carbon-rich interstellar species C(4)(X(3)Σ) with atomic He. This PES is subsequently used in quantum close-coupling scattering calculations. The collisional excitation cross-sections of the fine-structure levels of C(4) by He are calculated at low collisional energies. The thermal dependence of rate coefficients is calculated up to 50 K. The propensity rules between fine-structure levels are studied, and it is shown that F-conserving cross sections are much larger, especially for high-N rotational levels rather than F-changing cross sections, as expected from theoretical considerations. This is the first report on the collisional rate coefficients for this system and may have important implications for the astrophysical detection of C4 and modeling of carbon-rich media.

Rotational excitation of carbon monosulfide by collisions with helium

Context. Over the next few years, Alma and Herschel missions will open the universe to high spatial and spectral resolution studies at infrared and sub-millimeter wavelengths. Modeling of the observed spectra will require accurate radiative and collisional rates on species of astrophysical interest. Aims. The present paper focuses on the calculation of new rate coefficients among the 31 first rotational levels of the CS molecule in collision with He for temperatures ranging from 10 K to 300 K. Methods. A new 2D potential energy surface for the CS-He system, calculated at a CSr-distance frozen at its experimental equilibrium distance was obtained with accurate quantum chemistry methods. Quantum close-coupling calculations lead to collisional cross sections and rate coefficients. Results. The new rate coefficients are calculated up to 300 K. These new coefficients differ significantly from previously published ones. The consequences for astrophysical models are evaluated.

Rotational excitation of sulfur monoxide by collisions with helium at low temperature

We present two new two-dimensional potential-energy surfaces for the SO-He system calculated at SO r distance frozen at its experimental minimum-energy distance. Both are obtained at the RCCSD(T) level using two different basis sets (AVTZ and AVQZ) for the three atoms. Bond functions are placed at mid-distance between the SO center of mass and He for a better description of the van der Waals well. Close-coupling calculations of the collisional excitation cross sections of the fine-structure levels of SO by He are calculated at low energies. The exact level splitting is taken into account. It is found that the results obtained from the two surfaces are very similar, except for some small differences observed in the region of resonances at low energies. The propensity rules between fine-structure levels are studied, it is shown that F-conserving cross sections are much larger for high-N rotational levels than cross sections between F-changing levels, as expected from theoretical considerations. The use of infinite order sudden recoupling techniques from spin-free cross sections is investigated. Excitation rate coefficients among fine-structure levels are calculated at low temperatures.

Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN, and HNC isotopologues

Context. The 15 N isotopologue abundance ratio measured today in different bodies of the solar system is thought to be connected to 15 N-fractionation effects that would have occurred in the protosolar nebula. Aims. The present study aims at putting constraints on the degree of 15 N-fractionation that occurs during the prestellar phase, through observations of D, 13 C, and 15 N-substituted isotopologues towards B1b. Molecules both from the nitrogen hydride family, i.e. N2H + , and NH3, and from the nitrile family, i.e. HCN, HNC, and CN, are considered in the analysis. Methods. As a first step, we modelled the continuum emission in order to derive the physical structure of the cloud, i.e. gas temperature and H2 density. These parameters were subsequently used as input in a non-local radiative transfer model to infer the radial abundance profiles of the various molecules. Results. Our modelling shows that all the molecules are affected by depletion onto dust grains in the region that encompasses the B1-bS and B1-bN cores. While high levels of deuterium fractionation are derived, we conclude that no fractionation occurs in the case of the nitrogen chemistry. Independently of the chemical family, the molecular abundances are consistent with 14 N/ 15 N ∼ 300, a value representative of the elemental atomic abundances of the parental gas. Conclusions. The inefficiency of the 15 N-fractionation effects in the B1b region can be linked to the relatively high gas temperature ∼17 K, which is representative of the innermost part of the cloud. Since this region shows signs of depletion onto dust grains, we cannot exclude the possibility that the molecules were previously enriched in 15 N, earlier in the B1b history and that such an enrichment could have been incorporated into the ice mantles. It is thus necessary to repeat this kind of study in colder sources to test such a possibility.

Rotational excitation of CN(X Σ2+) by He: Theory and comparison with experiments

Rotational excitation of the CN(X (2)Sigma(+)) molecule with He is investigated. We present a new two-dimensional potential energy surface (PES) for the He-CN system, calculated at an internuclear CN distance frozen at its experimental equilibrium distance. This PES was obtained using an open-shell, coupled-cluster method including all single and double excitations, as well as the perturbative contributions of connected triple excitations [RCCSD(T)]. Bond functions were placed at mid-distance between the center of mass of the CN molecule and He atom for a better description of the van der Waals interaction. State-to-state collisional excitation cross sections of the fine-structure levels of CN by He are calculated for energies up to 2500 cm(-1), which yield after thermal averaging, rate coefficients up to 350 K. The exact spin splitting of the energy levels is taken into account. The propensity rules between fine-structure levels are studied, and it is shown that the rate constants for Deltaj=DeltaN transitions are much larger than those for Deltaj not equalDeltaN transitions, as expected from theoretical considerations. Our calculated rate coefficients are compared to experimental results at 295 K of Fei et al. [J. Chem. Phys. 100, 1190 (1994)]. The excellent agreement confirms the accuracy of the PESs and of the scattering calculations.

Low-energy inelastic collisions of OH radicals with He atoms and D2 molecules

We present an experimental study on the rotational inelastic scattering of OH (X {sup 2}{Pi}{sub 3/2},J=3/2,f) radicals with He and D{sub 2} at collision energies between 100 and 500 cm{sup -1} in a crossed beam experiment. The OH radicals are state selected and velocity tuned using a Stark decelerator. Relative parity-resolved state-to-state inelastic scattering cross sections are accurately determined. These experiments complement recent low-energy collision studies between trapped OH radicals and beams of He and D{sub 2} that are sensitive to the total (elastic and inelastic) cross sections [Sawyer et al., Phys. Rev. Lett. 101, 203203 (2008)], but for which the measured cross sections could not be reproduced by theoretical calculations [Pavlovic et al., J. Phys. Chem. A 113, 14670 (2009)]. For the OH-He system, our experiments validate the inelastic cross sections determined from rigorous quantum calculations.