Pascal Honvault

A KINETIC DATABASE FOR ASTROCHEMISTRY (KIDA)

We present a novel chemical database for gas-phase astrochemistry. Named the KInetic Database for Astrochemistry (KIDA), this database consists of gas-phase reactions with rate coefficients and uncertainties that will be vetted to the greatest extent possible. Submissions of measured and calculated rate coefficients are welcome, and will be studied by experts before inclusion into the database. Besides providing kinetic information for the interstellar medium, KIDA is planned to contain such data for planetary atmospheres and for circumstellar envelopes. Each year, a subset of the reactions in the database (kida.uva) will be provided as a network for the simulation of the chemistry of dense interstellar clouds with temperatures between 10 K and 300 K. We also provide a code, named Nahoon, to study the time-dependent gas-phase chemistry of zero-dimensional and one-dimensional interstellar sources.

Quantum mechanical and quasi-classical trajectory study of the C(1D)+H2 reaction dynamics

First accurate quantum mechanical (QM) calculations of integral and differential cross sections for the C(1D)+H2(v=0,j=0,1) insertion reaction have been performed on a newly developed ab initio potential energy surface [B. Bussery-Honvault et al., J. Chem. Phys. 115, 10701 (2001)]. These results have been compared with those obtained with a quasi-classical trajectory (QCT) method. A Gaussian-weighted binning procedure to assign product quantum states in the QCT calculations yields vibrational branching ratios and rotational distributions in better agreement with the QM calculations than those obtained when the usual histogramatic binning method is employed. This is the first time that the Gaussian-weighted binning procedure is used for an insertion reaction.

A quantum-mechanical study of the dynamics of the N(2D)+H2→NH+H reaction

We have studied the low energy quantum dynamics of the N(2D)+H2(X 1Σg+)→NH(X 3Σ−)+H(2S) reaction. We use the hyperspherical method and a recently published ab initio potential energy surface. We find a forward–backward symmetry in the differential cross sections which is characteristic of a complex formation. We also present rotational and vibrational integral cross sections.

The 2014 KIDA network for interstellar chemistry

Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These reacti ...

A quantum-mechanical study of the dynamics of the O(1D)+H2→OH+H insertion reaction

We have studied the quantum dynamics of the O(1D)+H2(X 1Σg+)→ OH(X 2Π)+H(2S) reaction at low collision energies. The hyperspherical method has been used in a time-independent formalism. We present rotational and vibrational distributions. OH vibrational distributions are found to be noninverted. Differential cross sections are almost symmetric in the forward and backward directions, with a slight preference for forward scattering. These features are consistent with the formation of an intermediate complex.

Quantum dynamics of ultracold Na + Na2 collisions

Ultracold collisions between spin-polarized Na atoms and vibrationally excited Na2 molecules are investigated theoretically, using a reactive scattering formalism (including atom exchange). Calculations are carried out on both pairwise additive and nonadditive potential energy surfaces for the quartet electronic state. The Wigner threshold laws are followed for energies below 10(-5) K. Vibrational relaxation processes dominate elastic processes for temperatures below 10(-3)-10(-4) K. For temperatures below 10(-5) K, the rate coefficients for vibrational relaxation (v=1-->0) are 4.8x10(-11) and 5.2x10(-10) cm(3) s(-1) for the additive and nonadditive potentials, respectively. The large difference emphasizes the importance of using accurate potential energy surfaces for such calculations.

A study of the C(1D)+H2→CH+H reaction: Global potential energy surface and quantum dynamics

The adiabatic global potential energy surface of the CH2 system for the first singlet state of A′ symmetry (a 1A′) has been computed. Ab initio, multireference, single and double configuration interaction calculations have been used to characterize this state. This potential energy surface has a calculated well depth of 99.7 kcal/mol relative to the C(1D)+H2 asymptote. The surface has no barrier for the perpendicular C2v geometry, but presents a large barrier (12.35 kcal/mol) for the collinear C∞v geometry. The ab initio calculations were carried out over 1748 geometries and the resulting energies were fitted to a many body expansion. Based on this surface, we have performed the first quantum reactive scattering calculations for the C(1D)+H2(X 1Σg+)→CH(X 2Π)+H(2S) reaction and total angular momentum J=0. The hyperspherical coordinates time-independent method has been used. We note that the state-to-state reaction probabilities as a function of the collision energy show a dense resonance structure which is...

Oxygen depletion in dense molecular clouds: a clue to a low O2 abundance?

Context. Dark cloud chemical models usually predict large amounts of O2, often above observational limits. Aims. We investigate the reason for this discrepancy from a theoretical point of view, inspired by the studies of Jenkins and Whittet on oxygen depletion. Methods. We use the gas-grain code Nautilus with an up-to-date gas-phase network to study the sensitivity of the molecular oxygen abundance to the oxygen elemental abundance. We use the rate coefficient for the reaction O + OH at 10 K recommended by the KIDA (KInetic Database for Astrochemistry) experts. Results. The updates of rate coefficients and branching ratios of the reactions of our gas-phase chemical network, especially N + CN and H + + O, have changed the model sensitivity to the oxygen elemental abundance. In addition, the gas-phase abundances calculated with our gas-grain model are less sensitive to the elemental C/O ratio than those computed with a pure gas-phase model. The grain surface chemistry plays the role of a buffer absorbing most of the extra carbon. Finally, to reproduce the low abundance of molecular oxygen observed in dark clouds at all times, we need an oxygen elemental abundance smaller than 1.6 × 10 −4 . Conclusions. The chemistry of molecular oxygen in dense clouds is quite sensitive to model parameters that are not necessarily well constrained. That O2 abundance may be sensitive to nitrogen chemistry is an indication of the complexity of interstellar chemistry.

Quantum Dynamics of UltracoldNa+Na2Collisions

Ultracold collisions between spin-polarized Na atoms and vibrationally excited Na2 molecules are investigated theoretically, using a reactive scattering formalism (including atom exchange). Calculations are carried out on both pairwise additive and nonadditive potential energy surfaces for the quartet electronic state. The Wigner threshold laws are followed for energies below 10(-5) K. Vibrational relaxation processes dominate elastic processes for temperatures below 10(-3)-10(-4) K. For temperatures below 10(-5) K, the rate coefficients for vibrational relaxation (v=1-->0) are 4.8x10(-11) and 5.2x10(-10) cm(3) s(-1) for the additive and nonadditive potentials, respectively. The large difference emphasizes the importance of using accurate potential energy surfaces for such calculations.

The O(1D)+H2 reaction at 56 meV collision energy: A comparison between quantum mechanical, quasiclassical trajectory, and crossed beam results

Quantum mechanical and quasiclassical trajectory reactive scattering calculations have been performed for the O(1D)+H2 (v=0,j=0) reaction on the Dobbyn–Knowles ab initio 1 1A′ and 1 1A″ potential energy surfaces (PES) at the mean collision energy Ecol=56 meV (1.3 kcal/mol) of a crossed beam experimental study based on H-atom Rydberg “tagging” time-of-flight detection. Novel data from this latter experiment are presented and compared with the theoretical results at the level of state-resolved integral and differential cross sections and product recoil energy distributions. A good overall agreement with small discrepancies is found between the experimental data and the results of the two theoretical approaches. The main conclusion of the present work is that the contribution of the ground state 1 1A′ PES to the global reactivity accounts for the experimental observations and that, at the title collision energy, the participation of the 1 1A″ PES in the reaction is negligible for all practical purposes.