Jean-Michel Launay

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.

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...

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.

Kinetics and Dynamics of the S(1D2) + H2 → SH + H Reaction at Very Low Temperatures and Collision Energies

We report combined studies on the prototypical S(1D2) + H2 insertion reaction. Kinetics and crossed-beam experiments are performed in experimental conditions approaching the cold energy regime, yielding absolute rate coefficients down to 5.8 K and relative integral cross sections to collision energies as low as 0.68 meV. They are supported by quantum calculations on a potential energy surface treating long-range interactions accurately. All results are consistent and the excitation function behavior is explained in terms of the cumulative contribution of various partial waves.

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.

A detailed quantum mechanical and quasiclassical trajectory study on the dynamics of the H++H2→H2+H+ exchange reaction

The H+ + H2 exchange reaction has been studied theoretically by means of a different variety of methods as an exact time independent quantum mechanical, approximate quantum wave packet, statistical quantum, and quasiclassical trajectory approaches. Total and state-to-state reaction probabilities in terms of the collision energy for different values of the total angular momentum obtained with these methods are compared. The dynamics of the reaction is extensively studied at the collision energy of E(coll)=0.44 eV. Integral and differential cross sections and opacity functions at this collision energy have been calculated. In particular, the fairly good description of the exact quantum results provided by the statistical quantum method suggests that the dynamics of the process is governed by an insertion mechanism with the formation of a long-lived collision complex.

Ultracold Li + Li2 collisions: bosonic and fermionic cases.

We have carried out quantum dynamical calculations of vibrational quenching in Li + Li(2) collisions for both bosonic (7)Li and fermionic (6)Li. These are the first ever such calculations involving fermionic atoms. We find that for the low initial vibrational states considered here (v < or = 3), the quenching rates are not suppressed for fermionic atoms. This contrasts with the situation found experimentally for molecules formed via Feshbach resonances in very high vibrational states.