Béatrice Bussery-Honvault

A bond–bond description of the intermolecular interaction energy: the case of weakly bound N2–H2 and N2–N2 complexes

The atom-bond pairwise additive approach, recently introduced by us to describe the potential energy surface for atom-molecule cases, is extended here for the first time to molecule-molecule systems. The idea is to decompose the van der Waals interaction energy into bond-bond pair contributions. This must be considered an improvement with respect to the familiar atom-atom pairwise additive representation since, still using a simple formulation, it indirectly accounts for three body effects. Such an approach also allows to include, in a straightforward way, the effect of the bond length on the intermolecular interaction energy. Cases of study are the weakly bound complexes involving the H(2) and N(2) molecules, namely N(2)-H(2) and N(2)-N(2), here described as a single bond-bond pair. For both systems ab initio calculations and experimental molecular beam scattering data, as well as second virial coefficients, have been employed to test the accuracy of the chosen representation of the interaction and to improve the obtained potential energy surfaces. The results of this work are important also for the generalization to the cases involving molecular ions and polyatomic molecules.

Linewidths of C2H2 perturbed by H2: experiments and calculations from an ab initio potential

In this work we present a theoretical and experimental study of the acetylene-hydrogen system. A potential surface considering rigid monomers has been obtained by ab initio quantum chemistry methods. This 4-dimensional potential is further employed to compute, using the close-coupling approach and the coupled-states approximation, pressure broadening coefficients of C2H2 isotropic Raman Q lines over a temperature range of 77 to 2000 K. Experimental data for the acetylene nu2 Raman lines broadened by molecular hydrogen are obtained using stimulated Raman spectroscopy. The comparison of theoretical values with experimental data at 143 K is promising. Approximations to increase the computational efficiency are proposed.

Time-dependent wave packet and quasiclassical trajectory study of the C(P3)+OH(X Π2)→CO(X Σ1+)+H(S2) reaction at the state-to-state level

The first calculations of state-to-state reaction probabilities and product state-resolved integral cross sections at selected collision energies (0.05, 0.1, 0.5, and 1.0 eV) for the title reaction on the ab initio potential energy surface of [Zanchet et al. J. Phys. Chem. A 110, 12017 (2006)] with the OH reagent in selected rovibrational states (v = 0-2, j = 0-5) have been carried out by means of the real wave packet (RWP) and quasiclassical trajectory (QCT) methods. State-selected total reaction probabilities have been calculated for total angular momentum J = 0 in a broad range of collision energies. Integral cross sections and state-specific rate coefficients have been obtained from the corresponding J = 0 RWP reaction probabilities for initially selected rovibrational states by means of a capture model. The calculated RWP and QCT state-selected rate coefficients are practically temperature independent. Both RWP and QCT reaction probabilities, integral cross sections, and rate coefficients are almost independent of the initial rotational excitation. The RWP results are found to be in an overall good agreement with the corresponding QCT results. The present results have been compared with earlier wave packet calculations carried out on the same potential energy surface.

The dynamics of the C(1D) + H2 reaction: a comparison of crossed molecular beam experiments with quantum mechanical and quasiclassical trajectory calculations on the first two singlet (11A' and 11A'') potential energy surfaces

The contribution of the first excited 11A″ state potential energy surface (PES) to the reactivity of the C(1D)+H2 system has been examined by comparing the simulations obtained with the theoretical state-to-state differential cross sections (DCSs) with experimental data from cross molecular beam experiments. The calculations were carried out using the quantum scattering and quasiclassical trajectory calculations on the ground and first excited PESs, and the DCSs were convoluted with the various apparatus functions in order to simulate the experimental laboratory angular distributions and time-of-flight distributions. It was expected that the participation of the excited PES could solve the relatively minor discrepancies resulting from the comparison between the experimental data and the simulations using the DCSs obtained on the ground state PES. Nevertheless, the addition of the contribution of the excited 11A″ PES worsens the agreement with the experimental results.

Born–Oppenheimer quantum dynamics of the C(D1)+H2 reaction on the CH2 ã A11 and b̃ B11 surfaces

We present the Born-Oppenheimer coupled-channel dynamics of the reaction (12)C((1)D)+(1)H(2)(X (1)Sigma(g) (+))-->CH(X (2)Pi)+H((2)S), considering the uncoupled CH(2) states ã (1)A(1) and b (1)B(1), the permutation-inversion symmetry, and Coriolis interactions. Using accurate MRCI potential energy surfaces (PESs), we obtain initial-state-resolved reaction probabilities, cross sections, and rate constants through the time-dependent, real wavepacket (WP) and flux methods, taking into account the proton-spin statistics for both electronic species. Comparing results on both PESs, we point out the role of the b (1)B(1) upper state on the initial-state-resolved dynamics and on the thermal kinetic rate. WP probabilities at J=0 and cross sections at E(col)=0.080 eV agree quite well with quantum-mechanical time-independent findings. Probabilities and WP snapshots show the different reaction mechanisms on the PESs, i.e., an ã (1)A(1) indirect perpendicular insertion and a b (1)B(1) direct sideways collision, associated with many and few sharp resonances, respectively. All cross sections are very large at low E(col), decrease at high energies, and that of the lowest reactant state presents some weak resonances. As the temperature increases from 100 to 400 K, the ã (1)A(1) rate constant increases slightly from 1.37x10(-10) to 1.43x10(-10) cm(3) s(-1), whereas the b (1)B(1) one decreases from 1.30x10(-10) to 0.98x10(-10) cm(3) s(-1). In this temperature range, the b (1)B(1) contribution to the total rate constant thus decreases from 49% to 41%. At 300 K, the WP and experimental rates are equal to (2.45+/-0.08)x10(-10) and (2.0+/-0.6)x10(-10) cm(3) s(-1), respectively.

Long-range multipolar potentials of the 18 spin-orbit states arising from the C(P3)+OH(X Π2) interaction

We present multipolar potentials at large intermolecular distances for the 18 doubly degenerate spin-orbit states arising from the interaction between the two open-shell systems, C((3)P) and OH(X (2)Pi). With OH fixed at its ground vibrational state-averaged distance r(0), the long-range potentials are two-dimensional potential energy surfaces (PESs) that depend on the intermolecular distance R and the angle gamma = CGH, where G represents the mass center of OH. The 18x18 diabatic potential matrix elements are built up from the perturbation theory up to second order and from a two-center expansion of the Coulombic interaction potential, resulting in a multipolar expansion of the potential expressed as a series of terms varying in R(-n). The expressions for the long-range coefficients of the expansion are explicitly given in terms of monomer properties such as permanent multipole moments, and static and dynamic polarizabilities. Accurate values for the monomer properties are used to properly determine the long-range interaction coefficients. The diagonalization of the full 18x18 potential matrix generates adiabatic long-range PESs in good agreement with their ab initio counterparts.

Ab initio potential energy curves, transition dipole moments and spin–orbit coupling matrix elements for the first twenty states of the calcium dimer

State-of-the-art ab initio techniques have been applied to compute the potential energy curves of the calcium diatom in the Born–Oppenheimer approximation for the first twenty singlet and triplet states dissociating into 3P+1S, 3D+1S, 1D+1S and 1P+1S atomic states. All the excited state potential curves were computed using a combination of the linear response theory within the coupled-cluster singles and doubles framework for the core–core and core–valence electronic correlation with the full configuration interaction for the valence–valence correlation. The electric and magnetic transition dipole moments governing the X and X , respectively, have been obtained as the first residue of the polarization propagator computed with the linear response coupled-cluster method restricted to single and double excitations. Spin–orbit coupling matrix elements have been evaluated within the complete active space self-consistent field framework using an effective core potential from the Cowan–Griffin relativistic ab initio model potential method. With these couplings, the spin–orbit coupled potential energy curves for the and 1 u states of the calcium diatom have been obtained. Our results are compared with other ab initio calculations and with the experimental data available for the calcium diatom. §Dedicated to Professor Andrzej J. Sadlej on the occasion of his 65th birthday.

Extensive ab initio study of the integrated IR intensity in the N2 fundamental collision-induced band

The present paper aims at ab initio investigation of the temperature variations of collision-induced absorption (CIA) intensity in the nitrogen fundamental. Recent measurements (Yu.I. Baranov et al., JMS 233, 160 (2005)) showed that the band integrated CIA intensity increases as soon as the temperature rises above room temperature. This phenomenon can be interpreted in terms of the increasing role played by the induced dipole at a nearly repulsive wall as far as the temperature increases. To check this idea a complete analysis of the CIA intensity based on ab initio calculation is required including evaluations of both the energy and dipole surfaces. We have used the coupled-cluster CCSD(T) technique to calculate the dipole surface from the electric-field perturbed molecular energies (finite-field method). This procedure is complemented with an accurate ab initio potential energy surface and by a semi-classical statistical averaging of the dipole. The calculated integrated intensities are shown to fit the observations within 15%. The increase of the integrated intensity with elevated temperatures is clearly seen.

Influence of ro-vibrational and isotope effects on the dynamics of the C(3 P)+ OD(X 2Π) → CO(X 1 Σ+) + D(2 S) reaction

The C(3 P)+OD(X 2Π) reaction has been studied by means of quantum mechanical real wave packet (RWP) and quasiclassical trajectory (QCT) methodologies on the ground potential energy surface of Zanchet et al. [J. Phys. Chem. A 110, 12017 (2006)]. Initial state selected total reaction probabilities at J = 0 total angular momentum have been calculated for a wide range of collision energies. Product state-resolved integral cross-sections at selected collision energies and excitation functions have been determined from the RWP calculations using the J-shifting approximation and from QCT calculations. State-specific and thermal rate coefficients have been calculated using both methodologies up to 500 K. The effect of reagent rotational excitation on the dynamics for the C(3 P)+OH(X 2Π) and C(3 P)+OD(X 2Π) reactions has been investigated and interesting discrepancies between the QCT and RWP results have been found. The RWP results are found to be in an overall good agreement with the corresponding QCT results, although the QCT integral cross-section and rate coefficients are slightly smaller than those obtained from the RWP calculations.