Guy Bergeron

Anharmonicity effects in the collinear collision of two diatomic molecules

Abstract The collinear collision of two diatomic molecules is studied. Six different systems - namely N 2 + N 2 , N 2 + CO, N 2 + OC, N 2 + O 2 , H 2 + H 2 , and H 2 + HBr - are selected as samples and compared to each other. In addition, the effect of anharmonicity on the vibrational excitation is investigated. Exact quantum-mechanical transition probabilities are calculated, the Morse and harmornic binding potentials being considered in each case. It is shown that anharmonicity often decreases the TV transition probabilities by a large factor, whereas it has only a small effect on VV transition probabilities. The results are interpreted on the basis of previous ideas on atom-diatom and diatom-diatom collinear collisions.

A quantum-mechanical collinear model study of the collision N2 + O2

Abstract A model close-coupling study of the translation-vibration-vibration (TVV) energy transfer in the collinear collision of two non-identical harmonic diatomic molecules is presented. The numerical application are for N 2 + O 2 driven by an exponential repulsion. Numerous transition probabilities are given, and the various dynamical processes are classified according to their importance. The classification is true whatever the collision energy. A resonable interpretation of the dynamics based on (i) our knowledge of the simpler atom-diatom collision, (ii) the results of auxiliary calculations using various truncated expansions of the interaction potential, and (iii) classical trajectory calculations for the same system, is finally presented. The main results are: (i) The TV transition probabilities of O 2 are greater than that of N 2 by several orders of magnitude; this is due to O 2 being the more excitable molecule and being acted on by the more exciting field; (ii) The secondary processes are interfering sequences of the two principal processes which are, respectively, the TV one-quantum jump in O 2 and VV one-quantum exchange between N 2 and O 2 . (iii) In the case of the TV one-quantum jump in N 2 the mechanism is not unique according as O 2 is vibrationally excited - or not - prior to collision.

A quantum-mechanical investigation of a collinear model for collision-induced dissociation

Abstract Quantum-mechanical wavepacket results are reported for a collinear A + BC → A + B + C dissociation model previously investigated semi-classically by Rusinek and Roberts. The quantum-mechanical and semi-classical probabilities are found to be in very good agreement.

On the dynamical behaviour of the collision between an atom and a highly asymmetric diatomic molecule

Abstract A detailed classical and quantum-mechanical study of the collinear collision between an atom and a highly asymmetric diatomic molecule (D + H-Br) is presented. The existence of a pronounced minimum in the variation with respect to the collision energy of some basic transition probabilities ( P 0→1 , P 1→2 ) and the overall energy transfer (Δ E ) is illustrated and interpreted.

A quantum-mechanical investigation of collinear models for collision-induced dissociation

Abstract Accurate quantum-mechanical wavepacket results are reported for two collinear A + BC → A + B + C dissociation models. Both systems are of the MO EXP (Morse oscillator, exponentially repulsive interaction) type and thus do not allow for the possibility of reaction. One of these models has been previously subjected to a semiclassical study by Rusinek and Roberts. In the present paper, dissociation probabilities from vibrational states υ = 0, 1 and 3 of the diatomic are reported in a wide energy range for both systems. Numerous state-to-state transition probabilities are also given and the results are compared to those of related studies.

On the fixed-nuclei approximation as applied to rotational excitation of molecules by atoms

Abstract The applicability of the fixed-nuclei approximation to the rotational excitation of a diatomic molecule by an atom is investigated. The approximation is shown to predict accurate quantum cross sections for the model system H 2 + N 2 at thermal collision energies. A quasi-classical Monte-Carlo study of the same problem is also performed, and the success of the fixed-nuclei approximation is interpreted by investigating in detail a number of coplanar classical trajectories.

A quantum-mechanical normal-mode approach to collinear collisions of identical diatomics. Perturbed stationary state formulation

Abstract A quantum mechanical perturbed stationary state study of the collinear collision of two identical diatomic molecules within the normal mode decomposition technique is presented. Various collision systems are investigated and are classified in three different groups according to their dynamical behaviour.

Classical study of the influence of induced rotation and anisotropy effects on energy transfer between an atom and a diatomic molecule

Abstract The lagrangian equations of motion for a mass 4 atom colliding with a mass 2 diatomic molecule are solved numerically in order to investigate the validity of some approximations often used in energy transfer theory. The results are shown to be very sensitive to the details of the intermolecular potential; the effect of induced rotation can change the vibrational energy transfer by an order of magnitude.

Exact quantum, quasiclassical forward and reverse reaction probabilities for a colldmear asymmetric model reaction

Abstract Detailed quasiclassical and time-independent quantum reaction probabilities are given for a surface on which large discrepancies between quasiclassical and wavepacket results have previously been found. The quasiclassical results are shown to agree relatively reasonably with the oscillation-averaged time-independent quantum ones if the quasiclassical reverse probabilities are chosen in the threshold region.

Isotope Effect on Vibrational Excitation Induced by Collision of H2 and D2 Molecules

Some effects of the rotational motion of a vibrating rotator on its collisionally induced vibrational excitation are studied for H2 and D2 molecules initially at rest colliding with a He atom. Whatever the collision energy, the vibrational energy transfers are almost equal, due to cancellation between two dynamically opposed effects (in terms of collisional adiabaticity): the rotational modulation effect on the vibrational coordinate favors the vibrational energy transfer in H2, the frequency effect in D2. Numerical results (derived from computed trajectories) are presented to confirm this interpretation.