Mr LARREGARAY Pascal

For the gas-phase elementary processes of interest in interstellar or atmospheric chemistry, typically involving a limited number of atoms, molecular beams techniques, associated with spectroscopy, allow to measure with an ever increasing precision the main observables from reaction dynamics, i.e. the quantum states and spatial distributions of the nascent products. The theoretical analysis of such experiments relies, on the one hand, on numerical simulation, mainly within the framework of classical mechanics, but also on the development of analytical models, of primary importance to unravel the concepts to rationalize the course of reactive collisions.

Concerning models, I apply and develop the statistical approach of reactivity. This theory, which might prove very powerful when the reaction involves a long-lived intermediate complex, has been much used lately. Nevertheless, it neglects the dynamics upon approach of the reactants and separation of the products (couplings between vibration, rotation and translation). We are thus developing, from both simulation and analytical models, a statistico-dynamical approach, based on the basic hypotheses of statistical theories (existence of transition states, statistical distribution of the complex phase space states) and which explicitly treats the dynamics of complex formation and dissociation. This approach is also applied to gas-surface processes (Langmuir-Hinshelwood process).

Concerning dynamics simulation, I contribute to the inclusion of semi-classical corrections into the classical formalism of reactivity to account for important quantum effects (quantization, tunneling). These developments might result important in that our research field is more and more interested in large molecules for which the simulation of quantum dynamics is today impossible.