Marc Joyeux

Laboratoire de Spectrometrie Physique, Universtie Joseph Fourier-Grenoble I, BP 87, F-38402 St. Martin d'Heres Cedex, FRANCE

Recent global surfaces fitted against ab initio points (and eventually slightly adjusted against experimental data) are able of describing the highly excited vibrational dynamics of small (triatomic) molecules up to the dissociation threshold or the isomerization barrier with reasonable accuracy. Classical studies at these high energies generally indicate wide spread chaos.The goal of this presentation is to lay emphasis on the fact that the quantum dynamics might be much more regular than the classical one. Two examples will be discussed.
The first example is 2-dimensional HOCl. The quantum spectrum will be shown to be governed by the classical stable POs up to the dissociation threshold. The connection between Husimi distributions, Poincare surfaces of section and Intramolecular Vibrational energy Redistribution (IVR) will also be discussed in some detail.
The second example is HCN/CNH. We have recently derived a modified version of canonical perturbation theory, which applies to floppy systems. Upon application of this procedure to two well-known ab initio surfaces for the HCN/CNH isomers, it will be shown that the three degrees of freedom can be considered to remain totally decoupled up to and above the isomerization barrier.