Vibrational dynamics up to the dissociation threshold: A case study of two-dimensional HOCl

This work is aimed at extending recent studies dealing with the highly exci ted vibrational dynamics of HOCl [J. Chem. Phys. 111, 6807 (1999); J. Chem. Phys. 112, 77 (2000)], by taking advantage of the fact that the OH-stretch remains largely decoupled from the two other degrees of freedom up to and above the dissociation threshold. The molecule is thus reduced to a two-dim ensional (2D) system by freezing the OH bond length to its equilibrium valu e. All of the calculated bound states of the 2D system, as well as the firs t 40 resonances, can be assigned with a Fermi polyad quantum number. The bi furcation diagram of the principal families of periodic orbits (POs) is ext ended to higher energies compared to 3D studies. In particular, the birth o f "inversion" states (states exploring two equivalent wells connected throu gh the linear HOCl configuration) is related to a period-doubling bifurcati on of the families of bending POs, while ''dissociation'' states (states fo r which the energy flows back and forth along the dissociation pathway) are shown to lie on top of three successive families of POs born at saddle-nod e bifurcations. Based on the derivation of a classical analogue of the quan tum Fermi polyad number, the energies of particular quantum states and clas sical POs are plotted on the same diagram for the 2D ab initio surface and are shown to agree perfectly. In contrast, comparison of classical Poincare surfaces of section and quantum Husimi distributions suggests that the cla ssical dynamics of 2D HOCl is much more chaotic than the quantum dynamics. This observation is discussed in terms of the quantum/classical corresponde nce, and particularly of the vague tori introduced by Reinhardt. It is neve rtheless shown that quantum and classical mechanics agree in predicting a s low intramolecular vibrational energy redistribution (IVR) between the OCl stretch and the bend degrees of freedom. (C) 2000 American institute of Phy sics. [S0021-9606(00)90945-4].