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MARK S CHILD, Physical and Theoretical Chemistry Laboratory, South Parks Rd, Oxford, OX1 3QZ, UK.
Changes in the eigenvalue structure in the vicinty of a saddle-point on the potential energy suface are illustrated by semi-classical and quantum mechanical studies on model potential energy surfaces for HCP. The following points are addressed:
(a) The connection between classical periodic orbits and Fermi resonace polyads, and the breakdown of the polyad model as the bending frequency tunes out of 2:1 resonance with the CP stretch.
(b) The observation of 'quantum mondromy' in the underlying spherical pendulum model, and its influence of the values of the spectroscopic vibration-rotation parameters, as the H atom approaches the P end of the molecule.
(c) A possible formulation of the spectroscopic theory at the saddle point in terms of spherical pendulum eigenstates, and the nature of the relevant matrix elements.