Highly Excited Motion in Molecules: Saddle-Node Bifurcations and Their Fingerprints in Vibrational Spectra

Abstract

The vibrational motion of highly excited molecules is discussed in terms of exact quantum and classical mechanics calculations, employing global potential energy surfaces, as well as in terms of a spectroscopic Hamiltonian and its semiclassical limit. The main focus is saddle-node bifurcations and their influence on the spectrum. The general features are illustrated by three examples, which despite their quite different intramolecular motions have several aspects in common: HCP, HOCl, and HOBr. In all three cases a 1:2 Fermi resonance is the ultimate cause of the complications observed in the spectra.