The bound state spectrum of HOBr up to the dissociation limit: Evolution of saddle-node bifurcations

Based on an accurate potential energy surface @J. Chem. Phys. 113, 4598 ~2000!# we calculated ca. 700 bound state energies and wave functions of nonrotating HOBr using the filter-diagonalization method. Similar to HOCl, a 1:2 anharmonic resonance between the HOBr bending and the OBr stretching mode determines the general structure of the level spectrum. One of the results of this resonance is a saddle-node bifurcation at which a new class of states ~‘‘dissociation states’’! comes into existence, which advance along the HO–Br dissociation path. Because the resonance condition at low energies is better fulfilled for HOBr, the bifurcation occurs at considerably lower energies than for HOCl. The results of the quantum mechanical calculations are interpreted in terms of classical periodic orbits ~continuation/bifurcation diagram! and a semiclassical analysis based on a spectroscopic Hamiltonian, which is fitted to the exact energy levels and also taking into account the wave functions. © 2003 American Institute of Physics. @DOI: 10.1063/1.1569914#