Saddle-node bifurcations in the spectrum of HOCl

A detailed analysis of the bound-state spectrum of HOCl ~hypoclorous acid! in the ground electronic state is presented. Exact quantum mechanical calculations ~filter diagonalization! are performed employing an ab initio potential energy surface, which has been constructed using the multireference configuration-interaction method and a quintuple-zeta one-particle basis set. The wave functions of all bound states up to the HO1Cl dissociation threshold are visually inspected in order to assign the spectrum in a rigorous way and to elucidate how the spectrum develops with energy. The dominant features are ~1! a 2:1 anharmonic resonance between the bending mode and the OCl stretching mode, which is gradually tuned in as the energy increases, and ~2! a saddle-node bifurcation, i.e., the sudden birth of a new family of states. The bifurcation is further investigated in terms of the structure of the classical phase space ~periodic orbits, continuation/bifurcation diagram!. It is also discussed how the spectrum of bound states persists into the continuum and how the various types of quantum mechanical continuum wave functions affect the state-specific dissociation rates. © 2000 American Institute of Physics. @S0021-9606~99!00901-0#