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 calcu lations (filter diagonalization) are performed employing an ab initio poten tial energy surface, which has been constructed using the multireference co nfiguration-interaction method and a quintuple-zeta one-particle basis set. The wave functions of all bound states up to the HO+Cl dissociation thresh old are visually inspected in order to assign the spectrum in a rigorous wa y and to elucidate how the spectrum develops with energy. The dominant feat ures are (1) a 2:1 anharmonic resonance between the bending mode and the OC l 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 s tates. The bifurcation is further investigated in terms of the structure of the classical phase space (periodic orbits, continuation/bifurcation diagr am). It is also discussed how the spectrum of bound states persists into th e continuum and how the various types of quantum mechanical continuum wave functions affect the state-specific dissociation rates. (C) 2000 American I nstitute of Physics. [S0021-9606(99)00901-0].