Quantum calculations of dynamics of triatomic and tetraatomic molecules

We investigate rotation-induced Fermi resonances in HOCl by the adiabatic rotation (AR), centrifugal sudden (CS), and exact methods, using a highly accurate potential energy surface. A simple two-state model is used to analyze the interaction, and interaction parameters are determined and compared with those of the experiment. The comparison between AR calculations and the experiment shows good agreement. We also study rotationally mediated coupling using exact and approximate methods. The resonance states of non-rotating HOCl are calculated for the sixth and seventh overtones of the OH-stretch, using a recent, adjusted potential energy surface based on high-quality ab initio calculations. The resonance calculations are done using a standard L2 approach, augmented by a localized negative imaginary potential which serves as an absorbing boundary in the asymptotic Cl + OH region. The widths of these two resonances are compared with results from a very recent experiment. In addition we present calculated ro-vibrational distributions of the OH fragment for these two resonances.; The isomerization of acetylene to vinylidene is examined in four degrees of freedom using a full dimensional potential. Eigenfunctions of the four degree-of-freedom Hamiltonian are obtained using a series of truncation/recoupling procedures that begins with the eigenfunctions of a two degree-of-freedom Hamiltonian. The double-well nature of the isomerization is clear in these calculations and is exploited to divide the eigenfunctions into symmetric and antisymmetric groups. The lowest energy doublet states of vinylidene at two levels of dimensionality reduction indicate a splitting of the order of a wavenumber. Franck Condon factors between these molecular eigenstates and a model, ground state vibrational wavefunction for the vinylidene anion are calculated to simulate the photodetachment spectrum of the vinylidene anion.; The isomerization of acetylene to vinylidene is also examined theoretically in full dimensionality (six degree-of-freedom) using a recent global potential energy surface. Eigenfunctions and eigenvalues of the corresponding exact Hamiltonian, for zero total angular momentum, are obtained using a series of truncation/recoupling procedures that begins with the eigenfunctions of a three degree-of-freedom Hamiltonian for the angular motion. By examining expectation values of the eigenfunctions and a number of states are definitely identified with vinylidene-like characteristics. (Abstract shortened by UMI.)...