BENDING DYNAMICS FROM ACETYLENE SPECTRA - NORMAL, LOCAL, AND PRECESSIONAL MODES

The bending dynamics of acetylene are analyzed starting from spectrosc opic fitting Hamiltonians used to fit experimental spectra. The possib ility is considered of a transformation in the dynamics from normal to local bending modes, as well as a new kind of correlated bending moti on called precessional modes. The spectroscopic fitting Hamiltonian of C2H2 is discussed with particular attention to the coupling interacti ons present due to Fermi and Darling-Dennison resonances. It is argued that for analysis of experiments in which the energy is initially pla ced in the bends,:many couplings can be neglected. Of the remaining co uplings, that responsible for the primary pathway of energy transfer o ut of the bends is a single Darling-Dennison coupling between the bend s. A Hamiltonian containing this coupling alone is analyzed to isolate the bending dynamics involved in the primary energy transfer pathway. The anharmonic modes born in bifurcations from the low-energy normal modes are determined from analysis of the classical form of the Hamilt onian. In addition to the usual normal modes, local and precessional m odes are found. Precessional modes have relative phases of pi/2 or 3 p i/2, with one local bend fully extended while the other has maximal ve locity. Sets of levels or ''polyads'' with the same total number of be nd quanta are plotted in phase space on the polyad phase sphere, allow ing a determination of the normal, local, or precessional character of a given quantum state. It is determined that local modes are found in the experimentally observed bend polyads with P greater than or equal to 14, and precessional modes are found in the, polyads P greater tha n or equal to 20. Polyads are classified on the molecular catastrophe map according to their structure of normal, local, and precessional mo des. Energy level spacing patterns within a polyad, shown previously t o be characteristic of phase space bifurcation structure, are determin ed and correlated with the phase sphere. A diabatic correlation diagra m analysis, previously applied to H2O, is suggested to extend the anal ysis here of normal, local, and precessional bending states to the ful l multiresonance, chaotic spectral fitting Hamiltonian. (C) 1996 Ameri can Institute of Physics.