Electronic signatures of large amplitude motions

Abstract

In this thesis, I demonstrate that measurements of electronic-structure-induced splittings in the rotational spectrum of a vibrationally excited state can identify the nature and extent of the distortion of the equilibrium electronic wavefunction and thereby provide a measure of progress along a reaction coordinate. One-dimensional models of the large amplitude bending reaction coordinates and their associated electronic signatures are constructed for two prototypical unimolecular isomerizations: acetylene<-->vinylidene (HCCH<-->CCH2), and hydrogen cyanide$hydrogen isocyanide (HCN<-->HNC). The nuclear quadrupole hyperne structures of HCN and HNC are distinct at their equilibrium geometries due to the dissimilar natures of bonding in the vicinity of the 14N nucleus. High resolution rotational spectroscopy has been used to determine the hyperne coupling parameters for the ground and excited vibrational levels of HCN and HNC, with up to ten quanta of bending excitation in HCN and up to four quanta in HNC. These spectra reveal the evolution of electronic structure along the isomerization path. Large amplitude local-bending vibrational eigenstates of the ... state of acetylene are shown to be unique in that they possess significant electric dipole moments as a result of the dynamical symmetry breaking in the local-mode limit. Stimulated emission pumping (SEP), through Franck{Condonforbidden vibrational levels of the A~ 1Au state, has been employed to populate the lowest few eigenstates that manifest large amplitude local-bending behavior.

(cont.) Locating appropriate SEP intermediate states has required thorough analysis of the A~-state level structure, particularly the overtones and combination levels involving the nearly degenerate low frequency bending modes, 04 and 06, that are directly related to two possible paths for trans{cis isomerization on the excited state surface. Recent developments in chirped-pulse rotational spectroscopy will enable identification of the higher energy local-bending eigenstates, which approach the acetylene<-->vinylidene transition state, based on their predicted Stark coefficients.