Spectroscopic signatures of isomnerization

Abstract

This thesis explores spectroscopic signatures of isomerization, especially new patterns that emerge and report on chemically relevant portions of the potential energy surface, such as the transition state. The most important new pattern discovered is the "isomerization dip", a sharp decrease in the vibrational level spacings, or effective frequency, in modes that project along the reaction coordinate as the transition state is approached. A pattern-breaking perturbation, a K-staggering analogous to a tunneling splitting, is also found in barrier-proximal states. These concepts are demonstrated by the experimental and theoretical analysis of cis-trans isomerization in S₁ acetylene (C₂H₂), including the near complete observation and assignment of the level structure from the potential minimum up to the transition state energy. The Ã-X̃ spectrum has been recorded in supersonic jets and molecular beams by laser induced fluorescence and H atom action detection, using double resonance techniques where appropriate to access and simplify portions of the Si manifold. Two major ab initio calculations have been undertaken to aid in understanding the experimental results, both relying on high-level electronic structure calculations. The large-amplitude dynamics and delocalized wavefunctions of the isomerizing system have been investigated by a reduced dimension discrete variable representation calculation. The anharmonic force fields of both conformers have been calculated by vibrational perturbation theory, and that of the trans conformer determined by fitting an effective Hamiltonian to the observed levels. The analysis of these results has led us to propose a method for extracting the transition state energy, and potentially other characteristics, from the frequency domain spectrum.