Exploring the effect of a potential barrier on the molecular rotation-vibration structure

Abstract

The goal of this thesis is to explore the effect of a potential barrier on the rotation-vibration structure of the sulfur dioxide (SO₂) C̃̃ state and the acetylene (HCCH) Ã state. The minimum-energy geometry of both electronically excited states is qualitatively different from their respective electronic ground state geometry. The SO₂ C state exhibits a barrier (~100 cm-¹) at the C₂u, geometry along the antisymmetric-stretching direction, separating two equivalent minimum-energy configurations with C, geometry. The HCCH A-state potential energy surface (PES) supports both trans- and cis-bent conformers (but not a linear configuration). The trans- and cis-conformer-wells are separated by a barrier of ~5000 cm-¹ (above the trans-bent minimum energy). For both the SO₂ C̃ state and the HCCH Ã-state, the presence of a potential barrier greatly complicates the rotation-vibration structure of the molecule. Interpretation of these barrier-related spectroscopic patterns requires both new experimental observations and new analysis tools, both of which are discussed in this thesis. For the SO₂ C̃ state, an IR-UV double-resonance excitation scheme enables direct observations of levels with odd quanta in the antisymmetric-stretching vibrational mode (v3). A new anharmonic force field is derived for the SO₂ C̃ state, which allows accurate determination of the shape of the barrier on the C̃-state PES. In addition, we develop tools, based on perturbation theory, the polyad model, and semiclassical analysis, to interpret the effect of the barrier on the C̃-state rotation-vibration structure. The cis-trans isomerization in the HCCH Ã-state has been the focus of the Field group acetylene project for the past ten years. However, the diminishing detection efficiency of the laser-induced fluorescence (LIF) scheme (due to acetylene predissociation), combined with a partial breakdown of the polyad fit model, has made it increasingly difficult to understand the HCCH A-state level-structure near the top of the cis-trans isomerization barrier. Two new sensitive and convenient action schemes are reported in this thesis to detect predissociated Ã-state rovibrational levels. The first scheme is based on detection of H-atoms by two-photon laser-induced (3d <-- 1s) fluorescence (3d --> 2p), and the second scheme is based on fluorescence detection of C₂ and C₂H fragments, photolyzed via resonance with the probed Ã-state levels. The photodissciation processes that give rise to the strong photofragment fluorescence signals are also studied in this thesis.