Exploring the effect of a potential barrier on the molecular rotation-vibration structure
Name
1023627499-MIT.pdf
Description
Full printable version
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27.63 MB
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Adobe PDF
Checksum (MD5)
281460acc27b4d40e965e1e64be6baa4
Author(s)
Jiang, Jun, Ph. D. Massachusetts Institute of Technology
Advisor(s)
Robert W. Field.
Date Issued
2017
Publisher
Massachusetts Institute of Technology
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.
Description
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 265-279).
Subjects
Chemistry.
MIT Department
Massachusetts Institute of Technology. Department of Chemistry
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