Resonance Raman spectroscopy utilizing tunable deep ultraviolet excitation for materials characterization

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Title: Resonance Raman spectroscopy utilizing tunable deep ultraviolet excitation for materials characterization
Author: Chadwick, Christopher Todd
Advisors: Dr. David E. Aspnes, Committee Member
Dr. Christopher M. Roland, Committee Member
Dr. C. Russell Philbrick, Committee Member
Dr. Hans D. Hallen, Committee Chair
Abstract: Resonance Raman spectroscopy offers some key benefits over other spectroscopy methods. In one facet, resonance Raman provides a level of specificity not present in non-resonant Raman scattering. In another facet, resonance Raman can provide increased scattering cross-sections that rival those associated with the intensities of species fluorescence. These features provide mechanisms for improved trace species detection in current Raman remote sensing applications; as well as signal level enhancement in tiny volume regimes, such as those typical in near-field optical microscopy. This dissertation presents three main thrusts that are not well documented in the previous resonance Raman studies. We demonstrate fine resolution (approx. 0.1 nm) resonance tuning of the excitation wavelength corresponding to sharp absorption bands in liquid benzene and liquid toluene. The Raman spectra for these materials show an appreciable increase in scattering intensity of fundamental vibrational modes and show significant enhancements in scattering intensities for overtone and combination vibrational modes not observed with non-resonant excitation. Resonantly excited fundamental modes are observed to be enhanced by 3 to 5 orders of magnitude over non-resonant excitation; and several resonantly excited overtone modes are observed for both liquid benzene and liquid toluene. We have observed, that for liquid benzene and liquid toluene, the maximum Raman scattering intensity is realized when the excitation wavelength corresponds to that of the vapor phase absorption maximum, not the liquid phase absorption maximum as expected. We present a simple model of the time-dependent energy accumulation in the scattering volume that suggest that the scattering medium is a highly disorganized fluid. The observed Raman scattering intensity originates from this metastable fluid observed during the liquid-vapor phase transition. Using different concentration solutions of liquid benzene in heptane, we illustrate the influence species absorption has on the potential level of signal enhancement offered by resonance Raman scattering. In low concentration environments, resonance Raman signal levels can be 1 to 3 orders of magnitude larger than those of non-resonant Raman. As concentration increases, the signal levels of both resonant and non-resonant Raman become comparable. Using the species absorption to normalize the number of scattering molecules, the resonance enhancement is shown to be 3 to 5 orders of magnitude over the non-resonant excitation.
Date: 2009-04-24
Degree: PhD
Discipline: Physics

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