Date of Award

Fall 2019

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Mark Berg

Abstract

Multiple population-period transient spectroscopy (MUPPETS) is a two dimensional ultrafast time resolved spectroscopy technique. MUPPETS uses three pairs of optical pulses over two time periods to separate homogeneous and heterogeneous causes of rate dispersion. This dissertation details improvements to the MUPPETS optical assembly enabling measurements on systems that previously fell outside of the MUPPETS 2-ns time window. In addition, the theoretical groundwork is laid to unveil the hidden coordinate controlling rate exchange using 2D and 3D correlation functions.

The first project details improvements made to the MUPPETS assembly. A detailed method was developed to eliminate astigmatism, coma, and spherical aberration from the MUPPETS optical assembly. A slow response photodiode was built to correct for long term power fluctuations in MUPPETS signals. A double modulation scheme was introduced by the addition of a second chopper to eliminate the detection of background signals. In addition, other concerns are discussed.

The second project develops a simple, but general, model that allows multidimensional correlation functions to be calculated for systems that exhibit nonexponential kinetics. Nonexponential kinetics imply the existence of at least one slow variable other than the observable, that is, the system has a “hidden” coordinate. Homogeneous and heterogeneous mechanisms are both included, and slow exchange of the rates is allowed. This model shows that 2D and 3D correlation functions of the observable measure the distribution and kinetics of the hidden coordinate controlling rate exchange. Both the mean exchange time and the shape of the exchange relaxation are measurable.

The fourth project utilizes the rotation of PM597 to probe the local properties of SDS micelles. In traditional 1D experiments, solute rotation in micelles is known to exhibit rate dispersion. This rate dispersion has commonly been attributed to local anisotropy or heterogeneity in the local viscosity. Polarized MUPPETS is used to resolve this conflict. This 2D technique shows that heterogeneity in the local viscosity is responsible for 87% of the rate dispersion in opposition of the more widely accepted “wobble-in-a-cone” model. This result suggests that on the subnanosecond timescale, the solute sees only one strong fluctuation of the micelle structure.

The final project measures the rotation anisotropy of PM597 with polarized pump- probe spectroscopy in a variety of ionic liquids and concentrations. The rotation of PM597 is measured in CnMIM:BF4 (χ = 0.2) and acetonitrile, where n = 2, 4, 8, and 12. In addition the rotation of PM597 was measured in acetonitrile and C12MIM:BF4 at χ = 0.0, 0.2, 0.4, and 0.6. Rate dispersion was measured in sample; however, the magnitude of rate dispersion was within the noise level of the MUPPETS experiment.

Rights

© 2019, Jason R. Darvin

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