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Some New Directions in Spectroscopy


The work presented in this thesis comprised two major projects. The first, a spectroscopic study of water clusters in the Terahertz vibrational region revealed a dramatic enhancement of tunneling motions involved in breaking and reforming hydrogen bonds. The second project involved constructing a second harmonic scattering (SHS) experiment which was used to probe buried interfaces.

The study of water clusters provides a useful route towards unraveling the many body terms of the water potential. High-resolution vibrational-rotational-tunneling (VRT) spectroscopy is a particular sensitive measure of the repulsive walls of the water potential. While this method is extremely powerful, the major challenge is assigning the complex spectra that is output from the experiment. To that end, the major contribution of this thesis is an automated pattern matching algorithm based on the symmetric-top, rigid rotor model. Using this algorithm, transitions of the water dimer, pentamer, hexamer, and octamer were identified and assigned from a back log of experimental data. These assignments revealed a dramatic enhancement of tunneling motions in the librational (300 – 600 cm-1) region. These enhancements were most dramatic for motions involved in breaking and reforming the hydrogen bond in the clusters, adding to the evidence that understanding this region is essential to understanding hydrogen bond breaking dynamics.

The use of 2nd order, nonlinear spectroscopy to probe surfaces is a well-established technique. Employing second harmonic generation in a scattering geometry enables probing buried interfaces in colloidal samples; these samples provide an appealing path forward towards studying real world polymers such as water purification membranes. We show that SHS can be used to probe resonant and non-resonant molecules at a polymer interface, and we extract free energies of adsorption from a variety of organic molecules. In the future, we plan to extend this technique to ionic species to supplement our existing studies of ion adsorption via sum frequency generation in reflection geometry.

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