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Exploring New Directions in Terahertz and X-ray Spectroscopy

  • Author(s): Shih, I-chih;
  • Advisor(s): Saykally, Richard J;
  • et al.

The terahertz region (30-200 cm-1) of the electromagnetic spectrum has historically been extremely difficult to work in due to the lack of suitable powerful light sources and limited detector sensitivity. Since the development of modern synchrotron light sources, soft X-ray spectroscopy (100-1000 eV) has no such issues regarding intensity of the light sources, but the fact that nearly all materials absorb soft X-rays has severely limited the application of spectroscopy to liquid samples in this region. In this dissertation, I describe a several new technical developments in both terahertz and X-ray spectroscopy,

and their application for the study of water clusters and ion-pairing in aqueous electrolytes.

In Chapter 2, the compatibility of two new terahertz sources, Schottky diode frequency multiplier chains and quantum cascade lasers, with the Berkeley Thz spectrometer, is investigated. With the multiplier chain source, new transitions of the methane-D2O dimer have been measured and compared with calculations. The goal of this work is to ultimately develop a more accurate potential surface for modeling of methane clathrates.

Chapter 3 presents a study of ion pairing by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy and DFT calculations, starting with a comparison between ammonium chloride and ammonium sulfate solutions. A spectral fingerprint with

sensitivity to the cation-anion distance and orientations is identified based on the electronic structure analysis. The resulting ion-pairing tendency is in accord with the "Law of Matching Water Affinities". Subsequently, the counterintuitive formation of like-charge guanidinium-guanidinium contact ion pairs in water is evidenced and characterized.

The difference in escape depths of ions and electrons from condensed phase samples could ideally yield surface-bulk contrast via the total ion yield (TIY) versus total electron yield (TEY) detection of X-ray absorption spectra. To examine this potential surface

sensitivity, the TIY and TEY spectra of aqueous salt solutions with different known surface affinities have been recorded and are discussed in detail in Chapter 4. Two new detection schemes, "downstream" and "upstream" detection, have been tested, aiming for the elimination of vapor signal interferences and possibly another surface-sensitive probe.

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