AbstractsPhysics

X-ray absorption spectroscopy (XAS) is a powerful technique to determine the structure and function of molecules. It provides element-specific information on geometry, chemical bonding, oxidation state, and spin state, and its applications range from biology to material science. For dilute samples, XAS is measured by partial fluorescence yield (PFY), where the intensity of a weak fluorescence line is recorded as a measure of absorption as the energy of the incident x-ray beam is scanned across an absorption edge of the element of interest. PFY increases the sensitivity for XAS if an x-ray detector is used that can efficiently separate the small fluorescence signal of interest from the x-ray background due to other elements in the sample. This dissertation describes the development of a high-resolution x-ray detector based on arrays of superconducting tunnel junctions (STJs). It is cooled to its operating temperature below 0.3 K with a liquid-cryogen-free adiabatic demagnetization refrigerator cryostat, and offers more than an order of magnitude improvement in energy resolution over conventional Ge- or Si-based solid state detectors. For operation in XAS experiments at a synchrotron, the STJ detector array is held at the end of a cold finger that can be inserted into an ultra-high vacuum endstation. This dissertation describes the design and performance of the STJ x-ray spectrometer, and demonstrates its use in PFY-XAS experiments in metallo-organic compounds at the Advanced Light Source synchrotron.