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Magnetic resonance studies of some silver halide-based model imaging systems.

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posted on 19.11.2015, 08:46 by Myra Toffolon. Olm
Band gap excitation of silver halides produces charge carriers which are subsequently localized at hydrogenic or deep traps. These intrinsic or extrinsic defects control the photographic behaviour of practical silver halide systems. This thesis describes the use of magnetic resonance techniques to study the structure of such traps in model systems. The lifetime of photoproduced conduction electrons can be greatly extended in silver-halide based materials designed to encourage the partitioning of free electrons and holes. Conduction Electron Spin Resonance was used to study the conduction processes of shallowly trapped and free carriers in several such materials, including Pb2+-doped AgCl, AgBr/AgCl, AgI/AgCl and AgCl/NaCl single crystals and powders. Mechanisms for the extension of the free electron lifetime are proposed, and the temperature and microwave power dependences of the CESR g-value and lineshape are discussed. In addition to studying the band structure and conduction processes as a function of silver concentration and Fermi level in AgCl/NaCl alloys, magnetic resonance studies were used to probe the microcrystalline structure and phase separation processes in these materials. Transition metal impurities, often associated with lattice defects, act as deep traps in the silver halides. The structure of the metal ion-defect site affects its trap cross-section and lifetime. The enhanced resolution provided by Electron Nuclear Double Resonance techniques was used to determine the structure of Rh2+-silver ion vacancy complexes in Rh3+-doped AgCl and NaCl single crystals and powders. The primary site in AgCl exhibited a static Jahn-Teller distortion at low temperature, which gave way to a two- dimensional and finally to a three-dimensional dynamic distortion as the temperature was raised. The application of ENDOR to glasses and powders was explored via studies of the DTBN radical in glassy matrices and of ?-irradiated sodium formate and silver: cyclohexene powders. The latter two studies provided, respectively, examples of "single-crystal-like" and "powder- type" ENDOR powder spectra.


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University of Leicester

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