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When hexagonal boron nitride is exposed to ionizing radiation two types of paramagnetic centers appear: three-boron centers and one-boron centers. Results of electron paramagnetic resonance, thermoluminescence, and thermally-stimulated-current measurements associated with these centers are described, and a model is proposed to explain these results. The model is consistent with views of others investigators on the role of carbon impurities in hexagonal boron nitride, and with the hypothesis that the three-boron centers are F centers. The three-boron centers and one-boron centers were found to introduce trapping levels at 1.0 and 0.7 eV, respectively, below the conduction band. It is suggested that carbon impurities produce luminescence centers, with an energy level at about 4. 1 eV below the conduction band. Ionizing radiation frees electrons from these levels into the conduction band. Some of the electrons may then be trapped either in three-boron centers or in one-boron centers.Some may fall back to the centers, thus emitting blue photoluminescence. The unpaired trapped electrons give rise to the EPR signals. When the samples are heated, the electrons escape from the traps (first from the one-boron centers and then from three-boron centers and give rise to the glow curves and at the same time also cause a decrease in the EPR signal. This model was supported by quantum-mechanical defect-model calculations, given in part II of this study {following paper).

A general theory for fiber-optic, evanescent-wave spectroscopy and sensors is presented for straight, uncladded, step-index, multimode fibers. A three-dimensional model is formulated within the framework of geometric optics. The model includes various launching conditions, input and output end-face Fresnel transmission losses, multiple Fresnel reflections, bulk absorption, and evanescent-wave absorption. An evanescent-wave sensor response is analyzed as a function of externally controlled parameters such as coupling angle, f number, fiber length, and diameter. Conclusions are drawn for several experimental apparatuses.

Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy using a special waveguide based on a silver halide fiber was used for probing the heat-induced secondary structure and conformation changes of bovine serum albumin (BSA). From the secondary derivative and the curve fitting of the obtained ATR-FTIR spectra, the changes of the BSA secondary structure with temperature were clearly identified. Two different thermal denaturation temperature ranges (i.e., 50-52 and 80-82 °C, at which a change of the protein structure occurred) were determined, while only one denaturation temperature was previously identified via classical FTIR measurements. Additionally, taking advantage of two-dimensional correlation spectroscopy more detailed information on changes of the protein secondary structure was revealed. The developed method facilitates in situ, sensitive, and more in-depth probing of protein secondary structures, which represents a significant advancement compared to conventional characterization methods.

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