Electron Transfer Chemistry in Optical Materials: An EPR Investigation of Radiation-Induced Defects in Chemically Modified Materials

Abstract

Abstract : The Final Technical Report for Grant F49620-96-1-0443 Electron Transfer Chemistry in Optical Materials: an EPR Investigation of Radiation-Induced Defects in Chemically Modified Materials details the construction, and performance-evaluation of a 94.9 GHz EPR (electron paramagnetic resonance) spectrometer, operating in both CW (continuous wave) and pulsed modes. The first application of this instrument in the study of radiation induced defects in optical materials; a neutron-irradiated alpha-Al2O3 single crystal fiber, and gamma-irradiated of vitreous silica samples; are also described. A major finding of this work is that, owing to excellent sensitivity for volume-limited samples, high-frequency EPR instrumentation makes it possible to characterize paramagnetic centers in optical fibers. This capability is illustrated by reported spectroscopic, line shape, and analytical studies of Cr3+, Fe3+, and color centers (electrons trapped at anion vacancies) at ppm levels in an alpha-Al2O3 fiber. Special characteristics of high-magnetic fields for improving and facilitating EPR spectroscopy and analysis (spin-counting) are detailed. An emergent feature of high-frequency EPR spectroscopy is the apparent ease of eliciting saturation and rapid passage effects. Pioneering efforts in the systematic characterization and theoretical simulation of these effects are reported. Preliminary results suggest that passage effects can be harnessed to provide enhanced sensitivity and pronounced relaxation-dependent selectivity in the EPR spectroscopy.