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We report results of a collaborative study of photon and charged-particle interactions with matter between the University of Vermont and Argonne and Brookhaven National Laboratories. A major goal was to extend the study of electromagnetic properties of selected materials to as wide a spectral range as possible. This broad approach discloses systematic trends not apparent in isolated measurements and exploits the power of dispersion analysis and sum-rule constraints. Emphasis was largely on UV and X-ray processes and capitalized on the wide range of photon energies available at NSLS. A key finding is that, under favorable circumstances, dispersion theory relates dispersive processes (e.g. refractive index, dielectric constant) to spectral moments of absorptive processes. This appears to be a new method in optics; it yields significant simplifications and provides a precise, model-independent characterization of optical materials. Problems addressed included a) x-ray magnetooptics; b) UV/soft-x-ray processes in insulators and their contribution to visible dispersion; c) demonstration of moments/dispersion analysis in glasses and applications to fiber-optic systems; d) the optical constants of silicon and their application to the stopping power of silicon for charged-particles. Results include: ● Resolution of a long-standing conflict over the relation between x-ray Faraday rotation and x-ray magnetic circular dichroism. Specifically, the Kramers-Kronig relations must be generalized to account for the breaking of time-reversal symmetry by magnetic fields. Experimental reports to the contrary were shown to be inconclusive. Reanalysis of x-ray Faraday rotation data supports the generalization. ● Demonstration that the optical properties of dielectrics in their region of transparency are determined by a series expansion in spectral moments of the dielectric’s infrared and ultraviolet absorption spectra. Application of this to silicate glasses clarifies the role of glass modifiers in introducing charge-transfer, intra-ionic and perturbed-exciton transitions that combine to determine visible optical properties. Roughly, the refractive index is determined by the total electronic absorption, while dispersion depends on how the absorption is distributed. Materials characterization using UV/soft-x-ray moments may have application to non-destructive testing of glasses and high-temperature glass melts. ● Application of moments methods to pulse propagation in optical fibers showed that signal distortion is minimized at the carrier-wave frequency for which dispersion in group velocity caused by IR processes just cancels the dispersion caused by UV and soft-x-ray processes. ● Construction of a composite set of optical constants for silicon employing a method of self-consistent dispersion analysis from the extensive measurements available in the literature. This provides a reliable set of optical constants in the IR and visible, and has clarified the issue of scaling relative measurements at the K edge and the accuracy of measurements at the L edge. ● Direct calculation of the stopping power of silicon for charged particles as outlined by Bethe using our composite set of optical constants. This task was not previously feasible for want of accurate dielectric function data over a wide spectral range. This study resolved a long-standing conflict between measured average excitation energy or I values that was an issue for radiation damage and shielding applications

Final Report, DOE Award Number DE-FG02-02ER45964, Electromagnetic Properties of Matter at X-ray Wavelengths