Adam P. Sorini

Many-pole model of inelastic losses in x-ray absorption spectra

Inelastic losses are crucial to a quantitative analysis of x-ray absorption spectra. However, current treatments are semi-phenomenological in nature. Here a first-principles, many-pole generalization of the plasmon-pole model is developed for improved calculations of inelastic losses. The method is based on the GW approximation for the self-energy and real space multiple scattering calculations of the dielectric function for a given system. The model retains the efficiency of the plasmonpole model and is applicable both to periodic and aperiodic materials over a wide energy range. The same many-pole model is applied to extended GW calculations of the quasiparticle spectral function. This yields estimates of multi-electron excitation effects, e.g., the many-body amplitude factor S 2 0 due to intrinsic losses. Illustrative calculations are compared with other GW calculations of the self-energy, the inelastic mean free path, and experimental x-ray absorption spectra.

Ab initio calculations of electron inelastic mean free paths and stopping powers

A method is presented for first-principles calculations of inelastic mean free paths and stopping powers in condensed matter over a broad energy range. The method is based on {it ab initio} calculations of the dielectric function in the long wavelength limit using a real-space Greens function formalism, together with extensions to finite momentum transfer. From these results we obtain the loss function and related quantities such as optical-oscillator strengths and mean excitation energies. From a many-pole representation of the dielectric function we then obtain the electron self-energy and inelastic mean free paths (IMFP). Finally using our calculated dielectric function and the optical-data model of Fernandez-Varea {it et al}., we obtain collision stopping powers (CSP) and penetration ranges. The results are consistent with semi-empirical approaches and with experiment.

Reducing radiation damage in macromolecular crystals at synchrotron sources

A new strategy is presented to reduce primary X-ray damage in macromolecular crystallography. The strategy is based on separating the diffracting and damaged regions as much as feasible. The source of the radiation damage to macromolecular crystals is from two primary mechanisms: the direct excitations of electrons by absorption, and inelastic scattering of the X-rays. The first produces photoelectrons with their accompanying Auger electrons from relaxation of the core hole and the second creates Compton electrons. The properties of these two mechanisms and calculations of primary X-ray damage quantify how to modify the spatial distribution of X-rays to reduce the deleterious effects of radiation damage. By focusing the incident X-rays into vertical stripes, it is estimated that the survival (the time during which quality diffraction data can be obtained with a given X-ray flux) of large crystals can be increased by at least a factor of 1.6, while for very small platelet crystals the survival can be increased by up to a factor of 14.

The Magic Angle Mystery in Electron Energy Loss Spectra: Relativistic and Dielectric Corrections

Recently it has been demonstrated that a careful treatment of both longitudinal and transverse matrix elements in electron energy loss spectra can explain the mystery of relativistic effects on the {it magic angle}. Here we show that there is an additional correction of order

Inelastic Losses and Multi‐Electron Excitations in X‐Ray Spectra

(Zalpha)^2

Ab initio Real Space Calculations of Electron Energy Loss Spectra

where

Ab initio theory and calculations of X-ray spectra

Z

Ab initio calculations of inelastic losses and optical constants

is the atomic number and

Narrowing of band gap in thin films and linear arrays of ordered TiO

alpha

_{2}

the fine structure constant, which is not necessarily small for heavy elements. Moreover, we suggest that macroscopic electrodynamic effects can give further corrections which can break the sample-independence of the magic angle.