Low-Energy Electron Inelastic Mean Free Path of Copper

A new method is presented for the measurement of electron inelastic mean free path (IMFP) of copper metal from the K-edge X-ray absorption fine structure (XAFS) using energies from 5 to 320 eV above the edge. The accuracy of theoretical determinations of electron IMFP at low energies is one of the key limiting factors in current XAFS modeling and Monte Carlo transport. Significant discrepancies between theoretical and experimental IMFP values have been revealed through recent studies, posing significant questions regarding the accuracy of key structural parameters extracted through XAFS analysis. Small molecules and organometallic systems, which often lack robust tabulations of key electron scattering data, are particularly susceptible to inconsistencies in the IMFP, requiring a new methodology to resolve these discrepancies. XAFS is determined using an advanced density functional theory (DFT) core of the finite difference method for XAFS (FDMX). The popular multiple-scattering approach, based on muffin-tin potentials, is shown to be inadequate for accurately calculating the fine structure. Experimental IMFP measurements are both consistent with past measurements and consistent with the latest plasmon theory. However, variation of measurements with temperature points to the need for fine spacing at room temperature and measured uncertainties of data points at low temperatures and also suggests significant temperature-dependent effects both of broadening and correlation from multiple sources. This work confirms both recent past experimental and theoretical works and points to new areas of challenge and discrepancy.