Accurate measurement of absolute experimental inelastic mean free paths and EELS differential cross-sections

Abstract

Methods are described for measuring accurate absolute experimental inelastic mean free paths and differential cross-sections using DualEELS. The methods remove the effects of surface layers and give the results for the bulk materials. The materials used are VC_0.83, TiC_0.98, VN_0.97 and TiN_0.88 but the method should be applicable to a wide range of materials. The data was taken at 200keV using a probe half angle of 29mrad and a collection angle of 36mrad. The background can be subtracted from under the ionisation edges, which can then be separated from each other. This is achieved by scaling Hartree-Slater calculated cross-sections to the edges in the atomic regions well above the threshold. The average scaling factors required are 1.00 for the non-metal K-edges and 1.01 for the metal L-edges (with uncertainties of a few percent). If preliminary measurements of the chromatic effects in the post-specimen lenses are correct, both drop to 0.99. The inelastic mean free path for TiC_0.98 was measured as 103.6±0.5nm compared to the prediction of 126.9nm based on the widely used Iakoubovskii parameterisation.

Keywords: Absolute elemental quantification; Absolute thickness measurement; Electron energy loss spectroscopy (EELS); Elemental cross section; Energy loss near edge fine structure (ELNES).