A data base for energy loss cross sections and mean free paths

Abstract

For quantitative elemental analysis by standard EELS techniques, we require inner-shell cross sections integrated over an energy range Δ beyond the ionization edge. These cross sections are conveniently parameterized in terms of a dipole oscillator strength f(Δ) which is independent of incident energy Eo and collection semi-angle β.Values of f(Δ) are obtainable from EELS measurements, from theoretical models and from photoabsorption data. Compilation of this data allows one to obtain a best estimate of a given cross section, together with an estimate of the probable error. Consistency is good for K-shells, allowing f(Δ) to be predicted to within 5% accuracy. At the opposite extreme, discrepancies of an order of magnitude exist for the N- and O-shells (Fig.1). In general, there is more consistency with regard to edge shape, as expressed by the ratio ϕ(Δ)=f(Δ)/f(100); see Fig.2.If microanalysis is carried out by matching to stored edge profiles, the convenient quantity is the differential oscillator strength df/dE. Such data is available directly from atomic calculations and, for some edges, from experimental measurements; see Fig.3.