Understanding energy loss in large-angle scattering of keV electrons from Ar and Ne

We present measurements of the spectra of electrons with energy between 0.6 and 2.25 keV elastically and inelastically scattered from Ar and Ne over large angles (from 3 deg. to 135 deg.). The intensity of the first loss feature [np→(n+1)s], relative to that of the elastic peak, was determined and compared with the results of relativistic distorted-wave calculations (for the energy loss part) and a relativistic optical potential method (for the elastic peak). Good agreement was found. The distorted-wave calculations are compared with first Born calculations. At small angles, both theories coincide and estimates of the optical oscillator strength are obtained. However, at large angles, the first Born approximation predicts negligible intensity, in strong contrast to the distorted-wave theory and the experimental data. The implications of these results for the interpretation of measurements of the generalized oscillator strength are discussed.