Splitting the plasmon peak in high-energy reflection electron energy loss experiments

Elastic scattering of energetic electrons over large angles (in this study 40 keV and 120°$120°$) implies momentum and hence energy transfer from an electron to a nucleus. Due to the large mass of the nucleus (relative to the mass of an electron) this energy transfer is small, but it has recently been shown that it can be resolved in a modern spectrometer. Hence the elastic peak of an overlayer/substrate system splits into different components corresponding to atoms with different mass. Here we extend this type of experiment to the plasmon part of a reflection energy loss spectroscopy (REELS) spectrum. It is shown that, for suitable systems, the plasmon peak of an overlayer/substrate system is split by the same amount as the elastic peak. This is a consequence of the fact that detection of an electron in REELS always requires a large-angle elastic scattering event. Moreover, we show that the relative intensity of the plasmon components contains information on the depth distribution of the scatterers.