EPES sampling depth paradox for overlayer/substrate system

An unexpected interface effect for the sampling depth of elastic peak electron spectroscopy (EPES) in applications to the overlayer/substrate system has been found. The sampling depths expressed as an information depth (ID) for Au/Ni and Rh/Al systems and selected energies in the range 200-10,000 eV were obtained from Monte Carlo (MC) simulations for typical for EPES cylindrical mirror analyser (CMA) configuration. It turned out that deep minimum in the ID dependence on the interface depth can exist. For example, for Rh/Al system at the energy of 2000 eV the ID can be smaller by a factor of two than the ID for the system elements. This effect can be explained in terms of the differential cross sections.