Modulated electron emission: The effect of elastic and inelastic electrons on core-level ionization

Abstract

We investigated the elastic and inelastic contributions to the focusing and defocusing properties of low-index Fe atomic chains on keV electron beams. To this end, we measured the intensity of the $2p$-ionization loss signal from a thin Co film buried in a Fe matrix as a function of the incidence angle of the electron beam for different values of the beam energy. The angular anisotropy of the measured signal, $A(z),$ directly measures the focusing effect at the depth $z$ where the Co marker is located. The maximum (102%) in $A(z)$ occurs at $z=5$ scatterers; at about 12 scatterers below the surface the anisotropy is still 86%. The intensity anisotropy of the $2p$-ionization loss we measured is compared to that of the Co $\mathrm{LMM}$ Auger signal and to single-scattering cluster calculations. It has been found that a finite spatial extension of the ionization region has to be taken into account to reproduce the experimental results. The relative weight of elastic and inelastic contributions to the $2p$ core-level ionization cross section has been evaluated as a function of depth and electron energy.