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

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 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.