Monte Carlo study of variations in the surface sensitivity of energy-resolved x-ray absorption spectra from Ni/NiO thin-film bilayers

The variation in the surface sensitivity of the x-ray absorption electron yield as a function of energy has been investigated for a Ni/NiO thin film bilayer sample. Experimental data was obtained using a recently developed detector capable of making energy-resolved electron yield x-ray absorption spectroscopy (XAS) measurements at ambient pressure. Datasets obtained using this technique contain information about the relative phase contribution to the XAS at each incident photon energy. Analysis of the data thus provides detailed information about the variation in surface sensitivity as a function of energy. This "surface sensitivity profile" is studied using Monte Carlo simulations which include inelastic scattering events and discrete energy loss processes. Consideration of the electron multiplication/propagation processes within the sample provides an insight into the principles behind electron-yield signal formation. The simulations accurately reproduce the experimentally determined fraction of NiO in the XAS down to sub-keV energies. The results provide a reliable method for the independent determination of layer thickness in stratified samples and will aid in the future development of the technique.