The prediction of metastable impact electronic spectra (MIES): perfect and defective MgO(001) surfaces by state-of-the-art methods

Abstract

We re-examine the theory of metastable impact electron spectroscopy (MIES) in its application to insulating surfaces. This suggests a quantitative approach which takes advantage of recent developments in highly efficient many-electron computational techniques. It gives a basis to the interpretation of experimental MIES spectra for perfect and defective surfaces. Our method is based on a static approach to predicting Auger de-excitation (AD) rates of He∗(1s2s) projectiles. A key quantity is the surface density of states (DOS) projected on the 1s orbital of the He∗ atom, which is calculated along its trajectory. We use density functional theory within both supercell geometry and embedded cluster models to calculate MIES spectra for the perfect MgO surface and for an MgO surface with different concentrations of adsorbed oxygen atoms. First we calculate the Auger de-excitation rates at various positions of the projectile above the surface. To predict MIES spectra, we integrate over projectile trajectories, with a subsequent weighted averaging with respect to various lateral positions of He∗ above the MgO surface unit cell. It is important to examine final-state effects for a correct comparison between theory and experiment, especially when there are localised defect states.