Time dependent density functional theory of X-ray absorption spectroscopy of alkaline-earth oxides

The time dependent density functional theory (TDDFT) has been employed to calculate the X-ray absorption spectra of the alkaline-earth oxides at the metal K and L and oxygen K edges. Cluster models to mimic the bulk are considered, embedded within an array of point charges to simulate the Madelung potential. Comparison with experimental data allows a precise assessment of the performances of the method, which appears competitive and suitable to reproduce the measurements. The configuration mixing explicitly included in the TDDFT scheme appears mandatory for a correct reproduction of the oscillator strength distribution in the metal 2p spectra. The origin of the theoretical spectral features is investigated with the help of the partial density of the virtual states (PDOS) calculated for each core hole considered. The trends of the spectral features along the series are discussed in terms of the nature of the virtual final states and related to the presence of the empty nd orbitals of the metal cations. The trend of the below-edge features in the O1s excitation spectra is discussed in terms of the metal-oxygen bonding interaction.