Electronic structure of the alkaline-earth fluorides studied by photoelectron spectroscopy

Precise values have been determined for the binding and separation energies of the outer electronic bands of the alkaline-earth fluorides using 40.81-eV ultraviolet and Al $K\alpha$ (1486.6 eV) x-ray photoelectron spectrometry. Excellent agreement is found between the experimental and Born-model theoretical values for the energy separation between the outermost levels of the alkaline-earth and fluorine ions when the experimental values are corrected for relaxation (polarization) effects and when the Madelung energy alone is taken to represent the electron-lattice interaction. Very good agreement also occurs between experimental and theoretical values for the absolute binding energies. These results are accounted for by the fact that the substantial repulsive energy existing in the alkaline-earth fluorides is stored predominantly as interionic elastic potential energy and has little effect on the electronic energy levels on the ions. Comparison of the bandwidths of the alkaline-earth fluorides from the present work and alkali-metal fluorides from previous work shows that the ${\mathrm{F}}^{-}$ $2p$ valence-band width is a function of the nearest-neighbor distance and independent of the particular compound.