Chemical effects in the X-ray emission (F Kα) and X-ray photoelectron spectra of alkali-metal and alkaline-earth-metal fluorides

Abstract

X-Ray emission (XE) and both core and valence-band X-ray photoelectron (XP) spectra have been measured for LiF, NaF, KF, BeF₂, MgF₂, CaF₂, SrF₂ and BaF₂. Both the F 1s ionisation energies and the F K_α X-ray energies decrease as the cation atomic number increases within a group of the Periodic Table. Knowledge of the F 1s binding energy enabled the valence-band XP spectra to be aligned on a common energy scale for direct comparison with the F K_α peak profile. Both the XE and XP spectra showed broader bands for the lighter cations, which for Li, Be, Mg and Ca could be resolved into two distinct components. Symmetric broadening of the valence-band peaks was rationalized as due to anion–anion interaction whilst additional structure on the low (kinetic) energy side and low (X-ray) energy side of the main XP and XE peaks was related to covalent character in the cation–fluoride bond. Differences in the relative intensities of these components for XE (reflecting F 2p character only) and the XP spectra were ascribed to the presence of F 2s and possibly cation s character in the more tightly bound orbitals of the valence band. Measurement of the BeK_α and Mg K_β XE spectra gave a direct indication of the covalent nature of the bonding in BeF₂ and MgF₂.