Core exciton energies of bulk MgO, Al2O3, and SiO2 from explicitly correlated ab initio cluster model calculations

Abstract

Ab initio cluster model wave functions are used to predict the existence of localized excited states in MgO, Al2O3, and SiO2 arising from metal 2p core-level excitations. Theoretical values obtained at different levels of theory result in a quantitative agreement with experiment, and the use of different models permits us to quantify the different contributions to the final excitation energy. The most important contribution is atomic in nature; a meaningful zero-order approximation is that in MgO and Al2O3 the exciton can be assigned to a M(2p6)→M(2p53s1)-like excitation, where M5Mg or Al. For the atomic models, the singlet-triplet exchange in the excited configuration is in good agreement with experiment. In addition, the solid-state effects on this exchange energy predicted by experiment are well reproduced by the cluster models representing MgO and SiO2, whereas a less clear situation appears in Al2O3. The open-shell orbital in the final state has, however, important contributions from the ions near the atomic site where excitation occurs. Nevertheless, the final state appears to be localized in space without any a priori assumption, the localization following from the holeparticle interaction implicitly induced in the final-state wave function. The Madelung field reduces the excitation energy with respect to the atomic value; the effect of neighboring atoms, mainly Pauli repulsion, acts in the opposite way; and electronic correlation effects decrease it again. In agreement with the covalent nature of SiO2, the exciton cannot be simply understood as arising from a Si(2p6)→Si(2p53s1) in a fully oxidized Si cation.