Core excitons at the boron K edge in hexagonal BN

High-resolution measurements of soft-x-ray absorption near the boron K threshold of hexagonal BN display a prominent peak at 192.0 eV. The angular dependence of this absorption for p-polarization geometry is consistent with a final state constructed mostly of π-like wave functions. Although higher-energy parts of the spectrum can be approximated by an effective-mass exciton theory, the 192.0-eV peak requires a substantial central-cell correction. This core exciton is thus highly localized in real space and delocalized in k→ space. It seems likely that a mixture of σ- and π-like wave functions is necessary to describe the final state; this could account for the observed line strength at normal incidence which is not zero.