Core excitons at the boron<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>K</mml:mi></mml:math>edge in hexagonal BN

Abstract

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 $\ensuremath{\pi}$-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 $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ space. It seems likely that a mixture of $\ensuremath{\sigma}$- and $\ensuremath{\pi}$-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.