Exciton spectroscopy of hexagonal boron nitride using nonresonant x-ray Raman scattering

We report nonresonant x-ray Raman scattering (XRS) measurements on the boron K edge of hexagonal boron nitride for transferred momentum (q) from 2 to 9 Å⁻¹ along directions both in and out of the basal plane. A symmetry-based argument, together with real-space full multiple scattering calculations of the projected density of states in the spherical harmonics basis, reveals that a strong pre-edge feature is a dominantly Y₁₀-type exciton with no other s, p, or d components. This conclusion is supported by a second, independent calculation of the q-dependent XRS cross section based on the Bethe-Salpeter equation. This study demonstrates methods which should be applicable to the determination of final-state symmetries for localized resonances in other q-dependent XRS studies of anisotropic single-crystal systems.