We report non-resonant x-ray Raman scattering (XRS) measurements from hexagonal boron nitride for transferred momentum from 2 to 9 $\mathrm{ \AA}^{ -1}$ 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 $Y_{ 10}$-type Frenkel exciton with no other \textit{ s}-, \textit{ p}-, or \textit{ d}- components. This conclusion is supported by a second, independent calculation of the \textbf{ q}-dependent XRS cross-section based on the Bethe-Salpeter equation.
Categories: cond-mat.mtrl-sci