Orientation Dependence of Core Edges in Electron-Energy-Loss Spectra from Anisotropic Materials

We have investigated the orientation dependence of the structure near the boron $K$ edge in electron-energy-loss spectra from hexagonal boron nitride. Angular distributions of spectra recorded from crystals with $c$ axis tilted at 45° to the beam allow us to determine unambiguously the $\pi$ and $\sigma$ character of the final states at different energies. Results are in quantitative agreement with a model based on hydridized atomic orbitals.