High-resolution x-ray absorption studies of core excitons in hexagonal boron nitride

In the K-shell excitation spectroscopy studies of hexagonal boron nitride, a sharp ^* resonance at 192.0 eV in the B K-edge region due to the B 1s-π ^* transition has been widely observed and accepted for three decades. However, our high-resolution near-edge x-ray absorption fine-structure studies disclose that this characteristic exciton peak actually consists of an asymmetric double-peak structure. Notable differences have been revealed in the polarization character of the two split peaks. These fine structures are explained by the coupling of core excitons to lattice vibrations with the inclusion of Jahn-Teller effect which breaks down Franck-Condon principle.