Vector part of optical activity probed with x-rays in hexagonal ZnO
Résumé
We discuss how measurements of x-ray circular intensity differentials (XCIDs) could be used to probe the Voigt-Fedorov vector part of optical activity (OA) due to electric dipole-electric quadrupole (E 1E 2) interferences. The first experiment was carried out on a single crystal of zincite (hexagonal ZnO: class 6mm). XCID spectra were recorded in the x-ray resonant diffraction regime near the Zn K edge using the (300) reflection at Bragg angles near 45°. In the x-ray range, the effective operator responsible for the vector part of OA can be assigned to the vector product \mathbf {L} \times {\bOmega }_{L}-{\bOmega }_{L} \times {\mathbf {L}} \propto {\mathbf {n}} , in which L and ΩL are time-reversal odd operators associated with the orbital angular momentum and the orbital anapole respectively, whereas n is a true time-reversal even electric dipole. This is consistent with the pyroelectric properties of zincite crystals.