Effect of x-ray polarization at the boronKedge in hexagonal BN

Abstract

High-resolution absorption measurements were made at the $K$ edge of boron in hexagonal boron nitride using polarized synchrotron radiation and different orientations of the crystal axis. The work made use of the highly focused beam at the exit slit of a new grazing-incidence ultrahigh-vacuum monochromator at the Stanford Synchrotron Radiation Project. An isolated exciton line was observed at threshold (192 eV) which did not increase as the electric vector became parallel to the $c$ axis. On the other hand, a second shoulder or line at 195 eV increased rapidly when the crystal was rotated in the beam. The results can be understood in terms of excitons formed from particular regions of the Brillouin zone with selection rules appropriate to the layer crystal.