Soft-x-ray emission insulators: Spectator versus normal emission.

We investigate the excitation energy dependence of the boron [ital K] soft-x-ray emission of B[sub 2]O[sub 3] and hexagonal-BN using synchrotron radiation. When exciting into the core exciton state the spectra are shifted to lower energy by 1.5 and 1.8 eV, respectively. This shift is found to be a result of phonon relaxation and differences in initial-and final-state electronic screening.