Electroreflectance of GaP to 27 eV

We report the first electroreflectance spectra obtained above 7 eV. The large number of new critical points observed includes a group in the energy range above 20 eV which occur between the $3d$ valence bands, derived from the deep-lying Ga $3d$ core levels, and the sp³ conduction bands. The resolution, currently limited by the monochromator band pass, enables us to resolve directly the 0.50 ±{} 0.03 eV spin-orbit splitting of the Ga $3d$ core levels for the first time.