Direct measurement of valence-charge asymmetry by x-ray standing waves

By monitoring valence-photoelectron emission under condition of strong x-ra y Bragg reflection, we have determined that a majority of GaAs valence ohar ge resides on the anion sites of this heteropolar crystal, in quantitative agreement with the GaAs bond polarity as calculated from the Hartree-Fock t erm values. In contrast, the valence-charge distribution in Ge is found to be symmetric. In both cases, the valence emission is found to be closely co upled to the atomic cores.