VARIATION OF MEAN EMITTER DEPTH WITH DIRECTION IN-CORE PHOTOELECTRON EMISSION FROM SINGLE-CRYSTALS

We have theoretically studied the variation of mean emitter depths wit h direction for core photoelectron emission from single crystals, incl uding the effects of both isotropic inelastic scattering and single an d multiple elastic scattering. Our calculations were carried out for e mission in the 1 keV energy range on both simple chains of atoms embed ded in an isotropic inelastic medium and on larger atomic clusters tha t should more realistically simulate emission from a semi-infinite sin gle-crystal substrate or epitaxial overlayer. The mean emitter depth i s found to vary by as much as +/-30% with direction. It is lowest just adjacent to low-index chains of atoms because of destructive interfer ences in photoelectron diffraction. It is highest along low-index dire ctions due to forward scattering, in spite of well-known reductions in intensity along such directions due to multiple-scattering defocusing effects. These variations of mean emission depth, due to photoelectro n diffraction effects (as well as analogous Auger diffraction effects) , should be taken into account in the quantitative characterization of surfaces, surface concentration profiles, and epitaxial surface struc tures using photoelectrons and Auger electrons.