DIFFRACTION AND HOLOGRAPHY WITH PHOTOELECTRONS AND FLUORESCENT X-RAYS

We consider studies of the atomic and magnetic structure near surfaces by photoelectron diffraction and by the holographic inversion of both photoelectron diffraction data and diffraction data involving the emi ssion of fluorescent x-rays. The current status of photoelectron diffr action studies of surfaces, interfaces, and other nanostructures is fi rst briefly reviewed, and then several recent developments and proposa ls for future areas of application are discussed. The application of f ull-solid-angle diffraction data, together with simultaneous character ization by low energy electron diffraction and scanning tunneling micr oscopy, to the epitaxial growth of oxides and metals is considered. Se veral new avenues that are being opened up by third-generation synchro tron radiation sources are also discussed. These include site-resolved photoelectron diffraction from surface and interface atoms, the possi bility of time-resolved measurements of surface reactions with chemica l-state resolution, and circular dichroism in photoelectron angular di stributions from both nonmagnetic and magnetic systems. The addition o f spin to the photoelectron diffraction measurement is also considered as a method for studying short-range magnetic order, including the me asurement of surface magnetic phase transitions. This spin sensitivity can be achieved through either core-level multiplet splittings or cir cular-polarized excitation of spin-orbit-split levels. The direct imag ing of short-range atomic structure by both photoelectron holography a nd two distinct types of x-ray holography involving fluorescent emissi on is also discussed. Both photoelectron and x-ray holography have dem onstrated the ability to directly determine at least approximate atomi c structures in three dimensions. Photoelectron holography with spin r esolution may make it possible also to study short-range magnetic orde r in a holographic fashion. Although much more recent in its first exp erimental demonstrations, x-ray fluorescence holography should permit deriving more accurate atomic images for a variety of materials, inclu ding both surface and bulk regions.