resolved photoemission has given the community a detailed picture of the electronic states at a surface. For clean surfaces, almost every form of surface state imaginable has been identified and understood theoretically. Dangling-bond or back-bond surface states on semiconductor surfaces, free-electron surface states on metals as well as magnetic surface states have been documented. Studies of adsorbed atoms or molecules, or impurities segregated to the surface, have revealed the nature of the adsorbate-adsorbate and adsorbate-substrate interaction.

Simple symmetry rules that exploit the polarized nature of the light source are used routinely to determine bonding geometry. The new insertion devices on the synchrotron, coupled with much more efficient detectors, will produce a sufficiently strong signal that spin-polarized angle-resolved photoemission will become common, enabling the experimentalist to observe with unprecedented detail the magnetic properties of the surface. The new sources will also enable experiments to be conducted with much higher energy and momentum resolution.

One of the most exciting applications of synchrotron radiation sources to surface science has been to the area of core-level spectroscopy. Conventional core-level photoelectron spectroscopy measures the binding energy shifts of a specific element in different environments. The intensity from undulator sources, coupled with the resolution from specially designed monochromators, will give a hundred to a thousand times more signal with less than 0.1 eV resolution. For the first time the inherent line shape of core excitations from chemisorbed atoms or molecules will be accessible, yielding valuable information about the dynamics of the excitation process. The tunability of the synchrotron has allowed experimentalists to probe the near edge or threshold region, as well as monitoring the extended x-ray absorption fine structure (EXAFS) oscillations above threshold. This technique has yielded important new information about the geometry and chemical nature of atoms and molecules bound to the surface. The next generation of undulator source, coupled with new monochromator designs, promises to make this type of spectroscopy even more useful. With better resolution and more intensity, the dynamics of the excitation and decay process can be monitored.

Recent experiments on an undulator have demonstrated the feasibility of coincidence experiments between electronic decay and ion fragmentation. The use of fluorescent detection will also provide a depth perception into the solid.

Inverse photoemission has, in the last few years, begun to yield as detailed a picture of the unoccupied surface states as photoemission has for the occupied states. The series of Rydberg-like surface states trapped in the image potential have been observed for many metals, presenting a more detailed picture of the nature of this long-range surface potential. The future will see higher-resolution spectra as well as the use of spin-polarized sources. The



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