The electronic structure of materials not only is fascinating from a theoretical point of view, but also has tremendous implications for technology. Electronic-structure studies, in fact, account for the majority of uses of uv radiation from synchrotron sources. Most of these studies involve photoelectron spectroscopy, a measurement technique that goes to the heart of many problems in atomic, molecular and condensed-matter physics. For example, photoelectron spectroscopy permits direct measurement of all quantum numbers of the electronic states in a material, and in particular determination of its band structure—that is, the relationship between energy and momentum of electrons in the material.