Surface and interface science at CHESS

Picture the atoms on the surface of a crystal. Their environment is different from that of the other atoms - they have no neighbors on one side and their bonding is incomplete. As a consequence, atomic arrangement near the surface differs from that of the crystal as a whole, and entirely different phase diagrams apply. Melting temperature, for instance, is different at surfaces than at deeper layers. And to add to the complexity, surface structure can change reversibly with temperature. These conditions are important to understand because surface structure has a major influence on the chemistry and physics of a material; catalytic properties, for example, depend heavily on surface characteristics. X-rays are useful for probing structure at buried surfaces including gas/solid, liquid/solid, solid/solid, or gas/liquid interfaces because they penetrate easily through layers of low-density matter before reaching a surface. The high-intensity x-radiation from a synchrotron is especially good because the scattering signals from surface layers are inherently weak in comparison with signals from the bulk