Recent Applications of X Rays in Condensed Matter Physics

ince their discovery by Wilhelm Conrad Rontgen, x rays «Jhave played a significant role in our lives: they make the unseen in our bodies visible; they enhance our security in air travel; they make possible nondestructive testing of a wide variety of materials. And through x-ray diffraction and spectroscopy they make it possible to probe the order of matter at the atomic level. The discovery of x-ray diffraction in crystals by Max von Laue, Walther Friedrich and Paul Knipping, and William Rragg's subsequent interpretation of the diffraction spots as proof of the wave nature of x rays, laid the foundation for the field of x-ray crystallography. X-ray crystallography proved essential in relating the atomic structure of solids to their functions and physical properties. The intensity distribution in the diffraction pattern was found to reveal how electrons were distributed within the atom and to give information about how the atoms in a solid vibrated about their equilibrium positions. In more complex crystals with unit cells built of molecules, the spot intensities in the regular diffraction pattern were modulated in a way that directly revealed the atomic structure of the molecule—a discovery that revolutionized chemistry and molecular biology. (See the article by Wayne Hendrickson on page 42 of this issue.) Diffraction patterns from the liquid state of matter, while not as detailed as those from the crystalline state, indicated the correlations in the positions of the atoms in the liquid, which formed the basis of the physics of liquids. Early in the history of x rays, Compton scattering revealed the energy and momentum distributions of electrons as well as vividly illustrating Heisenberg's uncertainty principle. But the real birth of x-ray spectroscopy came with the discovery that Bragg scattering could be used to select x rays of a particular wavelength. Using this technique, Moseley found that the discrete lines superimposed on the continuous bremsstrahlung spectrum varied systematically with the anode material's atomic number, making possible the completion of the periodic