Tiny Wiggles Pack a Wallop: X-Ray Absorption Fine Structure (XAFS)

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Sometimes knowledge sits and percolates for a while before someone comes along to develop its full potential for practical good. Such was the case for something called the "extended fine structure" effect in the absorption of x-rays by atoms, a phenomenon that was known for half a century. But it wasn't until UW physicist Edward A. Stern and colleagues Farrel W. Lytle and Dale E. Sayers came along in 1971 that scientists fully appreciated the potential of using the effect to analyze the chemical identities and environments of individual atoms in solid materials.

X-rays are very short-wavelength, high-energy light rays, invisible to the human eye. They can interact with the atoms in a solid material in two basic ways. They can be absorbed by an atom, causing its electrons to jump up to higher energy levels, or with enough energy, to be kicked out of the atom entirely. Or, x-rays can be scattered, which in the case of a crystalline compound, whose atoms are arranged in a tidy, repeating array, produces a decipherable scattering pattern. That pattern can be decoded to reveal the layout and spacings of atoms inside the crystal. X-ray crystallography, as the latter process is called, is used to deduce, for example, the structures of organic molecules and proteins.

In the first type of interaction--absorption--scientists had been aware for decades that when the x-ray absorption profile was graphed, some odd wiggles appeared on the trace. Stern and colleagues showed that these tiny wiggles in the characteristic x-ray absorption signature of an atom are packed with information about the identities and locations of the absorber's neighboring atoms. The information can be decoded for every type of atom in the solid, one at a time. And this analysis can be carried out whether or not the material is crystalline or amorphous.

Crystalline materials, like table salt, are compulsively tidy. They have the familiar geometric shapes with regular faces at defined angles, reflecting the underlying regular and repeating arrangement of their constituent atoms. Non-crystalline, or amorphous materials, like glass, or charcoal, lack such long-range order of atomic arrangement.

Now it happens that x-ray crystallography is limited to analyzing the tidy solids, so the new technique of XAFS, "x-ray absorption fine structure" developed by Stern and colleagues, opened up totally new horizons in structural analysis of amorphous materials. Scientists were especially keen to use the technique to analyze biological molecules, catalysts, and glasses, for example.

In one instance, Stern and colleagues were able to determine which of two proposed models for the structure of germania glass (GeOsubscript 2) was correct. And the technique has been used to study the economically important compounds called catalysts--solids that are used to carry out chemical reactions to make consumer and industrial products like gasoline, plastics, and myriad others. By understanding the relationship between the structure and the performance of catalysts, scientists have designed improved materials for the job.

New sources of high-energy x-rays now becoming available will make it possible to analyze smaller amounts of trace elements than ever before. What is soon to be the world's brightest source of x-rays, the Advanced Photon Source at Argonne National Laboratory in Illinois, will provide Stern and colleagues with a tool of greater power and finesse. The UW is the formal administrative center for the Pacific Northwest consortium which owns and operates some of the beam lines at the Advanced Photon Source at Argonne.

"The equipment will allow the focusing of the beam to a spot one-tenth of a micron in size—about four-millionths of an inch—which increases the intensity of the beam by 10,000 times," says Stern. That capability is important in the study of biological systems and pollutants, for example, and will assist in the development of advanced materials and new drugs, as well as new strategies for cleaning up the environment.

  1. "The Analysis of Materials by X-Ray Absorption," Edward A. Stern, Scientific American, 234, 96 (1976).