The EXAFS family tree: a personal history of the development of extended X-ray absorption fine structure.

This paper reviews the history of X-ray absorption spectroscopy (XAS) beginning with the first observation of an absorption edge, through the development of the modern theory and data inversion by the Fourier transform. I stop with my first trip to a synchrotron X-ray source. The study of XAS began at an exciting time for science. Wave mechanics, X-ray diffraction, X-ray scattering from non-crystalline materials experiments developed in parallel with XAS. However, the difficulty of obtaining data from conventional X-ray tubes limited the field to a potentially interesting minor subject. Only with the advent of synchrotron radiation and arrival of modern theory in the 1970s did XAS become widely applicable to fields ranging from environmental to biological sciences. Early developments in experimental technique and theory are emphasized. Since I worked in both the before-synchrotron and after-synchrotron time frames, I had the opportunity to meet some of the early scientists. A number of historical vignettes and photographs of the scientists involved in the development of EXAFS are presented.

Strontianite in coral skeletal aragonite.

An x-ray spectroscopic study of scleractinian coral skeletons indicated that, although some strontium substitutes for calcium in the aragonite structure, at concentrations of about 7500 parts per million, as much as 40 percent of the strontium resides in strontianite (SrCO3). A doublet peak in the Fourier transform of the extended x-ray absorption fine structure of the coral corresponded to six metal and 13 oxygen neighbors surrounding strontium at about 4.05 angstroms in strontium-substituted aragonite and at about 4.21 angstroms in strontianite. Thus, the mechanism of the temperature-sensitive partitioning of strontium between seawater and coral skeleton used for paleothermometry is unexpectedly complex.

X-ray Absorption Spectroscopic Investigation of Sulfur Sites in Coal: Organic Sulfur Identification.

High-resolution x-ray absorption spectroscopy was used to probe the chemical and structural environments of sulfur in coal. Measurement of the sulfur Kedge spectra down to 2472.0 electron volts under nonvacuum conditions was made possible in an all-helium path, and a Stern-Heald type ion chamber was used for fluorescence detection. For a number of selected sulfur-containing minerals and organic model compounds, results show that near-edge spectral features are diagnostic for sulfur in specific organic moieties such as thiols, disulfides, and various heterocyclics, as well as in mineral sulfide and sulfate phases. The spectrum of a model system containing 35 percent iron pyrite and 65 percent benzothiophene was found to match the observed spectral features of a bituminous coal from the eastern United States.

X-ray Absorption Spectroscopic Investigation of Trace Vanadium Sites in Coal.

X-ray absorption spectroscopy was used to probe the chemical and structural environment of vanadium in coal. It was found that vanadium exists in at least two environments, in both of which it was coordinated to oxygens. There was no evidence of vanadium in nitrogen (porphyrin) or sulfide environments. It was also found that the vanadium environments in the raw coal did not survive unchanged in a liquefaction process. These findings have implications for coal cleaning processes and for trace element release into the liquefaction process stream.