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Grazing-incidence X-ray Absorption and Emission Spectroscopy

Geochemistry Department, Earth Sciences Division, MS 70-108B, E. O. Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California, 94720, U.S.A.

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Grazing-incidence X-ray methods
Although the advantages of using grazing incidence (GI) techniques in X-ray scattering and spectroscopy have been known for a long time (Yoneda and Horiuchi 1971), the use of such geometry has only been developed fully with the advent of highly collimated X-ray sources, particularly synchrotrons. It is now possible to place the entire output of a synchrotron insertion device (wiggler or undulator) onto the surface of a flat crystal or analogous sample with minimal angular divergence at the critical angle for total external reflection. This allows the application of unprecedented X-ray intensity in a region confined within a few nm of the surface. Such intensity allows for measurement of the X-ray absorption and emission spectrum with sufficient counting statistics so that extremely small surface coverages of impurities, precipitates or sorbed species can be structurally probed. This is of value to environmental geochemistry, as the majority of toxic pollutants in the environment do not occupy mineral surfaces with anything like monolayer (ML) coverages. Hence the majority of X-ray experiments to date deal with unrealistically high surface loadings, relatively thick surface precipitates, or natural samples with high levels of contaminants. This is of special significance because small quantities of surface complexes may bind to defect or otherwise unrepresentative low density sites on a surface, and hence would not behave like species sorbed at higher densities. In addition, the use of single crystal samples allows the polarization of the synchrotron radiation to be used to probe specific directions with respect to the surface plane (for K and L₁ edges), hence potentially obtaining additional crystallographic information about oriented adsorbates, precipitates or epitaxial layers. One can select the type of substrate, the plane (hkl) of the substrate, and the orientation of that plane with respect to the propagation direction and electric vector polarization . . . [Full Text of this Article]