Bulk-sensitive XAS characterization of light elements: from X-ray Raman scattering to X-ray Raman spectroscopy

X-Ray absorption spectroscopy (XAS) is a powerful tool for element-specific characterization of local structure and chemistry Although application of XAS in the hard X-ray region is now routine, the soft X-ray region (containing light-element K-edges) presents a number of experimental problems. Most of the difficulties, including surface sensitivity, restricted sample environments, and radiation damage, stem from the submicron path lengths of soft Xrays and/or electrons. X-Ray Raman scattering (XRS) provides a means for obtaining the information content of soft X-ray spectra while maintaining the experimental benefits of hard X-ray techniques. In the XRS process, an incident photon is inelastically scattered and part of its energy is transferred to excite an inner shell electron into an unoccupied state. Under the dipole approximation, the resulting features are identical to the corresponding XAS spectrum. In the past, the extremely low cross-section of XRS has made this technique impractical, but intense new X-ray facilities and improvements in X-ray optics have helped to put XRS on the brink of becoming a routine spectroscopic tool. At present, high-quality X-ray Raman spectra can be obtained in minutes to hours. X-Ray Raman spectroscopy thus represents a hard X-ray alternative to conventional XAS techniques in the study of systems with light elements, including C, N and O. In this review we describe the technique, present examples of recent work, and discuss the prospects for the future. (C) 2002 Elsevier Science B.V. All rights reserved.