Enhanced reflectance X-ray absorption fine structure sensitivity using a whispering-gallery waveguide

A new technique of reflectance X-ray absorption fine structure (REFL-XAFS) utilizing waveguides where X-rays are reflected many times along the wavegu ide surface is discussed. The multiple total reflection (MTR) phenomenon hi ghly increases X-ray interaction with the waveguide surface and hence offer s higher sensitivity compared to conventional (single reflection) REFL-XAFS . On the one hand, this technique is a direct structural method for charact erizing waveguides (e.g. capillaries) where the application of other method s is very difficult. On the other hand, the conventional thin wafer can be transformed to a whispering-gallery (WG) waveguide by bending to a curved m irror. Ray tracing calculations demonstrate that the WG waveguide is very s uitable for REFL-XAFS measurements. This method was experimentally realized for a cylindrically bent silica wafer with the surface covered with a GeO2 monolayer. The Ge K-edge REFL-XAFS measurements were performed using both MTR and conventional techniques. The MTR technique allows us to achieve abo ut 20-fold gain in the signal-to-background ratio compared with the convent ional technique. The MTR phenomenon discussed in this paper can provide new possibilities to study clean surfaces, ultrathin films and adsorbed molecu les. (C) 2000 Published by Elsevier Science B.V. All rights reserved.