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 waveguide surface is discussed. The multiple total reflection (MTR) phenomenon highly increases X-ray interaction with the waveguide surface and hence offers higher sensitivity compared to conventional (single reflection) REFL-XAFS. On the one hand, this technique is a direct structural method for characterizing waveguides (e.g. capillaries) where the application of other methods 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 mirror. Ray tracing calculations demonstrate that the WG waveguide is very suitable 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 about 20-fold gain in the signal-to-background ratio compared with the conventional technique. The MTR phenomenon discussed in this paper can provide new possibilities to study clean surfaces, ultrathin films and adsorbed molecules.