Application of the Borrmann effect to x-ray monochromatization and to the overlayer versus substrate signal-ratio enhancement

In x-ray-excited surface spectroscopies one of the most relevant figures of merit is the overlayer versus substrate signal ratio. In this work we propose to utilize the Borrmann (or anomalous-transmission) effect, which takes place in perfect crystals, in order to have a weaker interaction of the incident beam with the substrate. In this way the overlayer versus substrate signal ratio can be enhanced, roughly by a factor varying between 3 and 20, depending on the kind of spectroscopy utilized. To demonstrate the feasibility of this method, the fluorescence signal intensity of a Ge overlayer (200 angstrom thick) is compared with the Si substrate scattering both in transmission and in reflection geometry. The synchrotron radiation from a wiggler insertion device was used as an x-ray source. An improvement of a factor about 3 is observed in the present experiment, in agreement with the expected value for this particular case. The monochromatization properties of such an arrangement are also presented.