Si1-xGex laterally graded crystals as monochromators for x-ray absorption spectroscopy studies

Using the BESSY I wavelength shifter (WLS) beamline we have examined the energy resolution of a laterally graded Si1-xGex crystal in the full divergent, white synchrotron beam. At the Fe-EC absorption edge (E = 7112 eV) we have measured an energy resolution of E/Delta E=7.1 . 10(4) for the (440) reflection and a vertical divergence of 0.63 mrad. For a pure Si(440) reference crystal and the same divergence we found E/Delta E=7.1 . 10(3) which means a factor of 10 improvement in the case of the laterally graded crystal. Similar results have been obtained at the Co-K absorption edge (E = 7710 eV) with an increase of 2.6 for E/Delta E. We present the experimental set-up and a method for the characterization of the absolute lattice parameter with a precision of Delta d/d = 2 . 10(-5). In addition, we discuss the application of the laterally graded Si1-xGex crystals in the crystal monochromator KMC-2 at the BESSY II storage ring.