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 en ergy resolution of a laterally graded Si1-xGex crystal in the full divergen t, white synchrotron beam. At the Fe-EC absorption edge (E = 7112 eV) we ha ve measured an energy resolution of E/Delta E=7.1 . 10(4) for the (440) ref lection and a vertical divergence of 0.63 mrad. For a pure Si(440) referenc e crystal and the same divergence we found E/Delta E=7.1 . 10(3) which mean s a factor of 10 improvement in the case of the laterally graded crystal. S imilar 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 wit h a precision of Delta d/d = 2 . 10(-5). In addition, we discuss the applic ation of the laterally graded Si1-xGex crystals in the crystal monochromato r KMC-2 at the BESSY II storage ring.