The energy-dispersive reflectometer diffractometer at BESSY-I

We describe the set-up of an energy-dispersive reflectometer installed at the 6 T wavelength shifter (WLS) of the storage ring BESSY-I. It can be used for reflectometry and in-plane diffraction studies of thin films under ambient conditions. Due to the exponential decay of the incident spectrum, the whole useful energy range between 3 keV < E < 35 keV is not available simultaneously. Absorbing the highly-intensive low-energy part of the spectrum, the specular reflectivity can be recorded up to q(z) > 12.5 nm(-1) using maximum angles of incidence and exit of alpha(i) = alpha(f) approximate to 2 degrees. Up to q(z) = 5 nm(-1) the intensity is high enough to detect small angle Bragg peaks of a fatty acid salt multilayer separated by Delta E = 2 pi/(d alpha(i)), (d is the multilayer period) in about 15 s with sufficient counting statistics. After normalization by the incident spectrum the energy-dispersive reflectivity spectrum provides information similar to that from an angular-dispersive experiment, but in a much shorter time. Besides the specular reflectivity, our set-up provides the possibility of simultaneous measurement of the in-plane momentum transfer (grazing incidence diffraction). For in-plane lattice parameter measurements a second detector is attached on a circle perpendicular to the plane of incidence. As in the angular-dispersive set-up, it enables investigation of the in-plane arrangement of a fatty acid salt multilayer in about 15 s. Keeping the in-plane Bragg angle (2 Theta approximate to 30 degrees) fixed, the respective in-plane Bragg peaks appear for different energies and can be selected by changing alpha(i).