The energy-dispersive reflectometer at BESSY II: a challenge for thin filmanalysis

Installed in 1999 the energy-dispersive reflectometer at the 13.2 bending m agnet employs the exponentially decaying white X-ray emission spectrum of t he 1.7 GeV storage ring of BESSY II outside the vacuum. Using an energy-dis persive detector specular and longitudinal-diffuse reflectivity spectra of thin films can be recorded simultaneously between 0.2 Angstrom (-1) <q(z) < 1.2 Angstrom (-1) within a few seconds. The capability of the experiment is demonstrated probing the correlation le ngth of Cd-arachidate and -stearate films at room temperature and its chang e during annealing. At T = 70 degreesC we observe an instantaneous decay of specular Bragg peaks accompanied with an increase of the diffuse scatterin g. This indicates the onset of the melting of 2D-ordered acid domains. The vertical diffusion coefficient is estimated to be about 2 x 10(-24) m(2)/s. (C) 2001 Elsevier Science BN. All rights reserved.