REX : a least-squares fitting program for the simulation and analysis of X-ray reflectivity data

Abstract A FORTRAN program REX, which has been developed to facilitate the interpretation of X-ray reflectivity data, is described. The program allows the simulation of reflectivity profiles as a function of either incident angle or of energy. Factors such as anomalous dispersion, and surface and interface roughness are taken into account in the model. In addition, experimental data of reflectivity as a function of incident angle can be matched to user-supplied theoretical parameters by a least-squares refinement procedure. Experimental reflectivity data recorded at several X-ray wavelenghts can be analysed simultaneously, thus eliminating certain experimental errors and increasing the probability of finding the correct physical model when fitting the data.