W0075

Model Independent Approaches to Analyzing Surface Biological Structures. N. F. Berk, NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899

Many systems of biological interest have thin film morphology, which are suitable for study by x-ray and neutron reflectometry. These include membrane mimetic, self assembled lipid layers and Langmuir-Blodgett films. The specular reflection from layered systems as a function of incident angle is determined by the scattering length depth profile of the film. To what extent is the inverse also true? Can density profiles be uniquely retrieved from reflection spectra? The answer depends on a mix of theoretical and practical concerns, which we discuss. It has been known for some time that the dynamical specular reflection problem can be inverted if both the magnitude and phase of reflection are measured. Several formal solutions to the phase problem for specular reflection have recently been given, as well as actual thin film implementations with neutron reflectometry. We briefly review work in this emerging field. We also plan to discuss an application to invert the depth profile of a lipid film from its measured complex neutron reflection amplitude using a ferromagnetic reference layer buried in the substrate.