An interferometric determination of the refractive part of optical constants for carbon and silver across soft x-ray absorption edges

Interferometric, direct determinations of the f1 scattering factor near absorption edges in the soft x-ray range is demonstrated. The interferometric system, which is based on wave front division (no beam splitter) with plane mirrors only, produces a linear fringe pattern. The principle consists in direct measuring of the fringe shift occurring upon insertion of a sample into one interferometer arm, by means of a dedicated detection system. This provides the optical thickness, which in turn gives the f1 factor, knowing either the sample mass per unit surface, or the sample thickness and density. With the sample being probed in transmission under near normal incidence, the determination of f1 is not perturbed by the absorption part of the complex scattering factor. Therefore, f1 data obtained here can be said new and independent with respect to those obtained previously, in the sense that they are obtained from a new, purely experimental technique, and are neither deduced from nor perturbed by absorption. The interferometer design used can be implemented in a very large spectral range. For demonstrating the ability of this interferometric system to provide such new f1 data, a thin free-standing carbon foil near the K edge (from 4.1 to 4.65 nm, 302–267 eV), and a membrane-supported silver layer near an M edge (from 2.6 to 3.8 nm, 477–326 eV) were used as test objects.