In resonant reflectivity experiments, the scattering power of iodine labels was used to retrieve selective structural information of the iodine positioning with respect to the membrane normal by varying the incidence of the X-ray radiation close to the iodine L III absorption edge around E = 4.5575 keV (Gullikson 1995 (Gullikson -2008 . The scattering length becomes explicitly dependent on the X-ray energy E near the absorption edge, according to