In GID mode the sample was tilted at a fixed angle of incidence close to the critical angle of total external reflection a c in order to optimize the lateral scattering intensity and to minimize background scattering caused by the substrate. The vertical scattering depth along z is, thereby, tuned by the angle of beam incidence a i and the angle of the scattered beam a f (compare to Fig. 3) . GID experiments were performed at 17 keV radiation using a Peltier cooled 2D CCD detector (Princeton Scientific Instruments, Princeton, NJ, USA) providing a resolution of 1,340 9 1,300 pixels (pixel size: 48 9 49 lm 2 ; active area: 64.3 9 63.7 mm 2 ) that was mounted at approximately 19.5 cm from the sample. The beam was cut by the entrance slits to 0.7 9 0.7 mm 2 in front of the sample; no beam restrictions were set on the detector side. Three different types of measurements were performed: