Effects of the Grazing Incidence Geometry on X-ray Photon Correlation Spectroscopy Measurements

X-ray photon correlation spectroscopy (XPCS) is a versatiletoolto measure dynamics on the nanometer to micrometer scale in bulk samples.XPCS has also been applied in grazing incidence (GI) geometry to examinethe dynamics of surface layers. However, considering GI scatteringexperiments more universally, the GI geometry leads to a superpositionof signals due to reflection and refraction effects, also known fromthe distorted-wave Born approximation (DWBA). In this paper, the impactof these reflection and refraction effects on the correlation analysisis determined experimentally by measuring grazing incidence transmissionXPCS (GT-XPCS) and grazing incidence XPCS (GI-XPCS) simultaneouslyfor a thin film sample, showing non-equilibrium dynamics. The resultsof the GI and GT geometry comparisons are combined within the frameworkof the standardly applied, simplified DWBA. These calculations allowidentifying the main contributions of the detected signal from theleading scattering terms along the out-of-plane direction q ( z ), which dominate the measuredintensity pattern on the detector. In combination with the calculationof the non-linear effect of refraction in GTSAXS and GISAXS, it ispossible to identify experimental conditions that can be chosen torun experiments and data analysis as close as possible to transmissionXPCS and to explain which limitations for data interpretations areobserved. Consequently, the beam exposure can be significantly reducedby using GI geometry only. Calculations of experimental settings priorto experiments are detailed to determine suitable q ( z ) regions for a variety of material systemsmeasured in bulk-sensitive GI-XPCS experiments, allowing us to determinethe scaling behavior of typical decay times as a function of q that is comparable to the scaling behavior obtained indistortion-free GT-XPCS or transmission XPCS experiments.