Improving Optical Models of Polarized R-SoXS for Quantitative Measurement of Molecular Orientation within Polymer Nanostructures*

Resonant soft X-ray scattering (RSoXS) with polarized X-rays is sensitive to local molecular orientation within nanostructures but it is difficult to interpret the scattering patterns due to a lack of appropriate optical models. Uniaxial optical tensors have been used successfully to measure average global molecular orientation in X-ray absorption spectroscopy (XAS) and ellipsometry. The model reduces into diagonal tensors whose elements correspond to parallel and perpendicular alignments of the incident electric field with respect to the transition dipole moment. It is uncertain, however, if such a model will work in polarized RSoXS as it assesses differences in local ordering. We show how to create such a model using XAS measurements and test it against polarized RSoXS measurements on pure films of a semicrystalline conjugated polymer. We show the model works for the pi* resonances but breaks down in regions that include other transitions. DFT calculations demonstrate how not all transitions follow uniaxial symmetry and require a lower symmetry model to reproduce measurements. Combining XAS and DFT into such a new model may enable further details of local molecular orientation to be extracted by pol-RSoXS that remove current limitations of XAS.