Theoretical study of inelastic X-ray scattering spectra for organic materials: Molecular excitation coupled with molecular exciton descriptions

Inelastic X-ray scattering (IXS) spectrum provides a powerful tool to investigate the electronic structure of organic materials, in complimentary to the conventional optical spectra. Starting from a single molecule, then a dimer, and finally a cluster of molecular aggregate, we developed a Frenkel exciton model to describe the IXS process based on the quantum chemical calculations for the molecular excitations in the organic crystal of open-ring photomerocyanine form of spirooxazine (Py-SO). It is shown that the quantum chemical calculation combined with Frenkel exciton model can well describe the experimental measurements in terms of (i) the overall features of the IXS spectra from 2 to 10 eV, (ii) the dispersion behavior of the lowest exciton band, and (iii) the momentum-transfer dependence of the peak intensity. The roles of molecular excited state and the intermolecular interactions have been discussed for the IXS spectra.