Azimuthal dichroism in near-edge X - ray absorption fine structure spectra of planar molecules

Azimuthal Dichroism in Near-Edge X - ray Absorption Fine Structure Spectra of Planar Molecules: The dependence of the near-edge x-ray absorption fine structure (NEXAFS) spectrum of molecules on the photon electric field direction is investigated by means of first principles simulations based on density-functional theory with the transition-potential approach. In addition to the well-known dependence of the NEXAFS resonances on the orientation of the electric field with respect to the molecular plane, we demonstrate that for planar molecules with sufficient in-plane anisotropy such as pentacene, a dichroic effect is computed with a splitting of the σ* resonance as a function of the azimuthal orientation of the photon electric field in the molecular plane. The σ* splitting is investigated as a function of the length of acenes and closely related molecules. A proper assignment of such spectral features guided by theory together with variable polarization experiments, may allow one to completely determine the orientation of molecules at interfaces. [1] G. Fratesi, V. Lanzilotto, L. Floreano, and G. P. Brivio, J. Phys. Chem. C (2013), vol. 117, pages 6632-6638