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 found with a splitting of the sigma* resonance as a function of the azimuthal orientation of the photon electric field in the molecular plane. The sigma* 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.