Theoretical core-level spectroscopy from adsorbed organic molecules

Hybrid interfaces between organic molecules and inorganic substrates are key systems in many technological applications, ranging from photovoltaics to organic field effect transistors and light emitting devices. Several properties of such interfaces can be effectively studied by electron core-level spectroscopies: X-ray photoemission (XPS) addresses changes in the chemical state of the atoms; near-edge X-ray absorption fine structure (NEXAFS) accesses molecular orientations and provides information on the unoccupied electronic levels; resonant photoemission spectroscopies (RESPES) can measure interfacial electron transfer times down to the femtosecond timescale. The potential of these experimental techniques is fully obtained by complementing them with theoretical investigations and reliable numerical modeling. The perturbation by charged atomic cores is efficiently taken into account within a density-functional theory (DFT) based approach, as will be exemplified here. Numerical simulation of the dependence of NEXAFS molecular spectra on the photon electric field direction for pentacene and perylene derivatives [1,2] demonstrates how one can completely determine the orientation (polar and azimuthal) of molecules at interfaces. Disentangling the contribution of specific atomic cores (initial states) and molecular orbitals (final states) to NEXAFS [2], is further used to demonstrate filling of the lowest-unoccupied molecular orbital of pentacene in the peculiar V-bent conformation it assumes on Al(001) [3]. Excitonic effects due to the presence of core-excited atoms in the molecule can be included in the modeling to understand the electron-transfer times as measured by RESPES, which are evaluated numerically for molecules on a semi-infinite TiO2(110) substrate whose continuum of conduction states is described by Green’s function techniques [4]. [1] J. Phys. Chem. C, 2013, 117 (13), pp 6632–6638 [2] Phys. Chem. Chem. Phys., 2014, 16 (28), pp 14834-14844 [3] J. Phys. Chem. C, 2015, 119 (7), pp 3624–3633 [4] J. Phys. Chem. C, 2014, 118 (17), pp 8775–8782