We investigate bidirectional femtosecond charge transfer dynamics using the core–hole clock implementation of resonant photoemission spectroscopy from 4,4′-bipyridine molecular layers on three different surfaces: Au(111), epitaxial graphene on Ni(111), and graphene nanoribbons. We show that the lowest unoccupied molecular orbital (LUMO) of the molecule drops partially below the Fermi level upon core–hole creation in all systems, opening an additional decay channel for the core–hole, involving electron donation from substrate to the molecule. Furthermore, using the core–hole clock method, we find that the bidirectional charge transfer time between the substrate and the molecule is fastest on Au(111), with a 2 fs time, then around 4 fs for epitaxial graphene and slowest with graphene nanoribbon surface, taking around 10 fs. Finally, we provide evidence for fast phase decoherence of the core-excited LUMO* electron through an interaction with the substrate providing the first observation of such a fast bidirectional charge transfer across an organic/graphene interface.
COBISS.SI-ID: 2895460
Electronic structure of 4-hydroxycyanobenzene in the gas phase, thick films, and single crystals has been investigated by X-ray photoemission spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). We have used resonant photoemission spectroscopy (RESPES) to identify the symmetry and atomic localization of the occupied and unoccupied molecular orbitals for the free molecule. Upon condensation into a thick film, we find XPS energy shifts in opposite directions for the oxygen and nitrogen core levels, consistent with the formation of an intermolecular hydrogen bond. This interaction is also accompanied by a significant spatial distortion of the lowest unoccupied molecular orbital that is displaced from the nitrogen atom, as indicated by the RESPES measurements. Thick films and single crystals display the same dichroism in polarization dependent NEXAFS, indicating that the intermolecular hydrogen bonding also steers the molecular assembly into a preferred molecular orientation.
COBISS.SI-ID: 2765412
Primary amines can interact with neighbor molecules or with a metal substrate via weak bonds involving the electron lone pair of their amino functional group. Near edge X-ray absorption spectra (NEXAFS) on the N 1s edge show that the structure of the empty molecular orbitals localized on the nitrogen atom is very sensitive to these interactions. Here we investigate the origin of these changes by means of theoretical calculations. NEXAFS spectra are simulated for the 1,4-benzenediamine (BDA) molecule in its free, crystalline, and monolayer on Au(111) forms. We identify the electronic states which are affected by these amino-based interactions. In the case of the molecular layer grown on the gold substrate, we show how the results of the calculations can be used to identify intermolecular interactions influencing adsorption geometries in molecular monolayers.
COBISS.SI-ID: 2772068