Abstract
Molecular reactivity is determined by the energy levels and spatial extent of the frontier orbitals. Orbital tomography based on angle-resolved photoelectron spectroscopy is an elegant method to study the electronic structure of organic adsorbates, however, it is conventionally restricted to systems with one single rotational domain. In this work, we extend orbital tomography to systems with multiple rotational domains. We characterise the hydrogen evolution catalyst Co-pyrphyrin on an Ag(110) substrate and compare it with the empty pyrphyrin ligand. In combination with low-energy electron diffraction and DFT simulations, we fully determine adsorption geometry and both energetics and spatial distributions of the valence electronic states. We find two states close to the Fermi level in Co-pyrphyrin with Co 3d character that are not present in the empty ligand. In addition, we identify several energetically nearly equivalent adsorption geometries that are important for the understanding of the electronic structure. The ability to disentangle and fully elucidate multi-configurational systems renders orbital tomography much more useful to study realistic catalytic systems.
Highlights
Molecular reactivity is determined by the energy levels and spatial extent of the frontier orbitals
We demonstrate that a combination of orbital tomography, low-energy electron diffraction (LEED) and density functional theory (DFT) can address the combined structural and electronic aspects, including orbital energies, their correct hierarchy and the molecular wave functions
We show a systematic analysis of LEED and Angle-resolved photoelectron spectroscopy (ARPES) data combined with DFT calculations that allows for the complete determination of adsorption geometries (AGs) of molecular adsorbates
Summary
Molecular reactivity is determined by the energy levels and spatial extent of the frontier orbitals. In order to understand or predict the reactivity and properties of adsorbed molecules, experimental access to their electronic structure is highly desirable Scanning probe methods such as scanning tunneling microscopy (STM) can in principle image the charge distribution with submolecular resolution[5,6], but give no direct access to the electronic wave function and are typically restricted to the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively). The pyrphyrin ligand (Pyr) was first synthesised by Ogawa[24] and is currently employed as carrier for catalytically active transition metals[25] Both CoPyr and Pyr adsorb on the Ag(110) surface in multiple rotational domains because the high-symmetry axes of the molecules are not aligned with the high-symmetry direction of the (110) substrate. The found orbital hierarchy differs from gas-phase DFT calculations and underlines the importance of high-level simulations and experimental data
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