Tomographic identification of all molecular orbitals in a wide binding-energy range

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Abstract
In the past decade, photoemission orbital tomography (POT) has evolved into a powerful tool to investigate the electronic structure of organic molecules adsorbed on surfaces. Here we show that POT allows for the comprehensive experimental identification of all molecular orbitals in a substantial binding energy range of more than 10 eV. Making use of the angular distribution of photoelectrons as a function of binding-energy, we exemplify this by extracting an orbital-resolved projected density of states for 15 π and 23 σ orbitals from the experimental data of the prototypical organic molecule bisanthene ( C 28 H 14 ) on a Cu(110) surface. These experimental results for an essentially complete set of orbitals within the given binding-energy range serve as stringent benchmarks for electronic structure methods, which we illustrate by performing density functional calculations employing four frequently used exchange-correlation functionals. By computing the respective molecular-orbital-projected densities of states, a one-to-one comparison with experimental data for an unprecedented number of 38 orbital energies became possible. The quantitative analysis of our data reveals that the range-separated hybrid functional HSE performs best for the investigated organic/metal interface. At a more fundamental level, the remarkable agreement between the experimental and the Kohn-Sham orbital energies over a binding-energy range larger than 10 eV suggests that—perhaps unexpectedly—Kohn-Sham orbitals approximate Dyson orbitals, which would rigorously account for the electron extraction process in photoemission spectroscopy but are notoriously difficult to compute, in a much better way than previously thought.
Funding Information
  • Austrian Science Fund (10.55776/I3731)
  • Deutsche Forschungsgemeinschaft (Po 2226/2-1, 223848855-SFB 1083, Ri 804/8-1)
  • European Commission (101071259)
  • H2020 European Research Council

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