Identification of Molecular Orbitals

Photoemission orbital tomography (POT) has rapidly developed into a powerful way to visualize and analyze the electronic structure of organic molecules on surfaces. In our latest work, we show that POT can identify all types of molecular orbitals—both π and σ—across a wide energy range of more than 10 eV. This is a major step forward, because σ orbitals, which are crucial for understanding how chemical bonds form or break at the edges of a molecule, have traditionally been much harder to access.

By measuring how electrons are emitted at different angles and energies, we reconstructed the orbital landscape of the organic molecule bisanthene (C₂₈H₁₄) on a Cu(110) surface. In total, we extracted the energies of 38 orbitals (15 π and 23 σ), resulting in a particularly comprehensive experimental orbital dataset for this system [1].

Experimental benchmarking of ab initio calculations
Theoretical and experimental energies of (a) σ and (b) π orbitals of bisanthene on Cu(110). For σ orbitals, averaged experimental energies are displayed. For π orbitals, the values from the fit of the hν = 35 eV data cube with all π orbitals are shown for reasons described in the articlel. For the four columns on the left of each panel, the adsorption process was split into successive steps and orbital energies were calculated for each of the steps. All theoretical energies were calculated with DFT, employing the HSE exchange-correlation functional. The different sets of calculated orbital energies were aligned at their vacuum levels, using the calculated work function of bisanthene/Cu(110). For clarity, the color of the orbitals alternates in a cyclic pattern from black to red to blue, following the orbital energies of the isolated molecule.
Copyright:
— Physical Review B

Experimental benchmarking of ab initio calculations
(A) σ(7,3) and σ(0,8) orbitals of bisanthene (top) and metalated bisanthene (bottom). (B and C) Band maps along the [1-10] and [001] directions. π and σ bands are labeled.
Copyright:
— Science Advances

These detailed measurements allow us to benchmark electronic structure theories more rigorously than ever before. Comparing our results with calculations using four widely used density functionals, we found that the HSE hybrid functional matches the experimental data best.

Importantly, we also show that σ orbitals—which reveal local chemical changes much more clearly than π orbitals—can be imaged with POT just as reliably. This means POT can directly track how molecules transform during reactions on surfaces. We demonstrate this by identifying the product of a dehalogenation and cyclodehydrogenation reaction [2].

Taken together, our findings highlight POT as a uniquely powerful tool for studying and understanding complex surface chemical processes, offering unprecedented insight into how molecular structures and orbitals evolve during reactions.

References

[1] A. Haags et al., Phys. Rev. B 111, 165402 (2025).

[2] A. Haags et al., Sci Adv. 8, eabn0819 (2022)

Last Modified: 04.12.2025