Servicenavigation

Project Details

Back

Projekt Print View

Exploring the Foundations of Photoemission Tomography

Subject Area Experimental Condensed Matter Physics
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Theoretical Condensed Matter Physics
Term from 2018 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 396769409
 
Final Report Year 2022

Final Report Abstract

Under the umbrella of the DACH project, we merged our efforts and expertise in surface science and photoemission spectroscopy, synchrotron-based metrology and theoretical solid state physics aiming at setting firm fundamental grounds for practical applications of photoemission orbital tomography (POT). To achieve this primary objective , we proposed five key questions to be answered: (i) How does the experimental geometry and the polarization of the incident ultraviolet light influence the applicability of the plane wave final state approximation (PWFSA)? (ii) Can POT be applied to molecular film geometries containing tilted molecules and/or to molecular orbitals other than π-orbitals of planar molecules? (iii) How important is the size of the molecule for POT to work, for instance, would it be applicable for smaller molecules such as benzene? (iv) Does the photon energy dependence of photoemission reveal information on absolute electron density distributions? (v) Is POT able to recognize new molecular species formed by on-surface chemical reactions? Despite severe obstacles caused by the COVID pandemic, we were able to fully realize the complex experimental plan requiring a use of big research facilities and achieved results strongly supported by state -of-the-art theoretical calculation. With these achievements in hand, we can confidently give positive answers to all questions listed above. Namely, (i) although the experimental geometry and relative light polarization obviously affect the angular distribution of photoemission to a definite extend, PWFSA remains sufficiently applicable for a broad range of experimental conditions and for a broad set of the target ed systems ranging from small molecules, like benzene, to extended 2D materials, like graphene. (ii) POT has successfully been proven to be applicable for strongly distorted organic molecules including a case of extreme bending of a fruit-fly organic dye, perylenetetracarboxylic dianhydride (PTCDA) on insulating substrates - the phenomenon discovered for a first time in our experiments and confirmed by theory. Moreover, POT has been successfully used to "image" π-orbitals of organic molecules synthesized at a surface. (iii) The method has been applied to molecules as small as CO, CN, and benzene providing a deeper insight into their electronic properties. For instance, for benzene it revealed a lifting of the degeneracy of initially degenerate electronic levels caused by the interaction with the substrate and not reported so far. (iv) Photon-energy dependent angular-distribution maps obtained in experiments have been successfully converted into spatial distribution of orbital-projected charge density. Our initial hypothesis regarding the origin of specific resonances beyond the behavior predicted in the framework of PWFSA was disproved in the course of the project. Instead, a correlation of experimental and theoretical data confirmed that they are attributed to interference of partial waves/channels of photoemission. Independently from this aspect, our experiments revealed a momentum-selective hybridization of wave -functions at a molecule/metal interface postulating in this way a new selection rule of chemical bonding conditioned on constructive/destructive interference of partial waves. (v) Applying POT to π and σ orbitals of molecules synthesized on surfaces, e.g., kekulene and bisanthene, we could describe in detail their native chemistry and the chemistry of their interaction with substrate thus demonstrating a stro ng potential of the technique for applications in catalysis. In the view of our recent success in the ultrafast POT and the POT of unoccupied states, this paves the way toward a direct access to "imaging" of molecular chemistry on the orbital level − to step-by-step monitor a transmutation of molecular orbitals in the course of a chemical reaction. The latter entices our scientific interest for the follow up research triggered by the success of this project and based on unambiguous verification of the POT as an analytical technique for widespread applications of the surface chemistry and physics.

Publications

 
 

Additional Information

© 2026 DFG Disclaimer / Copyright / Privacy Policy

Textvergrößerung und Kontrastanpassung