Exploring the Foundations of Photoemission Tomography

• Applicants: Dr. François Posseik; Professor Dr. Mathias Richter
• 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

Experimental methods to determine the characteristics of valence orbitals are ranging from femto-second laser spectroscopy to scanning probe techniques at ultra-cold temperatures. Although these approaches have attracted broad interest, there are several limitations, for instance, only rather simple molecules under restricted conditions, such as ultra-cold temperatures to prevent molecular diffusion, can be investigated. In contrast, the experimental approach proposed here, i.e. angle-resolved photoemission spectroscopy, can also be applied at technological relevant temperatures and for a large range of molecule/substrate combinations. To this end, a sample onto which the organic molecules have been deposited under ultra high vacuum is illuminated with UV light. The photoemitted electrons are then analyzed in terms of their energy and angular distribution. The method offers the possibility to obtain images of molecular orbitals in three dimensions and is, hence, also termed photoemission tomography. However, the interpretation of the experimental data is not straight forward. Specifically, certain assumptions have to be made about the quantum mechanical final state into which the electron is transferred from its initial bound state. The most simple ansatz is to use a free electron state here, i.e. a plane wave. It offers the advantage of interpreting the experimental data in a particularly simple manner, which allows to determine molecular geometries, to measure electron momentum distributions and to reconstruct orbital images.The aim of this project is to explore under which experimental conditions these simplifying assumptions lead to reliable results. Our team, consisting of surface scientists from the University of Graz and the Forschungszentrum Jülich and experts in the generation of metrologically characterized UV synchrotron radiation from the Physikalisch-Technische Bundesanstalt, will conduct a series of experiments to trace out the range of validity of the plane wave approximation. In order to interpret the experimental results and to theoretically predict to which extent the final state differs from such a free-electron state, the project team also comprises experts from the University of Graz in the field of quantum mechanical ab-initio calculations for the electronic structure of molecules and molecular interfaces. The possibility to image orbitals of technological relevant molecules will certainly widen our fundamental understanding of the concept of quantum mechanical electron orbitals. It will allow for the detailed investigation of physical and chemical processes and the interface between organic molecules and inorganic surfaces. Possible technological applications include the tailoring of catalytic surfaces, sensors, novel molecules and nano-structures to be used for energy harvesting (e.g. photovoltaics) or energy storage, or the identification and characterization of yet unknown molecular species.

• DFG Programme: Research Grants
• International Connection: Austria
• Co-Investigator: Professor Dr. Stefan F. Tautz
• Cooperation Partner: Professor Dr. Peter Puschnig, Ph.D.