Dominik BrandstetterMSc Student Karl-Franzens-Universität GrazInstitute of Physics – Solid State Theory
A recent study in photoemission orbital tomography [Science 371, 1056 (2021)] has demonstrated that transiently excited electrons can be traced in time. This has become possible through measuring their signature in the angle-resolved momentum distribution of photoelectrons released by a high-energy probe pulse. Further developing this exciting and powerful new technique, dubbed orbital cinematography, is a desirable track for future experimental development. In this work, we use time-dependent density functional theory to scout ahead and perform an ab-initio simulation of a sub-femtosecond pump-probe angle-resolved photoemission experiment. We investigate potential issues and possible remedies in the description of ARPES within the framework of TD-DFT and conclude with momentum maps of the frontier orbitals.
A. Haags, X. Yang, L. Egger, D. Brandstetter, H. Kirschner, F. C. Bocquet, G. Koller, A. Gottwald, M. Richter, J. M. Gottfried, M. G. Ramsey, P. Puschnig, S. Soubatch, F. S. Tautz
Momentum-space imaging of σ-orbitals for chemical analysis Journal Article
In: Sci. Adv., vol. 8, pp. eabn0819, 2022.
Tracing the modifications of molecules in surface chemical reactions benefits from the possibility to image their orbitals. While delocalized frontier orbitals with π character are imaged routinely with photoemission orbital tomography, they are not always sensitive to local chemical modifications, particularly the making and breaking of bonds at the molecular periphery. For such bonds, σ orbitals would be far more revealing. Here, we show that these orbitals can indeed be imaged in a remarkably broad energy range and that the plane wave approximation, an important ingredient of photoemission orbital tomography, is also well fulfilled for these orbitals. This makes photoemission orbital tomography a unique tool for the detailed analysis of surface chemical reactions. We demonstrate this by identifying the reaction product of a dehalogenation and cyclodehydrogenation reaction.
R. Wallauer, M. Raths, K. Stallberg, L. Münster, D. Brandstetter, X. Yang, J. Güdde, P. Puschnig, S. Soubatch, C. Kumpf, F. C. Bocquet, F. S. Tautz, U. Höfer
Tracing orbital images on ultrafast time scales Journal Article
In: Science, vol. 371, pp. 1056-1059, 2021.
Frontier orbitals determine fundamental molecular properties such as chemical reactivities. Although electron distributions of occupied orbitals can be imaged in momentum space by photoemission tomography, it has so far been impossible to follow the momentum-space dynamics of a molecular orbital in time, for example, through an excitation or a chemical reaction. Here, we combined time-resolved photoemission using high laser harmonics and a momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. We measured the full momentum-space distribution of transiently excited electrons, connecting their excited-state dynamics to real-space excitation pathways. Because in molecules this distribution is closely linked to orbital shapes, our experiment may, in the future, offer the possibility of observing ultrafast electron motion in time and space.
D. Brandstetter, X. Yang, D. Lüftner, F. S. Tautz, P. Puschnig
In: Comp. Phys. Commun., vol. 263, pp. 107905, 2021.
Ultra-violet photoemission spectroscopy is a widely-used experimental technique to investigate the valence electronic structure of surfaces and interfaces. When detecting the intensity of the emitted electrons not only as a function of their kinetic energy, but also depending on their emission angle, as is done in angle-resolved photoemission spectroscopy (ARPES), extremely rich information about the electronic structure of the investigated sample can be extracted. For organic molecules adsorbed as well-oriented ultra-thin films on metallic surfaces, ARPES has evolved into a technique called photoemission tomography (PT). By approximating the final state of the photoemitted electron as a free electron, PT uses the angular dependence of the photocurrent, a so-called momentum map or k-map, and interprets it as the Fourier transform of the initial state’s molecular orbital, thereby gaining insights into the geometric and electronic structure of organic/metal interfaces. In this contribution, we present kMap.py which is a Python program that enables the user, via a PyQt-based graphical user interface, to simulate photoemission momentum maps of molecular orbitals and to perform a one-to-one comparison between simulation and experiment. Based on the plane wave approximation for the final state, simulated momentum maps are computed numerically from a fast Fourier transform (FFT) of real space molecular orbital distributions, which are used as program input and taken from density functional calculations. The program allows the user to vary a number of simulation parameters, such as the final state kinetic energy, the molecular orientation or the polarization state of the incident light field. Moreover, also experimental photoemission data can be loaded into the program, enabling a direct visual comparison as well as an automatic optimization procedure to determine structural parameters of the molecules or weights of molecular orbitals contributions. With an increasing number of experimental groups employing photoemission tomography to study molecular adsorbate layers, we expect kMap.py to serve as a helpful analysis software to further extend the applicability of PT.
A. Haags, A. Reichmann, Q. Fan, L. Egger, H. Kirschner, T. Naumann, S. Werner, T. Vollgraff, J. Sundermeyer, L. Eschmann, X. Yang, D. Brandstetter, F. C. Bocquet, G. Koller, A. Gottwald, M. Richter, M. G. Ramsey, M. Rohlfing, P. Puschnig, J. M. Gottfried, S. Soubatch, F. S. Tautz
In: ACS Nano, vol. 14, pp. 15766-15775, 2020.
We revisit the question of kekulene’s aromaticity by focusing on the electronic structure of its frontier orbitals as determined by angle-resolved photoemission spectroscopy. To this end, we have developed a specially designed precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene, which allows us to prepare sufficient quantities of kekulene of high purity directly on a Cu(111) surface, as confirmed by scanning tunneling microscopy. Supported by density functional calculations, we determine the orbital structure of kekulene’s highest occupied molecular orbital by photoemission tomography. In agreement with a recent aromaticity assessment of kekulene based solely on C–C bond lengths, we conclude that the π-conjugation of kekulene is better described by the Clar model rather than a superaromatic model. Thus, by exploiting the capabilities of photoemission tomography, we shed light on the question which consequences aromaticity holds for the frontier electronic structure of a π-conjugated molecule.
Orbital tomography is a new method of analysis in surface science, developed in the last decade. It uses the data from angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) to clarify or even reconstruct orbitals of more complex organic molecules like bisanthene. The conservation of momentum parallel to the surface for an escaping photoelectron provides a basis for this. Using Fermi’s golden rule, it can be shown that the differential cross-section is proportional to the Fourier transformation of the initial state of the electron.
Thus, comparing experimental data and the data of a DFT-simulation, applying a ”plane wave” approximation for the final state of the electron, information about the initial state can be gained.
The practical part of this thesis was concerned with the extension of the functionality provided by a program written for said comparison. Besides implementing a “brute-force” algorithm to find the optimal orientation of the molecule in the simulation, multiple features to manipulate the data were added.