News

Congratulations, Arne Morlok, on the successful defense of your doctoral thesis!

Leonie Katharina Werner for her master’s thesis: “A novel action-detection scheme for wave packet interferometry of organic molecules in helium nanodroplets”

Supervisor: PD Dr. Lukas Bruder

Abstract of thesis:

A large proportion of photochemical processes in organic molecules happen non-radiatively, hence, it is of particular interest to develop experimental techniques that enable their investigation. Photoelectron spectroscopy is well-suited for probing non-radiative transitions. However, to date, the majority of photoelectron spectroscopy is conducted in the gas phase. Thus, the influence of an environment on the molecular dynamics is not considered. Embedding the molecule in helium nanodroplets enables the investigation of interaction dynamics between the dopant and the droplet. Helium nanodroplets act as a weakly-perturbing testbed to investigate single molecules in the gas phase. In this work, time-resolved photoelectron experiments have been conducted with the organic molecules tetracene and the prototypical photoswitch azobenzene exploiting helium nanodroplet isolation. An increase of low kinetic energy photoelectron yield is observed for both systems upon photoexcitation that induces non-radiative transitions. Evaluation of the droplet size-dependent dynamics yields the conclusion that two processes are taking place. A superposition of helium evaporation and dopant ejection is causing the increase of low kinetic energy photoelectron yield upon excitation, due to fewer interactions with the helium environment. First measurements of utilizing the change in photoelectron signal of azobenzene-doped helium nanodroplets for wave packet interferometric measurements are feasible. Excitation spectra show broad features that can not be distinguished from the excitation laser spectrum. This points to the fact that with the chosen pump wavelength range oligomers are predominantly excited.

See also here:
https://www.physik.uni-freiburg.de/aktuelles/preiseauszeichnungen/alumnipreis2025werner?set_language=de

F. Richter, U. Bangert, F. Landmesser, N. Gölz, F. Riedel, L. Bruder
Low dispersion phase-modulated rapid-scanning interferometry
Opt. Lett., 2025, 50, 3668-3671: abstract – pdf – arXiv

L. H. Coudert, N. L. Chen, B. Gans, S. Boyé-Péronne, G. A. Garcia, S. Hartweg, J.-C. Loison
The threshold-photoelectron spectrum of SiH₂: experiment and modeling with MCTDH method
Phys. Chem. Chem. Phys., 2025, 27, 13, 6628-6639: abstract – pdf

See also publications.

Sebastian Hartweg receives funding from the DFG’s Emmy Noether Programme to establish a junior research group to investigate photocatalytic reactions using XUV electron-ion coincidence spectroscopy

Nicolai Gölz für seine Master-Arbeit “Rapid Scanning in High-Resolution Coherent Spectroscopy”

Betreuer: Prof. Dr. Frank Stienkemeier

Coherent multidimensional spectroscopy (CMDS) is a powerful ultrafast spectroscopic technique to study dynamics of matter with a high spectro-temporal resolution otherwise only accessible in disjunct experiments. Extending the method to weakly perturbed molecular and cluster species in the gas phase permits very high spectral resolution [1]. Previous experiments have revealed valuable insights such as the homogeneous line profile of chromophores solvated in nanoclusters [2]. However, in this case, the attainable resolution is limited by the acquisition time. To solve this problem, a rapid scanning method, developed by the Oglivie group in 2021 [3], is implemented and extended in this thesis to inter-pulse delays in the nanosecond range. This reduces the acquisition time by up to two orders of magnitude. In addition, optical delay tracking allows the correction of non-constant delay stage speeds and movements in the interferometer, which further improves the spectral resolution of phase-modulated wave packet interferometry experiments.

[1] L. Bruder et al., Nat. Commun. 9 4823 (2018).

[2] U. Bangert et al., Nat. Commun. 13 3350 (2022).

[3] D. Agathangelou et al., J. Chem. Phys. 155 094201 (2021).

A team with participation from the University of Freiburg generates low-energy electrons using ultraviolet light. Freiburg, May 26, 2023

Solvated dielectrons are the subject of many hypotheses among scientists, but have never been directly observed. They are a pair of electrons dissolved in liquids such as water or liquid ammonia. A cavity forms in the liquid, which the two electrons occupy. An international research team led by Dr. Sebastian Hartweg, originally at the SOLEIL synchrotron in France and now at the Physics Institute of the University of Freiburg, and Prof. Dr. Ruth Signorell of ETH Zurich, with the participation of scientists from the SOLEIL synchrotron and Auburn University (USA), has now succeeded in discovering a formation and decay process of the solvated dielectron: In experiments at the SOLEIL synchrotron, supported by quantum chemical calculations, the team found direct evidence for the formation of these electron pairs through ultraviolet light excitation in tiny ammonia droplets containing a single sodium atom. The results have been published in the journal Science.

Traces of an Unusual Process

When dielectrons form through ultraviolet light excitation in ammonia droplets containing a sodium atom, they leave their traces in an unusual process that the scientists have now observed for the first time. One of the two electrons migrates to the surrounding solvent molecules, while the other electron is simultaneously ejected. “The astonishing thing is that similar processes have so far been observed mainly at significantly higher excitation energies,” says Hartweg. The team focused on this second electron because it could have interesting applications. Firstly, the ejected electron is generated with very low kinetic energy, meaning it moves very slowly. Secondly, this energy can be controlled via the irradiated UV light that initiates the entire process. Solvated dielectrons could therefore serve as a good source of low-energy electrons.

Specifically generated with variable energy

Such electrons can initiate a wide variety of chemical processes. For example, they play a role in the cascade of processes that lead to radiation damage in biological tissue. They are also important in synthetic chemistry, where they serve as effective reducing agents. By now being able to specifically generate these electrons with variable energy, the mechanisms of such chemical processes can be studied in more detail in the future. Furthermore, the controlled energy provided to the electrons could also be used to increase the effectiveness of reduction reactions. “These are interesting prospects for possible future applications,” says Hartweg. “Our work provides the foundation for this and contributes to a better understanding of these exotic and still enigmatic solvated dielectrons.”

Fact sheet:

Original publication: S. Hartweg, J. Barnes, B. L. Yoder, G. A. Garcia, L. Nahon, E. Miliordos, R. Signorell: Solvated dielectrons from optical excitation: An effective source of low-energy electrons, Science 0, eadh0184. DOI: https://doi.org/10.1126/science.adh0184

Dr. Sebastian Hartweg is a research associate at the Physics Institute of the University of Fribourg. His research focuses on the electronic structure and dynamics of molecules and molecular clusters. Prof. Dr. Ruth Signorell heads the Aerosols and Nanoscience research group at ETH Zurich. In addition to the University of Freiburg and ETH Zurich, the SOLEIL synchrotron in Saint-Aubin, France, and Auburn University, USA, were involved in the work.

The project was funded by the European Union’s Horizon 2020 research and innovation program of the European Research Council under grant agreement 786636, the Swiss National Science Foundation (project 200020_200306), and the United States National Science Foundation (Grant No. CHE-1940456).

Contact:
Hochschul- und Wissenschaftskommunikation
Universität Freiburg
Tel.: 0761/203-4302
E-Mail: kommunikation@zv.uni-freiburg.de

Emerging Investigator Award from the Journal of Chemical Physics 2022 for Katrin Erath-Dulitz

The Journal of Chemical Physics is awarding Katrin Erath-Dulitz one of two Best Paper Awards for the scientific article

“Spin-state-controlled chemi-ionization reactions between metastable helium atoms and ground-state lithium atoms.”

The scientific article was written as part of Tobias Sixt’s dissertation research at the University of Freiburg’s Physics Institute, under the supervision of Katrin Erath-Dulitz (Prof. F. Stienkemeier’s group), and is part of the Emerging Investigators Special Collection 2022, to which up-and-coming young scientists were invited. In the award-winning work, the authors use laser radiation to prepare the spin states of atoms. This allows them to manipulate the rate of chemical reactions between lithium and excited helium atoms. The ability to control such a reaction is a crucial step for capturing the two types of atoms over long periods of time and subsequently conducting precision physical measurements important for quantum science. Since September 2022, Katrin Erath-Dulitz has been working as an assistant professor of physics at the University of Innsbruck.

https://publishing.aip.org/about/news/two-early-career-researchers-capture-2022-jcp-emerging-investigator-awards/

Lukas Bruder, Markus Koch, Marcel Mudrich, Frank Stienkemeier

Ultrafast Dynamics in Helium Droplets

Part of the Topics in Applied Physics book series (TAP,volume 145, pp 447 – 511)

Freiburg physicists have developed a new spectroscopy method

Freiburg, 08.10.2021

A team of researchers from Freiburg led by Prof. Dr. Frank Stienkemeier and Dr. Lukas Bruder has succeeded in developing a new measurement method for investigating ultrafast processes in matter. These are processes at the atomic and molecular level that occur within a billionth of a second (10^-12 sec). The new method, which combines different spectroscopy techniques, enables, among other things, new insights into the energy structure in matter and the probability distribution of electrons. Fundamental molecular processes can now be understood more precisely, according to the researchers. The results of the research have been published in the scientific journal Optica and are expected to foster a variety of further developments in related scientific fields.

Investigating fundamental properties of matter

The Freiburg team has been working for several years on extending ultrafast, coherent, multidimensional spectroscopy in new directions. Put simply, spectroscopy involves studying the absorption of light in order to investigate important properties of matter. These include the mentioned ultrafast processes, as well as quantum coherence phenomena and interactions between atoms and other nanoscopic particles. “These are the fundamental properties of matter that drive the processes in nature at the nanoscopic level, and we want to better understand these properties through our experiments,” Stienkemeier reports.

A general problem in coherent, multidimensional spectroscopy is the complexity of the measurement data, which often makes a clear interpretation of the experimental results difficult or even impossible. The situation improves significantly when the experiment is combined with the use of, for example, a mass spectrometer. “This approach gives us the additional and very useful information about the chemical composition of the substance under investigation – a major advantage in the study of ultrafast chemical reactions,” Bruder explains.

A host of possibilities

Comparably, the Freiburg researchers have now succeeded in combining coherent, multidimensional spectroscopy with photoelectron spectroscopy. In this procedure, the substance is ionized and the energy of released electrons is measured. This procedure provides information about the energy structure and spatial probability distribution of electrons (orbitals) in matter. When photoelectron spectroscopy is combined with X-ray light sources, precise measurements with atomic selection are even possible, meaning that the energy distribution in a substance can be studied with extremely high resolution up to the atomic level.

“Our approach opens up a variety of exciting new developments,” Stienkemeier explains. “This ranges from extending our method for simultaneous energy- and angle-resolved electron measurements, to experiments with X-rays to obtain atom-specific information.” As another benefit of the Freiburg approach, the sensitivity of the coherent, multidimensional spectroscopy experiments has been improved by orders of magnitude. That is, signals that were previously a factor of 200 smaller than the noise in the measurement can now be detected. “The increased sensitivity allows us to study very clean samples in an ultra-high vacuum environment from which we can understand fundamental molecular processes more precisely,” Bruder adds.

The research project was funded within the framework of the international graduate school “CoCo,” established by the German Research Foundation, and by the European Research Council (ERC) through the project “COCONIS.”

Original publication:
Daniel Uhl, Ulrich Bangert, Lukas Bruder, and Frank Stienkemeier, “Coherent optical 2D photoelectron spectroscopy,” Optica 8, 1316-1324 (2021)

DOI: 10.1364/OPTICA.434853

The principle of the developed multidimensional electron spectroscopy: selective multidimensional frequency spectra of the investigated substance can be extracted from the measured kinetic energy distribution of electrons.

Contact:
Prof. Dr. Frank Stienkemeier
Physikalisches Institut
Albert-Ludwigs-Universität Freiburg
Tel.: 0761/203-7609
E-Mail: stienkemeier@uni-freiburg.de

 

Bastian Strauch
Hochschul- und Wissenschaftskommunikation
Albert-Ludwigs-Universität Freiburg
Tel.: 0761/203-4301
E-Mail: bastian.strauch@pr.uni-freiburg.de

 

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