24.10.23 – Matthias Wollenhaupt, University Oldenburg
07.11.23 – Daniel Keefer, MPI for Polymer Research, Mainz
14.11.23 – Sebastian Hartweg, University of Freiburg
21.11.23 – Giuseppe Sansone, University of Freiburg & Klaus Mecke, FAU Erlangen-Nuernberg
05.12.23 – Anne Lise Viotti, Lund University
12.12.23 – Peter Saalfrank, University Potsdam
16.01.24 – Grzegorz Kowzan, University Torun
06.02.24 – Oliver Kühn, University Rostock
05.12.23 – Anne-Lise Viotti, Lund University, Sweden
Making high-power lasers ultrafast
High power lasers operating on ultrashort fractions of time enable many applications in the industrial realm. They are also effective drivers for secondary photon and particle sources to explore extreme light-matter interactions at high repetition rates. Broadband optical parametric amplifiers have been extensively used to produce high peak and high average power ultrashort pulses but an efficient alternative is provided by direct post-compression of high-power diode-pumped ytterbium lasers. Recently, a novel spectral broadening approach, called the multi-pass cell technique, has emerged to achieve impressive pulse parameters such as sub-50 fs pulses at the kW average power level or with pulse energies beyond 100 mJ. In this talk, I will introduce the multi-pass cell method applied to pulse post-compression and present an overview of the current performances of the approach. I will also show how we implement it in the labs at the Lund Laser Centre and for which type of experiments it is used.
Chair: Giuseppe Sansone
21.11.23 – Giuseppe Sansone, University of Freiburg & Klaus Mecke, FAU Erlangen-Nuernberg
14.11.23 – Sebastian Hartweg, University of Freiburg
Solvated dielectrons from optical excitation and their decay via electron-transfer mediated decay
Low-energy electrons dissolved in liquid ammonia or aqueous media are powerful reducing agents that promote challenging reduction reactions and can cause radiation damage to biological systems. Knowledge of the underlying mechanistic processes remains incomplete, particularly with respect to the details and energetics of the electron transfer steps.
After giving a brief introduction of solvated electrons in aqueous systems and sodium ammonia solutions, I will present our recent work[1] on the ultraviolet (UV) photoexcitation and photoionization of sodium-ammonia clusters. Specifically, I will discuss how we identified the light–induced generation of spin-paired solvated dielectrons and their subsequent relaxation via an electron transfer–mediated decay as an efficient source of low-energy electrons.
Hartweg, S., J. Barnes, B.L. Yoder, et al. Science, 2023. 380(6650)
Chair: Frank Stienkemeier
07.11.23 – Daniel Keefer, MPI for Polymer Research, Mainz
Time-Resolved X-Ray Spectroscopy and Quantum Optimal Control of Molecular Photochemistry
Elementary processes in nature, chemistry, and functional materials critically rely on photochemical transformations. The primary steps in these transformations are facilitated by coupled nuclear and electronic motions on the femtosecond and sub-femtosecond timescale. Ultrafast X-ray sources from free-electron lasers and tabletop setups have opened new windows into these dynamics by providing unprecedented temporal and spectral resolutions. Their ultrabright intensities further allow for diffraction experiments from gas-phase molecular samples.
Due to the vast complexity of the primary steps in photo-induced molecular dynamics, and the partly uncharted territory enabled by ultrafast X-ray spectroscopy, theoretical proposals are highly valuable in explaining and designing next-generation measurements. In the past few years, we had designed new spectroscopic methods targeted at the direct detection of non-adiabatic molecular dynamics taking place at Conical Intersections (CIs).[1] These show up in any polyatomic system as regions of degeneracy between electronic states, facilitating a breakdown of the Born-Oppenheimer approximation and therefore enable non-radiative transitions between electronic states. The direct signatures of CI passages rely on coherences that emerge when a nuclear wavepacket bifurcates rather than the usually recorded ultrafast changes in transient absorption lines. We introduce novel spectroscopic techniques such as stimulated X-ray Raman,[2] time-resolved X-ray and electron diffraction,[3] the usage of vortex X-ray beams exhibiting specific polarizations, and time-resolved photoelectron spectroscopy, among others. Approaches to overcome the lack of phase control from stochastic free-electron laser sources will be discussed.[4]
I will further outline how quantum optimal control can be used as a tool to selectively amplify the intrinsically weak coherence-based signatures, which can be crucial in isolating them from congested spectra containing stronger, less interesting contributions. [5], [6]
[1] D. Keefer et al., Annu. Rev. Phys. Chem. 74, 73-97 (2023) [2] D. Keefer et al., Proc. Natl. Acad. Sci. U.S.A. 117, 24069 (2020) [3] D. Keefer et al., Proc. Natl. Acad. Sci. U.S.A. 118, e2022037118 (2021) [4] S. M. Cavaletto et al., Phys. Rev. X 11, 011029 (2021) [5] D. Keefer et al., Phys. Rev. Lett. 126, 163202 (2021) [6] D. Keefer et al., J. Am. Chem. Soc. 143, 13806 (2021)
Chair: Lukas Bruder
24.10.23 – Matthias Wollenhaupt, University Oldenburg
Coherent control of multi-photon ionization
In this talk, I will explain how we can manipulate the photoelectron momentum distribution by multiphoton ionization of atoms and molecules with polarization-shaped femtosecond laser pulses. After introducing the physical mechanisms of multiphoton ionization (MPI) and explaining the concepts of coherent control, I will show measurements of tailored 3D photoelectron momentum distribution obtained by photoelectron tomography. Our experimental results on the generation and manipulation of free-electron vortices with single-color and bichromatic polarization-shaped pulses will be presented along with a novel technique to extract the quantum mechanical phase of the photoelectron distributions by photoelectron holography. Finally, I will demonstrate the generation of molecular vortices on C60 fullerenes by MPI of Superatomic Molecular Orbitals (SAMOS) using counterrotating circularly polarized pulses.
