Tuesday 4:30 pm (Freiburg) / 7:30 am (Vancouver)

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, Poland

30.01.24 – Eran Rabani, UC Berkeley, USA

06.02.24 – Oliver Kühn, University Rostock

06.02.24 – Oliver Kühn, University of Rostock

Direct Optimal Control Theory For Laser-Driven Molecular Dynamics

Triggering chemical reactions with designed laser pulses has evolved from a dream to reality during the last decades such that even complex solution phase reactions can be manipulated. For instance, light has been used to accelerate polyurethane formulation allowing to write polymer patterns with a moving laser focus [1]. In fact this can be achieved even with simple laser pulses provided that a vibrational mode is excited, which is coupled to the reaction coordinate [2].

Theory has been keeping pace with experiment by developing a large number of control strategies and associated optimization methods. Among them optimal control theory is most popular. It builds on variational optimization of a control functional, taking into account a set of given constraints. Its practical realization follows an indirect approach, that is, the stationarity condition for the functional is casted into a two-point boundary problem to be solved numerically. Thus, changes of the functional require to adjust or even develop the numerical implementation. Recently, we have introduced an alternative approach, which is rather flexible concerning the choice of the functional. It is well-known in engineering and follows a direct control strategy. Here the control problem is first discretized and then the optimization is performed using nonlinear programming algorithms. In Ref. [3] direct control was used to drive the reaction coordinate in a bistable potential mimicking proton transfer. Comparing single Gaussian and exact wave packet dynamics it was shown that control fields can be used to manipulate the quantumness of the dynamics. In a second application we have focussed on intramolecular vibrational energy redistribution [4]. Its understanding and possible utilization is key to successful realization of laser control. Here it was demonstrated that direct control theory provides the flexibility to addressed various control targets such as the transient decoupling of the Fermi resonance states. Finally, laser control of cavity-catalyzed proton transfer will be discussed. It serves as an example for optimizing parameters of the Hamiltonian [5]

[1] T. Stensitzki, Y. Yang, V. Kozich, A.A. Ahmed, F. Kössl, O. Kühn, and K. Heyne, “Acceleration of a ground-state reaction by selective femtosecond-infrared-laser-pulse excitation,” Nature Chemistry 10, 126–131 (2018).
[2] K. Heyne, and O. Kühn, “Infrared Laser Excitation Controlled Reaction Acceleration in the Electronic Ground State,” J. Am. Chem. Soc. 141, 11730–11738 (2019).
[3] A.R. Ramos Ramos, and O. Kühn, “Direct optimal control approach to laser-driven quantum particle dynamics,” Frontiers in Physics 9, 615168 (2021).
[4] A.R. Ramos Ramos, and O. Kühn, “Manipulating the dynamics of a Fermi resonance with light. A direct optimal control theory approach,” Chem. Phys. 555, 111431 (2022).
[5] A.R. Ramos Ramos, E. Fischer, P. Saalfrank, and O. Kühn, „Shaping the Laser Control Landscape of a Hydrogen Transfer Reaction by Vibrational Strong Coupling. A Direct Optimal Control Approach“, arXiv:2401.01138 (2023).

Chair: Michael Thoss

30.01.24 – Eran Rabani, UC Berkeley, USA

Coherent electron transfer in interconnected quantum dot assemblies

Electron transfer stands as a cornerstone process in the intricate realms of chemistry, biology, and physics. Within this captivating landscape, a particularly fascinating aspect involves unraveling the nuanced transitions between nonadiabatic and adiabatic electron transfer regimes. Leveraging sophisticated computational simulations, we explore the interconnected quantum dot assemblies, demonstrating how precise adjustments to the hybridization energy (electronic coupling) can be achieved by manipulating the dimensions of connecting necks and/or the sizes of quantum dots. This deliberate fine-tuning emerges as a powerful tool for orchestrating electron transfer dynamics, steering the transformation from the inherently nonadiabatic Marcus regime to the more organized and coherent adiabatic regime within a single system.

To capture the intricate interplay of factors, we introduce an atomistic model that comprehensively considers multiple states and their intricate couplings to lattice vibrations. Employing the mean-field mixed quantum-classical method, we provide an in-depth portrayal of the charge transfer dynamics at the molecular level. In the wake of our explorations, a striking revelation emerges: charge transfer rates undergo a remarkable surge, spanning several orders of magnitude, as the system progresses towards the coherent, adiabatic limit, even when subjected to elevated temperatures. As an additional facet, we delineate the specific phonon modes that exhibit the most pronounced coupling to the dynamics of charge transfer, offering insights into the pivotal factors that govern these intricate molecular processes.

Chair: Michael Thoss

16.01.24 – Grzegorz Kowzan, University Torun, Poland

Towards cavity-enhanced two-dimensional infrared spectroscopy of gas-phase molecules

2DIR spectroscopy is a powerful and well-developed experimental technique, commonly used to study ultrafast molecular dynamics in optically thick liquid-phase and solid-state samples [1]. Newest advances in generation of high-power optical frequency combs in the mid infrared and in cavity enhancement of ultrafast nonlinear signals provide a way to extend 2DIR measurements to weakly absorbing samples, in particular to low-concentration gas-phase samples [2]. The high sensitivity of these techniques can be combined with the high resolution of multicomb spectroscopy or comb Fourier-transform spectroscopy to enable measurements of the shapes of individual resonances. In this talk, I will discuss our plan for the experimental realization of CE-2DIR spectroscopy, how it will be used to solve the problem of optical coherence transport in gas-phase molecules [3], and other future applications.

In 2DIR liquid-phase spectroscopy of coupled vibrational modes it is common to use sequences of pulse polarizations that eliminate parts of the molecular response and reveal weaker signals or use polarization dependence to constrain fit models. In general, these polarization conditions do not apply to the gas phase. I will present polarization conditions that enable control of the gas-phase molecular response [4] and demonstrate their effect on experimental 2DIR spectra of the asymmetric stretch of carbon dioxide.

[1] P. Hamm and M. Zanni, Concepts and Methods of 2D Infrared Spectroscopy, 1st ed. (Cambridge University Press, Cambridge, 2011).

[2] M. C. Silfies, A. Mehmood, G. Kowzan, E. G. Hohenstein, B. G. Levine, and T. K. Allison, J. Chem. Phys 159, 104304 (2023).

[3] G. Kowzan, H. Cybulski, P. Wcisło, M. Słowiński, A. Viel, P. Masłowski, and F. Thibault, Phys. Rev. A 102, 012821 (2020).

[4] G. Kowzan and T. K. Allison, J. Phys. Chem. Lett. 13, 11650 (2022).

Chair: Lukas Bruder

12.12.23 – Peter Saalfrank, University of Potsdam

Molecules coupled to photons and phonons

Molecules in contact with quantized photons and with (surface) phonons share some common features, and can be treated with similar theoretical models and methods to solve the underlying time-dependent, possibly multi-dimensional Schr¨odinger equation. In this talk, first of all the strong coupling of photons in a cavity to molecular vibrations will be considered, with special attention given
to: (i) The formation of vibro- and rovibro-polaritons and their signatures in vibrational spectra1,2, (ii) the influence of vibrational strong coupling (VSC) on rates and yields of chemical reactions and their possible control3,4, and (iii) the validity of the Born-Oppenheimer approximation in VSC5. In a second part of the talk, recent work of our group to treat the vibrational relaxation of adsorbates near surfaces due to vibration-phonon coupling6 will be presented. The following aspects will be addressed: (i) The “reduction” of multi-oscillator bath models by a hierarchical effective mode (HEM) approach7,8, and (ii) the quantification of non-Markovian effects in multi-dimensional system-bath problems6,8.

[1] Eric W. Fischer, Peter Saalfrank, Ground state properties and infrared spectra of anharmonic vibrational polaritons of small molecules in cavities. J. Chem. Phys. 154, 104311 (2021).
[2] Eric W. Fischer, Peter Saalfrank, Cavity-induced non-adiabatic dynamics and spectroscopy of molecular rovibrational polaritons studied by multi-mode quantum models. J. Chem. Phys. 157, 034305 (2022).
[3] Eric W. Fischer, Janet Anders, Peter Saalfrank, Cavity-altered thermal isomerization rates and dynamical resonant localization in vibro-polaritonic chemistry. J. Chem. Phys. 156, 154305 (2022).
[4] Eric W. Fischer, Peter Saalfrank, Cavity-catalyzed Hydrogen Transfer Dynamics in an Entangled Molecular Ensemble under Vibrational Strong Coupling. Phys. Chem. Chem. Phys. 25, 11771 (2023).
[5] Eric W. Fischer, Peter Saalfrank, Beyond Cavity Born-Oppenheimer: On Non-Adiabatic Coupling and Effective Ground State Hamiltonians in Vibro-Polaritonic Chemistry. J. Chem. Theor. Comput. 19, 7215 (2023).
[6] Foudhil Bouakline, Eric W. Fischer, Peter Saalfrank, A quantum-mechanical tier model for phonon-driven vibrational relaxation dynamics of adsorbates at surfaces. J. Chem. Phys. 150, 244105 (2019).
[7] Eric W. Fischer, Michael Werther, Foudhil Bouakline, Peter Saalfrank, A hierarchical effective mode approach to phonon-driven multilevel vibrational relaxation dynamics at surfaces. J. Chem. Phys. 153, 064704 (2020).
[8] Eric W. Fischer, Michael Werther, Foudhil Bouakline, Frank Grossmann, Peter Saalfrank, Non-Markovian vibrational relaxation dynamics at surfaces. J. Chem. Phys. 156, 214702 (2022).

Chair: Michael Thoss

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

Giuseppe Sansone (Freiburg)

The Ultrafast Odyssey: How Scientists Generated the First Attosecond Pulses to Probe Electrons in Motion

Klaus Mecke (FAU Erlangen-Nürnberg)

Numbers and Narratives:  Why should physicists care about poems?

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.

  1. 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.

Chair: Fabian Richter / Lukas Bruder