Tuesday 4:30 pm (Freiburg) / 7:30 am (Vancouver)
14.04.26 – Merline Cherukarathadathil Ulahannan
(University of Freiburg)
21.04.26 – Maria Popovic (University of Milan, Italy)
28.04.26 – Alexie Boyer (University of Strasbourg, France)
05.05.26 – Krzysztof Jachymski (University of Warsaw, Poland)
12.05.26 – Martin Pichotka (University of Freiburg)
19.05.26 – Manuel Gessner (University of Valencia, Spain)
02.06.26 – Frank Schlawin (University of Hamburg)
16.06.26 – Iva Brezinova (TU Vienna, Austria)
23.06.26 – Peter Ratzikis (IESL-FORTH, Greece)
30.06.26 –Madhusree Roy-Chowdhury (University of Kassel)
07.07.26 – Maxim Gelin (Hangzhou Dianzi University, China)
14.07.26 – Nicolas Treps (Sorbonne University, France)
21.07.26 – Thomas Parton (MPI of Colloids and Interfaces, Potsdam)
05.05.26 – Krzysztof Jachymski (University of Warsaw, Poland)
Stone skipping with ions in superfluids
The paradigm of superfluidity is the dissipationless flow of the fluid past an obstacle or defect, provided that the velocity remains smaller than the critical value. A similar scenario can be realized by studying the motion of a mobile impurity in a degenerate quantum gas. If the impurity is charged, its dynamics can be quite complex due to the long-range nature of the interactions. It turns out that the ion slows down by emitting waves, thereby changing momentum in a nonlinear way. I will describe the simplest theoretical approach to this problem based on the transformation to the co-moving frame. Dressing the impurity with the host atoms can result in formation of a polaronic state characterized by effective mass, which may be detectable in state-of-the art experiments.
Chair: Tobias Schätz
28.04.26 – Alexie Boyer (University of Strasbourg, France)
Investigating ultrafast internal conversion using extreme ultraviolet (EUV) time-resolved photoelectron spectroscopy
Absorption of light in molecules involves ultrafast processes that are often mediated by conical intersections (CIs), molecular geometries where potential energy surfaces intersect. At these intersections, the Born–Oppenheimer approximation breaks down, and nuclear motion drives transitions between electronic states. These non-adiabatic transitions ultimately lead to the formation of photoproducts. In such cases, photoinduced dynamics are governed by several factors, including the topography of the potential energy surfaces and conical intersections, the presence of dark states, and the nuclear degrees of freedom that facilitate electronic transitions. Together, these factors determine both the speed and efficiency of photochemical reactions. An accurate description of non-adiabatic dynamics therefore requires tracking the full evolution of the vibrational wavepacket created upon photoexcitation, from the initial Franck–Condon region to the formation of photoproducts.
In this context, the development of high-order harmonic generation (HHG) technologies, tabletop sources delivering extreme ultraviolet (EUV) pulses, has opened new opportunities to probe photoinduced dynamics. EUV time-resolved photoelectron spectroscopy (TRPES) has emerged as a powerful technique for investigating ultrafast processes, enabling the monitoring of vibrational wavepacket evolution along potential energy surfaces and through CIs.
In this seminar, I will introduce how EUV-TRPES can be used to track non-adiabatic dynamics and present several examples in which it has been applied to study ultrafast internal conversion in UV-photoexcited molecules, in both gas and liquid phases.
Chair: Pascal Pessier
21.04.26 – Maria Popovic (University of Milan, Italy)
Multiparameter quantum estimation of circulant graphs
In multiparameter estimation, compatibility conditions and Cramér-Rao bounds dictate the loss of information on individual parameters and the limits on estimation precision, while few systems allow for a fully analytical approach. Here, we present the analytical derivation of precision limits for the simultaneous estimation of vertex coupling strengths in circulant graphs, probed by the continuous-time quantum walk of a particle. We demonstrate that the weak compatibility condition is always satisfied, and show instances for which the estimations of the individual parameters are statistically independent. We analyse the classical Cramér-Rao bound derived from position measurements on the quantum walker, and evaluate how measurement time and experimental limitations – such as coarse-grained measurements – affect the estimation efficiency for two couplings. Furthermore, we derive the optimal initial states of the probe that achieve best estimation precision. Our findings highlight the role of graph symmetry in saturating multiparameter estimation precision bounds, and establish a benchmark for quantum metrology in networked systems.
Chair: Alessandra Colla
14.04.26 – Merline Cherukarathadathil Ulahannan (University of Freiburg)
Design for a Monochromatic, High-Repetition-Rate XUV Beamline for Time-Resolved Photoelectron–Photoion Coincidence Spectroscopy
In the first part of this talk, I will present my Master’s project carried out at TIFR Hyderabad, where I worked on building a laser-driven X-ray diffraction apparatus. Using a kHz femtosecond Ti:Sapphire laser (800 nm, 25 fs, 2.5 mJ, 1 kHz) focused onto a 20 μm methanol jet in vacuum, I generated a hot dense plasma producing highly energetic electrons (~2 MeV). The resulting highly energetic electrons was used for generating X-rays (~8 keV) which we tried to use for XRD.
In the next section I present our plans for a monochromatic XUV beamline with high-repetition rates of up to 120 kHz for time-resolved photoelectron photoion coincidence spectroscopy. The beamline will be based on a cascaded harmonic generation scheme [1] driven by an Yb Laser. The 800 μJ near IR pulses will be first spectrally broadened by self-phase modulation in a gas-filled multipass cell and compressed to ~40, after which the pulse is passed through a BBO crystal to generate the second harmonic (515 nm). The resulting pulses will be used to efficiently drive the HHG process in a gas jet producing XUV radiation in the range between 10 and 20 eV. After separation of the XUV from the driving field we will use a time-preserving monochromator [2] with a single plane diffraction grating in off-plane geometry for spectral selection while maintaining pulse durations of tens of femtoseconds.
A home-built nonlinear optical parametric amplifier pumped by the same Yb Laser will supply frequency-tunable ultrashort pump pulses in the visible and UV spectral range. This beamline will be used in performing time-resolved XUV photoelectron–photoion coincidence spectroscopy in molecular complexes.
References:
- Comby et al., Opt. Express 27, 20383 (2019).
- Frassetto et al., Opt. Express 19, 19169 (2011).
Chair: Sebastian Hartweg
