Speaker
Description
Many molecular light-energy conversion processes in nature occur on an ultrafast (sub-picosecond) timescale, such as retinal light harvesting, optical switching of green and yellow fluorescent proteins, and nucleobase photoprotection. The conversion process often occurs at conical intersections, which are regions in molecular phase space characterized by degenerate potential energy surfaces and a linear lifting of energy degeneracy.
To fully monitor and understand the ultrafast dynamics of a molecule, a combined knowledge of two subsystems—nuclear geometry and electronic structure—is required. Specifically, this involves understanding the molecular geometry changes that drive the molecule toward regions of strong coupling among electronic states, as well as the resulting changes in these states.
In this talk, I will show how time-resolved soft X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) of isolated molecules at free-electron lasers are used to obtain information on the changes in the molecular electronic structure during photoexcited molecular dynamics. The element- and site-selective nature of this method allows us to gather information about valence charge dynamics with angstrom precision in space and on a femtosecond timescale. I will highlight the power of these methods with a couple of molecular processes from internal conversion and photoinduced isomerization[1,2].
To investigate the light-induced changes in molecular geometry, we employ Coulomb-Explosion Imaging, a method that utilizes an intense and short X-ray pulse to highly ionize the molecules and subsequently resolve the momenta of the exploding molecular fragments. We use hydrogen atoms as messengers and discover the deplanarization of the molecular geometry on its path toward the conical intersection. The combination of both methods provides unprecedented experimental insight into the light-energy conversion dynamics in molecules[3].
In the last part of the talk, I will give an overview of the free-electron laser FLASH at DESY in Hamburg and its current upgrade process to highlight new opportunities for the ultrafast molecular and materials community at FLASH.
References
[1] Mayer D, Lever F, Picconi D et al., 2022 Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy Nat. Commun. 13 198
[2] Lever F, Picconi D, Mayer D et al., 2025 Direct Observation of the ππ to nπ Transition in 2-Thiouracil via Time-Resolved NEXAFS Spectroscopy J. Phys. Chem. Lett. 16 4038–46
[3] Jahnke T, Mai S et al., 2025 Direct observation of ultrafast symmetry reduction during internal conversion of 2-thiouracil using Coulomb explosion imaging Nat. Commun. 16 2074