Observing Ultrafast Molecular Energy-Conversion Using Free-Electron Lasers

Prof. Dr. Markus Gühr
DESY and University of Hamburg


At the center of this talk is the conversion of light energy into other forms of energy in molecules. Many 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. To fully monitor and understand the ultrafast dynamics of a molecule, combined knowledge of two realms—nuclear geometry and electronic structure—is required. Specifically, this involves understanding the molecular geometry changes that drive the molecule towards regions of strong coupling among electronic states and the resulting changes in these states.

After a brief introduction to molecular light harvesting, I will present new results we obtained on nucleobases. We use soft x-ray absorption and photoemission spectroscopy to gain insight into changes in the molecular electronic structure after (optical) light excitation. The element- and site-selective nature of this method allows us to gain information about valence charge dynamics with Angstrom precision in space and on a femtosecond timescale. To investigate the light-induced changes in molecular geometry, we use Coulomb-Explosion Imaging, a method that employs an intense and short x-ray pulse to highly ionize the molecules and then resolve the momenta of the exploding molecular fragments. The combination of both methods delivers unprecedented experimental insight into the light-energy conversion dynamics in molecules.

Seminarraum HI-Jena, Fröbelstieg 3