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Ultrafast Shell Games: Uncovering Charge Transfer Dynamics at FERMI

Friday, April 10, 2015

Understanding energy transfer and charge transport is the key to target design for artificial photosynthesis systems for generating a desired fuel from water and carbon dioxide. The idea for such solar fuel generators is inspired by the nature where the plant leaves capture the sunlight through their reaction centers to create and transport charges that lead to the conversion of water and carbon dioxide to carbohydrates. Learning from nature, for designing solar fuel systems we need to combine properly metallic centers that act as absorbers of sun light oxidation and reaction centers and are synchronized with each other.

Sketch of the FEL-based FWM experiment: kFEL1, kFEL2, kopt and kout are the wavevectors of the two crossed FEL excitation pulses, the optical probing pulse and the FWM signal. (a) Picture of the setup (close to the sample area) inside the DiProI end-station, the arrows are kFEL1, kFEL2 kopt and kout. (b) Image of the FWM signal

In a recent article in Nature, Fermi scientists report on a new experiment exploiting Four Wave Mixing (FWM) processes, based on third-order nonlinear light-matter interactions, to combine ultrafast time resolution with energy and wavevector selectivity to explore dynamics inaccessible by linear methods.

"We have demonstrated the possibility to generate coherent populations of excited states via FWM processes by shining two-colour FEL pulses interacting with an absorbing atom A." says the coordinator of the experiment Claudio Masciovecchio. "The charge associated with the excitation can move to a different atom B and the transfer of the charge can be then probed by a third pulse to uncover, as in a shell game, the dynamics of charge transfer mechanisms."

"We demonstrate", adds scientist Filippo Bencivenga "that extreme ultraviolet (EUV) FWM is already possible by using fully coherent Free Electron Laser (FEL) pulses, as those routinely delivered by FERMI and, in the foreseeable future, by other EUV/SXR FEL facilities."

In getting this knowledge these results are an important step in development of multidimensional spectroscopy in X-ray region, which opens the route to follow coherent charge and energy flow between different constituent atoms in materials on their fundamental time scales.

The research team smiles at the beamline.

This research was conducted by the following research team:
Filippo Bencivenga, Riccardo Cucini, Flavio Capotondi, Andrea Battistoni, Riccardo Mincigrucci, Erika Giangrisostomi, Alessandro Gessini, Michele Manfredda, Ivaylo P. Nikolov, Emanuele Pedersoli, Emiliano Principi, Cristian Svetina, Pietro Parisse, Francesco Casolari, Miltcho B. Danailov, Maya Kiskinova & Claudio Masciovecchio, Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy

Reference: F. Bencivenga, R. Cucini, F. Capotondi, A. Battistoni, R. Mincigrucci, E. Giangrisostomi, A. Gessini, M. Manfredda, I. P. Nikolov, E. Pedersoli, E. Principi, C. Svetina, P. Parisse, F. Casolari, M. B. Danailov, M. Kiskinova & C. Masciovecchio "Four wave mixing experiments with extreme ultraviolet transient gratings," Nature 520, 205 (2015), DOI: 10.1038/nature14341