Real-Time Dynamics of the Formation of Hydrated Electrons upon Irradiation of Water Clusters with Extreme Ultraviolet Light

The hydrated electron, a bound state of an electron in water, is of fundamental importance in nature and has been the subject of intense research for many decades. Due to its high reactivity, the hydrated electron plays a crucial role in cellular biology, radiation chemistry, and atmospheric science. However, the structure and properties of the hydrated electron still remains elusive. Recently, an international collaboration succeeded in tracing the formation of the hydrated electron in real time using tunable, XUV femtosecond pulses from the free electron laser. In this way, the initial steps of the hydration process and competing channels were observed. 
The hydration process is schematically shown in Fig. 1; an XUV FEL pulse initially ionizes the water clusters resulting in the creation of low-kinetic energy electrons. Through elastic and inelastic scattering, some of the electrons are trapped within the cluster forming bound, hydrated states. With a second UV pulse, the experiment probes the process in time recording the resulting electron kinetic energy distribution. From this, the hydration process is mapped out in time and the formation as well as decay times are determined. 

Figure 1.     Schematic of the solvation process
 

Additionally, the formation of free, excited hydrogen atoms as a fast, dominant radiation product was observed. An exemplary electron kinetic energy distribution is shown in Fig. 2 for a delay step of 20 ps showing hydrated electrons and electrons from excited hydrogen. The experimental proof that a hydrated electron can be created directly via radiation is an important step in understanding most ionization-induced processes in aqueous solutions.
 
The experiments were performed by an international group of researchers at the Low Density Matter beamline at FERMI. The work was financially supported by the Carl-Zeiss-Stiftung and the Deutsche Forschungsgemeinschaft (projects STI 125/19-1, GRK 2079).

Figure 2.  Electron kinetic energy distributions from UV probe pulse.

This research was conducted by the following research team:

A. C. LaForge^1,2, R. Michiels¹, M. Bohlen¹, C. Callegari³, A. Clark⁴, A. von Conta⁵, M. Coreno⁶, M. Di Fraia³, M. Drabbels⁴, M. Huppert⁵, P. Finetti³, J. Ma⁵, M. Mudrich⁷, V. Oliver⁴, O. Plekan³, K. C. Prince³, M. Shcherbinin⁷, S. Stranges⁸, V. Svoboda⁵, H. J. Wörner⁵, and F. Stienkemeier^1,9

¹ Institute of Physics, University of Freiburg, Freiburg, Germany 
² Department of Physics, University of Connecticut, Storrs, Connecticut, USA 
³ Elettra-Sincrotrone Trieste, Trieste, Italy 
⁴ Laboratory of Molecular Nanodynamics, EPFL, Lausanne, Switzerland 
⁵ Laboratorium für Physikalische Chemie, ETH Zürich, Zürich, Switzerland 
⁶ ISM-CNR, Istituto di Struttura della Materia, Trieste, Italy 
⁷ Department of Physics and Astronomy, Aarhus University, Aarhus C, Denmark 
⁸ Department of Chemistry, University Sapienza, Rome, Italy 
⁹ Freiburg Institute of Advanced Studies, University of Freiburg, Freiburg, Germany

Contact persons:

Aaron LaForge email: aaron.laforge@uconn.edu