Auger electron spectroscopy as a probe of ion pairing and protonation effects in aqueous solutions

• Applicant: Professor Dr. Lorenz S. Cederbaum
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2009 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 107745161

• Final Report Year: 2018

Final Report Abstract

Auger electron spectroscopy (AES) has been proven to be a powerful tool for probing electronic and geometric structures of various systems, e.g., atoms, molecules, clusters and particularly solid/air interfaces. In our project we clearly demonstrated high utility of this technique for exploring also liquid water and aqueous solutions. We paid particular attention to the high kinetic energy shoulders and separate peaks in Auger spectra which are domains of delocalized states, i.e., states where two final vacancies reside on two different monomers. In liquid water and in many aqueous solutions these delocalized states are populated via intermolecular electronic decay processes, such as ICD and ETMD, and processes involving ultrafast proton transfer. In our project we determined the relative efficiencies of all these processes and the radiolitic yields of the created neutral and cationic radicals. An important finding of our work is that the double charge created after electronic and nuclear relaxation in X-ray irradiated liquid water, aqueous hydroxide and likely some others aqueous solutions tends to be distributed between different units rather than sit on the same monomer. Through studying aqueous systems with remarkably different hydrogen bonding interactions we were able to determine many mechanistic details of the above processes and connect these details with the experimentally measured spectral shapes. For the first time we identified unambiguously ETMD processes in aqueous media and proposed ETMD spectroscopy as a very efficient mean for exploring local hydration structure in aqueous solutions. Compared to Auger spectra, ETMD spectra have a number of advantages, in particular they are considerably more sensitive to ion pairing. As demonstrated in our studies, different ion pairing situations have characteristic features in ETMD spectra. Also for the first time we applied tender X-rays for studying aqueous solutions and discovered a new phenomenon – ultrafast electron transfer processes from solvent molecules accompanying KLL Auger decay in solvated ions. These represent an additional source of charge delocalization in aqueous media following deep core hole creation. Overall, the discovered sensitivity of the Auger and ETMD spectroscopies to the local solvation structure can be used in many applications. Apart from aqueous solutions, also systems with organic or hybrid solvents, for example, Li-ion batteries can potentially be explored with the proposed spectroscopic techniques. Our results are relevant for the chemistry at biological surfaces and at the electrode-solution interfaces. Since all the above-mentioned processes create radicals and often also low-energy electrons, they should be of prime interest for radiation chemistry and radiotherapy. The ultrafast charge delocalization addressed in our project will surely attract attention of researchers performing experiments on object imaging and nanoplasma formation in various X-ray irradiated systems.

Publications

• Proton-Transfer Mediated Enhancement of Nonlocal Electronic Relaxation Processes in X-Ray Irradiated Liquid Water. J. Am. Chem. Soc. 136, 18170 (2014)
  Slavíček P., Winter B., Cederbaum L.S., Kryzhevoi N.V.
  
• Microhydration of LiOH: Insight from Electronic Decays of Core-Ionized States. J. Chem. Phys. 144, 244302 (2016)
  Kryzhevoi N.V.
  
• Relaxation Processes in Aqueous Systems upon X-Ray Ionization: Entanglement of Electronic and Nuclear Dynamics. J. Phys. Chem. Lett. 7, 234 (2016)
  Slavíček P., Kryzhevoi N.V., Aziz E.F., Winter B.
  
• Observation of Electron-Transfer-Mediated Decay in Aqueous Solution, Nature Chemistry 9, 708 (2017)
  Unger I., Seidel R., Thürmer S., Pohl M.N., Aziz E.F., Cederbaum L.S., Muchová E., Slavíček P., Winter B., Kryzhevoi N.V.
  
• Ultrafast Charge Transfer Processes Accompanying KLL Auger Decay in Aqueous KCl Solution. Phys. Rev. Lett. 119, 263003 (2017)
  Céolin D., Kryzhevoi N.V., Nicolas Ch., Pokapanich W., Choksakulporn S., Songsiriritthigul P. Saisopa T., Rattanachai Y., Utsumi Y., Palaudoux J., Öhrwall G, Rueff J.P.