Cluster models for solvation studied by core ionization
Final Report Abstract
Core level spectroscopy is element specific. Electronic and geometric structure alterations occurring in a system due to changes in its local environment are usually well reflected in the corresponding core level spectra. In our project we exploited core level spectroscopy to elucidate various aspects of the fundamental solvation phenomenon. In particular, we considered different microsolvated clusters to gain more insight into microscopic details of solvation. One of the most unexpected and important results of our project is the discovery that core level spectroscopy is sensitive to the binding motif of the excess electron(s) with the respective cationic cores of solvated metals. Remarkably, this is the satellite structure in the core level spectra rather than the chemical shift of the main line which indicates whether the excess electrons and the cations form contact or solvent separated ion-pairs. In our study we considered Li−(NH3 )n clusters where the transition from the contact ion-pair to a solvent separated one occurs already for n=4. Such small clusters are amenable for our computational approaches. In the clusters with n=1...3, local excitations within the anion and non-local excitations to charge-transfer-to-solvent states were identified which form a very complex satellite structure. The spectrum of the tetraammoniated lithium anion is contrastingly very simple. Its characteristic feature is the single very intense satellite peak which is attributed to an excitation of a solvated electron. In our study we explored close-shell systems only. We hope however that similar trends can be found in open-shell systems as well, in particular in microsolvated clusters of alkali metal atoms which are often used as model systems for studying solvated electrons in bulk solutions. Besides hydrogen bonded clusters we also studied systems embedded in rare-gas matrices and helium droplets. Here, we were especially interested in elucidating energy and charge transfer processes following inner-valence ionization. We have demonstrated that in systems containing heavy elements, the correct prediction of the relaxation pathways requires application of fully relativistic approaches. In the exemplary mixed krypton/xenon clusters considered in our work, the onset for double ionization was predicted to lie above the inner-valence single ionization potential prohibiting any electronic decay, if a non-relativistic theory is applied. By taking into account relativistic effects, in particular spin-orbit interaction, the double ionization threshold shifts to lower energies making electron transfer mediated decay possible. The latter is not only much faster than radiative decay but also leads to completely different fragmentation dynamics in clusters. Inner-valence ionized dopants in helium droplets can undergo interatomic Coulombic decay. We showed that the ICD processes of differently polarized inner-valence vacancies, like the 3p ones in Ca, have different characteristics. We proposed to exploit this fact to probe the interfacial layer in isotopically mixed droplets doped with Ca. Since Ca occupies the unique position in the 4 He/3 He interface, by ionizing the Ca 3p orbitals directed parallel and perpendicular to the droplet surface and then measuring distinct ICD signals, valuable information on the structure and properties of this interface can be obtained. Double core hole states are more sensitive to the local chemical environment than single core hole states, especially those where two holes are created on two different atomic sites in a system. In our project we continued explorations of various characteristics of the former states. In particular, we performed a systematic study to elucidate how the interatomic relaxation energy depends on the positions of two core holes relative to a polarazable unit present in the system and on the polarizability of this unit.
Publications
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Polarization and site dependence of interatomic relaxation effects in double core hole states. J. Phys. B: At. Mol. Opt. Phys. 46, 164012, (2013)
Tashiro M., Kryzhevoi N.V., Cederbaum L.S. Ehara M.
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Probing the interface of doped isotopically mixed helium droplets by the directional anisotropy of interatomic Coulombic decay. Phys. Chem. Chem. Phys. 15, 18167–18173, (2013)
Kryzhevoi N.V., Mateo D., Pi M., Barranco M., Cederbaum L.S.
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Communication: Electron transfer mediated decay enabled by spin-orbit interaction in small krypton/xenon clusters. J. Chem. Phys. 140, 161103, (2014)
Zobel J.P., Kryzhevoi N.V., Pernpointner M.
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Tracing electron solvation in Li- (NH3)n clusters with K-shell photodetachment spectroscopy. Chem. Phys. Lett., Volume 626, 17 April 2015, Pages 85-89
Kryzhevoi N.V., Tarantelli F., Cederbaum L.S.
