Interaction of long-living solvated electrons with adsorbates at the surface of ice
Zusammenfassung der Projektergebnisse
Water ice possesses the fascinating ability to solvate ions and electrons. At the ice surface solvated electrons facilitate electron-induced reactions via dissociative electron attachment. As compared to the gas phase the cross section of such processes is enhanced for molecules adsorbed at the ice surface and the reaction takes place at lower electron energy. In the reported project we investigated physical mechanisms at work and developed a molecular-scale picture of the underlying electron states and the interaction of the adsorbate with the electron solvation sites at the ice surface. As a model system we focused on ice structures grown on Cu(111) and halogenated benzenes C6 H5 X with X=F, Cl, or Br – i.e. a series of halogens with decreasing electron affinity. The topic was investigated by combining highresolution scanning tunneling microscopy (STM), two photon photoemission spectroscopy (2PPE), as well as theoretical modeling based on density functional theory and many body perturbation theory. Amorphous and crystalline ice structures of the so called ridge-type were grown on Cu(111) and characterized by high-resolution STM. On both types of structures submonolayer amounts of admolecules were adsorbed. In-depth structural analysis combined with theoretical modeling enabled to identify and understand structural models based on parallel hexagon adrows that are decorated with admolecules at their edges and connected by admolecule structures. Insight into the features of electron traps was obtained by modeling orientational and vacancy-related defects on the bilayerterminated surface. Strong electron traps are characterized by an excess of dangling OH-groups. Calculations showed that decorated adrows possess extended electron trap states, such that localized solvated electrons are expected on the admolecule covered ice structures at defects at adrows. STM experiments recorded images before and after illumination that populated such excess electron states. On crystalline ice structures collective changes of pairs or even rows of admolecules occurred while on amorphous ice single admolecules changed position. Electron-induced reactions of C6 H5 X on ice structures were investigated with 2PPE via work function changes. The evolution of these changes with time could be separated into two components, one specific for the adsorbed molecular species and another one that is attributed to changes at unspecific defect sites. The former component arises from the electron transfer to a transient ion [C6 H5 X]∗− with a finite lifetime and/or a dissociated species •[C6 H5 ] + X− . Analysis of the photon flux and energy dependence of the work function changes showed that C6 H5 Cl and C6 H5 Br dissociate while C6 H5 F remains undissociated reflecting similar results from gas phase studies. In STM experiments C6 H5 Cl and C6 H5 Br were adsorbed on amorphous and crystalline ice structures and illuminationinduced changes were identified as dissociation events. Stability of the reaction products against tip-manipulation indicates poisoning of reaction sites. This is corroborated by the specific evolution of work function changes. Theoretical modeling of C6 H5 X at prototypical electron traps on the bilayer termination unraveled the underlying mechanisms. The calculations show that C6 H5 X as an adsorbate prefers interaction via the halogen with dangling OH-groups at the surface or in vacancies. The adsorption energy is largest at sites with large electron affinity. According to our calculations, dissociative electron attachment to C6 H5 X in the gas phase or at the ice surface takes place via anti-bonding π*s, π*a and σ*C-X scattering resonances. Adsorption at the ice surface induces a shift of these resonances to lower energy which correlates with the electron affinity of the site. The insertion of an excess electron into σ*C-X resonance leads to barrierless dissociation in all cases. The occupation of the lower-lying π*s, π*a resonances creates a transient ion that may dissociate in a thermally activated process via conical intersections that occur between π*s, π*a, and σ*C-X energy surfaces as a function of the C-X bond distance. Compared to the gas phase, the DEA at the ice surface is facilitated in the following way: (i) the adsorption at the bilayer-terminated surface reduces the activation energy considerably and (ii) the energy of π*s, π*a resonances is further lowered at electron traps which mark the threshold energy for the excess electron. Calculated activation and threshold energies show a clear trend regarding the halogen, in particular for C6 H5 F the activation energy is not compatible with the available thermal energy. These and our experimental findings suggest that the same mechanisms are at work on crystalline and amorphous ice structures.
Projektbezogene Publikationen (Auswahl)
- Molecular-Scale Imaging of Water Near Charged Surfaces, ChemElectroChem 2, 431 (2014)
Michael Mehlhorn, Sebastian Schnur, Axel Groß, Karina Morgenstern
(Siehe online unter https://doi.org/10.1002/celc.201300063) - Using the first steps of hydration for the determination of molecular conformation of a single molecule, J. Am. Chem. Soc. 136, 13341 (2014)
Jörg Henzl, Konrad Boom, Karina Morgenstern
(Siehe online unter https://doi.org/10.1021/ja506762t) - Influence of water on supra-molecular assembly of 4,4’-dihydroxy azobenzene on Ag(111), J. Chem. Phys. 142, 101920 – 1-5 (2015)
Jörg Henzl, Konrad Boom, Karina Morgenstern
(Siehe online unter https://doi.org/10.1063/1.4907368) - Ultrafast electron dynamics at water covered, alkali adatoms adsorbed on Cu(111), Phys. Chem. Chem. Phys. 17, 8441 (2015)
Michael Meyer, Ishita Agarwal, Martin Wolf, Uwe Bovensiepen
(Siehe online unter https://doi.org/10.1039/c4cp05356g) - Admolecule Structures at the surface of ice, NIC Symposium 2014, Eds. K. Binder, G. Münster, and M. Kremer, Neuman Institute of Computing (Jülich, 2014); p 259
Michel Bockstedte, Anja Michl, Manuel Kolb
- Incomplete Bilayer Termination of the Ice (0001) Surface, J. Phys. Chem. C 120, 1097 (2016)
Michel Bockstedte, Anja Michl, Manuel Kolb, Michael Mehlhorn, Karina Morgenstern
(Siehe online unter https://doi.org/10.1021/acs.jpcc.5b10836) - The fractal dimension of ice on the nanoscale, Chem. Phys. Lett. 665, 1 (2016)
Sarah-Charlotta Heidorn, Cord Bertram, Karina Morgenstern
(Siehe online unter https://doi.org/10.1016/j.cplett.2016.09.064)
