Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S26: Focus Session: Electron & Ion Solvation in Clusters & the Condensed Phase II |
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Sponsoring Units: DCP Chair: J. Matthias Weber, University of Colorado at Boulder Room: Colorado Convention Center 205 |
Wednesday, March 7, 2007 2:30PM - 3:06PM |
S26.00001: Spectroscopy and Dynamics of Excess Electrons in Clusters Invited Speaker: Clusters in which excess electrons are bound to solvent molecules can provide important links with electrons solvated in liquids, most notably the hydrated electron in aqueous solution. These considerations have motivated a series of studies in our group on the spectroscopy and dynamics of excess electrons in water and methanol clusters, which have been investigated using a combination of one-photon and time-resolved photoelectron imaging and infrared photodissociation spectroscopy. Salient results are as follows. (i) Both (H$_{2}$O)$_{n}^{-}$ and (CH$_{3}$OH)$_{n}^{-}$ show evidence for multiple isomers with very different vertical detachment energies, suggesting multiple electron binding motifs to these clusters. (ii) The time-resolved experiments yield direct measurements of excited state lifetimes in these clusters. Extrapolation to the infinite-size limit yields lifetimes of 50 fs for the hydrated electron and 150 fs for electrons dissolved in methanol. These ultrafast lifetimes are in good agreement with so-called non-adiabatic solvation models for bulk solvated electrons. (iii) Recent infrared spectroscopy experiments on (H$_{2}$O)$_{n}^{-}$ (n$\le $50) clusters obtained using a tunable free-electron laser have provided new insights into how the electron binding in these clusters evolves with size. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:42PM |
S26.00002: Theoretical Studies of Negatively Charged Water Clusters: The Role of Polarization and Dispersion for Electron Binding Invited Speaker: A quantum Drude oscillator model is used to characterize negatively charged water clusters as large as (H$_{2}$O)$_{24}^{-}$. The Drude model allows for inclusion of electron correlation effects between the excess electron and the electrons of the water molecules, at a fraction of the computational cost of all-electron \textit{ab initio} methods. Application of the Drude model to (H$_{2}$O)$_{6}^{-}$ demonstrates that there are many isomers with small electron binding energies that are more stable than the species with double acceptor water monomers that dominate under experimental conditions and that have electron binding energies near 0.45 eV. The talk will also explore the connection between the Drude model and more traditional polarization models used in describing the interaction of excess electrons with water. We show that a series of ``polarization'' models can be derived from the Drude model, by carrying out an adiabatic separation between the excess electron and the Drude degrees of freedom. It is found that the polarization and Drude models give similar electron binding energies for species in which the excess electron experiences large electrostatic attraction, but that the polarization models significantly overbind the excess electron in cases where the electrostatics play only a small role. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 4:18PM |
S26.00003: Excess Electrons in Water: Clusters, Interfaces, and the Bulk Invited Speaker: The presence of charged species at interfaces plays a central role in a wide range of physical processes. Heterogeneous electron transfer is among the most notable examples with implications in electrochemistry, atmospheric chemistry, heterogeneous catalysis, or from a more general viewpoint, in biological through-space electron transfer reactions. An excess electron in an aqueous environment may be considered as a useful model for studying key energetic, structural and dynamic aspects of these complex phenomena. Excess electrons are known to stabilize in bulk water, as hydrated electrons. Hydrated electron systems with reduced dimensionality, such as negatively charged, finite size water clusters, and excess electrons at aqueous interfaces of infinite size, have also been studied for a while. In the present work we will overview the results of a series of mixed quantum-classical molecular dynamics simulations aimed to examine the physical properties of various aqueous excess electron systems. The investigated systems include finite size water cluster anions, infinite ambient water/air, supercooled water/air, Ih ice/air, amorphous ice/air interfaces, and the fully hydrated electron. The discussion will focus on the critical issue whether the excess electron localizes in interior-bound states completely surrounded by water molecules, or on the water surface (interface) with significant electronic amplitude appearing outside the molecular frame (surface-bound states). Correlations of the excess electron state with the size, internal energy, and the local molecular structure of the environment will be illustrated. We will also demonstrate the dramatic influence of the excess electron state on the observable physical properties. The possible interconnections of the finite size cluster anions, the electrons at the infinite size water/air interfaces, and the three-dimensional, fully hydrated electron are also explored in comparison with available experimental data. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S26.00004: Optical Spectrum of the Hydrated Electron in Supercooled and Supercritical Water and Ice David Bartels, Erica Price, Yiqui Du Simulation of the hydrated electron optical spectrum has been the goal of a generation of researchers, and was apparently achieved within the last decade using a one-quantum-electron/pseudopotential/classical water MD modeling strategy. The temperature dependence of the spectrum (red shift at elevated temperature) was reported to be actually the effect of water bulk density. The red shift in simulation was linear in the inverse density. Spectra of the hydrated electron recorded in our laboratory in supercritical water strongly disagree with the simulation result, in that there is very little spectral change for a factor of six change in water density, from 0.1 to 0.6 g/cc at 375$^{\circ}$C. A new result presented here concerns the spectra in supercooled water, which can be compared with spectra in water at higher temperature at the same bulk density. In this comparison, density of the water very clearly does not determine the position of the absorption maximum---the temperature does. The one-quantum-electron/pseudopotential/classical water MD methodology clearly lacks some critical aspects of the real water-electron interaction. A comparison of the electron solvated in supercooled water or in ice at the same temperature shows virtually the same shape on the blue side, but a much narrower bandwidth on the red side in ice relative to water. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S26.00005: Evidence for the Formation and Solvation of the (Na$^{+}$,e\={ }) Complex Pairs in Tetrahydrofuran (THF) Molly Cavanagh, Ross Larsen, Benjamin Schwartz Using ultrafast spectroscopy, we monitor the spectral relaxation of the solvated sodium atom created following the ultrafast excitation of the sodium anion (sodide) charge-transfer-to-solvent band in THF. Immediately following excitation, a sodium atom that has the characteristic gas-phase 590-nm D-line absorption is formed. By untangling the overlapping spectral dynamics of the sodide bleach and solvated electron, we are able to cleanly elucidate the dynamics of the Na atom, whose absorption spectrum eventually shifts to $\sim $900 nm. We observe a fast, $\sim $300 fs solvation of the immediately formed gas-phase-like Na atom species followed by a chemical interconversion in $\sim $800 fs, as characterized by an isosbestic point, into a new species. The new species, which we assign as a (Na$^{+}$;e\={ }) contact pair, undergoes slow solvation in $\sim $10 ps to ultimately form the equilibrium 900-nm absorber. In combination, our data offers the most complete picture of the dynamics of the sodide CTTS reaction and its spectral intermediates. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S26.00006: Single and Double Excess Electrons in Water Clusters Ying Li, Robert Barnett, Uzi Landman Excess electrons in polar solvents is a topic of continuing interest. Early theoretical research on this subject predicted formation of surface and internal hydrated electron states, depending on the size of the water cluster and the state of the cluster [1]. Evidence for these modes of electron hydration has been reported in recent experiments. We discuss here theoretical investigations of excess electrons states in water clusters as a function of cluster size and state (liquid and frozen) using hybrid quantum (DFT)/classical simulations. In addition we discuss dielectron hydration in clusters [2]. [1] R. N. Barnett, C. L. Cleveland, U. Landman, J. Jortner, J. Chem. Phys. 88, 4429 (1988). [2] H.-P. Kaukonen, R. N. Barnett, U. Landman, J. Chem. Phys. 97, 1365 (1992). [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S26.00007: Charge-transfer reactions, energy gaps, and electron-transfer diabatic surfaces Nicola Marzari, P. H.-L. Sit Density-functional theory in the LDA or GGA approximation has become the widely-used standard model of condensed matter theory. I will discuss shortcomings and solutions to some of the problems that arise when addressing complex chemical reactions. These challenges include the correct description of electron-transfer processes, where electrons become delocalized and shared between ions that should be in different oxidation states. An effective solution can be obtained by introducing a penalty functional that imposes the correct charge state on the ions involved in the reaction [1]. This approach is validated in a model system, showing that the ground state and the charge-transfer excited state can be calculated with negligible errors, and then applied to the determination of the diabatic free-energy surfaces for ferrous and ferric ions in solution. [1] P. H.-L. Sit, Matteo Cococcioni and Nicola Marzari, Phys. Rev. Lett. 97, 028303 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S26.00008: Isomers and the correlation between excess electron binding and the local H-bonding motif in hydrated electron clusters. Mark Johnson We describe a series of experimental results that address the origin of the isomeric classes of negatively charged water clusters that differ according to their electron binding energies. The molecular structure of the local electron binding site is revealed through the isomer-specific vibrational spectra in the intramolecular HOH bending and OH stretching regions for both H and D isotopomers. Isomer selection is accomplished with a photochemical population modulation scheme in which low electron binding isomers are sequentially and systematically removed from the mixed isomer ensembles created in free jet ion sources. The class of clusters (type I) that most strongly binds an excess electron exhibits a characteristic red-shifted band in the bending region that is assigned, based on the behavior of very small clusters, to electron attachment to a single water molecule held to the supporting network by a double H-bond acceptor (AA) motif. Isomers that bind the electron more weakly do not display this spectral signature, indicating that local H-bonding topology is a significant factor in controlling the overall work functions of the clusters. Isomer interconversion and growth mechanisms will also be addressed using Ar-mediated incorporation of hetero-isotopes and surface electron scavenging by reactive charge-transfer collisions. [Preview Abstract] |
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