Dynamics and Reactivity of Trapped Electrons on Supported Ice Crystallites

Abstract

The solvation dynamics and reactivity of localized excess electrons in aqueous environments have attracted great attention in many areas of physics, chemistry, and biology. This manifold attraction results from the importance of water as a solvent in nature as well as from the key role of low-energy electrons in many chemical reactions. One prominent example is the electron-induced dissociation of chlorofluorocarbons (CFCs). Low-energy electrons are also critical in the radiation chemistry that occurs in nuclear reactors. Excess electrons in an aqueous environment are localized and stabilized by the local rearrangement of the surrounding water dipoles. Such solvated or hydrated electrons are known to play an important role in systems such as biochemical reactions and atmospheric chemistry. Despite numerous studies over many years, little is known about the microscopic details of these electron-induced chemical processes, and interest in the fundamental processes involved in the reactivity of trapped electrons continues. In this Account, we present a surface science study of the dynamics and reactivity of such localized low-energy electrons at D(2)O crystallites that are supported by a Ru(001) single crystal metal surface. This approach enables us to investigate the generation and relaxation dynamics as well as dissociative electron attachment (DEA) reaction of excess electrons under well-defined conditions. They are generated by photoexcitation in the metal template and transferred to trapping sites at the vacuum interface of crystalline D(2)O islands. In these traps, the electrons are effectively decoupled from the electronic states of the metal template, leading to extraordinarily long excited state lifetimes on the order of minutes. Using these long-lived, low-energy electrons, we study the DEA to CFCl(3) that is coadsorbed at very low concentrations (∼10(12) cm(-2)). Using rate equations and direct measurement of the change of surface dipole moment, we estimated the electron surface density for DEA, yielding cross sections that are orders of magnitude higher than the electron density measured in the gas phase.

Coherent Electron Transfer at the Ag/Graphite Heterojunction Interface

“…The coherent charge transfer, which is completed by dephasing on ~10 fs time scale, is expected to be faster and more efficient than incoherent processes because it precedes electron energy relaxation. By contrast, charge transfer by internal photoemission from the photoexcited hot electron distribution occurs in competition with energy relaxation, and processes that generally belong to the Marcus theory can only be described when a local equilibrium exists [58]. For example, for Au particle, PbSe nanocrystal, and activated TiO 2 surfaces hot electron injection has been reported on <10 fs to hundreds of picoseconds time scales [8,13,16,27,33,59,60], with the fastest processes most likely being coherent.…”

Cosmic-Ray-Driven Reaction and Greenhouse Effect of Halogenated Molecules: Culprits for Atmospheric Ozone Depletion and Global Climate Change

“…This finding has revived the studies of electron-induced reactions of halogenated molecules including CFCs, particularly in polar media. This is evidenced by the observations first in ESD experiments 9,10,[15][16][17] , then in electron trapping experiments by Lu and Sanche [18][19][20] , and more recently in femtosecond time-resolved laser spectroscopic measurements in polar liquids 28,29,32,33 or on ice surfaces 21,24,26 . Interestingly, a very large DET cross section up to 4x10 12 cm 2 for CFCl 3 on D 2 O ice, measured most recently by Stähler et al 26 , is comparable to the values of ~1x10 14 and ~6x10 12 cm 2 for CF 2 Cl 2 adsorbed on H 2 O and NH 3 ice respectively, originally measured by Lu and Madey 9 .…”

Multistep and multiscale electron transfer and localization dynamics at a model electrolyte/metal interface

“…31,40,41 In molecular films with strong screening this can lead to states with lifetimes on the order of seconds to minutes, as observed in crystalline ice, crystalline ammonia, and amorphous solid water. 31,[42][43][44][45] The extremely long lifetimes of these normally unoccupied electronic states is sufficient for chemical reactivity both within molecular layers as well as with molecules exposed from the gas-phase. [43][44][45] In DMSO in electrochemical cells, transient interactions of the first few layers of solvent molecules due to changing electrode potentials can modify interface electronic states, their lifetimes, and coupling with reactants on ultrafast timescales, strongly determining the cell's functionality.…”

“…31,[42][43][44][45] The extremely long lifetimes of these normally unoccupied electronic states is sufficient for chemical reactivity both within molecular layers as well as with molecules exposed from the gas-phase. [43][44][45] In DMSO in electrochemical cells, transient interactions of the first few layers of solvent molecules due to changing electrode potentials can modify interface electronic states, their lifetimes, and coupling with reactants on ultrafast timescales, strongly determining the cell's functionality. 15,18 Therefore, for a complete description of the role of DMSO in electrochemical and battery systems it is important to understand the localization, coupling, and lifetime of DMSO electronic states near metal electrodes, and how DMSO electronic states near the interface could be precursors for long-lived and chemically reactive electronic states.…”