Published: July 14, 2011 r2011 American Chemical Society 13489 dx.doi.org/10.1021/ja204100j | J. Am. Chem. Soc. 2011, 133, 13489–13495 ARTICLE pubs.acs.org/JACS Cations Strongly Reduce Electron-Hopping Rates in Aqueous Solutions Niklas Ottosson,* ,† Michael Odelius,* ,‡ Daniel Sp  angberg,* ,§ Wandared Pokapanich, † Mattias Svanqvist, ^ Gunnar € Ohrwall, || Bernd Winter, # and Olle Bj€ orneholm † † Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden ‡ Fysikum, Albanova University Center, Stockholm University, SE-106 91 Stockholm, Sweden § Department of Materials Chemistry, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden ^ Fakult € at f€ ur Physik, Universit € at Freiburg, Stefan-Meier Strasse 19, D-79104 Freiburg, Germany ) MAX-lab, Lund University, Box 118, SE-221 00 Lund, Sweden # Helmholtz-Zentrum Berlin f€ ur Materialien und Energie, and BESSY, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany b S Supporting Information 1. INTRODUCTION Charge-transfer phenomena in aqueous environments are essential for numerous chemical and biochemical as well as technological processes and applications. 1 While charge-transfer rates in the electronic ground state of most redox systems are relatively well described by standard Marcus theory, 2 there lies great interest in improving models for charge transfer between neighboring sites initiated by photoinduced electronic transi- tions in solution. For example, such processes play a key role in the function of electrolyte-based dye-sensitized solar cells. 3 In most cases, the solvent is considered to only passively assist such processes, e.g., by enabling the redox couple to interact in favorable geometries, and is therefore not really involved in the charge transfer per se. Nevertheless, there exist several charge- transfer phenomena where the solvent molecules play a much more active role, such as charge-transfer-to-solvent (CTTS) absorption processes 4 and solvent-mediated proton-transfer reactions, which are ubiquitous in aqueous environments. 5 One important mechanism for charge transfer is electron- hopping, which for liquid water has recently been reported to extend down to attosecond time scales upon resonant excitation of a core electron into unoccupied orbitals. 6 The ultrafast time scale makes these processes extremely challenging to study experimentally directly in the time domain, and an attractive alternative is to use the short lifetime associated with the decay of the O 1s core hole (∼3.6 fs) itself to indirectly probe the concurrent ultrafast electron-hopping. The dynamical informa- tion can be accessed by analysis of the different channels in the associated Auger electron spectra, which reflect whether the excited electron has left the initial excitation site during the core-hole lifetime. 7 The principle is schematically illustrated in Figure 1, showing the relevant electronic decay processes of core- ionized/core-excited molecules. On a microscopic scale, the rate of electron-hopping depends on both the spatial and energetic overlap between the orbital into which the electron has been excited and the empty orbitals of the neighboring molecules. Due to the locally disordered nature of liquids, this is not as easily defined as in crystalline systems, for which hopping rates can be connected to bandwidth. Locally, however, the hopping rates in liquid water are determined by the same basic mechanisms. 8 Received: May 4, 2011 ABSTRACT: We study how the ultrafast intermolecular hopping of electrons excited from the water O1s core level into unoccupied orbitals depends on the local molecular environment in liquid water. Our probe is the resonant Auger decay of the water O1s core hole (lifetime ∼3.6 fs), by which we show that the electron- hopping rate can be significantly reduced when a first-shell water molecule is replaced by an atomic ion. Decays resulting from excitations at the O1s post-edge feature (∼540 eV) of 6 m LiBr and 3 m MgBr 2 aqueous solutions reveal electron- hopping times of ∼1.5 and 1.9 fs, respectively; the latter represents a 4-fold increase compared to the corresponding value in neat water. The slower electron-hopping in electrolytes, which shows a strong dependence on the charge of the cations, can be explained by ion-induced reduction of waterwater orbital mixing. Density functional theory electronic structure calculations of solvation geometries obtained from molecular dynamics simulations reveal that this phenomenon largely arises from electrostatic perturbations of the solvating water molecules by the solvated ions. Our results demonstrate that it is possible to deliberately manipulate the rate of charge transfer via electron-hopping in aqueous media.