Ultrafast charge transfer processes accompanying KLL Auger decay in aqueous KCl solution

X-ray photoelectron spectroscopy (XPS) and KLL Auger spectra of aqueous KCl solution were measured for the K$^+$ and Cl$^-$ edges. While the XPS spectra of potassium and chloride have similar structures, both exhibiting only weak satellite structures near the main line, the Auger spectra of these isoelectronic ions differ dramatically. A very strong satellite peak was found in the K$^+$ KLL Auger spectrum at the low kinetic energy side of the $^1$D state. Using equivalent core models and ab initio calculations this spectral structure was assigned to electron transfer processes from solvent water molecules to the solvated K$^+$ cation. Contrary to the potassium case, no extra peak was found in the KLL Auger spectrum of solvated Cl$^-$ indicating on a strong dependence of the underlying processes on ionic charge. The observed charge transfer processes are suggested to play an important role in charge redistribution following single and multiple core-hole creation in atomic and molecular systems placed into an environment

Observation of electron transfer mediated decay in aqueous solution

Photoionization is at the heart of X ray photoelectron spectroscopy XPS , which gives access to important information on a sample s local chemical environment. Local and non local electronic decay after photoionization in which the refilling of core holes results in electron emission from either the initially ionized species or a neighbour, respectively have been well studied. However, electron transfer mediated decay ETMD , which involves the refilling of a core hole by an electron from a neighbouring species, has not yet been observed in condensed phase. Here we report the experimental observation of ETMD in an aqueous LiCl solution by detecting characteristic secondary low energy electrons using liquid microjet soft XPS. Experimental results are interpreted using molecular dynamics and high level ab initio calculations. We show that both solvent molecules and counterions participate in the ETMD processes, and different ion associations have distinctive spectral fingerprints. Furthermore, ETMD spectra are sensitive to coordination numbers, ion solvent distances and solvent arrangemen

Charge Dependence of Solvent-Mediated Intermolecular Coster-Kronig Decay Dynamics of Aqueous Ions.

The 2s and 2p photoelectron spectra have been measured for Na(+), Mg(2+), and Al(3+) ions in aqueous solution. In all cases, the 2s lines are significantly broader than the 2p features, which is attributed to a shorter lifetime of the respective 2s hole. Since intraionic Coster-Kronig decay channels from the (2s)(-1) state are closed for free Na(+), Mg(2+), and Al(3+) ions, this is evidence for an intermolecular Coster-Kronig-like process, reminiscent of intermolecular Coulombic decay (ICD), involving neighboring water solvent molecules. The observed 2s Lorentzian line widths correspond to lifetimes of the (2s)(-1) state of 3.1, 1.5, and 0.98 fs for the solvated Na, Mg, and Al ions, respectively

Charge delocalization dynamics of ammonia in different hydrogen bonding environments: free clusters and in liquid water solution

Valence and core level photoelectron spectra and Auger electron spectra of ammonia in pure clusters have been measured. The Auger electron spectra of gas-phase ammonia, pure ammonia clusters and ammonia in aqueous solution are compared and interpreted via ab initio calculations of the Auger spectrum of the ammonia monomer and dimer. The calculations reveal that the final two-hole valence states can be delocalized over both ammonia molecules. Features at energies pertaining to delocalized states involving one, or more, hydrogen bonding orbitals can be found in both the ammonia cluster Auger electron spectrum and in that of the liquid solvated molecule. The lower Coulombic repulsion between two delocalized valence final state holes gives higher kinetic energy of the Auger electrons which is also observed in the spectra. This decay path-specific to the condensed phase-is responsible for more than 5% of the total cluster Auger intensity. Moreover, this interpretation is also applicable to the solid phase since the same features have been observed, but not assigned, in the Auger spectrum of solid ammonia

Bond Breaking, Electron Pushing, and Proton Pulling Active and Passive Roles in the Interaction between Aqueous Ions and Water as Manifested in the O 1s Auger Decay

A core ionized H2O molecule in liquid water primarily relaxes through normal Auger decay, leading to a two hole final state in which both valence holes are localized on the same water molecule. Electronic coupling to the environment, however, allows for alternative decays resembling Intermolecular Coulombic Decay ICD , producing final states with one of the holes delocalized on a neighboring water molecule. Here we present an experimental study of such minority processes, which adds to our understanding of dynamic interactions of electronically excited H2O molecules with their local surrounding in liquid water and aqueous solution. We show that the solvation of metal halide salts considerably influences these minority decay channels from the water O 1s 1 state. By breaking water water bonds, both the metal cations and halide anions are found to reduce the decay into water water delocalized states, thus having a amp; 8243;passive amp; 8243; effect on the Auger spectrum. The halide anions also play an amp; 8243;active amp; 8243; role by opening a new ICD like decay pathway into water halide delocalized states. The importance of this contribution increases from F to I , which we suggest to be caused by a directional polarization of the halide anion toward the core ionized H2O cation in the intermediate state of the Auger process. This increases the electronic overlap between the two centers and makes delocalized decays more probable. We furthermore show that F , the smallest and most strongly hydrated of the halides, plays an additional role as proton puller during the core hole lifetime, resulting in proton dynamics on the low femtosecond time scale. Our results represent a step forward toward a better understanding of how aqueous solutions, when exposed to soft X rays, channel excess energy. This has implications for several aspects of physical and radiation chemistry, as well as biolog

Adsorption of polar molecules on krypton clusters

The formation process of binary clusters has been studied using synchrotron based core level photoelectron spectroscopy. Free neutral krypton clusters have been produced by adiabatic expansion and doped with chloromethane molecules using the pickup technique. The comparison between the integrated intensities, linewidths, and level shifts of the cluster features of pure krypton and of chloromethane-krypton clusters has been used to obtain information about the cluster geometry. We have shown that most of the chloromethane molecules remain on the surface of the clusters

Cations Strongly Reduce Electron Hopping Rates in Aqueous Solutions

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 similar to 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 (similar to 540 eV) of 6 m LiBr and 3 m MgBr2 aqueous solutions reveal electron-hopping times of similar to 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

Large variations in the propensity of aqueous oxychlorine anions for the solution vapor interface

Core-level photoelectron spectroscopy measurements have been performed of aqueous solutions of NaCl codissolved with NaClOn (n=1-4). Each species has a distinct Cl 2p electron binding energy, which can be exploited for depth-profiling experiments to study the competition between Cl-and ClOn- anions for residing in the outermost layers of the solution/vapor interface. Strongest propensity for the surface is observed for n=4 (perchlorate), followed by n=3 (chlorate), n=2 (chlorite), n=0 (chloride), and n=1 (hypochlorite). Molecular dynamics simulations rationalize the greatest surface propensity of the most oxidized anions in terms of their larger size and polarizability. The anomalous behavior of hypochlorite, being less surface-active than chloride, although it is both larger and more polarizable, is suggested to arise from the charge asymmetry over the anion, increasing its efficiency for bulk solvation. (C) 2009 American Institute of Physics. [doi:10.1063/1.3236805

The influence of concentration on the molecular surface structure of simple and mixed aqueous electrolytes

We investigate various mechanisms contributing to the surface ion distributions in simple and mixed aqueous alkali-halide solutions depending on the total salt concentration, using a combination of photoelectron spectroscopy and molecular dynamics simulations. In simple solutions, the surface enhancement of large polarizable anions is reduced with increasing concentration. In the case of a NaBr/NaCl mixed aqueous solution, with bromide as the minority component, the situation is more complex. While the total anion/cation charge separation is similarly reduced with increasing salt content, this alone does not uniquely determine the ion distribution due to the co-existence of two different anions, Br- and Cl-. We show that bromide is selectively surface enhanced at higher concentrations, despite the fact that the total anion surface enhancement is reduced. This phenomenon, which can be viewed as "salting out'' of bromide by NaCl might have consequences for our understanding of the surface structure of mixed aqueous solutions subjected to concentration increase due to dehydration, such as seawater-born aerosols

Self-assembled heterogeneous argon/neon core-shell clusters studied by photoelectron spectroscopy

Clusters formed by a coexpansion process of argon and neon have been studied using synchrotron radiation. Electrons from interatomic Coulombic decay as well as ultraviolet and x-ray photoelectron spectroscopy were used to determine the heterogeneous nature of the clusters and the cluster structure. Binary clusters of argon and neon produced by coexpansion are shown to exhibit a core-shell structure placing argon in the core and neon in the outer shells. Furthermore, the authors show that 2 ML of neon on the argon core is sufficient for neon valence band formation resembling the neon solid. For 1 ML of neon the authors observe a bandwidth narrowing to about half of the bulk value. (c) 2007 American Institute of Physics