Localization of excess electrons in cubic NanClm clusters.

The localization and solvation of excess electrons in pure water have been resolved at the femtosecond time scale. Before it becomes solvated, the electron thermalizes and reaches in 110 fs a localized state absorbing in the infrared. This transient species with lifetime 240 fs has been postulated to exist but has not been observed previously in liquid water.

In view of the important implications of excess electrons (EEs) interacting with CO2-H2O clusters in many fields, using ab initio molecular dynamics simulation technique, we reveal the structures and dynamics of an EE associated with its localization and subsequent time evolution in heterogeneous CO2-H2O mixed media. Our results indicate that although hydration can increase the electron-binding ability of a CO2 molecule, it only plays an assisting role. Instead, it is the bending vibrations that play the major role in localizing the EE. Due to enhanced attraction of CO2, an EE can stably reside in the empty, low-lying π(*) orbital of a CO2 molecule via a localization process arising from its initial binding state. The localization is completed within a few tens of femtoseconds. After EE trapping, the ∠OCO angle of the core CO2 (-) oscillates in the range of 127°∼142°, with an oscillation period of about 48 fs. The corresponding vertical detachment energy of the EE is about 4.0 eV, which indicates extreme stability of such a CO2-bound solvated EE in [CO2(H2O)n](-) systems. Interestingly, hydration occurs not only on the O atoms of the core CO2 (-) through formation of O⋯H-O H-bond(s), but also on the C atom, through formation of a C⋯H-O H-bond. In the latter binding mode, the EE cloud exhibits considerable penetration to the solvent water molecules, and its IR characteristic peak is relatively red-shifted compared with the former. Hydration on the C site can increase the EE distribution at the C atom and thus reduce the C⋯H distance in the C⋯H-O H-bonds, and vice versa. The number of water molecules associated with the CO2 (-) anion in the first hydration shell is about 4∼7. No dimer-core (C2O4 (-)) and core-switching were observed in the double CO2 aqueous media. This work provides molecular dynamics insights into the localization and time evolution dynamics of an EE in heterogeneous CO2-H2O media.

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.

We performed a series of comparative quantum chemical calculations on various size negatively charged methanol clusters, CH3OHn-. The clusters are examined in their optimized geometries (n = 2-4), and in geometries taken from mixed quantum-classical molecular dynamics simulations at finite temperature (n = 2-128). These latter structures model potential electron binding sites in methanol clusters and in bulk methanol. In particular, we compute the vertical detachment energy (VDE) of an excess electron from increasing size methanol cluster anions using quantum chemical computations at various levels of theory including a one-electron pseudopotential model, several density functional theory (DFT) based methods, MP2 and coupled-cluster CCSD(T) calculations. The results suggest that at least four methanol molecules are needed to bind an excess electron on a hydrogen bonded methanol chain in a dipole bound state. Larger methanol clusters are able to form stronger interactions with an excess electron. The two simulated excess electron binding motifs in methanol clusters, interior and surface states, correlate well with distinct, experimentally found VDE tendencies with size. Interior states in a solvent cavity are stabilized significantly stronger than electron states on cluster surfaces. Although we find that all the examined quantum chemistry methods more or less overestimate the strength of the experimental excess electron stabilization, MP2, LC-BLYP, and BHandHLYP methods with diffuse basis sets provide a significantly better estimate of the VDE than traditional DFT methods (BLYP, B3LYP, X3LYP, PBE0). A comparison to the better performing many electron methods indicates that the examined one-electron pseudopotential can be reasonably used in simulations for systems of larger size.

Ring polymer molecular dynamics (RPMD) is used to directly simulate the injection and relaxation of excess electrons into supercritical helium fluid and ambient liquid water. A method for modulating the initial energy of the excess electron in the RPMD model is presented and used to study both low-energy (cold) and high-energy (hot) electron injections. For cold injection into both solvents, the RPMD model recovers electronically adiabatic dynamics with the excess electron in its ground state, whereas for hot electron injection, the model predicts slower relaxation dynamics associated with electronic transitions between solvent cavities. The analysis of solvent dynamics during electron localization reveals the formation of an outgoing solvent compression wave in helium that travels for over 2 nm and the delayed formation of water solvation shells on the timescale of 300 fs. Various system-size effects that are intrinsic to the simulation of excess electron injection are discussed. Comparison of the RPMD simulations with previous mixed quantum-classical dynamics simulations finds general agreement for both the mechanisms and timescales for electron localization, although the electron localization dynamics in the RPMD model is essentially completed within 400 fs in helium and 150 fs in water.

In terms of electronic structure change, surface anion vacancies in II–VI semiconductors could promote CO generation rates, which have been confirmed as active sites for photocatalytic CO2 reduction.

A cavity model has been developed for the localization of excess electrons in nonpolar media. The assumption of a continuum gives the analytical expression for the potential depth of a cavity, which is mainly determined by the short-range repulsive interaction between an excess electron and medium molecules. The shape of the cavity potential can be approximated adequately by a finite spherical well deep enough for the electron localization in alkane glassy matrices. Based on the present model, the broad optical absorption spectra due to the localized electrons in 3-methylpentane glassy matrix are attributed to bound-free transitions. The shape and the intensity of the electron spectra observed at 77 and 4.2 K can be reproduced by the theoretical spectra obtained by solving rigorously the spherical well problem. The energy of quasifree electrons, the total energy for the electron localization, and the cavity radius estimated from the simulation of the absorption spectra are in reasonable agreement with the available experimental results.

In this paper an attempt will be made to review the current state of the art in our understanding of excess electron states in non polar and polar liquids, with an emphasis on the following problems: The nature of quasi free and localized excess electron states. Localization criteria. Energetic stability of the localized excess electron state. Nature of electron solvent interactions. Configurational charges in the medium. A brief survey of excess electron states in non polar solvents such as He, Ar, Kr, Xe, H2, and D2 will focus attention on the general stability problem of an excess electron in a non polar dense fluid, configurational diagrams in ground and excited states, optical properties and non radiative relaxation processes. The basic problems involved are identical with those encountered in the study of excess electron states in polar solvents.Excess electron states in polar solvents will be handled by the application of continuum and molecular models. Information will be presented concerning the quasi free excess electron state in polar solvents, the electronic and medium rearrangement energy for the cavity model, the configurational stability of ground and excited state, configurational diagrams, optical properties, optical lineshapes and relaxation phenomena.

Graphene is one of the most promising materials for lithium-ion battery anodes due to its superior electronic conductivity, high surface area for lithium intercalation, fast ionic diffusivity and enhanced specific capacity. A reliable description of many battery processes requires an explicit description of electrochemical interactions involving electrons. A detailed atomistic modeling of electronic conduction and non-zero voltage simulations of graphitic materials require the inclusion of an explicit electronic degree of freedom. To enable large length- and time-scale simulations of electron conduction in graphitic anodes, we developed an eReaxFF force field concept describing graphitic materials with an explicit electron. The newly developed force field, verified against quantum chemistry-based data describing, amongst others, electron affinities and equation of states, reproduces the qualitative behavior of electron conductivity in pristine and imperfect graphitic materials at different applied temperatures and voltages. In addition, excess electron localization near a defect site estimated from eReaxFF simulations agree quite well with the corresponding density functional theory calculations. Our eReaxFF simulations show the initiation of lithium-metal-plating driven by electron transfer from the graphene surface to the exposed lithium ions demonstrating the method’s potential for studying lithium-graphene interactions with explicit electrons and explain many unresolved electrode and electrode-electrolyte interface processes.

We present the photoelectron spectra of negatively charged cesium iodide nanocrystals recorded using 2.540 eV photons. The species examined were produced using an inert gas condensation cluster ion source, and they ranged in size from (CsI)−n=13 to nanocrystal anions comprised of 330 atoms. Nanocrystals showing two distinct types of photoemission behavior were observed. For (CsI)−n=13 and (CsI)−n=36−165, a plot of cluster anion photodetachment threshold energies vs n−1/3 gives a straight line extrapolating (at n−1/3=0, i.e., n=∞) to 2.2 eV, the photoelectric threshold energy for F centers in bulk cesium iodide. The linear extrapolation of the cluster anion data to the corresponding bulk property implies that the electron localization in these gas-phase nanocrystals is qualitatively similar to that of F centers in extended alkali halide crystals. These negatively charged cesium iodide nanocrystals are thus shown to support embryonic forms of F centers, which mature with increasing cluster size toward condensed phase impurity centers. Under an alternative set of source conditions, nanocrystals were produced which showed significantly lower photodetachment thresholds than the aforementioned F-center cluster anions. For these species, containing 83–131 atoms, a plot of their cluster anion photodetachment threshold energies versus n−1/3 gives a straight line which extrapolates to 1.4 eV. This value is in accord with the expected photoelectric threshold energy for F′ centers in bulk cesium iodide, i.e., color centers with two excess electrons in a single defect site. These nanocrystals are interpreted to be the embryonic F′-center containing species, Cs(CsI)−n=41−65.

In this paper we present a theoretical study of the structure, energetics, potential energy surfaces, and energetic stability of excess electron bubbles in ((4)He)(N) (N=6500-10(6)) clusters. The subsystem of the helium atoms was treated by the density functional method. The density profile was specified by a void (i.e., an empty bubble) at the cluster center, a rising profile towards a constant interior value (described by a power exponential), and a decreasing profile near the cluster surface (described in terms of a Gudermannian function). The cluster surface density profile width (approximately 6 A) weakly depends on the bubble radius R(b), while the interior surface profile widths (approximately 4-8 A) increase with increasing R(b). The cluster deformation energy E(d) accompanying the bubble formation originates from the bubble surface energy, the exterior cluster surface energy change, and the energy increase due to intracluster density changes, with the latter term providing the dominant contribution for N=6500-2 x 10(5). The excess electron energy E(e) was calculated at a fixed nuclear configuration using a pseudopotential method, with an effective (nonlocal) potential, which incorporates repulsion and polarization effects. Concurrently, the energy V(0) of the quasi-free-electron within the deformed cluster was calculated. The total electron bubble energies E(t)=E(e)+E(d), which represent the energetic configurational diagrams of E(t) vs R(b) (at fixed N), provide the equilibrium bubble radii R(b) (c) and the corresponding total equilibrium energies E(t) (e), with E(t) (e)(R(e)) decreasing (increasing) with increasing N (i.e., at N=6500, R(e)=13.5 A and E(t) (e)=0.86 eV, while at N=1.8 x 10(5), R(e)=16.6 A and E(t) (e)=0.39 eV). The cluster size dependence of the energy gap (V(0)-E(t) (e)) allows for the estimate of the minimal ((4)He)(N) cluster size of N approximately 5200 for which the electron bubble is energetically stable.

We show that the energetics and electronic couplings for excess electron transfer (EET) can be accurately estimated by using unoccupied Kohn-Sham orbitals (UKSO) calculated for neutral pi stacks. To assess the performance of different DFT functionals, we use MS-PT2 results for seven pi stacks of nucleobases as reference data. The DFT calculations are carried out by using the local spin density approximation SVWN, two generalized gradient approximation functionals BP86 and BLYP, and two hybrid functionals B3LYP and BH&HLYP. Best estimations within the UKSO approach are obtained by the B3LYP and SVWN methods. TD DFT calculations provide less accurate values of the EET parameters as compared with the UKSO data. Also, the excess charge distribution in the radical anions is well described by the LUMOs of neutral systems. In contrast, spin-unrestricted DFT calculations of radical anions considerably overestimate delocalization of the excess electron. The excellent results obtained for the ground and excited states of the radical anions (excitation energy, transition dipole moment, electronic coupling, and excess electron distribution) by using UKSO of neutral dimers suggest an efficient strategy to calculate the EET parameters for DNA pi stacks.

Excess-electron and excess-hole states of charged alkali halide clusters

Far infrared magnetospectroscopic studies of negative donor ions (D -), and donors in the presence of many excess electrons in high magnetic fields in GaAs/AlGaAs quantum wells are reviewed.Both singlet and triplet transitions of well-center D --ions were observed and are in good agreement with recent theoretical calculations.For off-well-center D --ions evidence for a predicted magnetic-field-induced "unbinding" of the second electron was found.In the presence of many excess electrons the D --singlet and -triplet transitions are blue-shifted substantially and evolve into bound magnetoplasmon excitations.Cusps are observed at integral and fractional Landau-level filling factors (v) in a plot of normalized blue-shift of the D --singlet-like bound magnetoplasmon transition vs. v.For v < 1, the singlet-like bound magnetoplasmon transition continuously approaches the isolated D --singlet transition with increasing magnetic field, while the triplet-like transition loses strength, irrespective of the electron density.Exact diagonalization studies of a donor ion with a few electrons in a parabolic lateral confining potential show the importance of electron-electron interactions and localization due to the long-range fluctuating potential in explaining this behavior.High pressure studies in a specially designed diamond anvil cell exhibit a continuous evolution from bound magnetoplasmon transitions to isolated D --transitions to neutral donor transitions in a single sample as the pressure is increased and the electron density in the wells is decreased.

An analysis of isobutanol adsorption isotherms on alumina and of the IR spectra showed that isobutanol adsorbed irreversibly as a carboxylate species and as a coordinatively adsorbed molecule on an anion vacancy. Weakly bound coordinated molecules and hydrogen-bonded species both contributed to the reversible adsorption. The coordinated isobutanol apparently interacted with neighboring hydroxyl groups via the alcoholic hydroxyl proton; this interaction was favored by the polarizing power of the aluminum(III) cation to which the alcohol was coordinated. The hydrogen-bonding which occurred at higher coverage was apparently formed with the alcohol molecule as donor and the surface anion as acceptor groups. Cyclopropane isomerization proceeded in the presence, but not in the absence, of isobutanol, confirming that alcohol chemisorption provided protons due to strong polarization. Graphs, spectra, tables, and 46 references.