Electron Solvation Dynamics Research Articles

The nature of a floating electron

Dynamics of electron solvation at the water surface is studied using extensive ab initio simulations. Calculations have been performed on semi-classical water at 300 K temperature with an excess quantum electron on its surface. It is found that after a very fast 30–50 fs initial localization, there follow fast 50–70 fs rotationally mediated transitions of the excess electron between surface trap states with a lifetime of 150–400 fs. In less then 2 ps the excess electron gets trapped in an ordered “floating” electron state on the surface of water with a lifetime of more than 7 ps. The excess electron diffusion coefficient and spectrum of its velocity autocorrelation function change over time to reflect a transition from a very mobile phase (first 2 ps) to a trapped “floating” electron phase (2–7 ps).

Solvent rearrangement for an excited electron of I−(H2O)6: Analog to structural rearrangement of e−(H2O)6

The study of electron solvation dynamics is vital for understanding the phenomena related to the electron transfer process in solvents. On the basis of a recent femtosecond dynamics study of charge-transfer-to-solvent states in photoexcited iodide–water clusters [Lehr et al., Science 284, 635 (1999)], we have investigated the solvent rearrangement process for the excited electron in the iodide–water hexamer using ab initio calculations. Upon excitation of iodide–water hexamer, an electron transfers from the iodide anion to the water cluster. This results in release of the iodine atom and thereby formation of anionic water hexamer which undergoes rearrangement process toward the most stable conformation. The transformation pathway from the low-lying energy structures of the iodide–water hexamer to those of the electron–water hexamer is thus elucidated from the potential energy surface including the global and local minima and transition states of the electron–water hexamer.

Solvation Dynamics and Evolution of the Spatial Extent of Photoinjected Electrons in D2O/Cu(111)

Time- and angle-resolved two-photon-photoelectron (2PPE) spectroscopy is used to investigate the femtosecond dynamics of electron localization and solvation in ultrathin ice layers adsorbed on a Cu(111) single-crystal surface. An ultrashort UV pump pulse excites electrons in the Cu substrate to electronic states above the Fermi level (EF), whereby wave function overlap with the conduction band of the ice mediates electron transfer into the adlayer. Apart from relaxation back to the metal substrate, the conduction band electrons in the ice layer localize within the first 50 fs in an electronic state at 2.9 eV above EF. The localization process is revealed by a pronounced temporal change in the dispersion of the binding energy with electron momentum, ℏk∥. Subsequently, molecular reorientation stabilizes this localized electron on a picosecond time scale as observed by an increase of the binding energy with 270 meV/ps. Considering the absence of a temperature dependence in the spectra between 25 and 100 K, we exclude hopping processes of the localized electron between sites of different binding energy. We rather attribute the stabilization to electron solvation, in accordance with the line shape and the temporal evolution of the localized electronic state. By exploiting the angular dependence of the 2PPE spectra, we can separate the conduction band and the solvated, localized state in ice, which allows us to determine the position of the conduction band minimum to be 2.9 eV above EF. While the delocalized electrons in the conduction band exhibit a positive dispersion, the characteristic feature of the solvated state is an apparently negative dispersion at all time delays. As shown by model calculations of the electron momentum distribution as a function of binding energy, this observation is consistent with a solvated, localized electron with an increasing spatial confinement of its wave function evolving in parallel to electron solvation (i.e., increasing binding energy). From the width of the k∥ distribution an energy-dependent spatial extent of the probability density of the solvated electron is found which changes within 1.3 ps from about 20 Å to 10 Å. This finding thus corroborates the correlation of energetic stabilization and spatial extent of the electronic wave function during the process of electron solvation. On the basis of a simple model potential for a localized electron, we discuss the perturbation of the final state wave function and its influence on the analysis of angle-resolved photoemission spectra.

Ultrafast electron solvation dynamics in D 2O/Cu(1 1 1): influence of coverage and structure

Using femtosecond time-resolved two-photon photoelectron spectroscopy we have studied the dynamics of photoexcited electrons injected from a Cu(1 1 1) substrate into the conduction band of ultrathin ice layers. Ultrafast localization of the injected electrons within <50 fs is followed by a stabilization on a time scale of 0.1–1 ps due to local rearrangements of water dipoles. The dynamics of this electron solvation process are very similar for amorphous and crystalline ice, but exhibit a pronounced coverage dependence with two different regimes, which are attributed to solvation sites in the interior and at the surface of the adlayer. Additional information on the adsorbate structure is obtained from low-temperature scanning tunneling microscopy.

Ultrafast dynamics of electron localization and solvation in ice layers on Cu(111).

The femtosecond dynamics of localization and solvation of photoinjected electrons in ultrathin layers of amorphous solid H2O and D2O have been studied by time- and angle-resolved two-photon-photoelectron spectroscopy. After electron transfer from the metal substrate into the conduction band of ice, the excess electron localizes within the first 100 fs in a state at 2.9 eV above E(F), which is further stabilized by 300 meV on a time scale of 0.5-1 ps due to molecular rearrangements in the adlayer. A pronounced change of the solvation dynamics at a coverage of approximately 2 bilayers is attributed to different rigidity of the solvation shell in the bulk and near the surface of ice.

Temperature-dependent studies of solvated electrons in liquid water with two and three femtosecond pulse sequences

Two and three pulse sequences with an experimental time resolution of about 100 fs have been used to unravel the dynamics of solvated electrons in water at different temperatures. Both, geminate recombination and relaxation of the absorption spectrum can be distinguished using these pulse sequences. The relaxation process has been analysed applying shape stability of the absorption spectrum on an ultrafast time scale. A rigorous analysis of our data leads to strong evidence that solvated electrons in pure water are formed, after a delay, within the first few hundred fs after 9.32 eV photon absorption. Transferring optical sum rules (moment analysis of the absorption spectrum) to the ultrafast dynamics of solvated electrons we conclude that electronic ground state dynamics dominate the relaxation of solvated electrons.

Structural dynamics in quantum solids

Structural dynamics in semi-quantum and quantum solids (Ne, H 2 and D 2) is reported, and compared to results in classical solids such as Ar. The structural dynamics is driven by excitation of the lowest Rydberg state of the NO impurity. The resulting charge redistribution induces a local radial deformation of the medium (‘bubble’ formation) around the impurity. The steady-state spectroscopic signatures of this process are presented and analysed in the configuration coordinate model and harmonic approximation. Intermolecular potentials describing the impurity-medium interaction are obtained. The dynamics of ‘bubble’ formation and the ensuing medium response are probed in real-time by femtosecond pump-probe spectroscopy. In the very soft H 2 and D 2 environments, ‘bubble’ formation is a one-way process without recurrence of the cage motion and is complete in ∼1–2 ps. In the case of solid Ne, the dynamics are characterized by an initial expansion of the matrix cage around the impurity, followed by a low frequency recurrence. The results in solid Ne are complemented by preliminary molecular dynamics simulations. Overall the trend observed is that the expansion becomes slower in the sequence Ar–Ne–hydrogens, which is counterintuitive. This is discussed in terms of the quantum (delocalised) character of the lighter media, which introduces an additional microscopic friction. These results establish our experimental procedure based on the use of low- n Rydberg states as a novel method for probing structural and electronic solvation dynamics in non-polar media.

Time-resolved photoelectron spectroscopy of molecules and clusters.

Time-resolved photoelectron spectroscopy (TRPES) has become a powerful new tool in studying the dynamics of molecules and clusters. It has been applied to processes ranging from energy flow in electronically excited states of molecules to electron solvation dynamics in clusters. This review covers experimental and theoretical aspects of TRPES, focusing on studies of neutral and negatively charged species.

Instantaneous normal mode analysis of hydrated electron solvation dynamics

The instantaneous normal mode (INM) method is implemented in the context of mixed quantum-classical molecular dynamics (MD) simulations and applied to the analysis of the short-time solvation dynamics of the hydrated electron. Numerically suitable equations for computing the solvent dynamical matrix (Hessian) for both ground and excited adiabatic electronic states are derived using analytical derivative methods of quantum chemistry. Standard diagonalization of the Hessian leads to the sets of eigenfrequencies and eigenvectors that underlie the INM theory. Comparison of the hydrated electron and pure water INM spectra and the corresponding mode participation ratios shows that the quantum solute enhances the participation of collective low-frequency unstable modes (imaginary frequencies) at the expenses of stable ones. Distinct differential INM spectra, involving distinct solvent configurational averages, are introduced to describe the changes experienced by the solvent INMs upon the vertical excitation of the electron. The overall picture is that the INMs associated with lower frequency translational and rotational motions, as well as fast librational reorientations are markedly affected by the photoexcitation, as opposed to the localized internal vibrations of the individual water molecules. The INM solvation response for the upward transition calculated from the real modes agrees with the response obtained directly from the energy gap time correlation up to approximately 100 fs. The agreement extends over much longer times for downward transitions. The INM analysis of the solvation responses following vertical upward and downward transitions reveals that diffusive translational and librational motions are both important mechanisms for the early stages (≲50 fs) of the solvent response, with the latter dominating the first half of this time period. It is also shown that the short-time solvent relaxation involves the combined motion of molecules from the first and second hydration shells. In addition, the linearized INM solvation response calculated for D2O indicates a significant (∼36%) solvent isotope effect in the first 25 fs of the response, where the decay is Gaussian. These results are compared with previous studies of the hydrated electron solvation dynamics.

Mechanisms of the ultrafast production and recombination of solvated electrons in weakly polar fluids: Comparison of multiphoton ionization and detachment via the charge-transfer-to-solvent transition of Na− in THF

The processes by which solvated electrons are generated and undergo recombination are of great interest in condensed phase physical chemistry because of their relevance to both electron transfer reactions and radiation chemistry. Although most of the work in this area has focused on aqueous systems, many outstanding questions remain, especially concerning the nature of these processes in low polarity solvents where the solvated electron has a fundamentally different structure. In this paper, we use femtosecond spectroscopic techniques to explore the dynamics of solvated electrons in tetrahydrofuran (THF) that are produced in two different ways: ejection by multiphoton ionization of the neat solvent, and detachment via the charge-transfer-to-solvent (CTTS) transition of sodide (Na−). Following multiphoton ionization of the solvent, the recombination of solvated electrons can be well described by a simple model that assumes electrons are first ejected to a given thermalization distance and then move diffusively in the presence of the Coulombic attraction with their geminate cation. The short-time transient absorption dynamics of the THF radical cation in the visible region of the spectrum do not match the kinetics of the solvated electron probed at ∼2 μm, indicating that caution is warranted when drawing conclusions about recombination based only on the dynamics of the solvent cation absorption. With ∼4 eV of excess energy, geminate recombination takes place on the hundreds of picoseconds time scale, corresponding to thermalization distances ⩾40 Å. The recombination of solvated electrons ejected via CTTS detachment of Na−, on the other hand, takes place on two distinct time scales of ⩽2 and ∼200 ps with kinetics that cannot be adequately fit by simple diffusive models. The fraction of electrons that undergo the fast recombination process decreases with increasing excitation energy or intensity. These facts lead us to conclude that electrons localize in the vicinity of their geminate Na atom partners, producing either directly overlapping or solvent-separated contact pairs. The distinct recombination kinetics for the two separate electron generation processes serve to emphasize the differences between them: multiphoton ionization produces a delocalized electron whose wave function samples the structure of the equilibrium fluid before undergoing localization, while CTTS is an electron transfer reaction with dynamics controlled by the motions of solvent molecules adjacent to the parent ion. All the results are compared to recent experiments on the photodetachment of electrons in aqueous systems where contact pairs are also thought to be important, allowing us to develop a qualitative picture for the mechanisms of electron generation and recombination in different solvent environments.

Static and Dynamic Solvation at the Air/Water Interface

The electronic excited-state solvation dynamics of coumarin 314 (C314) adsorbed at the air/water interface was investigated by femtosecond time-resolved second-harmonic generation (TRSHG). The second-harmonic spectrum of C314 at the air/water interface was determined using a tunable optical parametric generator, giving a spectral peak at 419 ± 3 nm. The solvatochromic shift is in good agreement with the recently proposed interfacial polarity scale, which predicts that the empirical polarity parameter of the interface is the arithmetic mean of the two bulk phases. The time-resolved second-harmonic response to the excited-state solvation was measured at two fundamental wavelengths (840 and 900 nm) and with two pump polarizations (S and P). The results indicate that the static spectral peaks of the S- and P-pumped distributions are offset ∼5 nm, while the dynamic interfacial solvation proceeds with two time constants, τ1 = ∼200 fs and τ2 = 1.2 ps, which are independent of C314 interfacial orientation. The sl...

Ultrafast dynamics of solvated electrons in polar liquids

The temporal evolution of the optical absorption of solvated electrons in a pure water jet between 5 and 70 °C has been investigated in two pulse femtosecond experiments. A 60 fs (FWHM) UV pulse at 270 nm directly ionized pure water and the subsequent absorption was probed by a white light continuum at 12 different wavelengths in the range between 450 and 1000 nm. Due to the thickness of the water jet the time resolution is limited to about 150 fs. The transient absorption contains not only information on the temporal evolution of the absorption spectrum but also data on the time dependence of the concentration of the solvated electrons. We have used the optical sum rules to separate the temporal evolution of the absorption spectrum from the concentration of the electrons in the time interval between 300 fs and 100 ps. At ultra-short times the absorption spectra are displaced to the red and undergo a substantial blue-shift during the first few picoseconds. After about 5 ps the absorption spectrum of thermally equilibrated solvated electrons is recovered. Within our time resolution the data show no evidence of transient electronically excited states of solvated electrons. We interpret the temporal evolution of the absorption spectrum using the optical sum rules and deduce the time dependent decrease of the mean squared dispersion in position, r2(t), of the electrons. Certainly, r2(t) is related to the solvation process of electrons in polar fluids and therefore contains information on the solvation dynamics.

Femtosecond solvation dynamics of solvated electrons in neat water

We report on transient pump–probe spectroscopy of equilibrated solvated electrons in neat water in the visible/near-infrared using femtosecond pulses and polarization resolution. By excitation in the blue wing of the absorption band the e − are promoted from the 1s ground state to the highest of the 2p sub-states. First rapid solvent relaxation and/or population redistribution among the excited 2p sub-states is followed by the population of a modified excited state p′ with a lifetime of τ 2=190±40 fs. The second intermediate is assigned to a modified ground state s′′ while subsequent ground state recovery proceeds within τ 3=0.9±0.15 ps. The negligible net anisotropy <0.01 of the probe absorption indicates that the observed distribution of solvent cavities of hydrated electrons is close to spherical symmetry.

Stochastic-Liouville approach to optically induced electron transfer: electronic coherence in the reaction dynamics

Electron transfer (ET) dynamics in the excited state were investigated theoretically by means of the stochastic-Liouville equation. Time dependences of both the diagonal and off-diagonal elements of the density matrix of the three electronic states, the ground state, locally excited (LE) state and the charge-transfer (CT) state, were evaluated by solving the equation numerically. The formalism includes three major effects in the ET dynamics simultaneously: optical transition, electronic interaction between the LE and CT states and solvation dynamics. From results of the numerical calculation, roles of the electronic coupling and solvation dynamics in ET and effects of the electronic coherence created by the optical excitation on ET are discussed. Application of the present method to non-linear optical spectroscopy in briefly considered. Copyright © 2000 John Wiley & Sons, Ltd.

Electron solvation in finite systems: femtosecond dynamics of iodide. (Water)n anion clusters

Electron solvation dynamics in photoexcited anion clusters of I-(D2O)n=4-6 and I-(H2O)4-6 were probed by using femtosecond photoelectron spectroscopy (FPES). An ultrafast pump pulse excited the anion to the cluster analog of the charge-transfer-to-solvent state seen for I- in aqueous solution. Evolution of this state was monitored by time-resolved photoelectron spectroscopy using an ultrafast probe pulse. The excited n = 4 clusters showed simple population decay, but in the n = 5 and 6 clusters the solvent molecules rearranged to stabilize and localize the excess electron, showing characteristics associated with electron solvation dynamics in bulk water. Comparison of the FPES of I-(D2O)n with I-(H2O)n indicates more rapid solvation in the H2O clusters.

Femtosecond dynamics of solvation at the air/water interface

The electronic excited-state solvation dynamics of coumarin 314 (C314) adsorbed at the air/water interface was investigated by femtosecond time-resolved surface second-harmonic generation (TRSHG). This Letter presents the first use of TRSHG to measure solvation dynamics and presents the theoretical basis for monitoring electronic excited-state solvation with TRSHG. The interfacial electronic excited solvation time of C314 was measured to be 790±30 fs. This is close to bulk results and is in agreement with simple dielectric continuum models of solvation and the predictions of molecular dynamics simulations.

Ultrafast relaxation dynamics of solvated electrons in water

AbstractThe relaxation dynamics of excess electrons in a water jet between 5 and 70°C have been investigated on an ultrashort timescale. We have probed the transient absorption of the system immediately after UV multiphoton‐ionization with a 266 nm ultrashort laser pulse and a time resolution of about 50 fs. Probe wavelengths ranging from 450 to 1000 nm have been provided by a generated white‐light continuum. The data suggest a superposition of geminate recombination of the solvated electrons with their original counter‐ions and of relaxation into thermal equilibrium. Relaxation into thermal equilibrium is the much faster process and is completed after about 6 ps while the geminate recombination is much slower, temperature independent, and prevails for at least 100 ps. The here postulated interpretation of the data clearly shows that no other transient states than “hot solvated electrons” are required for an understanding of the observed ultrafast dynamics within our time resolution.

Alternative Mechanisms for Solvation Dynamics of Laser-Induced Electrons in Methanol

A multiphoton ionization study of neat methanol with subpicosecond 2-eV laser pulses has been previously reported. A hybrid electron solvation mechanism combining both a stepwise transition between two electronsolvent configuration states and a continuous first-order blue shift of the electron absorption spectra was found to closely fit the experimental data. If substantial absorption from free electrons is assumed in this spectral region, we find that another model comprising thermalization prior to a stepwise branching localization without blue-shifting spectra fits equally well at all wavelengths. However, these two models display considerable differences between their respective kinetic parameters, especially the electron localization time. Furthermore, for the nonshifting model, this calculated localization time is considerably longer than that for electron hydration in neat water. We suggest independent studies such as ultrafast electron scavenging experiments before adopting a particular mechanism for electron solvation in methanol.

Cavitation model of electron solvation dynamics: Effect of energy dissipation

The effect of energy dissipation on the electron solvation dynamics is studied within the framework of the cavitation model. Two different energy dissipation mechanisms, associated with the sound emission and viscous friction, are analysed. The radiational damping of the cavity boundary dynamics is described using the Herring equation. The model is applied to analysis of the electron solvation dynamics in water. Sound emission constitutes the most efficient mechanism of energy dissipation. Energy dissipation does not change the theoretical prediction concerning the small isotope effect on the electron solvation dynamics in water.

Phenomenology of Electron Solvation in Polar Fluids

solvated electron are studied as functions of the solvent density and of solvent molecular parameters which determine the electron solvent interaction and the solvent dynamical response. We find that electron localization in this solvent is caused primarily by the repulsive part of the electron -solvent interaction. Upon increasing the solvent molecular dipole from zero, the electron becomes more localized; however, this effect seems to saturate at moderate solvent polarities, and further increase of the polarity changes the ground (and excited) state energies without affecting strongly the electron size. In this regime the electron behaves approximately like a classical charge distribution as far as the dependence of its solvation energy on the solvent polarity is concerned. The dynamical response of the solvent to the solvated electron is investigated by studying the solvent-induced fluctuations of the electron’s energy levels. As expected we find that fluctuations in the ground and excited state energies are dominated by the electrostatic part of the electron -solvent interaction, and their dynamics therefore reflects the solvent rotational motion. Surprisingly, however, the electrostatic contributions mostly cancel in the fluctuations of the gap between the ground and first excited state. Consequently the gap fluctuations are dominated by the solvent translational motions. The implications of these observations on the dynamics of electron solvation are discussed.