Solvated electron dynamics

Femtosecond dynamics of the solvated electron in water studied by time-resolved Raman spectroscopy... 

When an electron is injected into a polar solvent such as water or alcohols, the electron is solvated and forms so-called the solvated electron. This solvated electron is considered the most basic anionic species in solutions and it has been extensively studied by variety of experimental and theoretical methods. Especially, the solvated electron in water (the hydrated electron) has been attracting much interest in wide fields because of its fundamental importance. It is well-known that the solvated electron in water exhibits a very broad absorption band peaked around 720 nm. This broad absorption is mainly attributed to the s- p transition of the electron in a solvent cavity. Recently, we measured picosecond time-resolved Raman scattering from water under the resonance condition with the s- p transition of the solvated electron, and found that strong transient Raman bands appeared in accordance with the generation of the solvated electron [1]. It was concluded that the observed transient Raman scattering was due to the water molecules that directly interact with the electron in the first solvation shell. Similar results were also obtained by a nanosecond Raman study [2]. This finding implies that we are now able to study the solvated electron by using vibrational spectroscopy. In this paper, we describe new information about the ultrafast dynamics of the solvated electron in water, which are obtained by time-resolved resonance Raman spectroscopy. 

Ethylene glycol is a very viscous liquid and the molecule presents two close OH groups. It has to be noticed that, among all the different solvents studied by pulse radiolysis, the transition energy of the solvated electron absorption band is maximum in liquid ethylene glycol. For these reasons, the electron in EG seems to have a special behaviour and it is of great interest to study the dynamics of the formation of equilibrated solvated electron. Within this context, the present communication deals with the dynamics of solvation in EG of electrons produced by photoionisation of the solvent at 263 nm. The formation of solvated electrons is followed by pump-probe transient absorption spectroscopy in the visible spectral range from 425 to 725 nm and also in near IR. For the first time, the absorption spectrum of the precursor of the equilibrated electron is observed in EG. Our results are shortly compared by those obtained in water and methanol. 

Assel A, Laenen R, Laubereau A (2000) Femtosecond solvation dynamics of solvated electrons in neat water. Chem Phys Lett 317 13-22... 

Solvation dynamics refers to the solvent reorganization or relaxation that accompanies the external excitation of a probe solute, usually a fluorescent organic dye or simply an excess solvated electron [55], Experimentally, the process of solvent reorganization can be time monitored by the time evolution of the fluorescence emission in time-dependent ultra-fast Stokes shift spectroscopy. 

In this chapter, we have reviewed some of our own work on solvation properties in supercritical fluids using molecular dynamics computer simulations. We have presented the main aspects associated with the solvation structures of purine alkaloids in CO2 under different supercritical conditions and in the presence of ethanol as co-solvent, highlighting the phenomena of solvent density augmentation in the immediate neighborhood of the solute and the effects from the strong preferential solvation by the polar co-solvent. We have also presented a summary of our results for the structure and dynamics of supercritical water and ammonia, focusing on the dielectric behavior of supercritical water as functions of density and temperature and the behavior of excess solvated electrons in aqueous and non-aqueous associative environments. 

The discussion above must be modified if the acceptor molecules adsorb onto the electrode surface. Under these circumstances, the electrons are captured by the acceptor directly rather than through an intermediary solvated electron, and such direct photoassisted electron transfer has been much studied recently with the advent of dye-sensitized solar cells and molecular electronic devices. The normal approach is to use two-photon excitation both to probe the existence of localized states at the surface and to explore their dynamics. In experiments of this nature, which have been mostly carried out on ad-... 

Ab initio and Monte-Carlo calculations. Attempts have appeared in pulse radiolysis to describe the dynamics of free electron production, recombination and solvation on a microscopic scale [31-34]. This requires the knowledge of a number of physical parameters solvated electron and free ion yields, electron and hole mobilities, slowing-down cross-sections, localization and solvation times, etc. The movement and fate of each reactant is examined step by step in a probabilistic way and final results are obtained by averaging a number of calculated individual scenarios. 

EJ Hart and M Anbar have detailed the characteristics and the chemistry of the solvated electron in water, otherwise known as the hydrated electron and denoted by e] y or e. A number of reviews on the solvated electron are also available.In this article, we will recall briefly the main steps of the discovery and the principal properties of the solvated electron. We will then depict its reactivity and focus on recent results concerning the effect of metal cations pairing with the solvated electron. At last, we will present results on the solvation dynamics of electron. Due to the development of ultrashort laser pulses, great strides have been made towards the understanding of the solvation and short-time reactivity of the electron, mainly in water but also in polar solvents. However, due to the vast and still increasing literature on the solvated electron, we do not pretend for this review to be exhaustive. 

Lately, quantum-classical molecular-dynamics simulations of an excess electron in water performed for wide ranges of temperature and pressure suggest that the observed red shift of the optical absorption spectrum is a density effect rather than a temperature effect. Indeed, by increasing the temperature, the mean volume of the cavity occupied by the solvated electron increases due to weakening of bonds between solvent molecules the electron is less confined in the cavity, and the potential well becomes less deep. 

Since its spectroscopic discovery, a large number of time-resolved experiments were carried out to clarify the relaxation dynamics of the solvated electron. The first experimental studies of the formation dynamics of electrons in liquids started with electron pulse radiolysis... 

Fig. 11. Time-resolved absorption spectra in neat methanol after two-photon absorption at 273 nm and energy level scheme used to account for the measmed dynamics during the generation of solvated electrons. [Reprinted ftom Ref. 94, Copyright 2003, with permission from Elsevier.]...

The solvated electron is a transient chemical species which exists in many solvents. The domain of existence of the solvated electron starts with the solvation time of the precursor and ends with the time required to complete reactions with other molecules or ions present in the medium. Due to the importance of water in physics, chemistry and biochemistry, the solvated electron in water has attracted much interest in order to determine its structure and excited states. The solvated electrons in other solvents are less quantitatively known, and much remains to be done, particularly with the theory. Likewise, although ultrafast dynamics of the excess electron in liquid water and in a few alcohols have been extensively studied over the past two decades, many questions concerning the mechanisms of localization, thermalization, and solvation of the electron still remain. Indeed, most interpretations of those dynamics correspond to phenomenological and macroscopic approaches leading to many kinetic schemes but providing little insight into microscopic and structural aspects of the electron dynamics. Such information can only be obtained by comparisons between experiments and theoretical models. For that, developments of quantum and molecular dynamics simulations are necessary to get a more detailed picture of the electron solvation process and to unravel the structure of the solvated electron in many solvents. 

Assel M, Laenen R, Laubereau A. (1998) Dynamics of excited solvated electrons in aqueous solution monitored with femtosecond-time and polarization resolution. /Phys Chem A 102 2256-2262. 

In this chapter, the recent progress in the understanding of the nature and dynamics of excess (solvated) electrons in molecular fluids composed of polar molecules with no electron affinity (EA), such as liquid water (hydrated electron, and aliphatic alcohols, is examined. Our group has recently reviewed the literature on solvated electron in liquefied ammonia and saturated hydrocarbons and we refer the reader to these publications for an introduction to the excess electron states in such liquids. We narrowed this review to bulk neat liquids and (to a much lesser degree) large water anion clusters in the gas phase that serve as useful reference systems for solvated electrons in the bulk. The excess electrons trapped by supramolecular structures (including single macrocycle molecules ), such as clusters of polar molecules and water pools of reverse micelles in nonpolar liquids and complexes of the electrons with cations in concentrated salt solutions, are examined elsewhere. 

This narrative echoes the themes addressed in our recent review on the properties of uncommon solvent anions. We do not pretend to be comprehensive or inclusive, as the literature on electron solvation is vast and rapidly expanding. This increase is cnrrently driven by ultrafast laser spectroscopy studies of electron injection and relaxation dynamics (see Chap. 2), and by gas phase studies of anion clusters by photoelectron and IR spectroscopy. Despite the great importance of the solvated/ hydrated electron for radiation chemistry (as this species is a common reducing agent in radiolysis of liquids and solids), pulse radiolysis studies of solvated electrons are becoming less frequent perhaps due to the insufficient time resolution of the method (picoseconds) as compared to state-of-the-art laser studies (time resolution to 5 fs ). The welcome exceptions are the recent spectroscopic and kinetic studies of hydrated electrons in supercriticaF and supercooled water. As the theoretical models for high-temperature hydrated electrons and the reaction mechanisms for these species are still rmder debate, we will exclude such extreme conditions from this review. 

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