Femtosecond relaxation dynamics of solvated electrons in liquid ammonia.

The ultrafast relaxation dynamics of the well-known solvated electron in liquid ammonia solutions are investigated with femtosecond near-infrared pump-probe absorption spectroscopy. Immediately after photoexcitation, the dynamic absorption spectrum of the electron is substantially red-shifted with respect to its stationary spectrum. A subsequent dynamic blue shift of the pump-probe spectrum occurs on a timescale of 150 fs. The data are understood in terms of ground-state "cooling" and can be quantitatively simulated by an intuitive temperature-jump model employing a dynamically evolving Kubo line shape for the electronic resonance. A simple estimate implies that, on average, the electron in the liquid is coordinated to six nearest-neighbor ammonia molecules. An equivalent analysis of the data based on a bubble-formation/cavity-contraction mechanism is briefly outlined.     

Kinetics of the reaction of solvated electrons with fluorobenzene in liquid ammonia

The rate of disappearance of solvated electrons by reaction with fluorobenzene in ammonia is accelerated by small concentrations of methanol; it also has a “negative activation energy” depending on the methanol concentration. The kinetic data suggest an exothermic electron attachment-detachment equilibrium with the fluorobenzene followed by a slower reaction of the electron adduct with the proton donating methanol.     

Photo-detrapping of solvated electrons in an ionic liquid

We studied the dynamics of photo-detrapped solvated electrons in the ionic liquid trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide (TMPA-TFSI) using laser flash photolysis. The solvated electrons were produced by the electron photodetachment from iodide via a 248 nm KrF excimer laser. The solvated electron decayed by first-order kinetics with a lifetime of about 240 ns. The spectrum of the solvated electron in the ionic liquid TMPA-TFSI is very broad with a peak around 1100 nm. After the 248 nm pulse, a 532 nm pulse was used to subsequently detrap the solvated electrons. After the detrapping pulse, quasi-permanent bleaching was observed. The relative magnitude of the bleaching in the solvated electron absorbance was measured from 500 to 1000 nm. The amount of bleaching depends on the probe wavelength. The fraction of bleached absorbance was larger at 500 nm than that at 1000 nm, suggesting that there are at least two species that absorb 532 nm light. We discuss the present results from viewpoint of the heterogeneity of ionic liquids.     

Resonance Raman spectra of electrons solvated in liquid alcohols

Resonance Raman spectra of electrons solvated in liquid methanol, ethanol, and n-propanol are presented. At least five distinct solvent modes exhibit resonantly enhanced scattering, including the OH torsion, CO/CC stretches, the OH in-plane bend, methyl deformations, and the OH stretch. The 200-350 cm-1 frequency downshift of the OH stretch indicates a strong H-bond interaction between the electron and the hydroxyl group. The multiple modes including alkyl vibrations that are coupled to the electronic transition of the solvated electron reveal the extension of the electron's wavefunction into the alkyl solvent environment.     

The absorption coefficient of a polar liquid with solvated electrons

An equation for the absorption coefficient of a polar liquid with excess (solvated) electrons is derived. It is taken into account that (1) each excess electron can for a certain time reside on only one liquid molecule (during this time, the molecule is in the anion-resonance state), and (2) a polar liquid is electrostatically nonuniform because it has different local potentials, which can be calculated for each molecule. The probabilities of quantum movements of excess electrons in a liquid from one molecule to another caused by the absorption of photons are considered.     

Time-resolved spectroscopy of solvated electrons

The theory of time-dependent transient spectra of solvated electrons is developed. It is shown that the evolution of the spectral line centre reproduces the time dependence of the classical correlation function of the random process of electronic energy level fluctuations. The results of this investigation are compared with experiment.     

Spectral moments of solvated electrons

A moment theory analysis is performed on the optical absorption spectra of surplus electrons localized in various media. The deduced attributes of the excess particle are compared with those obtained by a perturbation treatment of the exact ground state solution of a currently popular model potential for the species. Several frequency dependent observables are evaluated and a source of inadequacy in the current theory, the presence of a Coulomb tail, is revealed.     

Compton profiles of solvated electrons

The importance of applying a variety of experimental techniques to unravel the nature of solvated electrons is emphasized. Compton profiles are evaluated for these species from a range of models. Some comparisons are made with positron annihilation studies.     

Cleavage of carbon-carbon-bonds by solvated electrons

Benzylic C-C bonds in diarylmethanes and diarylethanes (Ar = phenyl, naphthyl) are cleaved by potassium in dimethoxyethane/octaglyme.     

Reactivity of tri-n-bytylphosphate towards solvated electrons

Decomposition yields of tri-n-butylphosphate in methanol solutions saturated with Ar or N₂O were determined. On the basis of difference between yields in Ar and N₂O saturated solutions the rate constant k_{/e s} ^– _+TBP/=5.0×10⁶ dm³.mol^–1.s^–1 was calculated.     

Electrochemical generation of solvated electrons from nanostructured carbon

Voltammograms for electrodes fabricated of nanostructured carbon of various morphology (nanotube paper, columnar and filament structures) in hexamethylphosphoric triamide (HMPA) solution have been obtained and analysed. Intensive dark-blue coloration near cathode surface at potentials as low as E ∼ −1.2 V (s.c.e.) has been observed. An ESR (electron spin resonance) spectrum of this frozen dark-blue solution was recorded. This spectrum coincides with one of free electrons. Experimental proofs of the existence of electron emission into electrolytic solutions at moderate cathodic potentials are present for all electrodes. This effect is established to be connected with the presence of atomically sharp areas on the electrode surfaces.     

Femtosecond spectroscopy of solvated electrons from sodium-ammonia-d3 solutions: temperature jump versus local density jump

The relaxation dynamics of solvated electrons from sodium-ammonia-d3 solutions was studied by femtosecond time-resolved near-infrared spectroscopy. The experimental pump-probe data reveal a pulse-width limited pump-induced redshift of the absorption spectrum of the ammoniated electron and a subsequent slower blueshift on a time scale of roughly 200 fs. The spectrotemporal response is interpreted using the nonadiabatic relaxation mechanism for cavity-bound solvated electrons in condensed phases. In particular, we develop a local density-jump model, which traces the dynamic spectrum back to a sequence of a pump-induced cavity expansion due to Pauli repulsion and a succeeding cavity contraction upon nonadiabatic return of the electron back to its ground state. Using the existing thermodynamic data of the solvent and experimental temperature and density-dependent absorption spectra of metal-ammonia solutions, an overall increase in the interparticle distance within the solvent cavity of 25% is crudely estimated. The density-jump model is compared to the temperature-jump model we proposed previously for the femtosecond relaxation dynamics of metal-NH(3) solutions.     

Time-resolved photoelectron spectroscopy of solvated electrons in aqueous NaI solution

Time-resolved photoelectron spectroscopy was used to study the energetics and dynamics of solvated electrons in aqueous solution. Solvated electrons are generated by ultrafast photodetachment in a 100 mM aqueous NaI solution. Initially, an ensemble of strongly bound ("cold") solvated electrons and an ensemble of weakly bound ("hot") electrons in an unequilibrated solvent environment are observed. We report an ultrafast recombination channel for the "hot" electrons with a rate of (800 fs)(-1) which is in competition with thermalization occurring with a rate of (1.1 ps)(-1). The thermalized electrons recombine with the iodide radical with a rate of (22 ps)(-1). About 35% of the thermalized electrons escape geminate recombination and form free, solvated electrons. The vertical detachment energy for the solvated electron is determined to be 3.40 eV. No indication for a surface-bound electron at lower binding energies was observed.     

Structure of solvated mercury(II) halides in liquid ammonia, triethyl phosphite and tri-n-butylphosphine solution

Liquid ammonia, trialkyl phosphites, and especially trialkylphosphines, are very powerful electron-pair donor solvents with soft bonding character. The solvent molecules act as strongly coordinating ligands towards mercury(ii), interacting strongly enough to displace halide ligands. In liquid ammonia mercury(ii) chloride solutions separate into two liquid phases; the upper contains tetraamminemercury(ii) complexes, [Hg(NH(3))(4)](2+), and chloride ions in low concentration, while the lower is a dense highly concentrated solution of [Hg(NH(3))(4)](2+) entities, ca. 1.4 mol dm(-3), probably ion-paired by hydrogen bonds to the chloride ions. Mercury(ii) bromide also dissociates to ionic complexes in liquid ammonia and forms a homogeneous solution for which (199)Hg NMR indicates weak bromide association with mercury(ii). When dissolving mercury(ii) iodide in liquid ammonia and triethyl phosphite solvated molecular complexes form in the solutions. The Raman nu(I-Hg-I) symmetric stretching frequency is 132 cm(-1) for the pseudo-tetrahedral [HgI(2)(NH(3))(2)] complex formed in liquid ammonia, corresponding to D(S) = 56 on the donor strength scale. For the Hg(ClO(4))(2)/NH(4)I system in liquid ammonia a (199)Hg NMR study showed [HgI(4)](2-) to be the dominating mercury(ii) complex for mole ratios n(I(-)) : n(Hg(2+)) > or = 6. A large angle X-ray scattering (LAXS) study of mercury(ii) iodide in triethyl phosphite solution showed a [HgI(2)(P(OC(4)H(9))(3))(2)] complex with the Hg-I and Hg-P bond distances 2.750(3) and 2.457(4) A, respectively, in near tetrahedral configuration. Trialkylphosphines generally form very strong bonds to mercury(ii), dissociating all mercury(ii) halides. Mercury(ii) chloride and bromide form solid solvated mercury(ii) halide salts when treated with tri-n-butylphosphine, because of the low permittivity of the solvent. A LAXS study of a melt of mercury(ii) iodide in tri-n-butylphosphine at 330 K resulted in the Hg-I and Hg-P distances 2.851(3) and 2.468(4) A, respectively. The absence of a distinct I-I distance indicates flexible coordination geometry with weak and non-directional mercury(ii) iodide association within the tri-n-butylphosphine solvated complex.     

Electrogenerated chemiluminescence with solvated electrons in hexamethylphosphoroamide (HMPA)

The electrogenerated chemiluminescence of N-toluenesulfonyl carbazole, 9-chlorofluorene derivatives and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) with solvated electrons in HMPA are discussed. The fairly bright emissions occurred simultaneously when solvated electrons were generated electrochemically. In the case of N-toluenesulfonyl carbazole and 9-chlorofluorene derivatives, the respective carbanions formed by “dissociative electron transfer reactions” are the fluorescents. The singlet state of TMPD seems to be directly formed by the electron transfer reactions between radical cations of TMPD and solvated electrons.     

Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state.     

A method for the generation of solvated electrons in polar solvents

A simple method for generating solvated electrons in a polar liquid by a rapid discharge of a capacitor bank is suggested. New theoretical concepts of excess electrons in polar liquids are used.