Search

Time-resolved x-ray absorption spectroscopy monitors the transient species generated by one-photon detachment of an electron from aqueous bromide. Hydrated bromine atoms with a lifetime of ca. 17 ns were observed, nearly half of which react with excess Br- to form Br2-. The K-edge spectra of the Br atom and Br2- anion exhibit distinctive resonant transitions that are absent for the Br- precursor. The absorption spectra indicate that the solvent shell around a Br0 atom is defined primarily by hydrophobic interactions, in agreement with a Monte Carlo simulation of the solvent structure., Comment: 6 pages, 4 figures + supplement, will be submitted to PRL

Femtosecond PTHB spectroscopy was expected to demonstrate the existence of distinct s-p absorption subbands originating from the three nondegenerate p-like excited states of hydrated electron in anisotropic solvation cavity. Yet no conclusive experimental evidence either for this subband structure or the reorientation of the cavity on the picosecond time scale has been obtained. We demonstrate that rapid reorientation of s-p transition dipole moments in response to small scale motion of water molecules is the likely culprit. The polarized bleach is shown to be too small and too short lived to be observed reliably on the sub-picosecond time scale., Comment: 10 pages + 3 figures + supplement, will be submitted shortly to Chem. Phys. Lett

Ionization of polar liquids and glasses often yields metastable electron centers collectively known as weakly bound (wb) electrons that absorb to the red of the ground state of the solvated/trapped electron. Formation of these species is thought to be the initial stage of electron localization in such media. Although these commonly occurring species have distinctive absorption spectra, no insight into their structure currently exists. In this Letter, we address the structure of the wb electron in low-temperature ice-Ih theoretically, using a combination of mixed quantum-classical (MQC) floating set of Gaussian orbitals (FSGO) - Metropolis Monte Carlo (MC) method and embedded cluster density functional theory (DFT) post treatment. The comparison with the observed properties of the wb electron in ice suggests that this species is an s-like wavefunction filling a water vacancy., Comment: Comments 12 pages, 2 figures + supplement; submitted to PRL

The radiation- and photo- chemistry of room temperature ionic liquids (ILs) composed of ammonium, phosphonium, pyrrolidinium, and imidazolium cations and bis(triflyl)amide, dicyanamide, and bis(oxalato)borate anions, have been studied using low-temperature Electron Paramagnetic Resonance (EPR). Several classes of radicals have been identified and related to reactions of the primary radiolytically generated electrons and holes. Large yields of terminal and penultimate C-centered radicals are observed in the aliphatic chains of the phosphonium, ammonium and pyrrolidinium cations, but not for imidazolium cation. This pattern can be accounted for by efficient deprotonation of a hole trapped on the cation (the radical dication) that competes with rapid charge transfer to a nearby anion. The latter leads to the formation of stable N- or O-centered radicals. The electrons either react with the protic impurity (for nonaromatic cations) yielding H atoms or the aromatic moiety (for imidazolium cations). Excitation of bis(triflyl)amide anion is shown to yield trifluoromethyl radical; the yield of this radical in radiolysis, though, is low (< 10% of the alkyl radical yield). In terms of their radiation chemistry, neat ILs appear to be intermediate between organic liquids and ionic solids. Addition of 10-40 wt% of trialkylphoshate (a common extraction agent for nuclear cycle separations) has relatively little effect on the fragmentation of the ILs. Radiation induced dealkylation of the phosphate is prominent, but the yield of the alkyl radical fragments derived from the phosphates is < 4% of the yield of the radical fragments drived from the solvent. We discuss the implication of these results for the radiation stability of nuclear cycle extraction systems based upon the IL diluents., Comment: 24 pages + 9 figures + supplement; submitted to J Am Chem Soc

We use transient absorption spectroscopy to monitor the ionization and dissociation products following two-photon excitation of pure liquid water. The two decay mechanisms occur with similar yield for an excitation energy of 9.3 eV, whereas the major channel at 8.3 eV is dissociation. The geminate recombination kinetics of the H and OH fragments, which can be followed in the transient absorption probed at 267 nm, provide a window on the dissociation dynamics at the lower excitation energy. Modeling the OH geminate recombination indicates that the dissociating H atoms have enough kinetic energy to escape the solvent cage and one or two additional solvent shells. The average initial separation of H and OH fragments is 0.7+-0.2 nm. Our observation suggests that the hydrogen bonding environment does not prevent direct dissociation of an O-H bond in the excited state. We discuss the implications of our measurement for the excited state dynamics of liquid water and explore the role of those dynamics in the ionization mechanism at low excitation energies., Comment: 25 pages, 5 figs, submitted to J Chem Phys

Adiabatic mixed quantum/classical molecular dynamics simulations were used to generate snapshots of the hydrated electron (e-) in liquid water at 300 K. Water cluster anions that include two complete solvation shells centered on the e- were extracted from these simulations and embedded in a matrix of fractional point charges designed to represent the rest of the solvent. Density functional theory and single-excitation configuration interaction methods were then applied to these embedded clusters. The salient feature of these hybrid calculations is significant transfer (ca. 0.18) of the excess electron's charge density into the O 2p orbitals in OH groups forming the solvation cavity. We used the results of these calculations to examine the structure of the molecular orbitals, the density of states, the absorption spectra in the visible and ultraviolet, the hyperfine coupling (hfc) tensors, and the IR and Raman spectra of the e-. The calculated hfc tensors were used to compute the EPR and ESEEM spectra for the e- that compared favorably to the experimental spectra of trapped e- in alkaline ice. The calculated vibrational spectra of the e- are consistent with the red-shifted bending and stretching frequencies observed in resonance Raman experiments. The model also accounts for the VIS and 190-nm absorption bands of the e-. Thus, our study suggests that to explain several important experimentally observed properties of the e-, many-electron effects must be accounted for., Comment: 68 pages, 12 figures + 16 more figures in the supplement (included) submitted to J Phys Chem A

A terawatt tabletop laser wakefield acceleration source of relativistic electrons has been developed in our Terawatt Ultrafast High Field Facility (TUHFF). The preliminary results for ultrafast radiolysis of liquid water using this femtosecond electron source are presented. A TUHFF based femtosecond x-ray source is proposed. Thomson scattering of the accelerated electrons off a counterpropagating terawatt laser beam will be used to generate keV x-ray photons. The expected parameters of this x-ray source have been estimated. The short pulse duration, high flux, and good collimation of the resulting x-ray beam would be conducive for ultrafast time-resolved x-ray absorption studies of short-lived transient species in gases, liquids, and solids. It is argued that the solvation dynamics of Br atoms generated in photoinduced electron detachment from aqueous bromide would make a convenient choice for the first pump-probe experiment using this x-ray source., Comment: published in Proc. SPIE Vol. 5920, "Ultrafast X-Ray Detectors, High-Speed Imaging, and Applications", S. Kleinfelder, D. L. Paisley, Z. Chang, J.-C. Kieffer, J. B. Hastings; Eds. (SPIE, Bellingham, WA, 2005) p. 1-10

We report the first ultrafast pulse radiolysis transient absorption spectroscopy measurements from the Terawatt Ultrafast High Field Facility (TUHFF) at Argonne National Laboratory. TUHFF houses a 20 TW Ti:sapphire laser system that generates 2.5 nC sub-picosecond pulses of multi-MeV electrons at 10 Hz using laser wakefield acceleration. The system has been specifically optimized for kinetic measurements in a pump-probe fashion. This requires averaging over many shots which necessitates stable, reliable generation of electron pulses. The latter were used to generate excess electrons in pulse radiolysis of liquid water and concentrated solutions of perchloric acid. The hydronium ions in the acidic solutions react with the hydrated electrons resulting in the rapid decay of the transient absorbance at 800 nm on the picosecond time scale. Time resolution of a few picoseconds has been demonstrated. The current time resolution is determined primarily by the physical dimensions of the sample and the detection sensitivity. Subpicosecond time resolution can be achieved by using thinner samples, more sensitive detection techniques and improved electron beam quality., Comment: submitted to J. Appl. Phys. 5 figures, 23 pages

We suggest that it might be possible to trap the electron in a cavity of a macrocycle molecule, in the same way this trapping occurs cooperatively, by several solvent molecules, in hydroxylic liquids. Such an encapsulated electron is a "molecular capacitor," in which the excess electron is largely decoupled from valence electrons in the trap. A specific design for such a trap that is based on calix[4]cyclohexanol is discussed in detail. It is shown theoretically, by ab initio and density functional theory (DFT) modeling, that one of the conformations of this molecule forms the optimum tetrahedral trap for the electron. The resulting "encapsulated electron" strikingly resembles the solvated electron in alcohols and water., Comment: 13 pages 2 tables 5 figures, submitted to Chem Phys Lett

Is it possible to design a supramolecular cage that would "solvate" the excess electron in the same fashion in which several solvent molecules do that co-operatively in polar liquids? Two general strategies are outlined for "electron encapsulation," viz. electron localization using polar groups arranged on the (i) inside of the cage or (ii) outside of the cage. The second approach is limited to polynitriles. We demonstrate that the latter faces a formidable problem: the electron attaches to the nitrile groups forming molecular anions with bent C-C-N fragments. Since the energy cost of this bending is very high, for dinitrile anions in n-hexane, the binding energies for the electron are very low and for mononitriles, these binding energies are lower still, and the entropy of electron attachment is anomalously small. Density functional theory modeling of electron trapping by mononitriles in n-hexane suggests that the mononitrile molecules substitute for the solvent molecules at the electron cavity, "solvating" the electron by their methyl groups. Such "solvated electrons" resemble multimer radical anions in which the electron density is shared (mainly) between C 2p orbitals in the solute/solvent molecules, instead of existing as cavity electrons. The way in which the excess electron density is shared by such molecules is similar to the way in which this sharing occurs in large di- and poly- nitrile anions, such as 1,2,4,5,7,8,10,11-octacyano-cyclododecane anion. The work thus reveals limitations of the concept of "solvated electron" for organic liquids. It also demostrates the feasibility of "electron encapsulation.", Comment: 44 pages & 12 fugures (low quality in some figures) + supplement w 3 tables, 10 fig, & Appendix; submitted to J Phys Chem A

Given the difficulty of obtaining compact analytical solutions for diffusion of interacting geminate pairs (such as electron-hole pairs generated by ionization of liquid) it is common, following the original method of Mozumder, to "prescribe" this diffusion. With this approach, the survival probability of the pair is represented as a product of the survival probability for a freely diffusing pair and a suitably defined weighting function. This approach gives the correct limiting survival probability for a pair in the Coulomb field. The same approach was used for simulation of reaction dynamics in radiolytic spurs ("independent reaction times" approach of Pimblott) and solve other vexing diffusion problems that do not have analytical solution. A reasonable question is, can the same method be used for any other interaction potential than Coulomb? Here we demonstrate that such a prescription is generally impossible. The correct result given by the prescribed diffusion approach for the Coulomb potential is, actually, purely accidental. The method is inherently contradictory and it should be used with caution., Comment: a short note (8 pages)

Photostimulated electron detachment from aqueous inorganic anions is the simplest example of solvent-mediated electron transfer. Here we contrast the behavior of halide anions with that of small polyatomic anions, such as pseudohalide anions (e.g., HS-) and common polyvalent anions (e.g., SO32-). Geminate recombination dynamics of hydrated electrons generated by 200 nm photoexcitation of aqueous anions (I-, Br-, OH-, HS-, CNS-, CO32-, SO32-, and Fe(CN)64-) have been studied. Prompt quantum yields for the formation of solvated, thermalized electrons and quantum yields for free electrons were determined. Pump-probe kinetics for 200 nm photoexcitation were compared with kinetics obtained at lower photoexcitation energy (225 nm or 242 nm) for the same anions, where possible. Free diffusion and mean force potential models of geminate recombination dynamics were used to analyze these kinetics. These analyses suggest that for polyatomic anions (including all polyvalent anions studied) the initial electron distribution has a broad component, even at relatively low photoexcitation energy. There seem to be no well-defined threshold energy below which the broadening of the distribution does not occur, as is the case for halide anions. Direct ionization to the conduction band of water is the most likely photoprocess broadening the electron distribution. Our study suggests that halide anions are in the class of their own; electron photodetachment from polyatomic, especially polyvalent, anions follows a different set of rules., Comment: to be submitted to J. Phys. Chem. A; 28 pages, 5 figs + Supplement

Monomers and small clusters of such molecules can reversibly trap conduction band electrons in dilute alkane solutions. The dynamics and energetics of this trapping have been studied using pulse radiolysis - transient absorption spectroscopy and time-resolved photoconductivity. Binding energies, thermal detrapping rates, and absorption spectra of excess electrons attached to monomer and multimer solute traps are obtained and possible structures for these species are discussed. "Dipole coagulation" (stepwise growth of the solute cluster around the cavity electron) predicted by Mozumder in 1972 is observed. Acetonitrile monomer is shown to solvate the electron by its methyl group, just like the alkane solvent does. The electron is dipole-bound to the CN group; the latter points away from the cavity. The resulting negatively charged species has a binding energy of 0.4 eV and absorbs in the infrared. Molecules of straight-chain aliphatic alcohols solvate the excess electron by their OH groups; at equilibrium, the predominant electron trap is a trimer or a tetramer; the binding energy of this solute trap is ca. 0.8 eV. Trapping by smaller clusters is opposed by the entropy which drives the equilibrium towards the electron in a solvent trap. For alcohol monomers, the trapping does not occur; a slow proton transfer reaction occurs instead. For acetonitrile monomer, the trapping is favored energetically but the thermal detachment is rapid (ca. 1 ns)., Comment: will shortly be submitted to J Phys Chem A; 53 pages w 12 figures; has a Supplement of 12 pages & 22 more figures

In common nonpolar liquids, such as saturated hydrocarbons, a dynamic equilibrium between trapped (localized) and quasifree (extended) states has been postulated for the excess electron (the two-state model). Using time-resolved dc conductivity, the effect of 1064 nm laser photoexcitation of trapped electrons on the charge transport has been observed in liquid n-hexane and methylcyclohexane. The light promotes the electron from the trap into the conduction band of the liquid, instantaneously increasing the conductivity by orders of magnitude. From the analysis of the two-pulse, two-color photoconductivity data, the residence time of the electrons in traps has been estimated as ca. 8.4 ps for n-hexane and ca. 13 ps for methylcyclohexane (at 295 K). The rate of detrapping decreases at lower temperature with an activation energy of ca. 200 meV (280-320 K); the lifetime-mobility product for quasifree electrons scales linearly with the temperature. We suggest that the properties of trapped electrons in hydrocarbon liquids can be well accounted for using the simple electron bubble (Wigner-Seiz spherical well) model. The estimated localization time of the quasifree electron is 20-50 fs; both time estimates are in good agreement with the "quasiballistic" model. This localization time is significantly lower than the value of ca. 300 fs obtained using time-domain terahertz (THz) spectroscopy for the same system [E. Knoesel et al., J. Chem. Phys. 121, 394 (2004)]. We suggest that the THz signal originates from the oscillations of electron bubbles rather than the free-electron plasma; vibrations of these bubbles may be responsible for the deviations from the Drude behavior observed below 0.4 THz. Various implications of these results are discussed., Comment: 37 page, 5 figures; w Supplement of 13 pages and 5 figures; accepted by J. Chem. Phys

The picosecond geminate recombination kinetics for hydrated electrons generated by 200 nm two photon absorption (12.4 eV total energy) has been measured in both light and heavy water. The geminate kinetics are observed to be almost identical in both H2O and D2O. Kinetic analysis based upon the independent reaction time approximation indicates that the average separation between the electron and its geminate partners in D2O is 13% narrower than in H2O (2.1 nm vs. 2.4 nm). These observations suggest that, even at this high ionization energy, autoionization of water competes with direct ionization., Comment: 10 pages + 2 figures, submitted to Chem. Phys. Letters

Single-shot ultrafast spectroscopy based on the frequency encoding of transient absorbance kinetics (FDSS) is demonstrated. These kinetics are sampled spectrally using linearly chirped pulses derived from a Ti:sapphire laser. A variable length grating pair compressor is used to provide group velocity dispersion out to -1.6 ps^2 and achieve the sampling of 512 channels per a 2-to-160 ps window with sensitivity > 5e-4. The possibilities of FDSS are illustrated with studies of three photon ionization of liquid water and one-photon excitation of a thin-film, Comment: 23 pages + 8 figures

"Single-shot" ultrafast spectroscopy based on the frequency encoding of transient absorbance kinetics using chirped probe pulses is analyzed theoretically. FDSS has an advantage over pump-probe spectroscopy in a situation where the "noise" is dominated by amplitude variations of the signal. Unlike "single-shot" techniques based on spatial encoding of the kinetics, no a priori knowledge of the excitation profile of the pump is needed. FDSS spectroscopy can be used for many types of samples, liquid or solid, including those comparable in thickness to the wavelength of the probe light. Another advantage is that due to the interference of quasimonochromatic components of the chirped probe pulse, an oscillation pattern near the origin of the FDSS kinetics emerges. This interference pattern is unique and can be used to determine the complex dielectric function of the photogenerated species., Comment: 25 pages + 5 figures + supplement

Recently, it became practicable to study geminate recombination dynamics of solvated electrons in polar liquids by using short pulses of light to reexcite these electrons back into the conduction band of the liquid and observe a change in the fraction of electrons that escape geminate recombination. In this Letter, the potential of this technique to provide additional insight into the recombination dynamics of electrons generated by charge-transfer-to-solvent (CTTS) photodetachment from monovalent anions in polar liquids is studied theoretically. The resulting expression accounts for the recent results from B.J. Schwartz's group at UCLA for electron photodetachment from Na- in tetrahydrofuran., Comment: 12 pages, 1 figure; to be submitted to Chem. Phys. Lett

Temporal evolution of transient absorption spectra for pre-solvated electron (e-) generated by biphotonic (200 nm) ionization of liquid H2O and D2O has been studied on femto- and pico- second time scales. These spectra were obtained in the intervals of 50 nm between 500 and 1700 nm. Two distinctive regimes of the spectral evolution were observed. In both of these regimes, the spectral profile changes considerably with delay time. For t<1ps, two new features (the 1150 nm band and 1400 nm shoulder) were observed in the spectral region where O-H overtones appear in the spectra of light water. These two features were not observed for the e- in D2O. Vibronic coupling to the modes of water molecules lining the solvation cavity is a possible origin of these features. On the sub-picosecond time scale, the absorption band of e- progressively shifts to the blue. At later delay times (t> 1ps), the position of the band maximum is "locked", but the spectral profile continues to change by narrowing on the red side and broadening on the blue side; the oscillator strength is constant within 10%. The time constant of this narrowing is ca. 0.56 ps for H2O and 0.64 ps for D2O, respectively. Vibrational relaxation and time-dependent decrease in the size and anisotropy of the solvation cavity are suggested as possible causes for the observed spectral transformations in both of these regimes. The geminate recombination kinetics for hydrated electrons generated by absorption of two 200 nm quanta (12.4 eV total energy) in light and heavy water are almost identical, suggesting that the average separation between the e- and its geminate partners in D2O is 13% narrower than in H2O. We suggest that autoionization of water competes with direct ionization even at this high photoexcitation energy., Comment: 35 pages, 9 figures with a supplement (9 pages + 7 figures); w references & many corrections; to be submitted to J. Phys. Chem. A

In this review (124 refs), several problems in radiolysis of saturated hydrocarbons are examined. Special attention is paid to the chemistry of radical cations, high-mobility holes, excited state and spur dynamics, magnetic field and spin effects, and optically detected magnetic resonance spectroscopy., Comment: modified review published in Radiation Chemsitry of Organic Liquids: Saturated Hydrocarbons, in Radiation Chemistry: Present Status and Future Prospects, in Radiation Chemistry: Present Status and Future Trends, edited by C. D. Jonah and B. S. M. Rao, Elsevier, Amsterdam, 2001; p. 175

Picosecond geminate recombination kinetics for electrons generated by multiphoton ionization of liquid water become power dependent when the radiance of the excitation light is greater than 0.3-0.5 TW/cm^2 (the terawatt regime). To elucidate the mechanism of this power dependence, tri- 400 nm photon ionization of water has been studied using pump-probe laser spectroscopy on the pico- and femtosecond time scales. We suggest that the observed kinetic transformations are caused by a rapid temperature jump in the sample. Such a jump is inherent to multiphoton ionization in the terawatt regime, when the absorption of the pump light along the optical path becomes very nonuniform. The heating of water is substantial (tens of degrees C) because the 3-photon quantum yield of the ionization is relatively low, ca. 0.42, and a large fraction of the excitation energy is released into the solvent bulk as heat. Evidence of the temperature jump is the observation of a red shift in the absorption spectrum of (thermalized) electron and by characteristic "flattening" of the thermalization dynamics in the near infra-red. The temperature jump in the terawatt regime might be ubiquitous in multiphoton ionization in molecular liquids. The implications of these observations for femtosecond pulse radiolysis of water are discussed., Comment: 30 pages + 8 figures; supplement of 7 pages + 9 figures; to be submitted to J. Phys. Chem. A

In concentrated solutions of NaClO4 and Na2SO4, the quantum yield for free electron generated by detachment from photoexcited anions (such as I-, OH-, ClO^4-, and [SO3]^2-) linearly decreases by 6-12% per 1 M ionic strength. In 9 M sodium perchlorate solution, this quantum yield decreases by roughly an order of magnitude. Ultrafast kinetic studies of 200 nm photon induced electron detachment from Br-, HO- and [SO3]^2- suggest that the prompt yield of thermalized electron does not change in these solutions; rather, the ionic strength effect originates in more efficient recombination of geminate pairs. Within the framework of the recently proposed mean force potential (MFP) model of charge separation dynamics in such photosystems, the observed changes are interpreted as an increase in the short-range attractive potential between the geminate partners. Association of sodium cation(s) with the electron and the parent anion is suggested as the most likely cause for the observed modification of the MFP. Electron thermalization kinetics suggest that the cation associated with the parent anion (by ion pairing and/or ionic atmosphere interaction) is passed to the detached electron in the course of the photoreaction. The precise atomic-level mechanism for the ionic strength effect is presently unclear; any further advance is likely to require the development of an adequate quantum molecular dynamics model., Comment: 40 pages, 10 figures + supplement 2 pages, 9 figures; will be submitted, in a modified form, to J. Phys. Chem A

The use of ultrafast pump-probe conductivity (PPC) for studies of photoelectron dynamics in nonpolar molecular liquids has been based upon the perturbation of geminate recombination dynamics of trapped electrons by their laser photoexcitation into the conduction band. Such a method is unsuitable for the studies of electron trapping dynamics of quasifree electrons per se. We demonstrate that the PPC method can be extended to study such dynamics, provided that the time resolution of the conductivity setup is better than the ratio mu[e_qf]tau[e_qf]/mu[e_trap] of the mobility-lifetime product for the quasifree electron and the mobility of the trapped electron. For some liquids (e.g., supercritical CO2) this time is sufficiently long (> 100 ns) and the standard conductivity equipment can be used. Even if the time resolution cannot be increased (due to the adverse effect on the sensitivity), the trapping dynamics can be studied provided that the trapping competes with cross recombination of quasifree electrons with holes in the solvent bulk. Since the mobility of these quasifree electrons is very large (10-10^3 cm^2/Vs), this recombination is facile even when the density of ionization events is fairly low (< 1 mM). Perturbation of the geminate electron-hole dynamics is not required for this method to work., Comment: 21 pages, 1 Appendix, 1 figure; to be submitted to J. Phys. Chem. A

Time resolved transient absorption spectroscopy has been used to determine quantum yields for electron photodetachment in 193 nm and (where possible) 248 nm laser excitation of miscellaneous aqueous anions, including hexacyanoferrate(II), sulfate, sulfite, halide anions (Cl-, Br-, and I-), pseudohalide anions (OH-, HS-, CNS-), and several common inorganic anions for which no quantum yields have been reported heretofore: SO3=, NO2-, NO3-, ClO3- and ClO4-. Molar extinction coefficients for these anions and photoproducts of electron detachment from these anions at the excitation wavelengths were also determined. These results are discussed in the context of recent ultrafast kinetic studies and compared with the previous data obtained by product analyses. We suggest using electron photodetachment from the aqueous halide and pseudohalide anions as actinometric standard for time-resolved studies of aqueous photosystems in the UV., Comment: 41 page, 6 figures; supplement: 3 pages, 12 figures; to be submitted to J. Phys. Chem. A

Charge-transfer-to-solvent (CTTS) reactions of hydroxide induced by 200 nm monophotonic or 337 nm and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, eaq-, have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component (ca. 13 ps) and a slower power "tail" due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% vs. 17.5% at 25 oC, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is < 4 cm TW-1 M-1 (assuming unity quantum efficiency for the photodetachment). For [OH-] between 10 mM and 10 M, almost no concentration dependence of the time profiles of solvated electron kinetics was observed. At higher temperature, the escape fraction of the electrons increases with a slope of 3x10-3 K-1 and the recombination and diffusion-controlled dissociation of the close pairs become faster. Activation energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics. The implications of these results for photoionization of water are discussed., Comment: 44 pages, 9 figures; supplement: 4 pages, 7 figures; to be submitted to J. Chem. Phys

Picosecond dynamics of hydrated electrons and hydroxyl radicals generated in 200 nm photodissociation of aqueous hydroxide and 400 nm (3-photon) ionization of water in concentrated alkaline solutions were obtained. No deprotonation of hydroxyl radicals was observed on sub-nanosecond time scale, even in 1-10 M KOH solutions. This result is completely at odds with the kinetic data for deprotonation of OH radical in dilute alkaline solutions. We suggest that the deprotonation of hydroxyl radical is slowed down dramatically in concentrated alkaline solutions., Comment: 12 pages; 2 figures; submitted to Chem. Phys. Lett

Rapid, efficient deactivation of the photoexcited PAH cations accounts for their remarkable photostability and have important implications for astrochemistry, as these cations are the leading candidates for the species responsible for the diffuse interstellar bands (DIB) observed throughout the Galaxy.Ultrafast relaxation dynamics for photoexcited PAH cations isolated in boric acid glass have been studied using femtosecond and picosecond transient grating spectroscopy. With the exception of perylene+, the recovery kinetics for the ground doublet (D0) states of these radical cations are biexponential, containing a fast (< 200 fs) and a slow (3-20 ps) components. No temperature dependence or isotope effect was observed for the fast component, whereas the slow component exhibits both the H/D isotope effect (1.1-1.3) and strong temperature dependence (15 to 300 K). We suggest that the fast component is due to internal Dn to D0 conversion and the slow component is due to vibrational energy transfer (VET) from a hot D0 state to the glass matrix., Comment: 26 pages, 3 figures; w supplement: 8 pages, 8 figures

While ionization of some polyatomic molecular liquids (such as water and aliphatic alcohols) yields so-called "solvated electrons" in which the excess electron density is localized in the interstices between the solvent molecules, most organic and inorganic liquids yield radical anions and cations in which the electron and spin densities reside on the solvent molecule or, more commonly, a group of such molecules. The resulting multimer ions have many unusual properties, such as high rates of diffusive hopping. The "solvated electron" can be regarded as a variant of a multimer radical anion in which the charge, while perturbing the solvent molecules, mainly resides in the space between these molecules. We give several examples of less known modes for electron localization in liquids that yield multimer radical anions (such as C6F6, benzene, acetonitrile, carbon disulfide and dioxide, etc.) and holes localization in liquids that yield multimer radical cations (such as cycloalkanes). Current understanding of the reaction properties for these high-mobility solvent radical anions and cations is discussed., Comment: truncated chapter "Radical Ions in Liquids", to appear in "Charged Particle and Photon Interactions with Matter. Chemical, Physicochemical, and Biological Consequences with Applications", eds. Y. Hatano and A. Mozumder, Marcel Dekker, NY, 2004

Single-shot ultrafast absorbance spectroscopy based on the frequency encoding of the kinetics is analyzed theoretically and implemented experimentally. The kinetics are sampled in the frequency domain using linearly chirped, amplified 33 fs FWHM pulses derived from a Ti:sapphire laser. A variable length grating pair compressor is used to achieve the time resolution of 500-1000 channels per a 2-to-160 ps window with sensitivity > 5x10-4. In terms of the acquisition time, FDSS has an advantage over the pump-probe spectroscopy in a situation when the "noise" is dominated by amplitude variations of the signal, due to the pump and flow instabilities. The possibilities of FDSS are illustrated with the kinetics obtained in multiphoton ionization of water and aqueous iodide and one-photon excitation of polycrystalline ZnSe and thin-film amorphous Si:H. Unlike other "single-shot" techniques, FDSS can be implemented for fluid samples flowing in a high-speed jet and for thin solid samples that exhibit interference fringes; no a priori knowledge of the excitation profile of the pump across the beam is needed. Another advantage is that due to the interference of quasimonochromatic components of the chirped probe pulse, an oscillation pattern near the origin of the FDSS kinetics emerges. This pattern is unique and can be used to determine the complex dielectric function of the photogenerated species., Comment: 29 pages, 10 figures

Single-shot ultrafast absorbance spectroscopy based on the frequency encoding of the kinetics is analyzed theoretically and implemented experimentally. In Part II of the series, arbitrary thickness sample is analysed theroretically. The model is then used to simulate the results for a-Si:H films., Comment: 12 pages, 14 figures