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Ilya A. Shkrob

Publications and source records attributed to Ilya A. Shkrob.

30 records · Page 2Linked to original sources
arXiv2004

Geminate recombination dynamics studied via electron reexcitation: Kinetic analysis for anion CTTS photosystems

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.

physics.chem-ph↗
arXiv2004

Radiation Chemistry of Organic Liquids: Saturated Hydrocarbons

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.

physics.chem-ph↗
arXiv2004

Light-induced temperature jump causes power-dependent ultrafast kinetics of electrons generated in multiphoton ionization of liquid water

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.

physics.chem-ph↗
arXiv2004

Electron Photodetachment from Aqueous Anions. II. Ionic Strength Effect on Geminate Recombination Dynamics and Quantum Yield for Hydrated Electron

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.

physics.chem-ph↗
arXiv2004

Rapid electron trapping studied by pump-probe photoconductivity: kinetic analysis

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.

physics.chem-ph↗
arXiv2004

Electron Photodetachment from Aqueous Anions. I. Quantum Yields for Generation of Hydrated Electron by 193 and 248 nm Laser Photoexcitation of Miscellaneous Inorganic Anions

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.

physics.chem-ph↗
arXiv2004

Ultrafast Dynamics for Electron Photodetachment from Aqueous Hydroxide

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.

physics.chem-ph↗
arXiv2004

Ultrafast studies on the photophysics of matrix-isolated radical cations of polycyclic aromatic hydrocarbons: implications for the Diffuse Interstellar Bands (DIB) problem

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.

astro-ph↗
arXiv2004

Multimer Radical Ions and Electron/Hole Localization in Polyatomic Molecular Liquids: A critical review

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.

physics.chem-ph↗
arXiv2004

Recombination of Geminate (OH,eaq-) Pairs in Concentrated Alkaline Solutions: Lack of Evidence For Hydroxyl Radical Deprotonation

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.

physics.chem-ph↗
arXiv2004

Frequency-domain "single-shot" (FDSS) ultrafast transient absorption spectroscopy using compressed laser pulses. Part I. Basic treatment

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.

physics.optics↗