20 research outputs found
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.Comment: 41 page, 6 figures; supplement: 3 pages, 12 figures; to be submitted
to J. Phys. Chem.
Electron Trapping by Polar Molecules in Alkane Liquids: Cluster Chemistry in Dilute Solution
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 figure
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Radical Cations in Radiation Chemistry of Liquid Hydrocarbons
The state of knowledge concerning radical cations in liquid alkanes is discussed with particular emphasis on those which exhibit high mobility. Uncertainty has existed in the interpretation of previous results with respect to the nature and reactivity of high mobility ions, especially for cyclohexane. Recent time-resolved studies on pulse radiolysis/transient absorption, photoconductivity, and magnetic resonance in these systems have led us to propose new mechanisms for the high mobility ions. In decalins, scavenging of these ions by solutes is a pseudo-first-order reaction. In cyclohexane, the behavior is more complex and is indicative of the involvement of two species. This bimodality is rationalized in terms of a dynamic equilibrium between two conformers of the solvent radical cation. Several experimental tests supporting these views include a recent study on two-color laser photoionization in cyclohexane
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.Comment: 40 pages, 10 figures + supplement 2 pages, 9 figures; will be
submitted, in a modified form, to J. Phys. Chem
Geminate recombination of hydroxyl radicals generated in 200 nm photodissociation of aqueous hydrogen peroxide
The picosecond dynamics of hydroxyl radicals generated in 200 nm photoinduced
dissociation of aqueous hydrogen peroxide have been observed through their
transient absorbance at 266 nm. It is shown that these kinetics are nearly
exponential, with a decay time of ca. 30 ps. The prompt quantum yield for the
decomposition of H2O2 is 0.56, and the fraction of hydroxyl radicals escaping
from the solvent cage to the water bulk is 64-68%. These recombination kinetics
suggest strong caging of the geminate hydroxyl radicals by water.
Phenomenologically, these kinetics may be rationalized in terms of the
diffusion of hydroxide radicals out of a shallow potential well (a solvent
cage) with an Onsager radius of 0.24 nm.Comment: 14 pages, 1 figur
Photo-Stimulated Electron Detrapping and the Two-State Model for Electron Transport in Nonpolar Liquids
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. Phy