1,497 research outputs found
Electron Photodetachment from Aqueous Anions. III. Dynamics of Geminate Pairs Derived from Photoexcitation of Mono- vs. Poly- atomic Anions
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 + Supplemen
Nematic crossover in BaFeAs under uniaxial stress
Raman scattering can detect spontaneous point-group symmetry breaking without
resorting to single-domain samples. Here we use this technique to study
, the parent compound of the "122" Fe-based
superconductors. We show that an applied compression along the Fe-Fe direction,
which is commonly used to produce untwinned orthorhombic samples, changes the
structural phase transition at temperature into a crossover
that spans a considerable temperature range above . Even in
crystals that are not subject to any applied force, a distribution of
substantial residual stress remains, which may explain phenomena that are
seemingly indicative of symmetry breaking above . Our results
are consistent with an onset of spontaneous nematicity only below
.Comment: 4 pages, 4 figure
-analysis of H3N3-2\textbf{}-based difference method for fractional hyperbolic equations
A novel H3N3-2 interpolation approximation for the Caputo fractional
derivative of order is derived in this paper, which improves
the popular L2C formula with (3-)-order accuracy. By an interpolation
technique, the second-order accuracy of the truncation error is skillfully
estimated. Based on this formula, a finite difference scheme with second-order
accuracy both in time and in space is constructed for the initial-boundary
value problem of the time fractional hyperbolic equation. It is well known that
the coefficients' properties of discrete fractional derivatives are fundamental
to the numerical stability of time fractional differential models. We prove the
related properties of the coefficients of the H3N3-2 approximate
formula. With these properties, the numerical stability and convergence of the
difference scheme are derived immediately by the energy method in the sense of
-norm. Considering the weak regularity of the solution to the problem at
the starting time, a finite difference scheme on the graded meshes based on
H3N3-2 formula is also presented. The numerical simulations are
performed to show the effectiveness of the derived finite difference schemes,
in which the fast algorithms are employed to speed up the numerical
computation
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