87 research outputs found
Effect of Anisotropic Reactivity on the Rate of Diffusion-Controlled Reactions: Comparative Analysis of the Models of Patches and Hemispheres
AbstractA comparative analysis of two models of anisotropic reactivity in bimolecular diffusion-controlled reaction kinetics is presented. One is the conventional model of reactive patches (MRP), where the surface of a molecule is assumed to be reactive over a certain region (circular patch) with the rest of the surface being inert. Another one is the model of reactive hemispheres (MRH), assuming that a molecule is reactive within a certain distance from a point on its surface. The accuracy of the known and newly derived simple analytical expressions for the reaction rate is tested by comparison with the simulation results obtained by the original Brownian dynamics method. These formulas prove to be quite accurate in the practically important limit of strong anisotropy corresponding to small size of the reactive patches or hemispheres. Numerical calculations confirm earlier predictions that the MRP rates are much smaller than the MRH rates for the same radii of the reactive regions, especially in the case where both reacting molecules are anisotropic
Generation of electron spin polarization in disordered organic semiconductors
The generation mechanisms of electron spin polarization (ESP) of charge
carriers (electrons and holes, called "doublets") in doublet-doublet
recombination and triplet-doublet quenching in disordered organic
semiconductors are analyzed in detail. The ESP is assumed to result from
quantum transitions between the states of the spin Hamiltonian of the pair of
interacting particles. The value of the ESP is essentially determined by the
mechanism of relative motion of particles. In our work we have considered the
cage and free diffusion models. The effect of possible attractive
spin-independent interactions between particles is also analyzed. Estimation
with obtained formulas shows that the proposed mechanisms can lead to a fairly
strong ESP much larger than the thermal one (at room temperatures)Comment: 10 pages, 3 figure
Magnetic field effects on electron-hole recombination in disordered organic semiconductors
Characteristic properties of magnetic field effects on spin selective
geminate and bulk electron-hole polaron pair (PP) recombination are analyzed in
detail within the approach based on the stochastic Liouville equation. Simple
expressions for the magnetic field (B) dependence of recombination yield and
rate are derived within two models of relative PP motion: free diffusion and
diffusion in the presence of well (cage). The spin evolution of PPs is
described taking in account the relaxation induced by hyperfine interaction,
anisotropic part of the Zeeman interaction induced, as well as -mechanism. A large variety of the -dependences of the recombination yield
and rate is obtained depending on the relative weights of
above-mentioned mechanisms. The proposed general method and derived particular
formulas are shown to be quite useful for the analysis of recent experimental
results.Comment: 12 pages, 3 figure
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
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