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  • 1
    Digitale Medien
    Digitale Medien
    On the mechanism of antiphase electron spin resonance spectra of radical pairs (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 8743-8746 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We have investigated the origin of the antiphase structure of the mw-induced ESR spectrum of a radical pair in a micelle. By applying an accurate numerical method we have examined various specially designed cases that clearly distinguish between the proposed mechanisms for the production of antiphase structure. It is demonstrated that neither of the previously proposed mechanisms can explain the observations. We argue that the only possible mechanism is a coherent, reencounter type mechanism involving two consecutive steps similar to the well established S–T0 mechanism. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.477541
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  • 2
    Digitale Medien
    Digitale Medien
    The spectral forms of transient ESR spectra of radical pairs and the origin of the antiphase structure (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 1595-1604 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We show that transient electron spin resonance (ESR) spectra of short lived radical pairs can be written formally as a superposition of Lorentzian-like lines and the corresponding dispersive lines. The coefficients to these spectral components depend strongly on the frequency and amplitude of the microwave (mw) field and consequently a variety of spectral forms may result. The coefficients, and thus the spectral form, are determined by the interradical interaction induced rate of change of the longitudinal and transversal electron spin polarization. The longitudinal polarization is responsible for the well known chemically induced dynamic electron polarization (CIDEP) which changes the intensity of the lines without affecting the line shape. The transversal polarization is shown to be responsible for the antiphase structure (APS) in accordance with our previous proposition [J. Chem. Phys. 109, 8743 (1998)]. A simple one re-encounter model calculation illustrates and explains qualitatively most of the observed APS characteristics. Numerical calculations are used to discuss anomalously large APS splitting. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.481947
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  • 3
    Digitale Medien
    Digitale Medien
    Green's function calculation of electron spin polarization. III. Freely diffusing radical pairs (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 8881-8888 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A general analytic expression is derived for the electron spin polarization (CIDEP) generated in neutral radicals pairs in liquid solutions at high magnetic fields. It incorporates arbitrary reactivities of the singlet and triplet states of the radical pair complex, additional dephasing processes at the contact distance, and arbitrary values of the singlet–triplet mixing parameter. The exchange interaction is assumed to decrease exponentially with the separation between the radicals and is characterized by a decay length lx and a maximum value J0. The only condition for the applicability of the expression is that the exchange interaction is locally weak (J0lx2/D)2(very-much-less-than)1, but it may be integral strong (D is the relative diffusion constant for the radicals). By comparison with numerical results, it is shown that the expression is very accurate; no deviations were found. The expression is obtained from our previously derived general expression by explicitly evaluating all the fundamental quantities from the Green's function of the free diffusive motion. It is shown that the previously published formulas correspond to special cases of the present expression. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.478897
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  • 4
    Digitale Medien
    Digitale Medien
    Green's function calculation of electron spin polarization. II. Approximation schemes (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 8869-8880 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We present two approximation schemes to the previously derived Green's function method that utilizes a gyroscopic representation of the spin state. First a consistent approximation scheme is developed in which the exact equations are expanded in terms of the small parameter lx/d, where lx is the decay length of the exchange interaction and d is the distance of closest approach. A general and explicit expression, correct to first order in the expansion parameter, is derived for spherical symmetric systems. Secondly, we introduce a modified kinematic approximation which for the first time accounts for recombination and dephasing processes. We show that for spherically symmetric systems the results of the modified kinematic approximation is equivalent to the first order results. This equivalence constitutes the first formal proof of the validity of a kinematic approximation. The derived expression depends only on the magnitude and decay length of the exchange interaction, the recombination and dephasing rate constants, and on the free Green's function. The problem of calculating electron spin polarization (CIDEP) is thus reduced to a calculation of the free Green's function, which describes the relative motion of the radicals in the absence of recombination. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.478896
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  • 5
    Digitale Medien
    Digitale Medien
    Green's function calculation of electron spin polarization. V. Quasi-equilibrium and fast mixing approximations for micellar radical pairs (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 3230-3240 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Simple analytical formulas are derived for the electron spin polarization (CIDEP) generated in neutral radicals pairs in micellar solutions at high magnetic fields. The formulas are applicable for a locally weak exchange interactions, which could be integrally strong, and are based on the general analytic expressions obtained in the preceeding article of this series. We introduce two approximation schemes, for slow and for fast singlet–triplet mixing. For practical purposes these two cover the complete range of mixing rates. The approximation for slow mixing is similar to the quasi-equilibrium approximation, but we have derived more accurate expressions and explicitly evaluated all quantities. For fast singlet–triplet mixing we suggest a new and highly accurate approximation, which replaces the micellar RP with an effective freely diffusing RP with a redefined value of the exchange interaction. The accuracy of the approximations is documented by a comparison with numerically exact results. A specific feature for micelles is that the polarization as a function af the singlet–triplet mixing rate (q) has two maxima, where the low field maximum can be as large as 1/2 of the maximun possible value even for very small values of the exchange interaction. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1385556
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  • 6
    Digitale Medien
    Digitale Medien
    Reversible electron transfer in photochemistry and electrochemistry (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2652-2663 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A complete set of integral equations is used to describe the kinetics of reversible photoionization after instantaneous excitation, including geminate and bimolecular charge recombination, to either the ground or excited states of neutral products. The normalized distribution of ionization products, calculated taking into account reverse electron transfer, differs from that for irreversible photoionization. At low reorganization energy and slow diffusion, the reversibility of the quasiresonant ionization reduces its quantum yield, but does not affect the charge separation quantum yield. The excitations restored by bimolecular recombination of ions produce the delayed fluorescence which goes to zero as a second power of time. The quantum yield of the electroluminescence detected after injection of ions into solution has a sharp free energy dependence, like that observed experimentally. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1385161
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  • 7
    Digitale Medien
    Digitale Medien
    Green's function calculation of electron spin polarization. IV. Radical pairs in micelles (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 3219-3229 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: An analytic expression is derived for the electron spin polarization (CIDEP) of neutral radicals pairs in micellar solutions at high magnetic fields. It is valid for arbitrary reactivities of the singlet and triplet states of the radical pair complex, additional dephasing processes at the contact distance, and arbitrary values of the singlet-triplet mixing and the micellar parameters. The expression is obtained from our previously derived general formula by explicitly evaluating all the fundamental quantities, that are expressed in terms of the Green's function of the diffusive motion in a micelle and which apparently has not been given before. The exchange interaction is assumed to decrease exponentially with the separation between the radicals and to be characterized by a decay length lx and a maximum value J0. The micelle is modeled by a potential well. The only condition for the applicability of the expression is that the exchange interaction is locally weak, i.e., (J0lx2/Dm)2(very-much-less-than)1, but it may be integrally strong; Dm is the relative diffusion constant for the radicals in the micelle. A comparison with numerical results showed that the expression is very accurate, no deviations were found. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1385555
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  • 8
    Digitale Medien
    Digitale Medien
    Viscosity dependence of geminate recombination efficiency after bimolecular charge separation (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 2472-2479 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The efficiency of geminate recombination after bimolecular ionization of an excited electron donor (or acceptor) is studied. For ions starting from inside or outside the recombination layer, the recombination efficiency has the opposite viscosity dependence. As a result the latter becomes nonmonotonous, provided the kinetic controlled ionization gives way to a diffusional one, creating the more remote ions the higher the solvent viscosity. This effect, first discovered experimentally, receives its explanation here, which is conceivable only on the basis of the Integral Encounter Theory of remote electron transfer in the liquid state. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1433746
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  • 9
    Digitale Medien
    Digitale Medien
    Fluorescence and phosphorescence resulting from electrochemical generation of triplet excitations (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 10464-10471 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The biexcitonic annihilation of triplet products of ion recombination is incorporated in the kinetic description of electrochemiluminescence. This process resulting in excitation and fluorescencence of singlets is the dominant mechanism of the initial quenching of triplets, while their concentration is still high. Therefore, the contribution of triplets in subsequent phosphorescence is never as high as expected even at a large exergonicity of inter-ion electron transfer when the recombination to the triplet state prevails on recombination to the ground state of neutral products. The kinetics and quantum yields of fluorescence and phosphorescence are calculated by means of integral encounter theory. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1413965
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