Comparison of exciplex generation under optical and X-ray excitation.

Exciplex generation under optical and X-ray excitation in identical conditions is experimentally compared using a specially chosen model donor-acceptor system, anthracene (electron acceptor) and N,N-dimethylaniline (electron donor) in non-polar solution, and the results are analyzed and interpreted based on analytically calculated luminescence quantum yields. Calculations are performed on the basis of kinetic equations for multistage schemes of bulk exciplex production reaction under optical excitation and combination of bulk and geminate reactions of radical ion pairs under X-ray excitation. These results explain the earlier experimentally found difference in the ratio of the quantum yields of exciplexes and excited electron acceptors (exciplex generation efficiency) and the corresponding change in the exciplex generation efficiency under X-irradiation as compared to the reaction under optical excitation.

Analysis of the methods for the derivation of binary kinetic equations in the theory of fluorescence concentration quenching.

In the framework of unified many-particle approach the familiar problem of fluorescence concentration quenching in the presence of pumping (light pulse) of arbitrary intensity is considered. This process is a vivid and the simplest example of multistage bulk reaction including bimolecular irreversible quenching reaction and reversible monomolecular transformation as elementary stages. General relation between the kinetics of multistage bulk reaction and that of the elementary stage of quenching has been established. This allows one to derive general kinetic equations (of two types) for the multistage reaction in question on the basis of general kinetic equations (differential and integro-differential) of elementary stage of quenching. Relying on the same unified many-particle approach we have developed binary approximations with the use of two (frequently employed in the literature) many-particle methods (such as simple superposition approximation and the method of extracting pair channels in three-particle correlation evolution) to the derivation of non-Markovian binary kinetic equations. The possibility of reducing the obtained binary equations to the Markovian equations of formal chemical kinetics has been considered. As an example the exact solution of the problem (for the specific case) is examined, and the applicability of two many particle methods of derivation of binary equations is analyzed.

CPMG echo amplitudes with arbitrary refocusing angle: explicit expressions, asymptotic behavior, approximations.

Exact explicit analytical expression for echoes in the Carr-Purcell-Meiboom-Gill sequence with arbitrary excitation and refocusing angles and resonance offset of RF pulses was obtained, employing the generating functions formalism developed earlier by authors. Asymptotic form and analytical approximation for echoes were derived in an elegant way and analyzed in details. In particular, it was shown that depending on T(1), T(2) and parameters of the pulse sequence, oscillatory behavior of echoes can take place. Accuracy of asymptotic forms and approximations were tested by comparison with exactly calculated echo amplitudes. Besides, it was shown, that the generating function approach can be applied to the consideration of terminated pulse sequences, when after-pulses echoes are registered.

Analysis of lipid peroxidation kinetics. 1. Role of recombination of alkyl and peroxyl radicals.

The kinetics of the lipid peroxidation reaction is only partly understood. Although the set of reactions constituting the overall reaction are believed to be known, it has not been possible to predict how the reaction will respond to a change of one or more of the parameters, e.g., initial concentrations of reactants or different ways of initiating the reaction, nor has it been possible to predict the time dependence of the products. The reason for these problems is the complicated structure of the kinetic scheme, which includes a chain reaction. In this work we perform an in-depth analysis of the importance of the individual reaction steps, and we introduce a new quasi-stationary concentration method based on the assumption that one or more concentrations vary much slower than the others. We show that it is justified to use a quasi-stationary concentration approximation for the alkyl radical L*, but not for the peroxyl radicals LO2 * as assumed in previous works. The method allows us to derive manageable analytical expressions. On the basis of literature values of the rate constants, we are able to introduce specific simplifications that allow us to obtain simple analytical expressions for the time dependence of all concentrations involved in the process.

General kinetic laws of monomolecular-bimolecular reaction A+B <= => C in solutions.

Non-Markovian kinetic equations of the reversible monomolecular-bimolecular reactions of the type A+B right arrow over left arrow C (at arbitrary ratio between A and B concentrations) derived earlier are used in the calculation of kinetics on macroscopic space-time scales. It is found that the kinetics of the systems with different structure of reactants is universal, and it is the direct generalization of the kinetic law of mass action of formal chemical kinetics. The analysis of the kinetics allows one to establish the time range of the applicability of the law of mass action. It is shown that beyond these limits the usual kinetic law of mass action becomes invalid, and correct description of the kinetics even in the most rough approximation calls for the non-Markovian corrections to usual kinetic laws.