The pulsed version of the reaction yield detected magnetic resonance (RYDMR) method is theoretically considered for the reaction scheme according to which the annihilation of triplet excitons in a molecular crystal occurs. Analytical expressions are obtained that describe the kinetics of prompt fluorescence decay with time under the conditions of its excitation by an ultrashort laser pulse. The corresponding shape of the decay curves of prompt fluorescence is determined. It is shown that the obtained curves can be used for a more detailed determination of the reaction rate constants than in the case of the stationary version of the RYDMR method.

The contribution to the RYDMR spectrum from an elementary event of collision of two triplet excitons during their mutual annihilation is calculated, and the reverse process of photogeneration of a pair of triplet excitons in a molecular crystal are considered. The interaction between the excitons in the cell is assumed weak in comparison with the Zeeman splitting and the fine-structure parameters for the triplet state of each of the molecules. For both the annihilation of excitons and the photogeneration of a pair, the RYDMR signal from a single-crystal sample disappears at certain orientations of the crystal relative to the external magnetic field. This effect was previously experimentally observed for the anthracene−tetracyanobenzene crystal and was explained in the case of stationary optical excitation of the molecule. The results obtained in this paper make it possible to consider an arbitrary, not only stationary, mode of irradiation sample. An interval of angles between the static magnetic field and the crystallographic axes of the crystal within which the lines in the spectrum can disappear is determined. The possibility of using the RYDMR method for studying nanosized objects, for which additional peaks in the RYDMR spectrum may arise, is briefly discussed.

The LCAO MO self-consistent field method in combination with the 6-31** basis set is used to calculate sections of the potential energy surface for a proton in the NH3…HCl system in the presence of an external electrostatic field. The field strength is varied in the range of 0.000 to 0.017 a. u. In the absence of the field, the potential of the proton in the isolated complex has one well, the N–Cl distance is equal to 2.92 Å, and the NH3…H–Cl bond is a hydrogen bond. With increasing distance between the N and Cl atoms, a second well appears in the potential, with the well near the Cl atom remaining deeper. In the presence of an external electric field, with increasing its strength, the depth of the well near the N atom increases, while the height of the barrier for proton transfer from the chlorine to the nitrogen atom decreases. At a certain field strength, the well near the nitrogen atom becomes deeper than that near the chlorine atom, so the proton moves to the nitrogen, making the complex ionic. Thus, the external electric field can influence the type of chemical bond in NH3…HCl system.

Characteristic features of net chemically induced dynamic electron spin polarization (CIDEP) P n in triplet–radical (TR) quenching are analyzed in detail within the framework of a general model that enables one to analyze CIDEP both numerically and analytically. This model also makes it possible to accurately describe the nonadiabatic transitions between the terms of the TR-pair spin Hamiltonian that lead to CIDEP generation. The proposed theory yields a simple analytical dependence of P n on the parameters of the model. In particular, it is shown that, within a wide region of parameters, the dependence of P n on the coefficient of relative TR diffusion D r is described by a simple linear relation: \(P_n^{ - 1}\left( {{D_r}} \right) = {Q_0} + \overline {{q_n}} {D_r}\) (with Q 0 and \({{q_n}}\) independent of D r ). It is also demonstrated that the numerical and analytical results obtained are very useful in analysis of experimental data, as demonstrated by analyzing the experimental dependence of P n on D r .

The properties of topological defects representing local regions of contraction and extension in the Frenkel—Kontorova chains are described. These defects exhibit the properties of quasi-particles—solitons that possess certain effective masses and are capable of moving in the Peierls—Navarro potential field having the same period as that of the substrate on which the chain is situated. The energy characteristics related to soliton motion in the chain are discussed. The dynamics of highly excited solitons that can appear either during topological defect formation or as a result of thermal fluctuation is considered. The decay of such an excitation resulting in soliton thermalization under the action of a fluctuating field generated by atomic vibrations in the chain and substrate is described in terms of the generalized Langevin equation. It is shown that soliton motion can be described using a statistically averaged equation until the moment when the soliton attains the state of thermodynamic equilibrium or is captured in one of the Peierls—Navarro potential wells, after which the motion of soliton in the chain acquires a hopping (activation) character. Analytical expression describing the curve of soliton excitation decay is obtained.

Теоретически проанализированы особенности аномальной (дисперсионной) миграции поляронов P (т.е. электронов и дырок) в неупорядоченных органических полупроводниках и проявление этих особенностей в магнитных эффектах в проводимости полупроводников. В соответствии с результатами исследований, наиболее важной причиной влияния слабых магнитных полей на фотопроводимость является их влияние на PP-рекомбинацию (рекомбинацию электронов и дырок), которая представляет собой спин-селективный процесс. В рамках подхода, основанного на модели непрерывных во времени случайных блужданий (CTRW-модели), показано, что аномальная (дисперсионная) миграция, которая в CTRW-модели описывается медленно спадающим распределением времен ожидания прыжка t-(1 + ), существенно проявляется в магнитозависимости выхода PP-рекомбинации и, таким образом, проводимости. Особенности этого влияния пояснены на примере зависимости выхода рекомбинации от магнитного поля, предсказываемой в рамках g-механизма магниточувствительности.

The specific features of the anomalous (dispersive) migration of polarons P (electrons and holes) in disordered organic semiconductors and manifestations of these features in the magnetic field effects on the conductivity of semiconductors are theoretically examined. According to the results, the most important reason for the influence of weak magnetic fields on the photoconductivity is their impact on PP-recombination (recombination of electrons and holes), a spin-selective process. Within the framework of an approach based on the continuous time random walk (CTRW) model, it is shown that anomalous (dispersive) migration, which is described in the CTRW model by a heavy-tailed hop waiting time distribution ψ( t )∼ t -(1+α) , clearly manifests itself in the magnetic field dependence of the PP-recombination yield and, hence, the conductivity. The specific features of this influence are illustrated by the example of the dependence of the recombination yield on the magnetic induction, which is predicted by the Δ g -mechanism of magnetic sensitivity.

An approximate analytical expression describing the motion of a topological soliton along a Frenkel-Kontorova chain at relatively high speeds is derived with the help of the Langevin equation. Comparison with numerical solutions shows a good quality of the resulting approximation.

The influence of an external varying field on the non-Markovian migration of particles described in the continuous-time random walk model (CTRWM) was analyzed theoretically. In terms of the Markovian representation for the CTRWM suggested earlier, a rigorous method for describing the influence of an external force was developed. This method reduced the problem to solving the non-Markovian stochastic Liouville equation (SLE) for the particle distribution function. An analysis of the derived SLE and its comparison with the earlier equations were performed. The method was used to study the characteristic features of the time dependence of the first and second moments of the distribution function for particles involved in subdiffusion motion in a uniform varying external field. Both oscillating and fluctuating fields were considered. In both cases, anomalously strong field effects on the second particle distribution moment (variance) were observed. This influence was especially strong for a fluctuating field, and in the limit of anomalously slow fluctuations at that.

To describe the motion of a soliton in ideal Frenkel-Kontorova systems at energies above the amplitude of the Peierls-Navarro pinning potential (PNP), a dynamic Langevin equation with stochastic friction force was proposed: ÿ + αẏ − βsin2π y = 0, where y is the dimensionless coordinate of the soliton in units of the substrate potential period, which is determined by the first moment of the soliton shape according to the standard procedure; differentiation is performed with respect to the dimensionless time τ = t ( κ / m ) 1/2 , where κ is the force constant of the chain bond and m is the mass of a chain element. A procedure for determining the friction coefficient α is discussed. Numerical calculations of the dependences of the coordinate and velocity of the soliton on the time over wide ranges of initial conditions and parameters α and β. An analysis of the calculation results showed that the motion of the soliton slows down exponentially, being modulated as it does by the action of the PNP, and end up at one of the minima of the PNP.

A non-Markovian version of the Liouville stochastic equation was used to analyze spin relaxation in a pair of particles with spins 1/2 and the dipole-dipole interaction between the spins. The particles were involved in anomalously slow stochastic relative rotation described by the angular correlation function K(t) ∼ 1/t α with α < 1. The Liouville stochastic equation could be used to describe memory effects in the kinetic dependence of rotational relaxation resulting in a very slow descent of K(t). Our analysis showed that the anomalous relative motion of radicals manifested itself in special features of the shape of the magnetic resonance line of the particle pair.

The notion of a soliton as a quasiparticle in the Frenkel–Kontorova system is substantiated. This allows the use of the diffusive model for the description of the processes of soliton creation and disappearance at the chain ends. The Frenkel–Kontorova model is modified through the addition of barriers with certain height and concentration to the pinning profile of the potential energy surface for the soliton motion along the chain. The ratio of the relaxation times in ideal and defective systems is estimated, and it is shown that the relaxation rate can be reduced by ten and higher orders of magnitude under quite reasonable assumptions.

The non-Markovian variant of the stochastic Liouville equation (SLE) based and the continuous time random walk approach (CTRWA) is proposed. This SLE turns out be represented by the conventional Markovian SLE. The non-Markovian SLE is applied to the analysis of anomalous long-tailed CTRW processes and, in particular, anomalous rate fluctuations in some chemical reactions. These fluctuations are shown to strongly manifest themselves in the reaction kinetics.

Temperature dependence of the proton spin-lattice relaxation time (T1) in powdered benzoic acid dimer and in its deuterated analog is calculated. The model assumes that two protons (deuterons) synchronously move in the double-minimum potential of the dimer. The two-dimensional potential energy surface was constructed previously, which adequately describes the static properties of the hydrogen-bonded complex. The important characteristics of this potential are a very strong mode coupling and a very high proton potential barrier (≳25 kcal/mol), whereas the experimental activation energy for the proton transfer is known to be on the order of 1 kcal/mol only. This apparent discrepancy is removed by our suggestion that the proton transfer is driven by the transitions between OHO fragment vibrational levels under the action of random forces of the surrounding. The excitation of the low-frequency intermolecular vibrations assists such transfer mechanism strongly. Using four fitting parameters to allow for the medium repolarization, the calculated T1 temperature dependence is found to be in good agreement with the experiments in the natural and deuterated benzoic acid dimer. The agreement is best at high temperature where the apparent activation energy for proton transfer was found to be 2.3 kcal/mol.