Volume 125, Nº 4 (2017)

Some features of the dynamics of particles and fields at cyclotron resonances have been discussed with the focus on chaotic dynamical regimes. It has been shown that the known criterion of the transition of the regular dynamics of particles to chaotic dynamics at cyclotron resonances sometimes describes this transition incorrectly. The reason for such a feature of the criterion has been revealed. The anomalous sensitivity of the dynamics of particles to external fluctuations at autoresonance has been analyzed. A theory of excitation of electromagnetic waves by a beam of phased oscillators under the conditions of isolated nonlinear cyclotron resonance has been developed. It has been shown that the chaotic dynamical regime is due to the periodic change in the structure of the phase portrait of particles in the wave field. It has been shown that higher moments can play a more significant role than lower moments in almost all chaotic dynamical regimes at cyclotron resonances. In this case, the known kinetic diffusion equations should be generalized with the inclusion of these higher moments.

We describe the general analytic derivation of the dielectronic recombination (DR) rate coefficient for multielectron ions in a plasma based on the statistical theory of an atom in terms of the spatial distribution of the atomic electron density. The dielectronic recombination rates for complex multielectron tungsten ions are calculated numerically in a wide range of variation of the plasma temperature, which is important for modern nuclear fusion studies. The results of statistical theory are compared with the data obtained using level-by-level codes ADPAK, FAC, HULLAC, and experimental results. We consider different statistical DR models based on the Thomas–Fermi distribution, viz., integral and differential with respect to the orbital angular momenta of the ion core and the trapped electron, as well as the Rost model, which is an analog of the Frank–Condon model as applied to atomic structures. In view of its universality and relative simplicity, the statistical approach can be used for obtaining express estimates of the dielectronic recombination rate coefficients in complex calculations of the parameters of the thermonuclear plasmas. The application of statistical methods also provides information for the dielectronic recombination rates with much smaller computer time expenditures as compared to available level-by-level codes.

The properties of the avalanche processes that develop on a dynamical lattice, the structure of links in which changes due to a specific characteristic of each lattice node, namely, its “activity,” which determines the probability of connection of a certain node with neighboring nodes in one step of lattice evolution. The statistics of the sizes of the avalanches appearing in the lattice system is studied as a function of the node activity and the link lifetime (the lifetime of the links formed in the system). It is analytically and numerically shows that the type of avalanche dynamics in the system changes as a function of these parameters. The following three regimes can take place in the system: (1) avalanches of any sizes, from small to catastrophic, can appear, which is reflected in the power-law behavior of the probability density function of the appearance of avalanches of certain sizes; (2) avalanches of a certain average size mainly appear in the system, and the probability density is close to that of a normal distribution; and (3) transient regime, where the probability density function of the appearance of avalanches of certain sizes is close to an exponential function. These results open up the possibilities of controlling the behavior of a complex system; in particular, they can be used to prevent catastrophic avalanches by changing the link lifetime and the average node activity.

Heterochiral islands, in which topological dipoles are oppositely directed, are observed in freestanding antiferroelectric (SmC_A^*) films. The topological dipoles in films with a transverse electric polarization and a planar molecule orientation at island boundaries are coplanar with an electric field. The topological dipoles in films with a longitudinal polarization and a planar orientation at island boundaries are perpendicular to an electric field. For a radial director orientation at island boundaries, the topological dipoles in films with a longitudinal polarization are coplanar with a field. Changing the orientation of an electric field, we can control the position of a topological defect at an island boundary and the orientation of a topological dipole. Heterochiral islands can form dimers with an anomalously small interisland distance.

The control of clustering in molecular beams with the help of IR lasers is experimentally studied. Studies were performed with a molecular CF₃I gas diluted with argon or xenon using a cw CO₂ laser. The control of clustering is based on the resonance vibrational excitation of molecules or clusters by the IR laser radiation near the nozzle exit. Depending on the distance from the excitation region to the nozzle cut, the irradiation of the molecular beam either suppresses the clustering of resonantly excited molecules (when the beam is irradiated near the nozzle exit where clustering occurs) or causes the dissociation of small clusters (when the beam is irradiated away from the nozzle where clustering is at the growth stage). The suppression of molecular clustering and the dissociation of clusters in beams were studied by measuring and analyzing the integrated intensities of ion peaks of cluster fragments with a time-of-flight spectrometer. The efficiencies of clustering suppression and cluster dissociation were studied as functions of exciting laser radiation parameters, the beam irradiation geometry, gas parameters over the nozzle, and the nozzle construction. It is shown that the efficiencies of clustering suppression and cluster dissociation strongly depend on the exciting laser radiation intensity, the nozzle construction, and the distance from the irradiation region to the nozzle cut. Parameters providing the most efficient control of clustering suppression and cluster dissociation are found.

A consistent quantum approach is used to study the influence of intraatomic spin–orbit and hyperfine interactions on the character of a resonance dipole–dipole interatomic interaction and, hence, collective effects. For this purpose, the collective spontaneous decay of excited states and the spectral dependence of the total scattering cross section of a monochromatic light wave are analyzed in the system consisting of two rubidium-87 atoms. The modification of the radiation properties and the interaction of the atoms with external radiation are studied as functions of the interatomic distance. The presence of a complex structure of the sublevels of both the ground and excited states is shown to modify the collective effects substantially as compared to the case when this structure is absent.

The nonlocal dipole–dipole interaction is studied between excitations in chromophores forming a bunch or a tube of J-aggregates and closely spaced quantum dots (QDs). Equations describing the evolution of exciton pulses in a quasi-one-dimensional medium are derived taking into account the interaction with the transition resonant to nanoparticles. It is shown that the efficient controllable resonance energy transfer can occur in the system between QDs and an exciton pulse. The efficiency of this process significantly increases if the bunch of aggregates is deformed to bend nanoparticles round. It is shown that the interaction of permanent dipole moments of QDs and chromophores leads to the formation of a potential barrier or a well. It is found that the combined influence of these factors can be used to efficiently control the dynamics of pulses in aggregates.

The spontaneous emission of an atomic system in the field of a high-intensity femtosecond laser pulse is considered within the framework of a consistent quantum-mechanical approach based on an examination of the interaction of a quantum system with a set of quantized field modes in a vacuum state. Both even and odd harmonics of the driving field are shown to appear in the atomic emission spectrum, and the mechanisms of their generation are elucidated. A comparison with the semiclassical theory of laser harmonic generation is made. The semiclassical approach in describing the spontaneous emission in strong laser fields, especially under conditions of significant depletion of the ground (initial) state in a laser field, is shown to be limited.

We have studied rotating magnetohydrodynamic flows of a thin layer of astrophysical plasma with a free boundary in the β-plane. Nonlinear interactions of the Rossby waves have been analyzed in the shallow-water approximation based on the averaging of the initial equations of the magnetic fluid dynamics of the plasma over the depth. The shallow-water magnetohydrodynamic equations have been generalized to the case of a plasma layer in an external vertical magnetic field. We have considered two types of the flow, viz., the flow in an external vertical magnetic field and the flow in the presence of a horizontal magnetic field. Qualitative analysis of the dispersion curves shows the presence of three-wave nonlinear interactions of the magnetic Rossby waves in both cases. In the particular case of zero external magnetic field, the wave dynamics in the layer of a plasma is analogous to the wave dynamics in a neutral fluid. The asymptotic method of multiscale expansions has been used for deriving the nonlinear equations of interaction in an external vertical magnetic field for slowly varying amplitudes, which describe three-wave interactions in a vertical external magnetic field as well as three-wave interactions of waves in a horizontal magnetic field. It is shown that decay instabilities and parametric wave amplification mechanisms exist in each case under investigation. The instability increments and the parametric gain coefficients have been determined for the relevant processes.

Angular and energy distributions of prompt neutrons from the spontaneous fission of ²⁵²Cf have been measured. The yield of “scission” neutrons has been estimated by comparing the measured distributions with calculations within the model of emission of neutrons from completely accelerated fragments. It has been shown that the best description of measured angular and energy distributions of fission neutrons is achieved under the assumption that neutrons with a higher (about 6–8%) probability are emitted along the fission axis in the center-of-mass system of fission fragments.

Within the Heisenberg model in the presence of uniaxial magnetic anisotropy, formulas are obtained that allow one to estimate both the spontaneous magnetization reversal time of an antiferromagnetic chain and the magnetization reversal time due to interaction with the conducting tip of a scanning tunneling microscope (STM). Corrections due to a possible difference between the properties of the end and inner atoms of the chain are calculated. Numerical estimates are obtained for typical parameter values of the Heisenberg Hamiltonian.