Light-induced drift of ions in a magnetic field

The effect of a magnetic field on the ion drift in a weakly ionized gas under the combined action of the light-induced drift and light pressure is studied theoretically. It is shown that, under the action of light, a component of ion drift velocity transverse to the direction of propagation of radiation may appear in a weakly ionized gas upon the application of an external magnetic field. It is shown that the Lorentz force acting on ions in the magnetic field radically changes the dependence of the ion drift velocity on the radiation frequency detuning. It is predicted that the ion drift velocity component along the direction of radiation must reverse its sign upon an increase in the magnetic field and an anomalous light-induced drift may be observed.     

Role of the velocity frame of reference in thermodiffusion in liquid mixtures

The role and significance of the velocity frame of reference in the interpretation, modeling and formulation of thermodiffusion in multicomponent liquid mixtures were investigated, focused on the nonequilibrium thermodynamics modeling approach. The effect of the velocity frame of reference on the phenomenological equations and thermodiffusion coefficients and expressions is explored. Theoretically and also by the aid of representative calculations, it is shown that while in binary mixtures transformation from one frame to another does not affect the sign and magnitude of the thermodiffusion coefficients, in multicomponent mixtures (ternary and higher), even the sign of the thermodiffusion coefficients may change when an alternative frame is used. This implies that in multicomponent mixtures for either experimental data or model estimations, the employed velocity frame for the thermodiffusion coefficients plays an important role in calculations. The Soret coefficients and the thermodiffusion factors are independent of the frame of reference.     

Influence of a magnetic field on the light-induced ion drift

It is shown that the velocity component transverse to the radiation propagation direction can arise in the light-induced ion drift (LIID) after applying an external magnetic field to a weakly ionized gas. It is predicted that the projection of the ion drift velocity onto the radiation direction changes its sign with an increase in the magnetic field, resulting in the anomalous LIID.     

激发光线宽对原子光致漂移速率的影响

在光致漂移效应的研究中,激发光线宽会改变原子激发的速度选择性,进而影响漂移速率的大小.本文以原子光致漂移速率方程理论为基础,利用强碰撞模型描述原子与缓冲气体的碰撞作用,运用数值方法对速率方程进行求解计算,研究了激发光线宽对原子漂移速率的影响.研究结果表明,其他条件相同时,随着线宽的增大,漂移速率的值呈现先增大后减小的趋势.存在一个最佳的激发光线宽,使得原子的漂移速率达到最大值.最佳线宽与激发光功率密度、温度和缓冲气体压强有关.为了获得最佳的光致漂移效果,激发光应工作在最佳线宽条件下.当激发光线宽在最佳线宽附近波动时,设置激发光线宽略大于最佳线宽可减少线宽波动对漂移速率的影响,对获得较大漂移速率更为有利.     

Multiphoton avalanche generation of free carriers in a multiband crystal

A new mechanism for generating nonequilibrium electron-hole pairs in transparent wide-gap crystals under high-power picosecond light pulses of prebreakdown intensity is considered. The kinetics of free carrier generation is investigated by the numerical solution of the system of balance equations for particle concentrations within the multiband model of electron energy spectrum of the crystal. It is shown that in the intensity range of 10¹⁰–10¹² W/cm² the proposed way of nonequilibrium carrier generation is more effective in the case of picosecond pulses than the conventional multiphoton absorption. It is shown that the nonequilibrium carrier generation considered here may initiate a multiphoton avalanche.     

Nonlinear effects in gases in the Couette problem

The nonlinear processes of heat and mass transfer in a rarefied gas confined between two infinite parallel plates maintained at different temperatures and moving at a relative velocity are considered. The profiles of the gas macroscopic flow velocity, density, temperature, heat fluxes, and shear stress were calculated on the basis of kinetic equations by the discrete velocity method in a wide range of Knudsen numbers at different values of temperature difference between the plates and plate velocities. It was shown that under certain conditions, the direction of gas flow near the “hot” plate can change to the opposite. It was discovered that the longitudinal and normal components of heat flux at a certain temperature difference between the plates change their orientation to the opposite in transition and nearly free molecular regimes.     

Kinetics of CO Molecules Taking into Account Resonant VE Exchanges in a Nonequilibrium Nozzle Flow

The kinetics of CO molecules with excited electronic levels for a nonequilibrium flow in nozzles is studied. Three electronic energy levels are taken into account; the VV exchanges of the vibrational energy within each electronic energy level, VT transitions from the vibrational energy to the translational energy, and VE exchanges between the vibrational energy and the electronic terms occur in the system. The system of equations of vibrational level kinetics and gasdynamics, which is written in the zeroth approximation of the Chapman–Enskog method, is solved numerically. Special attention is paid to estimating the effect of the VE exchanges of the vibrational energy on the formation of nonequilibrium populations of the vibrational levels. It is shown that resonant VE transitions considerably affect the relaxation process and noticeably change the population of the upper vibrational levels even in the absence of recombination and dissociation processes; VE exchanges also affect the time dependence of the vibrational temperature.     

Effect of the bulk viscosity on the sound propagation in nonequilibrium suspensions of microparticles in gas

The sound propagation in a mixture of gas with uniformly dispersed solid particles, whose temperature is maintained above that of the gas by an external source, is considered. The dispersion properties of this kind of suspensions are studied, and expressions for the second viscosity and the sound velocity in such suspensions are derived. It is shown that, in a nonequilibrium suspension, the second viscosity may be negative. The ranges of the suspension parameters, for which the propagation of low-frequency sound is impossible, are determined.     

Sound amplification in inhomogeneous flows of nonequilibrium gas

The conditions of the acoustic instability of flows of thermodynamically nonequilibrium gas are determined. It is shown that the growth increments of the velocity, temperature, pressure, and density disturbances are different. When the Mach numbers are small, only the velocity and temperature disturbances grow along the flow.     

Role of confined Bloch waves in the near field heat transfer between two photonic crystals

The near field heat transfer between two finite size one-dimensional photonic crystals separated by a small vacuum gap and maintained in nonequilibrium thermal situation is theoretically investigated. The main features of this electromagnetic transfer are discussed and compared with what is generally observed with media that support surface polaritons. It is shown that the presence of surface Bloch waves can significantly enhance heat transfers beyond the far field limit for both polarization states of electromagnetic field at subwavelength separation distances. A specific attention is addressed to the consequence of the slopes of surface Bloch waves dispersion curves on the heat transfer. In particular, it is shown that the localization of surface Bloch waves close to the light line allows to observe a transfer exaltaion at larger separation distances than the Wien wavelength. These results could open new possibilities for the development of innovative near-field technologies such as near-field thermophotovoltaic conversion, plasmon assisted nanophotolitography or near-field spectroscopy.     

Photovoltaic effect in unipolar multivalley semiconductors

Abstract

The theory of the nonequilibrium charge carrier transport in unipolar multivalley semiconductors is developed. It is shown that the diffusion of photoexcited nonequilibrium heavy and light electrons in multivalley semiconductors is a correlated process, like the ambipolar diffusion in the case of the electron-hole plasma. The light-induced intervalley transitions, resulting in the imbalance between the subsystems of the light and heavy electrons, give rise to the electromotive force (emf) through the mechanism of the Dember photovoltaic effect. The value of the emf occurring in the ‘metal-semiconductor-metal’ structure is calculated in the linear approximation in terms of the light intensity as a small parameter. It is shown that the emf is determined by the conductivity of heavy and light electron subsystems, as well as by the surface conductivity of the metal-semiconductor interface.     

Tagged particle fluctuations in uniform shear flow

The nonlinear Boltzmann and Boltzmann-Lorentz equations are used to describe the dynamics of a tagged particle in a nonequilibrium gas. For the special case of Maxwell molecules with uniform shear flow, an exact set of equations for the average position and velocity, and their fluctuations, is obtained. The results apply for arbitrary magnitude of the shear rate and include the effects of viscous heating. A generalization of Onsager's assumption of the regression of fluctuations is found to apply for the relationship between the equations for the average dynamics and those for the time correlation functions. The connection between fluctuations and dissipation is described by the equations for the equal-time correlation function. The source term in these equations indicates that the “noise” in this nonequilibrium state is qualitatively different from that in equilibrium, or even local equilibrium. These equations are solved to determine the velocity autocorrelation function as a function of the shear rate.     

Dissipation of vortex disturbances in a vibrationally nonequilibrium diatomic gas

The influence of the vibrational relaxation on suppression of the Kelvin??Helmholtz instability in an evolving shear layer of a vibrationally nonequilibrium diatomic gas is studied numerically on the basis of equations of two-temperature aerohydrodynamics. Planar waves with the maximum growth rates, which were computed within the framework of a linearized system of equations of inviscid two-temperature gas dynamics, are used as the initial disturbances. It is shown that relaxation of the nonequilibrium vibrational mode at excitation levels, which can be obtained in diatomic gases in nozzle flows, in underexpanded jets, or in flows with moderate laser pumping, is accompanied by noticeable suppression of vortex disturbances. The associated relative enhancement of dissipation of kinetic energy of a large vortex structure averaged over its ??lifetime?? reaches approximately 13 %.     

Thermodynamic parameterization

A new method of succesive construction of a solution is developed for problems of strongly nonequilibrium Boltzmann kinetics beyond normal solutions. Firstly, the method provides dynamic equations for any manifold of distributions where one looks for an approximate solution. Secondly, it gives a successive procedure of obtaining corrections to these approximations. The method requires meither small parameters, nor strong restrictions upon the initial approximation; it involves solutions of linear problems. It is concordant with the H-theorem at every step. In particular, for the Tamm-Mott-Smith approximation, dynamic equations are obtained, an expansion for the strong shock is introduced, and a linear equation for the first correction is found.     

Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.     

Calculating the nonequilibrium carrier relaxation kinetics in AlGaAs

The effect of the initial density of nonequilibrium electron–hole pairs on the kinetics of their relaxation is analyzed. The influence of cooling, formation, ionization, and radiative recombination of excitons is discussed. It is shown that the exciton lifetime increases with the optical excitation density and, as a result, maximum exciton density is attained for a longer time.