The Electron Relaxation to Stationary States in Collision Dominated Plasmas in Molecular Gases

The temporal collision dominated relaxation of electrons to new stationary states, starting from initial stationary states and due to jump-like changes of the electric field, was studied in the plasmas of the molecular gases N₂ and CO. Numerical solving of the time dependent Boltzmann equation for the electrons yields the temporal evolution of their energy distribution function and of resulting macroscopic quantities. The varying relaxation due to different values of the field strength in the final stationary state has been investigated considering the molecules of the plasma only as vibrationally non-excited and, in another case, including the additional impact of collisions with vibrationally excited molecules. The results obtained are discussed and, in particular, the relaxation times found for the transitions to the new stationary states are analysed on the basis of the energy transfer effectiveness by the collision processes. An approximative microphysical basis for the understanding of the main features of the relaxation in such complex molecular gas plasmas could be obtained.     

Die Berücksichtigung der Ionisation durch Elektronenstoß in der Elektronenkinetik des schwachionisierten anisothermen Plasmas III. Das instationäre Verhalten bei großer impulsförmiger Feldstörung

The Influence of Ionization by Electron Collisions on the Electron Kinetics of the Low Ionized Anisothermal Plasmas The time behaviour of the electron component was calculated during the additional application of a single pulse to the electric field in the plasma. The investigations were performed for the weakly ionized Ne-plasma as a typical example taking into account supplementarily the direct ionization due to electron collisions and an electron loss term with a constant life time besides elastic and exciting collisions. Using the instationary Boltzmann equation we determined the time behaviour of the essential macroscopic quantities. Besides the caluclation of the marked temporal development of such quantities as the electron concentration, the electron collision frequencies for excitation and ionization and the different energy transfer rates especially the relaxation of the electron component was analysed after switch on and switch off the additional rectangular pulse.     

Modeling phase transition for compressible two-phase flows applied to metastable liquids

The seven-equation model for two-phase flows is a full non-equilibrium model, each phase has its own pressure, velocity, temperature, etc. A single value for each property, an equilibrium value, can be achieved by relaxation methods. This model has better features than other reduced models of equilibrium pressure for the numerical approximations in the presence of non-conservative terms. In this paper we modify this model to include the heat and mass transfer. We insert the heat and mass transfer through temperature and Gibbs free energy relaxation effects. New relaxation terms are modeled and new procedures for the instantaneous temperature and Gibbs free energy relaxation toward equilibrium is proposed. For modeling such relaxation terms, our idea is to make use of the assumptions that the mechanical properties, the pressure and the velocity, relax much faster than the thermal properties, the temperature and the Gibbs free energy, and the ratio of the Gibbs free energy relaxation time to the temperature relaxation time is extremely high. All relaxation processes are assumed to be instantaneous, i.e. the relaxation times are very close to zero. The temperature and the Gibbs free energy relaxation are used only at the interfaces. By these modifications we get a new model which is able to deal with transition fronts, evaporation fronts, where heat and mass transfer occur. These fronts appear as extra waves in the system. We use the same test problems on metastable liquids as in Saurel et al. [R. Saurel, F. Petitpas, R. Abgrall, Modeling phase transition in metastable liquids: application to cavitating and flashing flows, J. Fluid Mech. 607 (2008) 313–350]. We have almost similar results. Computed results are compared to the experimental ones of Simões-Moreira and Shepherd [J.R. Simões-Moreira, J.E. Shepherd, Evaporation waves in superheated dodecane, J. Fluid Mech. 382 (1999) 63–86]. A reasonable agreement is achieved. In addition we consider the six-equation model with a single velocity which is obtained from the seven-equation model in the asymptotic limit of zero velocity relaxation time. The same procedure for the heat and mass transfer is used with the six-equation model and a comparison is made between the results of this model with the results of the seven-equation model.     

Relaxation model of the orientational phase transition in fullerite C60

The model of thermal behavior of a thermoelastic medium is developed in the context of the Landau theory of phase transitions. In the framework of this model, two different problems are considered with allowance for order parameter relaxation: the problem of relatively slow uniform heating (cooling) of the medium under external hydrostatic pressure and the problem of order parameter relaxation at thermal isolation. A finite value of the relaxation constant τ of the order parameter is demonstrated to bring about the heating (cooling) rate dependence of the physical quantities, such as specific heat. The relaxation time of the order parameter is shown to be twice larger than the temperature relaxation time, as a consequence of the Landau expansion of the free energy.     

可激发气体振动弛豫时间的两频点声测量重建算法

振动弛豫时间是可激发气体分子内外自由度能量转移速率的宏观体现,它决定了声吸收谱峰值点对应的弛豫频率.本文给出了等温、绝热定压和绝热定容三种不同热力学过程下振动弛豫时间的相互关系;基于Petculescu和Lueptow[2005 Phys.Rev.Lett.94 238301]的弛豫过程合成算法,推导了单一压强下两频点声测量值的弛豫时间重建算法.该算法可应用于等温、绝热定压、绝热定容弛豫时间和弛豫频率的重建测量,并避免了弛豫时间传统声测量方法需要不断改变气体腔体压强的问题.仿真结果表明,对于室温下CO_2,CH_4,Cl_2,N_2和O_2组成的多种气体,重建的弛豫时间和弛豫频率与实验数据相符.     

多元可激发气体声弛豫频率的环境影响分析

推导多元可激发气体中声弛豫频率和环境温度、压强的解析关系.理论分析和仿真计算表明：声弛豫频率线性反比于主弛豫过程的弛豫时间,正比于主弛豫过程的振动耦合热容,反比于外自由度热容;温度升高导致振动耦合热容增加、内外自由度能量转移速率增大引起弛豫时间减少,进而造成声弛豫频率正比于环境温度;压强增加使得分子碰撞速率增加引起弛豫时间减少,进而使得声弛豫频率线性正比于环境压强.     

Stoßbestimmte Relaxation des Elektronenensembles im Plasma bei zusätzlicher Aufheizung durch ein elektrisches Feld. III. Das periodische Verhalten der Elektronenkomponente im elektrischen Feld großen Modulationsgrades

Collision Dominated Relaxation of the Electron Ensemble in a Plasma with Additional Heating by an Electric Field. III. The Periodic Behaviour of the Electron Component in an Electric Field with a Large Modulation Amplitude With the aid of the non-stationary Boltzmann-equation the periodic behaviour of the isotropic part of the velocity distribution of electrons and thereby determined macroscopic quantities is calculated for periodic electric fields with large modulation amplitude. The investigations concern a weakly ionized column plasma in neon under typical low and medium pressure conditions. Based on the numerical results for typical ranges of field strength and cycle times of the electric field a qualitative physical interpretation for the periodic behaviour of the electron component is obtained. The introduction of special field-dependent adjustment times allows the formalution of conditions which characterize the case of quasi-stationary behaviour and also the case of small amplitudes of modulation in the macroscopic quantities determined by the isotropic distribution function. The periodic states between these two limiting cases can be interpreted as due to two competing processes. The first one is the energy input controlled by the electric field and the second one is the energy loss in binary collisions of the electrons with the atoms.     

Die Relaxation der Geschwindigkeitsverteilungsfunktion der Elektronen im homogenen elektrischen Feld mit elastischen und anregenden Stößen. II. Das Verhalten des Elektronenensembles bei aperiodischer Feldänderung zwischen zwei stationären Zuständen

Recently the relaxation of the electron component was investigated under field-free conditions after sudden switch-off the electric field and otherwise after sudden changing the electric field to a new value of field strength. To continue these papers we consider now the relaxation process of the electron ensemble in the presence of a monotonous time variable electric field. The investigations are based on a computation of time change in the isotropic part of the velocity distribution function of the electrons and of the macroscopic parameters determined by the distribution function. The start-ing-point is the non-stationary Boltzmann equation with stationary initial states taking into consideration elastic and exciting collisions. Besides the representation of the received numerical results a physical interpretation is obtained for the duration of the whole relaxation process, for its initial stage as well as for the momentary stage of the relaxation by introduction of normalized characteristic time quantities. Further characterizing conditions are found related to the relaxation after quasi-jumplike change of the electric field and in the case of quasistationary field alteration respectively. With the introduced characteristic time quantities statements about the degree of realization of one of this limiting cases are possible for any given monotonous field. The investigations are per-formed in a low ionized non-thermal neon plasma.     

The polaronic nature of intraband relaxation in InAs/GaAs self-assembled quantum dots

Polaron relaxation processes in a series of n-type InAs quantum dots (QDS) have been investigated using energy-dependent far-infrared pump–probe spectroscopy. For energies up to 53 meV, polarons decay to 2 longitudinal acoustic phonons; above this energy additional decay channels open resulting in a reduction of the decay time. Inter-state transfer has been observed between closely spaced p-like excited states, with the measured transfer times in good agreement with calculations assuming acoustic phonon assisted transfer. Finally, for QDs containing 2 electrons we observe evidence of a spin-flip process resulting in long (700 ps) relaxation times.     

Time‐resolved random laser spectroscopy of inhomogeneously broadened systems

The understanding of energy transfer processes in biological systems occurring among optical centres which exhibit inhomogeneously broadened spectral bands is of paramount importance to determine time constants and spatial distribution of energy flow. A new time resolved‐spectroscopy based on the random laser generation of the optical probes is reported. As an example, the excited state relaxation of Rhodamine B molecules in an organic‐inorganic hybrid material is investigated. This kind of spectroscopy may resolve not only the spectral features of the system but also provide a high speed picture of the energy transfer and excited state relaxation of the involved optical probes. The results could be applied to other kind of efficient interacting chromophore pairs embedded in inhomogeneous scattering structures such as biological tissues.     

Superradiance and energy transfer within a system of atoms

We investigate cooperative effects in energy relaxation and energy transfer for N atoms in a thermal radiation field with superradiance master equations as well as a closed set of coupled moment equations. Both spatially large and spatially small systems are considered. For small systems nonlinear rate equations for the energy are related to the moment equations. Symmetry of the small system to interchanging atoms is used to incorporate off-diagonal solutions of the superradiance master equation in expressions for the probability of the transfer of energy from one group of atoms to another. The long time excitation probability for initially unexcited atoms is large and strongly correlated. Cooperative processes in a large system which fall off with the distance between a cooperating pair of atoms include energy loss and transfer terms in the master equation. The energy transfer is oscillatory in time. Energy relaxation is shown by numerical solution to become cooperative in a very sudden manner as the scale of the atomic system is decreased through the resonant wavelength.     

Local nonequilibrium mass transfer in a two-component system under external pulsed irradiation with energy fluxes

The nonisothermal mass transfer in metal materials under irradiation with concentrated energy fluxes is studied in the one-dimensional approximation. Local nonequilibrium equations of extended irreversible thermodynamics are used to describe the transfer phenomena. It is established that, for short times (on the order of the time required for relaxation of the diffusion flow to its local-equilibrium value), the wave mechanism for mass transfer is dominant over the diffusion one, ensuring that the impurity-concentration profiles have a nonmonotonous form. The degree of influence of the space-time nonlocality of the transfer processes on the formation of concentration profiles is estimated, and the model results are compared with the experimental data.     

卟啉酞菁二元分子光谱及激发态弛豫的非简并四波混频研究

通过对卟啉酞菁二元分子TTPP-O-Pc和TTPP-O(CH_2)_5-O-Pc的发射光谱和激发光谱的系统研究,证实了(tetratolylphenyl porphyrin)TTPP到Pc的能量转移,区分了后一种材料中发射光谱的两组谱带的来源.利用非简并四波混频(NDFWM)方法研究了二元分子激发态弛豫过程,获得了这两种材料的激发态弛豫时间分别为3.6ps和4ps,以及电荷弛豫时间分别为18.5ps和82ps.     

气体声弛豫过程中有效比热容与弛豫时间的分解对应关系

声在多原子分子气体中传播所引起的弛豫过程是探索气体特性的重要方面. 本文通过研究气体声弛豫过程中振动自由度与平动自由度(V-T)以及振动自由度之间(V-V)的分子能量转移模型, 给出了有效比热容与弛豫时间的分解对应关系及其通用获得方法. 该分解模型与现有的声弛豫模型相比, 反映了分解后的V-T 和V-V弛豫过程中振动比热容与弛豫时间的对应关系, 并发现了较高能级是引起对应声弛豫过程的决定因素. 将基于该分解模型获得的气体声弛豫衰减谱经碰撞直径微调改进后, 比现有理论更接近实验数据, 其结果证明了该分解对应关系的正确性和合理性.     

Effect of energy transfer on the kinetics of the decay and anisotropy of luminescence of prodan molecules

The temporal characteristics of the decay and polarization of luminescence of prodan solutions with different concentrations, excited by a picosecond laser radiation, are studied. The kinetics of the emission decay and polarization depend on the spectral range of luminescence recording and on the luminophore concentration and reflect the relaxation processes (leading to a long-wavelength shift of the entire emission band) and, in the case of sufficiently high concentrations, nonradiative energy transfer between the prodan molecules. It was found that the evolution of luminescence anisotropy in a diluted solution depends on the recording wavelength, which allowed us to calculate the average times of the Brownian rotational diffusion for prodan in glycerol, whose maxima were determined to be about 40 ns in the spectral region near 520 nm and to decrease to 14 and 17 ns at the edges of the emission region at 450 and 560 nm, respectively.     

Solution of the Boltzmann equation by expanding the distribution function with several time and coordinate scales in the enskog series in knudsen parameter

A method to solve the Boltzmann equation is analyzed in the case when the distribution function depends on slow and fast time and coordinate scales. Basic relationships for calculating the nonequilibrium multiscale distribution function are shown to differ substantially from those found in the framework of the Chapman-Enskog method: the transfer equations are complemented by the contributions of relaxation processes. The heat and momentum transfer equations derived from the general solution to the Boltzmann equation involve additional terms accounting for relaxation effects. The relaxation effects included in the energy equation result in both a hyperbolic heat conduction equation and a finite rate of heat transfer. In the viscous stress tensor, the Newtonian term of the transfer equation turns out to be supplemented by relaxation terms.     

Femtosecond optical diagnostics of interlevel energy relaxation in the band gap of silver iodide nanocrystals

The kinetics of the relaxation processes in silver iodide nanocrystals synthesized in a nanoporous silicate glass matrix and excited by femtosecond laser pulses has been experimentally studied. A short-lived characteristic transmission band is found to be formed in the spectral range of exciton absorption of nanocrystals. The specific features of photoexcited-carrier relaxation, which are due to the energy redistribution of captured carriers over levels in the band gap of silver iodide nanocrystals, are considered. The energy transfer relaxation rates are estimated and possible mechanisms of the energy transfer are discussed.     

热散焦效应对甲烷后向受激喇曼散射的影响

采用单纵模Nd:YAG二倍频激光泵浦,研究了CH4气体中的受激喇曼散射．实验发现,当激光的重复频率为2 Hz,1．1 MPa CH4中,泵浦能量为95 mJ时,后向一级斯托克斯(BS1)的量子转化效率可达73％,并且由于激光脉冲的张弛振荡,BS1的脉宽被压窄到1．2 ns,峰值功率达到了泵浦光功率的2．7倍,其光束质量大大优于泵浦光．在同样条件下,而重复频率为10 Hz时,BS1的量子转化效率降低为36％,但对光束质量影响不大,这是因为BS1呈现为与泵浦光波前翻转的复制光波,可以补偿热畸变．研究结果表明,如果设计一个CH4／He混合气体的带封闭循环冷却系统的高性能630 nm喇曼激光器,可能有其实际应用价值．     

Anomalous magnetic relaxation in thin Pd<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript> films

The magnetic relaxation in Pd_0.99Fe_0.01 films, which have the thicknesses that are practically important for cryoelectronics (25 and 40 nm), is detected and experimentally studied. The relaxation is shown to be substantial only in thin films. The magnetization relaxation is found to be well described by the sum of two exponential functions with characteristic times that differ by an order of magnitude from each other. The characteristic relaxation time and the ratio of the contributions of two relaxations depend on temperature. The activation energies of the relaxation processes are determined. The activation volume is shown to correspond to a 20-nm ferromagnetic cluster. The results obtained agree with the model of two-component magnetization in thin PdFe films [6].