Hot carrier microwave conductivity in the non-zero collision duration regime

We examine the small signal a.c. conductivity under hot electron conditions when the collision duration is a significant fraction of the mean-time between collisions. It is found that the high frequency conductivity is enhanced by the non-zero collision duration and that the plasma edge is shifted significantly.     

(1+2+1)双共振多光子电离概率的理论研究

共振增强多光子电离光谱技术已成为研究原子、分子高激发态能级结构的重要方法。运用光和物质相互作用的速率方程理论,推导出四能级物质系统1+2+1双共振增强多光子电离概率的解析表达式,以此为基础,理论模拟了电离概率随激发光强、激光脉冲宽度和碰撞弛豫速率的变化,发现在1+2+1多光子电离机制中,电离概率随光强的增加而增大,继而出现单步、双步激发饱和的现象,直至饱和值1;继续增大光强,电离概率将围绕饱和值1窄幅振荡,振荡幅度随光强增加而增大。随激光脉冲宽度的增大,电离概率从零开始逐渐增大直至饱和值1。而随碰撞弛豫速率的增大电离概率以线性规律减小。     

Photon physics at ultrarelativistic-energy heavy-ion collisions

Photons are discussed as potential probes of the dynamics of nucleus-nucleus reactions at ultrarelativistic energies. The signals conveyed by photons produced in the early partonic phase of the collision are explored. The discussion is illustrated by presently existing calculations which include photon production during the entire duration of the collision.     

Binary collision expansion and partial green's functions of Kadanoff-Baym

A new formula for the binary collision expansion of the unitary operator U (t₂, t₁) is proposed. The formula is applied to the expansion of the partial Green's functions of Kadanoff-Baym in powers of the correct binary scattering amplitude. It is shown that certain linked diagrams of left-multidentate structure should be taken into account. The duration of the binary collision is seen to play an important role in the rigorous formulation. Upon neglecting this duration, a useful approximation is found for the analysis of correlations on a macroscopic time scale.     

A theory of Doppler and binary collision broadening by foreign gases—I: Formal theory

The quantum theory of combined Doppler and foreign gas pressure broadening at the binary collision level is developed from the point of view of statistical mechanics. A kinetic equation is first derived which is linearized in the radiation field. For line-broadening purposes, this equation is reduced to a steady-state equation for collisions localized in position space. The equation takes into account the effects of photon recoil momentum, binary collision correlations, collisions of finite duration and line-coupling.     

The collision integral in nuclear matter at zero temperature

The collision rate in an infinite Fermi system having a deformed Fermi surface is computed in the limit of small deformation. For a given amount of internal energy, the damping rate is nearly independent of temperature. The calculation is applied to thermalization in heavy ion collisions. We find that the collisional damping time is comparable to the duration of the collision for medium-weight nuclei. Thus the predictions of mean field theory, such as the presence of a fusion window at small impact parameters, will only have validity for lighter ions.     

Collisions of few-cycle dissipative solitons in active nonlinear fibers

The collision of two oppositely directed few-cycle dissipative solitons in a single-mode fiber is numerically studied. The possibility of a purely electric or purely magnetic field with an anomalously high amplitude and small duration that results from the collision is demonstrated. Long-lived structures of the field and medium that correspond to the continuous generation of weak oppositely directed waves are excited due to the collisions of solitons (video pulses).     

Theoretical investigation of 2+2 resonance enhanced multi-photon ionization probability

The analytic formula of the ionization probability in the process of 2+2 resonance enhanced multi-photon ionization (REMPI) is derived from the dynamic rate equation about the interaction of photon and material. Based on the formula, the influence of laser intensity, laser pulse duration and collision relaxation rate on the ionization probability is investigated theoretically. It is shown that the ionization probability increases with the laser intensity and laser pulse duration. The ionization probability will get to one until two steps saturation with the increase of laser intensity. After that, the ionization probability will oscillates narrowly around the saturation value if the laser intensity increases further. The results also show that the ionization probability will decrease in linear manner nearly with the increase of collision relaxation rate. But the variation is small. So the influence of the collision relaxation rate (or even sample pressure) on the ionization probability can be ignored. In the practical use of REMPI, higher sample pressure will reduce the intensity of the detection signal is due to the collision complex of the positive and negative ions. This will reduce the collection efficiency of the ions, and influence the magnitude of the ion signal further.     

HeH+体系光缔合反应的动力学研究

采用含时量子波包方法研究基电子态HeH⁺体系优化的光缔合几率,即He+H⁺→HeH⁺随碰撞粒子对初始动量的变化,并研究伴随该过程的多光子跃迁和解离.选取v=6振动能级为目标态,通过优化激光场参数得到碰撞粒子对的最优光缔合几率.结果表明：随着碰撞粒子对初始动量的增加,光缔合所用的最优激光场的持续时间逐渐减少,但光场的强度和失谐逐渐增加.在某些初始碰撞动量附近,由于共振多光子跃迁过程的存在,最优的光场不能严格地将碰撞粒子对缔合到目标态.此外,研究表明随着初始动量的增加,系统的超阈值解离几率近似线性地增加到一个最大值,直至饱和强度.并且,在碰撞粒子对的初始动量较大时,超阈值解离主导解离过程.     

一种M2M网络中的能量有效媒体访问控制协议

为了减小M2M网络中MAC层的接入冲突,提高接入数据传输效率和节点能量利用效率,提出了一种新的混合分组协议HG-MAC(Hybrid and Grouping MAC)。HG-MAC混合利用基于调度与基于竞争的两种接入方式;对节点进行分组分层管理,减少接入冲突和提供QoS保障;采用休眠机制提高能量利用效率;引入动态调整机制和可变帧长度提高协议灵活性。通过建立二维马尔可夫链,对竞争时长与数据传输速率之间的关系进行了理论分析。利用OPNET仿真工具,将HG-MAC与CSMA/CA和TDMA的性能进行了仿真比较。结果表明HG-MAC在数据传输速率、能量效率和信道利用率上具有一定的优势。     

Collision of two short laser pulses in plasma and the generation of short-lived Bragg mirrors

We consider the nonlinear excitation of small-scale electron-density perturbations when two identical short laser pulses propagating toward each other collide in plasma. Pulses with duration τ of the order of the plasma-oscillation period ω_pτ ≤ 1, ω_p is the plasma frequency) are shown to excite long-lived localized plasma oscillations in the collision region. The energy conservation laws for the nonlinear mixing of short laser pulses in plasma are analyzed. We investigate the scattering of a sounding wave by the electron-density perturbations produced in the pulse collision region (short-lived Bragg mirror).     

Collision of a structurally composite particle with a barrier

We suggest a simple model of a structurally composite particle with internal degrees of freedom and study the simplest kinematical and dynamical properties of such a particle. The collision of a structurally composite particle with one internal degree of freedom with a barrier is analyzed in detail. We show that both total cooling and “heating” of the internal degree of freedom can be observed during the reflection of such a particle. We calculate such basic parameters of the collision as its duration, the number of collisions in the interaction time, and the velocities of the envelope and the internal particle after the collision. Properties characteristic of chaotic scattering are shown to appear when a structurally composite particle collides with a barrier.     

A Monte Carlo technique for quantum transport in semiconductors

A Monte Carlo technique to solve the Liouville's equation for an electron-phonon system in semiconductors is presented. The numerical procedure accounts for quantum features such as collision duration and intracollisional field effect. The semiclassical Boltzmann formalism can be easily recovered. A comparison of the results obtained with the proposed technique and with a traditional ensemble Monte Carlo method is presented.     

The collisional stabilization of excited β-naphthylamine molecules by the paraffin hydrocarbons in the gas phase

The transfer of vibrational energy from molecules of β-naphthylamine excited by the mercury lines at 2804 Å and 2652 Å to the homologous series of paraffin hydrocarbons up to n-hexane has been investigated in the gas phase at 180°C. Although the average amount of energy transferred collisionally increases with the complexity of the added gas by a factor of 5, the transfer efficiency expressed as an accommodation coefficient remains virtually unchanged.

A transfer mechanism based on the internal redistribution of vibrational energy within the collision complex is examined, in terms of which it is unnecessary to invoke vibration-vibration transfer except for pentane and hexane. The collision duration estimated on the basis of this model is well within an order of magnitude of that expected from collision diameters and relative velocities of the molecules concerned.     

Balance equation formulation and memory effects in retarded high field semiconductor transport

The role of the finite, non-zero collision duration in high electric fields is examined for its effect on transient and overshoot response of carrier velocity and energy. A quantum kinetic equation approach is used to develop modifications of the balance equations obtained in a drifted Maxwellian approach for treating high-field transport. These result in corrections which require convolution integrals to be evaluated on the short-time scale.     

Non-Markovian dynamics in a dense potassium vapor

Transient four-wave mixing experiments on a dense potassium vapor, which has a dephasing time long compared to the collision duration, reveal distinct signatures of non-Markovian dynamics. Theoretical fits assuming stochastic fluctuation of the excited-state frequencies confirm that the two-time correlation function has a finite temporal width.     

Resetting and annihilation of reentrant activity in a model of a one-dimensional loop of ventricular tissue

Resetting and annihilation of reentrant activity by a single stimulus pulse (S1) or a pair (S1-S2) of coupled pulses are studied in a model of one-dimensional loop of cardiac tissue using a Beeler-Reuter-type ionic model. Different modes of reentry termination are described. The classical mode of termination by unidirectional block, in which a stimulus produces only a retrograde front that collides with the activation front of the reentry, can be obtained for both S1 and S1-S2 applied over a small vulnerable window. We demonstrate that another scenario of termination-that we term collision block-can also be induced by the S1-S2 protocol. This scenario is obtained over a much wider range of S1-S2 coupling intervals than the one leading to a unidirectional block. In the collision block, S1 produces a retrograde front, colliding with the activation front of the pre-existing reentry, and an antegrade front propagating in the same direction as the initial reentry. Then, S2 also produces an antegrade and a retrograde front. However, the propagation of these fronts in the spatial profile of repolarization left by S1 leads to a termination of the reentrant activity. More complex behaviors also occur in which the antegrade fronts produced by S1 and S2 both persist for several turns, displaying a growing alternation in action potential duration ("alternans amplification") that may lead to the termination of the reentrant activity. The hypothesis that both collision block and alternans amplification depend on the interaction between the action potential duration restitution curve and the recovery curve of conduction velocity is supported by the fact that the dynamical behaviors were reproduced using an integro-delay equation based on these two properties. We thus describe two new mechanisms (collision block and alternans amplification) whereby electrical stimulation can terminate reentrant activity. (c) 2002 American Institute of Physics.     

Dynamics of the fields of counterpropagating light pulses of a few oscillations in nonlinear dielectric media

Evolution equations have been derived for the fields of counterpropagating light pulses of a few oscillations interacting with dielectric media with nonresonant dispersion and inertialess electron nonlinearity. Changes in the field structure and spectrum of a low-intensity pulse colliding with a high-intensity counterpropagating light pulse are illustrated. It has been shown that such a collision can be accompanied by the generation of radiation at multiple frequencies with a duration smaller than the durations of the initial pulses.     

A new closure hypothesis for the BBGKY system of equations

The first part of this paper deals with the Frey-Salmon irreversibility hypothesis, the kinetic equation that is deduced therefrom, and the comparison with experiment for the viscosity and thermal conductivity coefficients. The relaxation time which has been introduced is the average value of the duration of a collision. The second part deals with a new irreversibility hypothesis which leads to a kinetic equation of the same form as the first but in which the relaxation time is deduced from the interparticle potential.     

Proximity networks and epidemics

Disease spread in most biological populations requires the proximity of agents. In populations where the individuals have spatial mobility, the contact graph is generated by the “collision dynamics” of the agents, and thus the evolution of epidemics couples directly to the spatial dynamics of the population. We first briefly review the properties and the methodology of an agent-based simulation (EPISIMS) to model disease spread in realistic urban dynamic contact networks. Using the data generated by this simulation, we introduce the notion of dynamic proximity networks which takes into account the relevant time-scales for disease spread: contact duration, infectivity period, and rate of contact creation. This approach promises to be a good candidate for a unified treatment of epidemic types that are driven by agent collision dynamics. In particular, using a simple model, we show that it can account for the observed qualitative differences between the degree distributions of contact graphs of diseases with short infectivity period (such as air-transmitted diseases) or long infectivity periods (such as HIV).