The intervalley X6→Γ6, L6 scattering time in GaAs measured by ultrafast pump-probe infrared absorption spectroscopy

A direct measurement of the dynamics of electrons in the X₆ valley for a GaAs crystal by time-resolved absorption spectroscopy is reported for the first time. IR picosecond probe pulses were used to monitor the growth and decay of the population in the X₆ valley subsequent to excitation by a 527 nm pump pulse. The intervalley X₆→Γ₆, L₆ scattering time t_x of 0.70 ± 0.50 ps is determined and the crossection for the X₆→X₇ transition is estimated to be 1.8 × 10⁻¹⁶ cm².     

相干光被粗糙表面散射后衍射谱的分布探讨

利用傅立叶光学的基本原理，对相干光波被粗糙表面散射后衍射谱的分布进行探讨。     

二次散射对脉冲激光雷达回波功率的影响

理论上分析了二次散射对大气脉冲激光雷达回波功率的影响并提出一种计算方法。采用某型大气激光雷达的测试结果与理论计算具有较强的一致性,证明了理论结果的正确性。根据二次散射回波信号的特点,选用适当的滤波方法将其影响降至最低。二次散射的理论计算解释了实测中发现的＂纹波＂现象,纹波的幅度和频率与雷达自身特性及粒子特性相关,其衰减速度较单次散射慢得多。因此对于大气探测具有进一步研究的重要价值。     

基于多通路融合网络的高速公路雾天能见度等级识别

雾天是影响高速公路交通安全的重要因素。研究从监控图像进行高速公路雾天能见度的自动识别方法可以为交通管理部门的智能管理和决策提供技术支持。根据大气散射模型分析出与雾浓度相关的多个物理因素,提出了综合这些物理因素的多通路融合识别网络。该网络使用三个通路联合学习深度视觉特征、传输矩阵特征和场景深度特征,并设计注意力融合模块来自适应地融合这三类特征以进行能见度等级识别。同时构建了一个合成数据集和一个真实的高速公路场景数据集,用于网络参数学习和性能评估。实景数据集中的图像是从中国多条高速公路的监控视频中收集的。在这两个数据集上的实验表明,所提方法可以适应不同的监控拍摄场景,能够比现有方法更准确地识别能见度等级,有效提升了识别精度。     

Decay rate of critical fluctuations in steam and in dilute steam-NaCl mixtures

The decay rate of critical fluctuations in steam and in a steam-NaCl mixture has been investigated experimentally with the aid of photon correlation spectroscopy. For pure steam, the measurements have been performed along seven isochores [(¦– _c¦)/_c<0.09] as a function of the temperatureT for (T–T_t)<1 K. The results have been compared with the values predicted by the renormalization-group theory written as a modification of the classical mode coupling theory. The agreement between experiment and theory is satisfactory along the critical isochore, but larger deviations are noted for _c when approching the transition temperatureT _t. The decay rate of a 0.1% (molar) dilute mixture of NaCl in H₂O has been measured along some near-critical isochores as a function of temperature. Its behavior, which is very different from that observed for pure steam, is dicussed.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Ultrasonic crack characterization via a constrained inversion algorithm

The Kirchhoff approximation is used to show that the time domain impulse response of an isolated flat crack can be given a simple geometrical interpretation in terms of the derivative of a projected length function. For an elliptical crack, this derivative can be obtained explicitly to yield the two edge-diffracted waves which originate from the flashpoints of the crack. An explicit coordinate invariant expression is obtained from this elliptical crack solution which relates the time difference, t, between the arrival of these edge-diffracted waves and the crack size and orientation. Previously, we have proposed that this expression, together with t measurements in different scattering directions, could be used in a regression analysis as the basis for performing a constrained inversion of crack scattering data (i.e., where we attempt to obtain the best equivalent flat elliptical crack that fits the scattering measurements). Here we will demonstrate some results of applying the proposed algorithm using noisy synthetic data. The sensitivity of the results to both, number of measurements and transducer orientation, will be discussed.     

Thermal diffusivity of fluids in a broad region around the critical point

Dynamic light scattering is a suitable method for the investigation of transport properties such as the thermal diffusivity of optically transparent fluids. The main advantages of the method are its quickness, the fact of the thermodynamic state of equilibrium of the sample (gradients are not required), and the relatively simple evaluation of data without the necessity of calibration. However, an insufficient production of intensity of scattered light may be a limiting effect. For that reason the vicinity of the gas-liquid critical point represents the classical range of application. In this paper, it is shown that by means of an appropriate choice of experimental apparatus, measurements are also feasible in an extended range of states. Broad regions around critical points of three pure fluids (sulfur hexafluoride, SF₆; ethane, C₂H₆; nitrous oxide, N₂O) over temperature ranges ¦T-T _c¦ of 0.02 to 50 K and density ranges (/_c) of 0.2 to 2 were investigated. In this region the thermal diffusivity shows great variations with temperature and density and cannot be described by means of ideal-gas behavior or relations for liquids. The measurements were carried out along the coexistence curve for both phases, along the critical isochore and along some isotherms with TT _c. The measured or calculated density, pressure, and thermal diffusivity data as well as some correlations are presented.     

低碳钢显微结构的EBSD表征

近年来,新开发的电子背散射衍射技术将多晶材料的显微组织、微区成分与结晶学数据分析联系起来,能对晶界类型、取向、位向差和结构及其分布进行观察、统计测定和定量分析,从而建立了晶界结构、取向和织构等与多晶材料性能的定量和半定量关系,成为现代材料研究的重要实验技术。笔者结合低碳钢生产中的一些实际问题,扼要介绍了电子背散射衍射技术中的晶体取向图在多晶材料显微结构表征中的应用。     

Edge diffraction correction for 3D scattering center modeling based on the shooting and bouncing ray technique

In the traditional scattering center extraction method based on the shooting and bouncing ray(SBR) technique,only the physical optics contribution of the target is considered.However,the physical optics method is unable to describe the contribution of the actual edge diffraction.By comprehensively considering the specular reflection and edge diffraction contribution of the target,an edge diffraction correction method for 3D scattering center modeling based on the SBR technique is proposed.Using the SBR technique and combining the image domain ray tube integration and equivalent edge currents method,the edge diffraction correcting formula for the 3D inverse synthetic aperture radar image is derived.Simulation results show that the proposed method is able to effectively improve the modeling accuracy of the 3D scattering center.     

The Importance of Free Fatty Chain Length on the Lipid Organization in the Long Periodicity Phase

The skin’s barrier ability is an essential function for terrestrial survival, which is controlled by intercellular lipids within the stratum corneum (SC) layer. In this barrier, free fatty acids (FFAs) are an important lipid class. As seen in inflammatory skin diseases, when the lipid chain length is reduced, a reduction in the barrier’s performance is observed. In this study, we have investigated the contributing effects of various FFA chain lengths on the lamellar phase, lateral packing. The repeat distance of the lamellar phase increased with FFA chain length (C20–C28), while shorter FFAs (C16 to C18) had the opposite behaviour. While the lateral packing was affected, the orthorhombic to hexagonal to fluid phase transitions were not affected by the FFA chain length. Porcine SC lipid composition mimicking model was then used to investigate the proportional effect of shorter FFA C16, up to 50% content of the total FFA mixture. At this level, no difference in the overall lamellar phases and lateral packing was observed, while a significant increase in the water permeability was detected. Our results demonstrate a FFA C16 threshold that must be exceeded before the structure and barrier function of the long periodicity phase (LPP) is affected. These results are important to understand the lipid behaviour in this unique LPP structure as well as for the understanding, treatment, and development of inflammatory skin conditions.