飞秒激光-薄膜靶相互作用中超热电子产额和激光转化效率

 在激光能量130 mJ（靶面），脉宽60 fs，波长800 nm，对比度1∶10^-6，激光与靶法线成45°夹角，P偏振，靶面激光峰值功率密度约为7.0×10¹⁷ W·cm^-2，无预脉冲的条件下，采用电子谱仪与经γ标准源标定的LiF热释光探测器（TLD）相配合，测量了飞秒激光-薄膜靶相互作用中产生的超热电子能谱。根据所测的能谱，推算出超热电子的产额和激光能量转化为超热电子能量的效率，在靶法线方向分别为1.19×10¹⁰/sr和4.55%/sr，在激光反射方向分别为1.83×10⁹/sr和0.76%/sr。结果显示，不同方向的超热电子产额和激光转化效率有所不同，原因在于激光-等离子体相互作用产生的超热电子构成各向异性的分布。     

飞秒激光-薄膜靶相互作用中超热电子产额和激光转化效率

在激光能量130 mJ（靶面）,脉宽60 fs,波长800 nm,对比度1∶10-6,激光与靶法线成45°夹角,P偏振,靶面激光峰值功率密度约为7.0×1017 W·cm-2,无预脉冲的条件下,采用电子谱仪与经γ标准源标定的LiF热释光探测器（TLD）相配合,测量了飞秒激光-薄膜靶相互作用中产生的超热电子能谱。根据所测的能谱,推算出超热电子的产额和激光能量转化为超热电子能量的效率,在靶法线方向分别为1.19×1010/sr和4.55%/sr,在激光反射方向分别为1.83×109/sr和0.76%/sr。结果显示,不同方向的超热电子产额和激光转化效率有所不同,原因在于激光-等离子体相互作用产生的超热电子构成各向异性的分布。     

Probing phonon emission via hot carrier transport in suspended graphitic multilayers

We study hot carrier transport under magnetic fields up to 15 T in suspended graphitic multilayers through differential conductance () spectroscopy. Distinct high-energy anomalies have been observed and shown to be related to intrinsic phonon-emission processes in graphite. The evolution of such anomalies under magnetic fields is further understood as a consequence of inter-Landau level cyclotron-phonon resonance scattering. The observed magneto-phonon effects not only shed light on the physical mechanisms responsible for high-current transport in graphitic systems, but also offer new perspectives for optimizing performance in graphitic nano-electronic devices.     

近阈值下He原子的双电子电离实验中出射电子研究

利用适用于低能电子入射的反应显微成像谱仪，对电子入射He原子近阈值下的双电离过程进行了研究，实验测量了反应后3个粒子的全部动量，获得了出射电子间的关联信息.主要介绍近阈值下的双电离实验装置及实验技术，集中分析反应后出射电子的动量能量关系，对描述近阈值双电离的Wannier理论进行了检验，发现在入射电子能量为106eV时，实验结果具有Wannier理论预言的性质.     

Characteristics of wall sheath and secondary electron emission under different electron temperatures in a Hall thruster

In this paper, a two-dimensional physical model is established in a Hall thruster sheath region to investigate the influences of the electron temperature and the propellant on the sheath potential drop and the secondary electron emission in the Hall thruster, by the particle-in-cell(PIC) method. The numerical results show that when the electron temperature is relatively low, the change of sheath potential drop is relatively large, the surface potential maintains a stable value and the stability of the sheath is good. When the electron temperature is relatively high, the surface potential maintains a persistent oscillation, and the stability of the sheath reduces. As the electron temperature increases, the secondary electron emission coefficient on the wall increases. For three kinds of propellants(Ar, Kr, and Xe), as the ion mass increases the sheath potentials and the secondary electron emission coefficients reduce in sequence.     

Initial oxidation of Ni(1 1 1) observed by electron emission microscopy: PEEM and MEEM

The initial oxidation of Ni(1 1 1) in the temperature range of 550-700 K has been monitored by photoelectron emission microscopy (PEEM) and metastable-atom electron emission microscopy (MEEM). The PEEM and MEEM images show uniform patterns for the chemisorbed overlayer, reflecting the electronic homogeneity as seen at the μm scale. During the nucleation and lateral growth of oxide, however, the μm-scale pattern due to the formation of oxide domains appears and its evolution depends strongly on the substrate temperature and dose pressure of gaseous O₂. Our data indicate that the high-temperature oxidation is regarded as a successive multi-nucleation process in a reaction-diffusion field.     

Transmission electron microscopy observations of CNT morphology before and after heating in vacuum

Heating of multi-walled carbon nanotubes is often required to obtain clean patterns in the field electron emission microscope (FEEM). A transmission electron micrograph study of morphological changes in the cap structure of multi-walled carbon nanotubes due to heating in vacuum is presented. The lack of significant structural change in the cap structure when specimens were heated to 1925 K for 2.5 h determines an upper bound for the diffusivity of multi-walled carbon nanotube surface atoms, of the order of 10⁻¹⁶ cm²/s at 1925 K.     

Structural characterization of Al_(0.55)Ga_(0.45)N epitaxial layer determined by high resolution x-ray diffraction and transmission electron microscopy

Structural characteristics of Alo.55 Gao.45N epilayer were investigated by high resolution x-ray diffraction(HRXRD)and transmission electron microscopy(TEM);the epilayer was grown on GaN/sapphire substrates using a high-temperature A1 N interlayer by metal organic chemical vapor deposition technique.The mosaic characteristics including tilt,twist,heterogeneous strain,and correlation lengths were extracted by symmetric and asymmetric XRD rocking curves as well as reciprocal space map(RSM).According to Williamson-Hall plots,the vertical coherence length of AlGaN epilayer was calculated,which is consistent with the thickness of AlGaN layer measured by cross section TEM.Besides,the lateral coherence length was determined from RSM as well.Deducing from the tilt and twist results,the screw-type and edge-type dislocation densities are 1.0×10~8 cm~(-2) and 1.8×10~(10) cm~(-2),which agree with the results observed from TEM.     

Transmission electron microscopy observation of a deformation twin in TWIP steel by an ex situ tensile test

The formation of deformation twins in twinning-induced plasticity steels was observed in transmission electron microscope by an ex situ tensile test. The twinning process initially includes formation of extended dislocations at primary slip plane, then cross-slip to a conjugate slip plane with dissociation of the leading partial into a stair-rod dislocation and an emitted partial, and finally un-faulting of the original faults and formation of Frank partials. Repetition of the operation of the process on successive conjugate planes results in the formation of deformation twins. The formation mechanism of deformation twins can thus be explained by improving the stair-rod cross-slip model.     

Numerical analysis for verifying the performance of lens system in a scanning electron microscope

This work presents an analysis of the lens system in a thermionic scanning electron microscope (SEM) using a numerical computation and an optics-based calculation. The behaviors and characteristics of the electron beam are analyzed and the focusing capability is investigated. The beam spot size is estimated by calculation and proved by experiment. The characteristics and properties determining the SEM performance are investigated for various design parameters through a numerical analysis and an optics-based calculation. Particularly, a combination of two approaches gives more detailed information than a single approach in investigating an extremely small beam spot by demagnification through the electromagnetic lens system in a SEM column.     

Direct observations of La ordering and domain structures in La0.61Li0.17TiO3 by high resolution electron microscopy

La_0.61Li_0.17TiO₃ microstructures have been studied by high resolution electron microscopy. A local lattice distortion occurs in the vicinity of the domain boundary region due to the twin with an angle of 89°. The average domain size of La_0.61Li_0.17TiO₃ is greater than 20 nm. The domain size and structures of La_0.61Li_0.17TiO₃ differ greatly from those of La-poor compounds, such as La_0.55Li_0.35TiO₃. At a nanoscopic level, microdomains of 20–100 nm in size construct a two-dimensional structure in La-rich compounds, while microdomains of 5–10 nm in size construct a three-dimensional structure in La-poor compounds. In addition, the Li-ion conduction mechanisms for La-rich and La-poor compounds are two- and three-dimensional, respectively.     

Impact of electron irradiation on particle track etching response in polyallyl diglycol carbonate (PADC)

In the present work, attempts have been made to investigate the modification in particle track etching response of polyallyl diglycol carbonate (PADC) due to impact of 2 MeV electrons. PADC samples pre-irradiated to 1, 10, 20, 40, 60, 80 and 100 Mrad doses of 2 MeV electrons were further exposed to 140 MeV²⁸ Si beam and dose-dependent track registration properties of PADC have been studied. Etch-rate values of the PADC irradiated to 100 Mrad dose electron was found to increase by nearly 4 times that of pristine PADC. The electron irradiation has promoted chain scissioning in PADC, thereby converting the polymer into an easily etchable polymer. Moreover, the etching response and the detection efficiency were found to improve by electron irradiation. Scanning electron microscopy of etched samples further revealed the surface damage in these irradiated PADCs.     

A comparative study of nanoparticles in premixed flames by scanning mobility particle sizer, small angle neutron scattering, and transmission electron microscopy

Scanning mobility particle sizer (SMPS) and transmission electron microscopy (TEM) studies were conducted for TiO₂ and soot particles. The TiO₂ particles were produced from a premixed stagnation ethylene-oxygen-argon flame (? = 0.36) doped with titanium tetraisopropoxide. Soot was generated from a burner-stabilized premixed ethylene-oxygen-argon flame (? = 2.5). The close agreement among SMPS, TEM, and X-ray diffraction results for TiO₂ nanoparticles demonstrates that the probe sampling/mobility measurement technique is accurate for on-line analysis of the size distribution of particles as small as 3 nm in diameter. In the case of soot, notable disagreement between the SMPS and TEM sizes was found and attributable to the fact that the soot taken from the flame studied herein is liquid-like and that upon deposition on the TEM grid, the primary particles do not retain their sphericity. This interpretation is supported by measurements with photo ionization aerosol mass spectrometry, small angle neutron scattering, and thermocouple particle densitometry.