Effect of inner-surface roughness of conical target on laser absorption and fast electron generation

The effect of inner-surface roughness of conical targets on the generation of fast electrons in the laser-cone interaction is investigated using particle-in-cell simulation. It is found that the surface roughness can reduce the fast-electron number （in the energy range E 〉 1 MeV） and energy, as compared to that from a cone with smooth inner wall. A scaling law for the laser reflectivity based on the vacuum-heating model is derived. Both theory and simulation indicate that laser reflection increases with the height-to-width ratio of the periodic inner surface structure and approaches that of a smooth cone as this ratio becomes zero.     

Behaviour of Self-Standing CVD Diamond Film with Different Dominant Crystalline Surfaces in Thermal-Iron Plate Polishing

Self-standing CVD diamond films with different dominant crystalline surfaces are polished by the thermal-iron plate polishing method. The influence of the dominant crystalline surfaces on polishing etfficiency is investigated by measuring the removal rate and final roughness. The smallest rms roughness of 0.14 μm is measured with smallest removal rate in the films with the initial （220） dominant crystalline surface. Activation energy for the polishing is analysed by the Arrhenius relation. It is found that the values are 170kJ/mol, 222kJ/mol and 214kJ/mol for the film with three different dominant crystalline surfaces. Based on these values, the polishing cause is regarded as the graphitization-controlling process. In the experiment, we find that transformation of the dominant crystalline surfaces from （111） to （220） always appears in the polishing process when we polish the （111） dominant surface.     

Rarefied Gas Flow in Rough Microchannels by Molecular Dynamics Simulation

The molecular dynamics simulation method is applied to investigate the rarefied gas flow in a submicron channel with surface roughness which is modelled by an array of triangle modules. The boundary conditions are found to be determined not only by the Knudsen number but also the roughness, which implies that the breakdown of the Maxwell slip model under the conditions that the surface roughness is comparable to the molecular mean free path. The effects of the rarefaction and the surface roughness on the boundary conditions and the flow characteristics are strongly coupled. The flow friction increases with increasing roughness and with decreasing Knudsen number.     

Size- and Temperature-Dependent Thermal Expansion Coefficient of a Nanofilm

The thermal expansion coefficient （TEC） of an ideal crystal is derived by using a method of Boltzmann statistics. The Morse potential energy function is adopted to show the dependence of the TEC on the temperature. By taking the effects of the surface relaxation and the surface energy into consideration, the dimensionless TEC of a nanofilm is derived. It is shown that with decreasing thickness, the TEC can increase or decrease, depending on the surface relaxation of the nanofilm.     

Influence of roughness on the detection of mechanical characteristics of low-k film by the surface acoustic waves

The surface acoustic wave （SAW） technique is a precise and nondestructive method to detect the mechanical charac- teristics of the thin low dielectric constant （low-k） film by matching the theoretical dispersion curve with the experimental dispersion curve. In this paper, the influence of sample roughness on the precision of SAW mechanical detection is inves- tigated in detail. Random roughness values at the surface of low-k film and at the interface between this low-k film and the substrate are obtained by the Monte Carlo method. The dispersive characteristic of SAW on the layered structure with rough surface and rough interface is modeled by numerical simulation of finite element method. The Young＇s moduli of the Black DiamondTM samples with different roughness values are determined by SAWs in the experiment. The results show that the influence of sample roughness is very small when the root-mean-square （RMS） of roughness is smaller than 50 nm and correlation length is smaller than 20 μm. This study indicates that the SAW technique is reliable and precise in the nondestructive mechanical detection for low-k films.     

Measurement of inner surface roughness of capillary by an x-ray reflectivity method

The inner surface roughness of a capillary is investigated by the reflectivity of x-rays penetrating through the capillary. The results are consistent with the data from atomic force microscope (AFM). The roughness measured by this new method can reach the order of angstroms with high quality capillaries.     

Ultrasonic backward radiation profile on painted rough interface

Generalized Lamb surface waves are generated in a surface region when ultrasonic waves are incident to the layered substrates such as painted specimen in liquid. Then, backward radiated waves are returned to the direction of incidence by scattering and energy radiation of the surface waves. Hence, the backward radiation could be used in assessing the interracial state between layer and substrate because the surface wave is sensitive to the change of physical properties in a surface region. Painting surface treatment of commercial products and articles occasionally hide the surface region trouble such as roughness and crack. The evaluation of interfacial state under painting layer is very important in the prevention of great accidence, hence the evaluation technique should be nondestructive, fast and easy applicable to the fields. The backward radiations were measured for the painted glass with periodic interfacial roughness immersed in water tank. The effect of interfacial roughness on the angular pattern and frequency spectrum of the ultrasonic backward radiation was investigated to develop the nondestructive technique for interfacial roughness evaluation.     

Diffusion of Pt dimers on a Wulff polyhedral surface

The surface diffusion of Pt dimers on Wulff polyhedral clusters with 586-12934 atoms was studied by the embedded atom method. The minimum energy diffusion path and the corresponding energy barrier for dimer diffusion on cluster surfaces were determined through a combination of the quenched Molecular Dynamics and the Nudged Elastic Band method. It was found that the diffusion of a dimer across the step-edge by dissociation and consecutive single-atom exchange with the edge atoms, rather than diffusion over t...     

Azimuthal anchoring of a nematic liquid crystal on a grooved interface with anisotropic polar anchoring

Zhang Y J et al.[Zhang Y J,Zhang Z D,Zhu L Z and Xuan L 2011 Liquid Cryst.38 355] investigated the effects of finite polar anchoring on the azimuthal anchoring energy at a grooved interface,in which polar anchoring was isotropic in the local tangent plane of the surface.In this paper,we investigate the effects of both isotropic and anisotropic polar anchoring on the surface anchoring energy in the frame of Fukuda et al.’s theory.The results show that anisotropic polar anchoring strengthens the azimuthal anchoring of grooved surfaces.In the one-elastic-constant approximation(K11 = K22 = K33 = K),the surface-groove-induced azimuthal anchoring energy is entirely consistent with the result of Faetti,and it reduces to the original result of Berreman with an increase in polar anchoring.Moreover,the contribution of the surface-like elastic term to the Rapini-Papoular anchoring energy is zero.     

New expressions for the surface roughness length and displacement height in the atmospheric boundary layer

An alternative model for the prediction of surface roughness length is developed. In the model a new factor is introduced to compensate for the effects of wake diffusion and interactions between the wake and roughness obstacles. The experiments are carried out by the use of the hot wire anemometry in the simulated atmospheric boundary layer in a wind tunnel. Based on the experimental data, a new expression for the zero-plane displacement height is proposed for the square arrays of roughness elements, which highlights the influence of free-stream speed on the roughness length. It appears that the displacement height increases with the wind speed while the surface roughness length decreases with Reynolds number increasing. It is shown that the calculation results based on the new expressions are in reasonable agreement with the experimental data.     

Energy loss straggling measurements for helium ions in Mylar and polypropylene polymeric foils

In the present contribution, we report results on energy straggling of He ions penetrating Mylar and polypropylene thin polymeric foils. The measurements were performed in the 900–3000 keV incident particle energy range by using the indirect transmission technique developed previously. The experimental straggling data are corrected to consider the roughness effects due to target thickness inhomogeneity. As expected, the roughness contribution to straggling is more important for helium than for proton ions and decreases as the ion energy increases. At low velocities, (<500 keV/amu), the variation of the experimental energy straggling results differs strongly from predictions based on Bohr’s formalism, and with increasing energies, the experimental results approach gradually the Bohr values.     

MeV能量离子在生物样品中的能量损失与能量离散

为了研究 MeV能量离子在生物样品中的能量损失与能量离散, 分别使用1.0, 1.8和2.8 MeV质子和4.5 MeV氦离子分别辐照不同质量厚度的洋葱内表皮膜。 当质子穿过该生物样品后, 可以利用透射能谱测量透射离子的能量损失和能量离散。 实验结果显示, 在以上的生物样品中, MeV能量离子的能量损失值和TRIM程序模拟的结果相吻合, 但是透射离子的能量离散值却与TRIM程序模拟结果有很大的不同。 结合生物样品的结构不均匀的特性, 对Bohr能量离散理论进行了修正, 并发现修正后的Bohr能量离散理论计算结果与实验值符合得很好。     

Stopping power and straggling of energetic light ion beams in yttrium

Energy loss and straggling for protons, deuterons and α-particles in yttrium were measured in the energy region between 0.15 MeV and 2.5 MeV. The measured stopping powers of the hydrogen ions agree reasonably well with the semi-empirical values of Andersen and Ziegler, although better agreement was found with the values of Janni. For helium ions there are a marked difference with the values of Ziegler. Below 200 keV/amu the stopping ratios of the helium and hydrogen ions are lower than Ziegler's master curve. The straggling values of the hydrogen ions are about 15% lower than the Bohr estimate, while the straggling of the helium ions reach the Bohr value at about 0.2 MeV/amu.     

Energy loss straggling of fast charged particles

Energy loss straggling of fast charged particles colliding with atoms have been considered in the eikonal approximation. The result is represented in the form of the Fano formula with a nonperturbative correction. The known nonperturbative Titeica formula (which is transformed to the Fano formula when perturbation theory is applicable) is obtained only under certain approximations in eikonal calculations. It has been shown that straggling calculated with allowance for nonperturbative effects at large charges of the projectile can be significantly different (by an order of magnitude) from the results obtained by Titeica, Fano, and Bohr. Energy loss straggling of fast highly charged ions on hydrogen and copper atoms have been calculated. The latter results are compared to experimental data.     

Description of Ion Energy Straggling after Penetration through Thin Target

The energy loss straggling for fast ions is simulated using the Monte Carlo method in the case of large and small transferred momenta. The interval of applicability of the Bohr approximation is determined for frequently used solid targets. The asymmetry in the distribution of ion energy losses is related to the influence of large transferred momenta and is observed in wide intervals of ion energy and target thicknesses.     

Antiproton and proton energy loss straggling at keV energies

The slowing-down process of point-like charged particles in matter has been investigated by measuring the energy straggling for antiprotons and protons in Al, Ni and Au. A comparison with binary theory shows good agreement for Al and Au. For Ni, experimental data are not as convincing. In particular for the aluminum target, the Barkas-like effect of reduced energy straggling for antiprotons compared to protons is visible in the experimental data and a nearly velocity-proportional straggling is found, in good agreement with binary theory.     

Energy loss straggling in collisions of fast finite-size ions with atoms

The influence of ion size on straggling of energy losses by fast partially stripped ions is studied using the nonperturbative approach based on the eikonal approximation. It is shown that such a consideration of collisions of ions with complex atoms can lead to considerable corrections in calculating root-mean-square straggling of energy losses by fast ions compared to the results obtained for point ions. The root-mean-square straggling of energy losses are calculated for bromide and iodine ions in collisions with copper, silver, and aluminum atoms. It is shown that allowance for the size of the electron “coat” of an ion noticeably improves the agreement with experimental data.     

Space distribution and energy straggling of charged particles via Fokker-Planck equation

Summary The Fokker-Planck equation describing a beam of charged particles entering a homogeneous medium is solved here for a stationary case. Interactions are taken into account through Coulomb cross-section. Starting from the charged-particle distribution as a function of velocity and penetration depth, some important kinetic quantities are calculated, like mean velocity, range and the loss of energy per unit space. In such quantities the energy straggling is taken into account. This phenomenon is not considered in the continuous slowing-down approximation that is commonly used to obtain the range and the stopping power. Finally the well-known Bohr or Bethe formula is found as a first-order approximation of the Fokker-Planck equation. The authors of this paper have agreed to not receive the proofs for correction.     

Ranges of implanted atoms under polyatomic cluster bombardment of the Cu (111) surface

A computer simulation of the bombardment of the Cu (111) surface with Cu_N and Au_N polyatomic clusters differing in size (N = 1, 6, and 13) with energies of 0.5 and 5 keV/atom has been performed in the framework of classical molecular dynamics. The spatial distribution of the implanted atoms, their ranges, and range fluctuations (straggling) depending on the size N and energy E/N of the incident cluster has been investigated. It has been shown that an increase in the mean range and range straggling is observed at a fixed energy per incident atom as the cluster size N increases. At the same time, the effect of an increase in the range (at a specified value of E/N) gradually disappears with increasing cluster energy, whereas the effect of an increase in straggling is retained. These tendencies qualitatively agree with the available experimental observations. It has been shown that the dominating contribution to the increase in the atom range of the implanted cluster is made by the so-called clearing-the-way effect, which is weakened with increasing the incident cluster energy. The effect of the range straggling increasing is significantly due to the presence of nonlinear “spike” effects at the bombarded target.     

Surface radiative recombination in GaAs with surphon participation

Surface photoluminescence spectra of GaAs-Si₃N₄ (Ge₃N₄) systems excited by a He-Ne laser were investigated in the 4.2–50 K temperature range. Two phonon replicas (PR) of the zero phonon surface emission band at 1.477 eV were observed. This permits a calculation of the surface phonon energy (33 ± 0.5 meV). The PR behaviour agrees with Hopfield's theory on the basis of which information is given on the dynamic characteristics of the lattice at the surface, the Bohr electron radius of the radiative centre localized on the surface, etc. From the mean number of emitted surface phonons (surphons), the Franck-Condon shift, and the widening of the surface emission band, we conclude that there is an intensification of electron-phonon interactions at the surface in comparison with the bulk. The band of near-edge surface luminescence is discussed, which we associate with a manifestation of collective effects at the surface.