Measurement of the energy penetration depth into solid targets irradiated by ultrashort laser pulses

The energy penetration depth of a short (100 fs) Ti-sapphire laser pulse (0.8 &mgr;m) of intensity 3x10(16) W/cm(2), in solid density materials has been measured. High-Z (BaF2) and low-Z (MgF2) solid layers targets were used. The penetration depth was determined from the measurement of the x-ray emission spectra, as a function of the target thickness. The investigation of these spectra showed that in the low-Z case, solid density material to a depth of 50 nm was heated to a peak electron temperature of approximately 150 eV. For the high-Z material, the penetration depth corresponding to this temperature exceeded 100 nm. This is evidence of a larger heat penetration depth in a high-Z material in comparison to a low-Z material. A model based on electron heat conduction is used to estimate the energy penetration depth. It is suggested that the larger heat penetration in high-Z material is due to heating of the material, caused by the radiation flux, generated by the electron heat conduction.     

Kinetic theory of surface waves in a semibounded plasma flow

A study is made of the spectrum of surface waves in a semibounded plasma flow. The frequency spectra and damping rates of the waves propagating along the flow are analyzed both in the high-frequency range (in which the spatial dispersion is weak and the wave damping is governed primarily by electron collisions) and the low-frequency range (in which the spatial-dispersion effects dominate), with focus on the effect of the flow velocity on the propagation of ion-acoustic waves. Special attention is paid to the penetration of a static field into a plasma flowing at a supersonic velocity.     

Initial stages in the intercalation of 1T-TiS2(0001) single crystals by potassium

A study is reported of spontaneous intercalation of 1T-TiS₂(0001) by potassium, which takes place when an alkali metal is deposited on the surface of this layered material. The experiments were carried out in ultrahigh vacuum at room temperature within the 0–10 adsorbate monolayer range. An analysis of electron diffraction patterns visualizing the crystalline structure of the nanometer-scale surface layer of the studied samples showed that penetration of the intercalant into TiS₂ stimulates a 1T → 3R(I) structural phase transition and brings about a substantial (by 2.2 Å) increase in the interlayer separation in this compound. It was found that the process passes through stages of gradual filling of the interlayer gaps and that this is accompanied by a lateral displacement of the titanium and sulfur layer sandwiches in the original crystal.     

Numerical simulation of laser-generated ultrasound in non-metallic material by the finite element method

The use of a pulsed laser for the generation of the elastic waves in non-metallic materials in the thermoelastic regime is investigated by using finite element method (FEM), taking into account not only thermal diffusion and the finite spatial and temporal shape of the laser pulse, but also optical penetration and the temperature dependence of material properties. The optimum finite element model is established based on analysis of two important parameters, meshing size and time step, and the stability of solution. Temperature distributions and temperature gradient fields in non-metallic material for different time steps are obtained, this temperature field is equivalent to a bulk force source to generate ultrasonic wave. The laser-generated ultrasound waveforms at the epicenter and surface acoustic waveforms (SAWs) are obtained and the influence of optical penetration into the material on the temperature field and the ultrasound waveforms are analyzed. The numerical results indicate that the heat penetration into non-metallic material is caused mainly by the optical penetration, and the ultrasound waveforms, especially the shape of the precursor, are strongly dependent on the optical penetration depth into non-metallic material.     

A TEM study of nanoparticles in lustre glazes

Transmission electron microscopy (TEM) has been used to investigate the nanoscale morphology of some contemporary lustre glazes. High-resolution TEM, electron energy-loss spectroscopy and energy-dispersive X-ray analysis data imply that two kinds of nanoparticles are present in the lustre layer, namely metallic Ag and metallic Cu particles. Moreover, these particles appear separated in the material. The dense top layer consists of Ag particles and the particles occurring below this upper layer are metallic Cu. A depth profile of the sizes of the nanoparticles with respect to their penetration depths has been drawn. The particle sizes are mainly situated in the range of 5 nm to 15 nm, though smaller and larger particles occur frequently. PACS 68.37.Lp; 61.46.+w; 81.05.Pj; 79.20.Uv     

n型硅微尖场发射电子能谱的模拟计算

结合金属的场发射电子能谱,模拟计算了场渗透对n型半导体硅微尖的场发射能谱的影响,并与n型硅微尖的场发射能谱实验结果进行了比较,讨论了模拟计算误差的来源。计算结果表明电场渗透现象导致硅的场发射能谱向低能方向偏移,表面电场越高,能谱的偏移量越大,其偏移程度可超过1eV。导致硅微尖的场发射能谱偏移的主要因素是半导体的场渗透现象。     

Development of a 3-D, multi-nuclear continuous wave NMR imaging system

The development of a 3-D, multi-nuclear continuous wave NMR imaging (CW-NMRI) system is described and its imaging capability is demonstrated on a range of materials exhibiting extremely short T(2) relaxation values. A variety of radiofrequency resonators were constructed and incorporated into a new gradient and field offset coil assembly, while the overall system design was modified to minimise microphonic noise which was present in an earlier prototype system. The chemically combined (27)Al in a high temperature refractory cement was imaged, and the CW-NMRI system was found to be sensitive to small differences in (27)Al content in these samples. The penetration of (23)Na in salt water into samples of ordinary Portland cement (OPC) was investigated, with enhanced uptake observed for samples with larger pore size distributions. The solid (13)C component in a carbonated cement sample was also imaged, as were the (7)Li nuclei in a sample of powdered Li(2)CO(3). A spatial resolution of 1mm was measured in an image of a rigid polymeric material exhibiting a principal T( *)(2) value of 16.3 micros. Finally, a high-resolution 3-D image of this rigid polymer is presented.     

Radiation loss of electrons under scattering by a Thomas-Fermi atom

A universal theory and calculation results for the bremsstrahlung of electrons on complex atoms are presented. The theory accounts for the dynamic polarization of the core in the energy range from 0.5 to 10 keV, which is characteristic of radiation energy losses in a hot plasma with heavy ions. The treatment is based on the statistical atom model and the quasi-classical approximation of the incident electron. The model accounts for the penetration of the incident electron into the atomic core, which affects the relationship between the polarization and static radiation channels. The contribution of the polarization channel in both the spectral and the total radiation loss of electrons at various frequencies, nucleus charges, and energies of the incident particle is analyzed. It is shown that the contribution of the polarization channel is comparable with that of the static channel (which was calculated elsewhere) in a wide range of parameters. The results obtained are in a reasonable quantitative agreement with the detailed quantum-mechanical calculation carried out for individual atoms.     

Penetration kinetics of a cumulative jet into brittle materials

The experimental results on the penetration of cumulative jets into brittle materials are analyzed to substantiate the assumption that continuous hydrodynamic penetration is violated. The penetration of a cumulative jet into a brittle material has a jumplike character and consists of hydrodynamic penetration, the collapse of the cavity, and secondary penetration into the collapsed material. For a continuous supply of a cumulative jet, this process is repeated at the penetration depth. The necessary conditions of the secondary penetration consist in a high strength of the brittle material and a high fracture rate, which should provide the spallation and collapse of the cavity walls. Jumplike penetration ends when a rarefaction wave passes to the zone of primary penetration.     

岩石、混凝土和土抗侵彻能力数值计算与分析

利用LS-DYNA3D软件数值计算了弹体侵彻岩石、混凝土和土问题,分析在不同碰撞速度条件下的弹体响应和靶体抗侵彻能力。碰撞速度小于900 m/s时,弹体侵彻岩石的减加速度峰值约是侵彻混凝土的2倍,而侵彻混凝土的减加速度峰值约是侵彻土的6倍。减加速度峰值高则稳态侵彻过程短,弹体能量消耗很快。碰撞速度超过1.5 km/s时,随靶体材料的强度、密度逐渐减小,侵彻深度和孔径逐渐缓慢增加,岩石、混凝土和土3种靶体材料相比,最大侵彻深度增加41%~62%,最大扩孔口径增加16%~25%。     

Anomalous capacitive sheath with deep radio-frequency electric-field penetration

A novel nonlinear effect of anomalously deep penetration of an external radio-frequency electric field into a plasma is described. A self-consistent kinetic treatment reveals a transition region between the sheath and the plasma. Because of the electron velocity modulation in the sheath, bunches in the energetic electron density are formed in the transition region adjacent to the sheath. The width of the region is of order V(T)/omega, where V(T) is the electron thermal velocity, and omega is the frequency of the electric field. The presence of the electric field in the transition region results in a collisionless cooling of the energetic electrons and an additional heating of the cold electrons.     

Photoluminescence spectra of doped GaAs films

We have studied the dependence of the photoluminescence (PL) spectrum on the doping level and the film thickness of n-GaAs thin films, both experimentally and theoretically. It has been shown theoretically that modification of the PL spectrum of p-type material by p-type doping is very small due to the large valence-band hole effective mass. The PL spectrum of n-type material is affected by two factors: (1) the electron concentration which determines the Fermi level in the material; (2) the thickness of the film due to re-absorption of the PL signal. For the n-type GaAs thin films under current investigation, the doping level as well as the film thickness can be very well calibrated by the PL spectrum when the doping level is less than 2×10¹⁸ cm^-3 and the film thickness is in the range of the penetration length of the PL excitation laser. PACS 78.20.-e; 78.55.Cr; 78.66.Fd     

Small-scale fracture toughness of ceramic thin films: the effects of specimen geometry,ion beam notching and high temperature on chromium nitride toughness evaluation

For the implementation of thin ceramic hard coatings into intensive application environments, the fracture toughness is a particularly important material design parameter. Characterisation of the fracture toughness of small-scale specimens has been a topic of great debate, due to size effects, plasticity, residual stress effects and the influence of ion penetration from the sample fabrication process. In this work, several different small-scale fracture toughness geometries (single-beam cantilever, double-beam cantilever and micro-pillar splitting) were compared, fabricated from a thin physical vapour-deposited ceramic film using a focused ion beam source, and then the effect of the gallium-milled notch on mode I toughness quantification investigated. It was found that notching using a focused gallium source influences small-scale toughness measurements and can lead to an overestimation of the fracture toughness values for chromium nitride (CrN) thin films. The effects of gallium ion irradiation were further studied by performing the first small-scale high-temperature toughness measurements within the scanning electron microscope, with the consequence that annealing at high temperatures allows for diffusion of the gallium to grain boundaries promoting embrittlement in small-scale CrN samples. This work highlights the sensitivity of some materials to gallium ion penetration effects, and the profound effect that it can have on fracture toughness evaluation.     

Soft X‐ray refractive index by reconciling total electron yield with specular reflection: experimental determination of the optical constants of graphite

The complex refractive index of many materials is poorly known in the soft X‐ray range across absorption edges. This is due to saturation effects that occur there in total‐electron‐yield and fluorescence‐yield spectroscopy and that are strongest at resonance energies. Aiming to obtain reliable optical constants, a procedure that reconciles electron‐yield measurements and reflection spectroscopy by correcting these saturation effects is presented. The procedure takes into account the energy‐ and polarization‐dependence of the photon penetration depth as well as the creation efficiency for secondary electrons and their escape length. From corrected electron‐yield spectra the absorption constants and the imaginary parts of the refractive index of the material are determined. The real parts of the index are subsequently obtained through a Kramers–Kronig transformation. These preliminary optical constants are refined by simulating reflection spectra and adapting them, so that measured reflection spectra are reproduced best. The efficacy of the new procedure is demonstrated for graphite. The optical constants that have been determined for linearly polarized synchrotron light incident with p‐ and s‐geometry provide a detailed and reliable representation of the complex refractive index of the material near π‐ and σ‐resonances. They are also suitable for allotropes of graphite such as graphene.     

Stable nitroxyl radicals as pH, thiol and electron transfer probes

In this review article original applications of stable nitroxyl radicals (SNRs) developed by the author with his students and colleagues are presented. ESR (electron spin resonance) spectra of SNRs of the imidazoline and imidazolidine types display high pH sensitivity. Mechanisms of proton exchange in such SNRs were studied in detail and made possible their wide application as pH probes (range of pH 0-13.5) in a variety of chemical and biological systems, including in vivo assays. A stable biradical, bis (2,2,5,5-tetramethyl-3-imidazoline-1-oxyl-4-il)-disulfide, was synthesized and used to monitor thiol-disulfide exchange with free thiols in solution. This approach permitted us to develop an extremely sensitive, quantitative and noninvasive method for determining the concentrations of glutathione and cysteine in living cells. The biradical was used to clarify the topography of cysteines in enzymes and to study their involvement in folding and stability of some proteins. We proposed and implemented a novel ESR approach to study electron transfer: the kinetics of photooxidation of an SNR (with concomitant production of an ESR-silent oxoammonium ion) by a Ru(III)-bipyridyl complex. Recovery of the ESR signal was observed when illumination was terminated. The rate of recovery depends strongly on the presence of organic electron donors, including some amino acids, and on pH. We used site-directed spin-labeled mutants of bacteriorhodopsin, to which a Ru(III) complex was also bound at a well-defined locus, to show the applicability of the proposed method to study the electron transfer and effects of local neighboring and water penetration in proteins. *** DIRECT SUPPORT *** A04RK041 00002     

Experimental verification of the effects of optical wavelength on the amplitude of laser generated ultrasound in polymer-matrix composites

Laser ultrasound is now integrated into the manufacturing process of some of the most modern aircraft for the inspection of composite parts. Unfortunately, for some material and process combinations, laser-ultrasound suffers from a lack of sensitivity. In laser-ultrasound generation, optical penetration depth plays a very important role. It was shown that changing the generation wavelength from the 10.6 microm of the CO2 laser to the 3-4 microm range can significantly improve generation efficiency. In this paper, ultrasonic displacements are compared to measurements of optical penetration depth in different polymer-matrix composites. Ultrasonic waves were generated using an optical parametric oscillator operating in the 3.0-3.5 microm band and optical penetration depth spectra were evaluated using quantitative photoacoustic spectroscopy. The relative amplitudes of the generated ultrasonic waves track closely the optical penetration depth spectra. These results experimentally demonstrate the importance of optical penetration in the laser-ultrasound generation process.     

Substrate influence in electron-phonon coupling measurements in thin Au films

Accurate understanding and measurement of the energy transfer mechanisms during thermal nonequilibrium between electrons and the surrounding material systems is critical for a wide array of applications. With device dimensions decreasing to sizes on the order of the thermal penetration depth, the equilibration of the electrons could be effected by boundary effects in addition to electron-phonon coupling. In this study, the rate of electron equilibration in 20 nm thick Au films is measured with the Transient ThermoReflectance (TTR) technique. At very large incident laser fluences which result in very high electron temperatures, the electron-phonon coupling factors determined from TTR measurements deduced using traditional models are almost an order of magnitude greater than predicted from theory. By taking excess electron energy loss via electron-substrate transport into account with a proposed three temperature model, TTR electron-phonon coupling factor measurements are more in line with theory, indicating that in highly nonequilibrium situations, the high temperature electron system looses substantial energy to the substrate in addition to that transferred to the film lattice through coupling.     

Magnetic and superconducting properties of doped (Sr,Ca)10Cu17O29-type single crystals

Magnetization, resistivity and electron spin resonance (ESR) measurements have been performed on single crystals of A₁₀Cu₁₇O₂₉ (A=Ca_5.9, Sr_3.5, Bi_0.3, Pb_0.1, Y_0.1, Al_0.1) of the S=1/2 quasi-one-dimensional system, which has both simple chains and two-leg ladders of copper ions. Substantial hole doping has been achieved in the studied crystals, which led to superconductivity with a high critical temperature (T_c≈80 K). The values of the penetration depth have been estimated for temperatures in the range 30–60 K using the reversible magnetization data. A rough estimation of the Ginzburg–Landau parameter, κ, indicates that the superconductivity in the investigated ladder material should be described as an extreme type-II limit. It has been suggested that the superconductivity in the studied system should be related to the two-leg ladders rather than to the chains.     

Dynamic diffusion of helium into various types of solids during their deformation and dispersion

Quantitative relations governing the penetration of helium atoms into various types of solids in the course of their plastic deformation in liquid ³He (T = 0.6–1.8 K) and ⁴He (T = 4.2 K) and dispersion in gaseous helium at 300 K were obtained and analyzed. Experiments were carried out on metals with different lattice types, ionic single crystals, amorphous alloys, and barite and titanium dioxide powders dispersed in helium. Curves illustrating helium extraction from deformed specimens under dynamic annealing were obtained. The temperature range of helium extraction was found to correlate with the melting temperature and the initial and deformed structures of a material, which determine the number and character of helium traps present in the material. The dependence of helium penetration intensity on the type of defects forming under plastic deformation for various materials, as well as the formation of chemical bonds of helium atoms to the defected structure of these materials, is discussed.     

Transformation of the dimensionality of excitonic states in quantum wells with asymmetric barriers in an electric field

A transformation of the dimensionality of excitonic states from 2D to 3D with increasing external electric field is observed in single GaAs/Al_xGa_1−x As quantum-well structures with asymmetric barriers. The binding energy of a 2D exciton remains constant over a wide range of variation of the field, since the decrease in the binding energy is compensated by increasingly larger penetration of the electronic wave function into the barrier layer, where the exciton binding energy is higher because the effective mass is larger and the dielectric constant of AlGaAs is lower than that of GaAs. When the maximum of the electron wave function is displaced into the barrier as the field increases, the exciton binding energy decreases. As the field increases further, a 2D exciton transforms into a quasi-3D exciton, with a heavy hole in the quantum well and an electron in a resonant above-barrier state. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 207–211 (10 February 1998)