Characteristic features of deformation of electron spectra behind two-layer barriers

A study is made of the characteristic features of changes in electron spectra behind two-layer barriers for small variations in the thickness of the layers. It is shown that the high-energy region of the spectrum of electrons emerging from the barrier into a small solid angle in the forward direction contains the most information about the barrier thickness and the new spectrum characteristics introduced make it possible, in transilluminating two-layer objects with a beam of fast monoenergetic electrons, to determine the layer which changes its thickness.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 49–53, July, 1978.     

Angular distribution of electrons behind barriers comparable to the extrapolated range

A study is reported of the angular distribution of monoenergetic electrons from a betatron which have traversed barriers of Al, Cu, and Pb with thicknesses reaching the extrapolated range. The initial electron energy is in the range 3–8 MeV. The experimental results agree well with Monte Carlo calculations carried out for identical calculations and with the results of earlier work for smaller barriers. The half-width of the electron angular distribution, increasing with increasing barrier thickness, reaches a nearly constant value, while the distribution itself corresponds to the laws of complete diffusion, i. e., converts into a distribution similar to cos². As the thickness of the absorbers is increased further, the half-width decreases and the distribution becomes narrower, due to the penetration far into the absorbers of electrons which have been scattered predominantly in the forward direction.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 35–38, May, 1975.     

Excitation of helium atoms in collisions with plasma electrons in an electric field

The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical electron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a self-consistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. Therefore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 2¹ P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.     

太赫兹量子级联激光器中有源区上激发态电子向高能级泄漏的研究

利用热力学统计理论和激光器输出特性理论,建立了太赫兹量子级联激光器(THz QCL)有源区中上激发态电子往更高能级电子态泄漏的计算模型,以输出功率度量电子泄漏程度研究分析了晶格温度和量子阱势垒高度对电子泄漏的影响.数值仿真结果表明,晶格温度上升会加剧电子泄漏,并且电子从上激发态泄漏到束缚态的数量大于泄漏到阱外连续态,同时温度的上升也会降低激光输出功率.增加量子阱势垒高度能抑制电子泄漏,并且有源区量子阱结构中存在一个最优量子阱势垒高度. THz QCL经过最优量子阱势垒高度优化后,工作温度得到提升,其输出功率相比于以往的结果也有所提高.研究结果对优化THz QCL有源区结构、抑制电子泄漏和改善激光器输出特性有指导作用.     

Monto Carlo simulation of the behaviour of electrons during electron—assisted chemical vapour deposition of diamond

The behaviour of electrons during electron-assisted chemical vapour deposition of diamond is investigated using Monte Carlo simulation. The electron energy distribution and velocity distribution are obtained over a wide range of reduced field E/N (the ratio of the electric field to gas molecule density) from 100 to 2000 in units of 1Td=10^-17Vcm². Their effects on the diamond growth are also discussed. The main results obtained are as follows. (1) The velocity profile is asymmetric for the component parallel to the field. The velocity distribution has a peak shift in the field direction. Most electrons possess non-zero velocity parallel to the substrate. (2) The number of atomic H is a function of E/N. (3) High-quality diamond can be obtained under the condition of E/N from 50 to 800Td due to sufficient atomic H and electron bombardment.     

Highway noise barriers: new shapes

This paper describes the relative acoustical performances established by scale model testing of a number of relatively novel noise barriers in typical highway situations. The various barriers were thin, wide, T-profiled, cylindrically topped, corrugated, inclined, Y-profiled, arrow-profiled and of the thnadner principle, and some were treated with sound absorptive material. The highway situations involved a single barrier with a protected receiver (i.e., a receiver behind the barrier), a single barrier with a receiver on the opposite side of the highway, and parallel barriers, one on each side of the highway. In the single barrier, protected receiver case, higher noise reduction was found for wide top barriers, especially those of T-profile, and especially T-profile absorptive top barriers with cap widths of 0·6 m (2 ft) or more and of small cap thickness. Absorptive side treatment was effective in reducing a small, but measurable sound increase found when a reflective sided barrier is installed on the opposite side of the highway to receivers, and in reducing the degradation in performance that occurs when there is a barrier on each side of the highway rather than on just one.     

Diagnostics of a hollow relativistic electron beam from the transfer function of a converting target

Methods for measuring angular, geometrical, and energy characteristics of a hollow relativistic electron beam using the transfer function of a converting target are developed. A relationship between the anisotropy parameters of bremsstrahlung behind the target, angular characteristics of electrons, and geometrical parameters of the electron beam in the target plane is established. The energy balance equation that describes the interaction between the electron beam and converting target is studied. The transfer function of the target and its behavior versus electron energy are found. The time and integral characteristics of the electron beam are examined.     

Most probable and average energy losses of the beam of monoenergetic charge particles with average and low energies at multiple scattering

The results of statistical modeling of the discrete process of multiple inelastic scattering are presented. This process is modeled to find the most probable and average energy losses of a beam of charged particles (electrons and protons) passing through a material layer with a given thickness. The proposed approach is based on determining the most probable energy loss at single small-angle scattering, on including the effect of the statistical probability on this quantity at multiple scattering, and on determining the average number of inelastic interactions for particles in a film with a known thickness. The dependence of the particle energy lost during interaction with atomic electrons on their relative motion is taken into account for low-energy particles. A new interpretation is offered for the parameter J in the logarithmic term in the formulas for the average and most probable energy losses of charged particles. A computational scheme for this parameter as an average potential energy of atomic electrons is given.     

Transition behavior from uncoupled to coupled multiple stacked CdSe/ZnSe self-assembled quantum-dot arrays

Transition behavior from uncoupled to coupled multiple stacked CdSe/ZnSe quantum-dot (QD) arrays grown by molecular beam epitaxy were investigated. Transmission electron microscopy showed that vertically stacked self-assembled CdSe QD arrays were embedded in the ZnSe barriers. The results for the photoluminescence (PL) data at 18 K demonstrated clearly that the transition behavior from uncoupled to coupled peaks depended on the ZnSe barrier thickness. The temperature-dependent PL measurements showed that the activation energy of the electrons confined in the CdSe QDs increased dramatically with decreasing ZnSe spacer layer thickness due to the strong coupling between CdSe/ZnSe QD arrays. The present observations can help improve understanding of the dependence of the coupling behavior and activation energy in CdSe/ZnSe QDs on the spacer layer thickness.     

Radiation produced by electrons making multiple passes through thin internal targets in the Tomsk Synchrotron

It is shown experimentally and theoretically that the use of thin (approximately 10^–3 r.l.) internal targets in a synchrotron results in electrons making multiple passes through them during each shot. The number of passes the electrons make is determined by their energy, the synchrotron parameters, and the thickness, structure, and material of the target, and gives rise to an increase in the effective thickness of the target. Using a scraper (a thick movable target at a given azimuth of the synchrotron) allows the electrons to make a single pass, which permits the spectral and angular properties of the -radiation from the electrons to be studied in connection with thin single crystals in which the effect of dechanneling can be ignored.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 81–87, June, 1991.     

Electron kinetics in collisionless shock waves

We study the kinetic model of the formation of the energy spectrum of nonthermal electrons near the front of a quasilongitudinal, supercritical, collisionless shock wave. Nonresonant interactions of the electrons and the fluctuations generated by kinetic instabilities of the ions in the transition region inside the shock front play the main role in the heating and preacceleration of electrons. We calculate the electron energy spectrum in the vicinity of the shock wave and show that the heating and preacceleration of electrons occur on a scale of the order of several hundred ion inertial lengths in the vicinity of the viscous discontinuity. Although the electron distribution function is significantly nonequilibrium near the shock front, its low-energy part can be approximated by a Maxwellian distribution. The effective electron temperature T _eff ² behind the front, obtained in this manner, increases with the Mach number of the shock wave slower than it would if it followed the Hugoniot adiabat. We determine the condition under which the electron heating is ineffective but the electrons are effectively accelerated to high energies. The high-energy asymptotic behavior of the distribution function is that of a power law, with the exponent determined by the total compression ratio of the plasma, as in the case of acceleration by the first-order Fermi mechanism. The model is used to describe the case (important for applications) of acceleration of electrons by shock waves with large total Mach numbers, with the structure of these waves modified by the nonlinear interaction of nonthermal ions and consisting of an extended prefront with a smooth variation of the macroscopic parameters and a viscous discontinuity in speed with a moderate value of the Mach number. Zh. éksp. Teor. Fiz. 115, 846–864 (March 1999)     

A numerical method to calculate dwell time for electron in semiconductor nanostructure

To some extent,the operational quickness of semiconductor devices depends on the transmission time of an electron through semiconductor nanostructures.However,the calculation of transmission time is very difficult,thanks to both the contentious definition of the transmission time in quantum mechanics and the complicated effective potential functions experienced by electrons in semiconductor devices.Here,based on an improved transfer matrix method to numerically solve the Schr?dinger equation and...     

Single electron charging effects in semiconductor quantum dots

We have studied charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas (2 DEG) of a GaAs/AlGaAs heterostructure. The gate structure allows an independent control of the conductances of the two tunnel barriers separating the quantum dot from the two 2 DEG leads, and enables us to vary the number of electrons that are localized in the dot. We have measured Coulomb oscillations in the conductance and the Coulomb staircase in current-voltage characteristics and studied their dependence on the conductances of the tunnel barriers. We show experimentally that at zero magnetic field charging effects start to affect the transport properties when both barrier conductances are smaller than the first quantized conductance value of a point contact at 2e ²/h. The experiments are described by a simple model in terms of electrochemical potentials, which includes both the discreteness of the electron charge and the quantum energy states due to confinement.     

Two-electron excitation of calcium, strontium, and barium atoms on exposure to electron impact

The absolute effective excitation cross-sections for the spectral transitions of alkaline-earth atoms under the action of electron impact are determined, which correspond to simultaneous excitation of s²-valence electrons. The results were obtained on a setup with intersecting atomic and electron beams. It is established that the effective cross-sections of one- and two-electron excitation of valence subshells into states with the same effective principal and orbital quantum numbers are equal. The results obtained are compared with the available theoretical data. Institute of Electron Physics, National Academy of Sciences of Ukraine, 21 Universitetskaya St., Uzhgorod, 294016, Ukraine. Translated from Zhurnal Prikladnoi, Spektroskopii, Vol. 65, No. 3, pp. 433–436, May–June, 1998.     

Dynamics of electrons in gradient nanostructures (exactly solvable model)

A flexible multi-parameter exactly solvable model of potential profile, containing an arbitrary number of continuous smoothly shaped barriers and wells, both equal or unequal, characterized by finite values and continuous profiles of the potential and of its gradient, is presented. We demonstrate an influence of both gradient and curvature of these potentials on the electron transport and spectra of symmetric and asymmetric double-well (DW) potentials. The use of this model is simplified due to one to one correspondence between the algorithms of calculation of the transmittance of convex barriers and energy spectra of concave wells. We have shown that the resonant contrast between maximum and minimum in over-barrier reflectivity of curvilinear barrier exceeds significantly the analogous effect for rectangular barrier with the same height and width. Reflectionless tunneling of electrons below the bottom of gradient nanostructures forming concave potential barriers is considered. The analogy between dynamics of electrons in gradient fields and gradient optics of heterogeneous photonic barriers is illustrated.     

Level shifts and inelastic electron scattering in dense plasmas

A completely quantum mechanical formalism has been developed to describe the high density plasma effects on fundamental atomic parameters. Both the bound and free electrons are treated by a method which in principle is similar to Hartree's self-consistent field method. The free plasma electrons' wavefunction is obtained from the Schrödinger equation with the effective pottential representing the spherically averaged Coulomb interaction with bound and free electrons. Results are given for level shifts, coefficients of transition probabilities, and electron collision cross sections of Ne⁺⁹ for temperatures of 200 and 500 eV for an electron density range of 1–6 × 10²⁴ cm^?3.     

Secondary electron emission from thin carbon films

For the purpose of investigating how secondary electrons are produced in carbon, the correlation between energy-loss events and secondary electrons was studied experimentally by using the coincidence method. If a secondary electron is detected in coincidence with an electron transmitted through a thin film which has lost an amount of energy E, then the process causing this energy loss results in the production of secondary electrons. We established the existence of these coincidences and have taken inelastic and coincidence spectra for films of different thickness. We found that in carbon secondary electrons are predominantly produced as a result of energy losses of about 20 eV, with an efficiency of about 5%. The escape depth of secondary electrons was also estimated to be approximately 30 Å.     

Effects of atomic number Z on the energy distribution of hot electrons generated by femtosecond laser interaction with metallic targets

The effects of atomic number Z on the energy distribution of hot electrons generated by the interaction of 60fs, 130mJ, 800nm, and 7×10^17W/cm^2 laser pulses with metallic targets have been studied experimentally. The results show that the number and the effective temperature of hot electrons increase with the atomic number Z of metallic targets, and the temperature of hot electrons are in the range of 190-230keV, which is consistent with a scaling law of hot electrons temperature.     

Mechanisms for second harmonic generation in the interaction of a superintense ultrashort laser pulse with cluster plasma

The effects of the interaction of an intense femtosecond laser pulse with large atomic clusters are considered. The pulse intensity is of the order of 10¹⁸ W cm^?2. New effects appear when the magnetic component of the Lorentz force is taken into account. The second harmonic of laser radiation is generated. The second harmonic generation (SHG) efficiency is proportional to the square of the number of atoms in a cluster and the square of the laser radiation intensity. The resonance increase in the SHG efficiency at the Mie frequencies (both at the second harmonic frequency and fundamental frequency) proved to be insignificant because of the fast passage through the resonance during cluster expansion. The mechanisms of the expansion and accumulation of energy by electrons and ions in the cluster are discussed in detail. The energy accumulation by electrons mainly occurs due to stimulated inverse bremsstrahlung upon elastic reflection of the electrons from the cluster surface. The equations describing the cluster expansion take into account both the hydrodynamic pressure of heated electrons and the Coulomb explosion of the ionized cluster caused by outer shell ionization. It is assumed that both inner shell and outer shell ionization is described by the over barrier mechanism. It is shown that atomic clusters are more attractive for the generation of even harmonics than compared to solid and gas targets.     

高能电子束韧致辐射特性的理论研究

高能电子束轰击金属靶会产生韧致辐射X射线,为优化韧致辐射X射线品质,需要研究如何获取最佳辐射效率等韧致辐射规律.结合理论分析,并采用MCNP/4C对10,20?MeV电子的韧致辐射规律进行了模拟研究.讨论了不同靶材料产生的韧致辐射效率、角分布、能谱分布、准直锥孔内辐射效率等问题.通过对不同靶材料韧致辐射的模拟研究,给出了不同厚度靶与光子效率、注量分布、出射电子与角分布的关系与规律.由此得到不同靶材料对于10,20?MeV电子在最优韧致辐射效率下的一些边界条件与规律. 关键词： 韧致辐射 最佳效率 角分布 能谱