Angular and energy dependences of the surface excitation parameter for semiconducting III-V compounds

Sum-rule-constrained extended Drude dielectric functions were used to study surface excitations generated by energetic electrons moving across surfaces of semiconducting III-V compounds. Parameters in the dielectric functions were determined from fits to experimental optical data and electron energy-loss spectra. Electron inelastic mean free paths (IMFPs) in GaN, GaP, GaAs, GaSb, InAs and InSb were calculated for electron energies between 200 and 2000 eV, and the results were found to follow the simple formula, i.e., λ = kE^p, where λ is the IMFP and E is the electron energy. Surface excitation parameters (SEPs), which describe the total probability of surface excitations by electrons crossing the surface and travelling in vacuum, were also calculated for different electron energies and crossing angles. The SEP was found to follow the simple formula, i.e., , where P_s is the SEP and α is the crossing angle relative to the surface normal.     

Surface excitation probabilities in surface electron spectroscopies

The phenomenon of surface excitation is competitive in nature for elastic and other inelastic scattering processes in surface electron spectroscopies; the knowledge of influence of surface excitations in electron energy loss spectra is then essential for quantitative surface analysis with these spectroscopies. The inelastic scattering of an electron moving in the vicinity of a surface is considered in a self-energy formalism to estimate the contribution of surface excitation in electron-solid interactions via the total surface excitation probability. The formulation uses the optical bulk dielectric function and provides the spatial and angular dependence of the differential and total inelastic cross-sections. The kinetic energy range of probing electrons considered is 100-5000 eV and the numerical evaluation of total surface excitation probabilities are performed for several metals, Au, Ag, Cu, Ni, Fe and Ti; empirical formulae for the surface excitation probability are given for each of these materials and compared with experimental results for the surface excitation parameter. The total surface excitation probability is higher in Ag as compared to other metals under consideration, for identical conditions of electron-solid interactions.     

Quantification of surface effects: Monte Carlo simulation of REELS spectra to obtain surface excitation parameter

The surface excitation effect is investigated by using the quantum mechanical frame work of complex self-energy of electrons which interact with a bounded semi-infinite medium. In the self-energy formalism, differential inverse inelastic mean free path (DIIMFP) has contributions from bulk and surface plasmons. Monte Carlo simulation of the interaction of electrons with a solid medium and surface has been performed. The surface excitation parameter (SEP) is then obtained from the simulated reflection electron energy loss spectroscopy (REELS) spectra. The calculated SEP results by Monte Carlo simulation are compared with the previous calculations of total surface excitation probability, which was estimated by a numerical integration of surface term of DIIMFP. The contribution merely due to surface excitations towards REELS spectra is extracted by subtracting the two Monte Carlo simulated REELS spectra that based on the two models of electron inelastic scattering, i.e. a full surface model (SM) and a pure bulk model (BM). The surface excitations found to be significant at low energy losses and diminish at higher energy losses whereas the bulk plasmon contributions show opposite behavior and are negligible at lower energy losses. The average number of surface excitations is then evaluated by the computation of ratio of the integrated surface contribution to the elastic peak. The calculated results for Ag are found to be reasonably in agreement with our previous results for total probability of surface excitations and other reported experimental data for SEP. Surface correction factor (SCF) is calculated using SEP for several metals and is compared with the reported ratio of SCF with Ni sample as reference.     

Theoretical determination of the surface excitation parameter for Ti, Fe, Cu, Pd, Ag, and Au

The angular dependence of the surface excitation parameter (SEP) is theoretically determined for different materials (Ti, Fe, Cu, Pd, Ag, and Au) for angles to the surface normal between 10° and 70° and for energies between 300 and 3400 eV. The method is based on analysis of energy-differential inelastic electron scattering cross sections calculated from the dielectric function with the software QUEELS (quantitative analysis of electron energy losses at surfaces) valid for reflection-electron-energy-loss spectroscopy (REELS). The calculations presented here are closely related to a method previously developed [N. Pauly, S. Tougaard, F. Yubero, Surf. Interf. Anal. 37 (2005) 1151] for materials (like Al and Si) with a clear surface plasmon peak. However, for the materials studied here the excitations are broad and there is no clear visual distinction between surface and bulk excitations. It is therefore not possible, as in the previous studies of Al and Si, to isolate the intensity of the surface excitation directly. Therefore the volume component for an infinite medium is subtracted from the calculated cross section and in this way the surface excitation component of the cross section is determined and the SEP calculated.     

Surface plasmon excitation at a Au surface by 150-40,000 eV electrons

Spectra of electrons with energies between 5 and 40 keV reflected from a homogeneous Au surface have been measured and analyzed to give the normalized distribution of energy losses in a single surface and volume excitation, as well as the total probability for excitation of surface plasmons. The resulting single scattering loss distributions compare excellently in (absolute units) with data from previous work taken at lower energies (150-3400 eV). An empirical relationship is derived for the total surface excitation probability as a function of the energy. For high energies the surface scattering zone represents only a small fraction of a typical electron trajectory and hence interference effects should be small at these energies. Since we find that both the energy dependence of the surface plasmon excitation probability and the shape of the single scattering loss distributions are the same at high and low electron energies, we conclude that there is no evidence for interference effects in the entire energy range studied.     

Inelastic interactions of electrons with cylindrical interfaces

A theory was developed to deal with inelastic interactions for an electron moving parallel to the axis of a cylindrical structure. Formulas for the differential inverse inelastic mean free path (DIIMFP) and the total inverse inelastic mean free path (IIMFP) were derived using dielectric response theory. A sum-rule-constrained extended Drude dielectric function with spatial dispersion was applied to calculate DIIMFPs and IIMFPs for a solid wire and a cavity in solid. The calculated results showed that surface excitations occurred as the electron moved near the boundary either inside or outside the solid, whereas volume excitations arose only for electron moving inside the solid. It was found that the probability for surface excitations increases and that for volume excitations decreases for an electron moving close to the surface. Near the surface, the decrease in volume excitations is compensated by the increase in surface excitations. For a cavity in solid, the IIMFP inside the solid can be approximated by a constant value equal to the IIMFP for the infinite solid, except in the immediate vicinity of the cavity boundary.     

Angular distributions of electrons and light ions elastically reflected from a solid surface

The angular distributions of elastically reflected electrons are described on the basis of the transfer equation for a radiation particle. The exact solution of the problem is obtained. This solution is compared with experimental spectra of elastically reflected electrons and with the approximate solutions.     

Persistent excitation in adaptive parameter identification of uncertain chaotic system

This paper studies the parameter identification problem of chaotic systems.Adaptive identification laws are proposed to estimate the parameters of uncertain chaotic systems.It proves that the asymptotical identification is ensured by a persistently exciting condition.Additionally,the method can be applied to identify the uncertain parameters with any number.Numerical simulations are given to validate the theoretical analysis.     

Inelastic mean free path, surface excitation parameter, and differential surface excitation parameter in Au for 300-3000 eV electrons

An analytical approach for simultaneously determining an inelastic mean free path (IMFP), a surface excitation parameter (SEP) and a differential SEP (DSEP) with absolute units was applied for the analysis of absolutely measured reflection electron energy loss spectra for Au. The IMFP, SEP and DSEP in Au for 300-3000 eV electrons are successfully obtained. The obtained DSEPs show a reasonable agreement with those theoretically calculated. The present SEPs were compared with those calculated by several empirical equations, revealing that the present SEPs are close to those calculated using the Oswald's equation. The IMFPs for Au determined by the present analysis were compared with those calculated by the TPP-2M predictive equation, revealing that the present IMFPs are in fairly good agreement with those calculated by the TPP-2M equation. The results confirmed that the present approach is effective for experimentally determining the SEP, DSEP, and IMFP for electrons in solids.     

Monte Carlo simulation for Multi-Mode Elastic Peak Electron Spectroscopy of crystalline materials: Effects of surface structure and excitation

The Multi-Mode Elastic Peak Electron Spectroscopy (MM-EPES) analysis is confined to incoherent electron elastic scattering and the use of variable primary energy. This experimental method is very sensitive to the surface region of the sample. However, for quantitative interpretation, the MM-EPES method needs jointly a Monte Carlo (MC) computer simulation of electron trajectories in the solid. In the present work, we proposed a new approach to calculate the percentage η_e of elastic reflected electrons by the surface of a sample. This simulation takes into account the surface effects (i.e. surface plasmon), and the atoms arrangement in the substrate. The concept of the surface excitation parameter (SEP) is also presented. Computer simulations were performed on the three low index single crystals of Cu, Au, Si and Ag. The results confirm that the distribution of substrate atoms, according to the crystallographic structure, influences the intensity measured by EPES.A simple prediction formula was proposed to calculate η_e for elastic electrons entering in a Retarding Field Analyzer (RFA) spectrometer which is the apparatus giving experimentally numerical values of the percentage η_e.     

Wettability and surface energy of parylene F deposited on PDMS

For the first time, the wettability and surface energy of parylene F were investigated. The results showed that parylene F had a hydrophobic surface with an eigen water contact angle of 104.7 ± 0.6°. We found that 3.5 μl probe liquid was an optimal value for the contact angle measurement of parylene F. Moreover, we found that the Lifshitz–van der Waals/acid–base approach was unsuitable for determining the surface energy of parylene F, whereas the Owens–Wendt–Kaelble approach and the limitless liquid–solid interface wetting system were compatible. The surface energy of parylene F was estimated to be 27.06 mJ/m² (Owens–Wendt–Kaelble) and 40.41 mJ/m² (Limitless liquid-solid interface wetting system). Furthermore, this investigation also provided a reference for the applications of empirical and physics-based semi-empirical approaches for the estimation of surface energy.     

Angular and energy dependences of cross sections for deuteron electrodisintegration at subbarrier energies

A method is proposed for analyzing a large set of spectra calculated for the Coulomb breakup of deuterons. The regularities in the behavior of the respective cross section as a function of the emission angles and energies of particles constituting the deuteron are studied on the basis of this method. The deuteron-breakup reaction on lead nuclei at an energy of E _d = 8 MeV is considered by way of example. It is shown that the approximate Landau-Lifshitz formulas for this process are unable to predict some of its features. An experiment is proposed for studying the physical nature of the Nemets effect.     

Effect of finite energy of intravalley acoustic phonons on the temperature of non-equilibrium electrons in a quantum surface

The effect of finite energy of intravalley acoustic phonons on the electric field dependence of the temperature of the non-equilibrium carriers in a quantum surface has been studied here. The calculations have been made, for a rather pure material, at low lattice temperature. Numerical results are obtained for GaAs and Si. The results are interesting being significantly different from what one obtains by neglecting the phonon energy.     

Dissociative excitation cross sections of the lead atom in collisions between electrons and PbI2 molecules

Dissociative excitation of the lead atom in e-PbI₂ collisions has been studied experimentally. 27 excitation cross sections were measured for an energy of the exciting electrons equal to 100 eV. Nine optical excitation functions were recorded with variation of the electron energy in the range 0–100 eV. The most probable reaction channels for low electron energies are discussed, as well as the ratio of the dissociative excitation cross sections for the lead atom in e-PbI₂ and e-PbCl₂ collisions. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 701–706, November–December, 2006.     

Radiative rates and electron impact excitation rate coefficients for H-like Fe XXVI

In this paper we report on calculations on energy levels, radiative rates, collision strengths, and effective collision strengths for transitions among the lowest 36 levels of the n≤6 configurations of H-like Fe XXVI. Flexible atomic code (FAC) is adopted for the calculation. Energy levels and radiative rates are calculated within relativistic configuration-interaction method. Direct excitation collision strength is calculated using relativistic distorted-wave approximation. Resonance contributions through the relevant He-like doubly excited n^′l^′n^″l^″ configurations with n^′≤7 and n^″≤75 are explicitly taken into account using the isolated process and isolated resonances approximation. We present the radiative rates, oscillator strengths, and line strengths for all electric dipole (E1), magnetic dipole (M1), electric quadrupole (E2), and magnetic quadrupole (M2) transitions among the 36 levels. Furthermore, collision strengths and effective collision strengths are reported for all the 630 transitions among the above 36 levels over a wide energy (temperature) range up to 25 keV . Extensive comparisons are made with earlier available results and the accuracy of the data is assessed.     

Dissociative excitation of a cesium atom in collisions of slow electrons with CsF molecules

The method of extended intersecting beams is used for studying inelastic collisions of electrons with cesium fluoride molecules. Forty-six cross-sections for dissociative excitation of the spectral lines of a cesium atom located in the spectral region 387–825 nm were used. The behavior of the sections in the spectral series of a cesium atom is investigated, which is reflected by a power law. The values of constants that are parameters in the power-law formula are determined. Moscow Power Engineering Institute, 14, Krasnokazarmennaya St., E-250, Moscow, 105835, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 20–25, January–February, 1997.     

Electron impact ionization and excitation rate coefficients in the negative glow region of a glow discharge

Some easy to use reasonable approximations for electron impact rate coefficients have been considered. The most important rate coefficients for electron collisions in noble gases are electron-neutral ionization and electron impact excitation. Electron-neutral ionization besides electron impact excitation of some states of the argon and helium atom in direct current (dc) glow discharge plasma has been calculated. The plasma parameters of electron are significant factors for computing the rate coefficients. We present first results of probe diagnostic that includes the double probe measurements of the plasma parameters, namely, electron temperature (T_e) and electron density (n_e). Electron properties obtained from the double probe characteristic curves including T_e and n_e as well as the calculated rate coefficients (ionization and excitation) were studied as a function of the axial distance from the cathode while the discharge operating parameters of voltage and pressure were varied. Two regions of the glow discharge were investigated: cathode fall region and negative glow. Particular emphasis was placed on the negative glow region.     

近地面层大气折射率结构常数的模式研究

 运用Bulk方法对东南沿海和合肥一水库附近的折射率结构常数进行了估算,为避免海边海盐对金属丝的污染,用高精度超声风速仪替代温度脉动仪测量折射率结构常数。模式主要输入的参数有气温、地表面温度、湿度、风速及测量点高度。结果表明：运用Bulk方法估算折射率结构常数是可行的,相似性函数分别采用Frederickson公式与Thiermann公式,差别不大;直接测量海面上折射率结构常数比较困难,通过输入海表面温度、海面上的粗糙度长度以及适合的相似性函数,可用于估算海面上的折射率结构常数。     

Susceptibilities of nano-particles at the surface of a solid

The self-consistent approach is proposed for calculation of an effective susceptibility of single nano-particle and nano-particle layer at a surface of a solid. This approach is based on exact summation of iteration procedure series, which is applied to solution of self-consistent equation for a local field. The condition when the layer can be considered as dilute one is discussed.     

High order Correlation polarization potential for vibrational excitation scattering of diatomic molecules by low-energy electrons

This paper introduces a correlation--polarization potential with high order terms for vibrational excitation in electron--molecule scattering. The new polarization potential generalizes the two-term approximation so that it can better reflect the dependence of correlation and polarization effects on the position coordinate of the scattering electron. It applies the new potential on the vibrational excitation scattering from N₂ in an energy range which includes the ²П_g shape resonance. The good agreement of theoretical resonant peaks with experiments shows that polarization potentials with high order terms are important and should be included in vibrational excitation scattering.