弗兰克-赫兹实验中温度与电子平均自由程的关系

运用原子非弹性碰撞模型来测量F-H曲线峰谷间距随峰谷数的线性关系,研究温度对电子平均自由程的影响,并尝试探讨一种较为准确的测量原子第一激发能的实验方法.     

Monte Carlo方法模拟低能电子在Al中的散射

基于文献[1]的工作,电子在固体中的弹性散射用Mott微分截面计算;非弹性散射分为单电子激发和等离子激发并由Streitwolf、Gryzinski及Quinn的截面描述.模拟了低能电子在Al块样及薄膜中的散射过程,对不同能量低能电子作用下Al的背散射系统、能谱又透射系数作了计算,结果与实验符合较好.也对背散射电子、低能损背散射电子表面分布作了计算,结果表明低能损背散射电子具有较好的空间分辨率.     

表面等离激元诱导的非线性非弹性电子散射

<正>电子与样品的非弹性散射过程是电子能谱技术的基础。然而非弹性散射电子通常只占极少的比例,大多数电子是没有能量损失的弹性散射电子。最新的实验研究发现,银纳米结构激发出的局域等离激元场可以导致非线性的电子散射现象,使得非弹性电子的强度显著增强。这一新的发现未来可以用于研究吸附在金属纳米结构上的原子、分子,非线性电子散射过程会大大提高信号     

12C+27Al深部非弹性散射研究

用△E-E 计数器望远镜鉴别反应产物,研究71.5兆电子伏的¹²C 离子在²⁷Al 靶子上的深部非弹性散射,得到了产物 B 和 Be 在质心系 E-θ平面的截面等高线图。由图可以看出准弹性散射和深部非弹性散射的一般特征。有两点值得特别指出:第一,从淮弹性峰到深部非弹性散射岭呈现某些精细结构.第二,完全阻尼部份的平均动能不等于终态库伦能.     

介观体系和介观物理

20世纪80年代初中期开始兴起的介观物理是凝聚态物理中人们十分关注的研究领域,“介观”一词大家已听得很多了.一段时间以来,常读到介观尺度是由电子非弹性散射平均自由程决定的说法,觉得不太确切.想到这一说法可能和我早先发表在《物理》杂志的文章 ［1］,以及我们主编的《介观物理》一书第一版的前言 ［2］有关,特写此短文说明.这里的说明也许仍有不妥之处,请读者指正. 1 介观尺度 介观体系的大小是由介观尺度来刻划的.介观尺度是载流子保持相位记忆的长度,一般记为Lφ,称为退相位长度.相位记忆的丢失源于载流子的非弹性散射,因此Lφ必定是与非弹性散射有关的尺寸.电子和声子的散射,电子吸收或放出一个声子,能量发生改变,是电子最常碰到的非弹性散射.如果把介观尺度理解为非弹性散射的平均自由程,又把后者理解为vFτφ,其中vF是电子的费米速度,τφ是非弹性散射(例如电子和声子散射)的弛豫时间,那么当温度下降,体系中声子数越来越少,τφ越来越长时,介观尺度就变得很大了.对于普通金属,如果τφ取为10-9s,因为vF≈108 cm/s,介观尺度可以达到0.1cm左右,显然这个尺寸过大了. 实际上,在液体氦的温度下,如果普通金属的剩余电阻率ρ≈1 μΩcm,则弹性散射的弛豫时间τ0≈10-13s,电子在两次非弹性散射之间会经受上万次弹性散射,走无规行走的路径,以扩散的方式运动,Lφ是相继两次非弹性散射间电子扩散运动的距离.由于扩散系数D=(vFl)／3,其中l为弹性散射的平均自由程,在τφ时间内的扩散距离为(Dτφ)1／2,约10μm,远小于上述的“平均自由程”长度.这一简单的对扩散距离的估算,给出了介观体系大小正确的数量级.     

PtSi超薄膜厚度的一种检测方法研究

介绍了采用角分辨X-射线光电子解谱(angle resolved X-ray photoelectric spectrum(ARXPS))测试薄膜不同角度光电子能谱强度,计算电子平均自由程,从而计算出PtSi超薄膜厚度的方法,并给出其透射电子显微镜(TEM)晶格象验证结果.实验表明该方法简单易行,适用于其他超薄膜厚度的测量 关键词： PtSi超薄膜 小角度X射线电子能谱 TEM晶格象     

58,60,62,64Ni25.1MeV质子非弹性散射研究及其低激发态能谱和B(E2)比率的IBA分析

使用上海原子核研究所1.4m等时性迴旋加速器提供的25.1MeV质子束,测量了偶镍同位素弹性散射和第一激发态2+1的非弹性散射微分截面.用零程扭曲波玻恩近似(DWBA)作理论分析,得到^58,60,62,64Ni解决2+1态的四极形变参数β₂分别为0.23、0.28,0.25,0.21.采用相互作用玻色子近似(IBA)计算了这些核低激发态能谱及其约化电跃迁几率B(E2).并对IBA唯象参数作了初步的准粒子无规位相近似(QPRPA)微观计算.     

18O+148Nd的准弹性碰撞研究和耦合道分析

用Q3D磁谱仪及其焦面探测器系统,在90.72MeV处测量了¹⁸O+¹⁴⁸Nd的弹性散射和非弹性散射角分布,利用DWBA和耦合反应道计算程序FRESCO,对实验结果进行了拟合.在对弹性散射分析中,引入极化势的DWBA计算与耦合道计算有类似的结果.讨论了非弹性散射激发过程中的核–库仑激发的相干效应,得出了描述相干强度的近似因子.     

弗兰克－赫兹实验中电子与汞原子的碰撞机理

Ｈｇ原子６￣１Ｓ＿０→６￣３Ｐ＿０．１．２的电子碰撞激发，不仅仅是简单的非弹性碰撞过程的能量转移，还包含电子交换以及Ｈｇ原子俘获电子形成极短寿命（≤１Ｏ￣１３Ｓ）的Ｈｇ￣－离子态（６ｓ６ｐ￣２）而共振这样两种强烈影响激发截面的重要机制，电子与Ｈｇ原子的弹性碰撞存在量子效应，弹性散射截面σ＿０与电子初动能大小有关。     

~(58,60,62,64)Ni25.1MeV质子非弹性散射研究及其低激发态能谱和B(E2)比率的IBA分析

使用上海原子核研究所1.4m等时性迥旋加速器提供的25.1MeV质子束,测量了偶镍同位索弹性散射和第一激发态2_~+的非弹性散射微分截面。用零程扭曲波玻恩近似(DWBA)作理论分析,得到~(58,60,62,64)Ni 2_1~+态的四极形变参数β_2分别为0.23、0.28,0.25,0.21。采用相互作用玻色子近似(IBA)计算了这些核低激发态能谱及其约化电跃迁几率B(E2)。并对IBA唯象参数作了初步的准粒子无规位相近似(QPRPA)微观计算。     

Determination of the inelastic mean free paths (IMFPs) in Ti by elastic peak electron spectroscopy (EPES): Effect of impurities and surface excitations

Surface excitations are important in surface sensitive electron spectroscopes, especially in elastic peak electron spectroscopy (EPES) since they may distort quantitative information. This phenomenon is more pronounced at low electron energy and glancing emission angles and should be appropriately corrected.In the present work we investigate quantitatively the role of contaminations, density and surface excitations on electron inelastic mean free paths (IMFPs) in Ti determined by elastic peak electron spectroscopy (EPES) using Cu standard. In the Monte Carlo algorithm the new NIST 3.1 database of electron elastic scattering cross sections was applied. It has been also shown that accounting for surface excitations, as well as for appropriate input parameters (surface composition, density, hydrogen) in the EPES method, is important for accuracy of evaluated IMFPs. Due to high reactivity of Ti, the IMFPs for contaminated Ti may be of interest. The authors indicate the magnitude of various corrections on the IMFPs derived by EPES.     

Surface excitation correction of the inelastic mean free path in selected conducting polymers

In earlier works, the inelastic mean free path (IMFP) of electrons was determined by elastic peak electron spectroscopy (EPES) using Ni and Ag reference standard samples, but fully neglecting surface excitation. Surface excitation that is characterized by the surface excitation parameter (SEP), and may affect considerably the elastic peak for the sample and the reference material. The SEP parameters of selected conducting polymers (polythiophenes, polyaniline and polyethylene) were determined by EPES using Si and Ge reference samples. Experiments were made with a hemispherical analyzer of energy resolution 100-200 meV in the E = 0.2-2.0 keV energy range. The composition of the sample surfaces was determined by in situ XPS, their surface roughness by AFM. The experimental SEP parameter data of eight polymer samples were determined by our new procedure, using the formulae of Chen and Werner et al. in the E = 0.2-2.0 keV energy range. The trial and error procedure is based on the best approach between the experimental and calculated IMFPs, corrected on surface excitation. The improvement in the SEP correction appears in the difference between the corrected and Monte Carlo calculated IMFPs, assuming Gries and Tanuma et al. IMFPs for polymers and standard, respectively. The term describing the improvement by SEP resulted in 50-72% (good correction for five polymers) 24% (poor correction for one polymer), 1-6% (no correction for two polymers). The 100% correction was not achieved, indicating that the difference between experimental and calculated IMFP cannot be entirely explained by surface excitation. Using the SEP data of Si and Ge reference samples based on Chen's and Werner's material parameter values resulted in similar SEP corrections for the polymer samples.     

Determination of the inelastic mean free path of electrons in GaAs and InP after surface cleaning by ion bombardment using elastic peak electron spectroscopy

The inelastic mean free path (IMFP) of electrons is an important material parameter needed for quantitative AES, EELS and non-destructive depth profiling. The distinction between the terms for IMFP and the attenuation length (AL) has been established by ASTM standards. A practical experimental method for determining values of the IMFP is elastic peak electron spectroscopy (EPES). In this method, experimentally determined ratios of elastically backscattered electrons from test surfaces and from a Ni reference standard are compared with the values evaluated theoretically.The present paper reports systematic measurements of the IMFP by EPES for GaAs and InP. They are carried out in two laboratories using two different electron spectrometers: a CMA in Budapest and DCMA in Warsaw. Prior to measurements, the samples were amorphized by high-energy Ar⁺ ions (100–400 keV), and the surface composition was determined by quantitative XPS. Argon cleaning produces enrichment of samples in the surface layer in Ga (80%) and In (70%), respectively. The experiments refer to a such modified sample surface that was considered in Monte Carlo calculations. The experimental data were analyzed using calibration curves from Monte Carlo calculations which account for multiple elastic scattering events. This approach has been used previously for elemental solids and is now extended to amorphized binary compounds. The experimental values of IMFP obtained in both laboratories exhibited a reasonable agreement with the available literature data in the 0.1–3.0 keV energy range. With respect to the information depth of EPES, the experimental results refer to the bulk composition within a reasonable extent.     

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.     

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.     

Evaluation of the inelastic mean free path (IMFP) of electrons in polyaniline and polyacetylene samples obtained from elastic peak electron spectroscopy (EPES)

The inelastic mean free path (IMFP) of electrons was determined experimentally for selected polyaniline and polyacetylene samples with Ag and Ni references using elastic peak electron spectroscopy (EPES). The surface composition was determined by XPS and density by helium pycnometry. The high resolution hemispherical ESA-31 and ADES-400 spectrometers were used for measurements in the energy range E = 0.5–3.0 keV and E =0.4 − 1.6 keV, respectively. The integrated elastic peak intensity ratios for sample and reference were calculated using the Monte Carlo (MC) algorithm based on the electron elastic scattering cross-sections database NIST SRD64 version 3.1 and applying TPP-2M IMFPs for polymers. Surface excitation parameters (SEP) and material parameters ( _ach ) for polymers were determined, using the model of Chen, from comparison of measured and MC calculated elastic peak intensity ratios. These corrections proved to be efficient in decreasing the percentage deviations between the obtained IMFPs and the TPP-2M formula IMFPs. The elastic peak of hydrogen was observed in the EPES spectra of polymers. The experimental contribution of the hydrogen to the total elastic peak was 0.58%, while this value obtained from the MC simulations was 1.98%.      

Monte Carlo simulation of photoelectron angular distribution

A practical simulation method has been performed for studies of the influence of surface excitations on the angular distributions of photoelectron peak intensities. The surface effects have been incorporated into simulations by using the surface excitation parameters (SEPs) which have been calculated with the extended Drude dielectric function. Also, elastic scattering cross sections are calculated using the finite difference method for a Hartree-Fock-Wigner-Seitz potential in the Dirac equation to take into account the solid-state effect. Results of Monte Carlo simulations reveal that surface effects lead to a reduction of the intensities at small detection angles and a sharp decrease at large angles since the surface excitation is most probable for glancing electrons. The calculated results taking into account surface effects are in better agreement with the experimental data.     

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.     

Backscattered electron spectra from graphite

The backscattered electron spectra from graphite sample were studied both experimentally and theoretically at impact energies between 500 and 5000 eV. The angle of the incident electron beam was 50° and the detection angle was 0° with respect to the surface normal, respectively. Monte Carlo (MC) simulations were performed based on the Classical Transport Theory (CTT) model to mimic the experimental spectra. In our simulations, both elastic and inelastic scattering of primary electrons and secondary electron emission from graphite are taken into account. There is found satisfactory agreement between measured and calculated electron spectra.     

Direct Monte Carlo simulation of scattering processes of kV electrons in aluminum; comparison of theoretical N(E) spectra with experiment

A Monte Carlo simulation of the scattering processes of kV electrons penetrating into aluminum was performed. The simulation is based on the use of different types of differential cross-sections for individual elastic and inelastic scattering: (i) Elastic scattering; the differential cross-sections derived by partial wave expansion method. (ii) Inelastic scattering; Gryzinski's excitation function for inner-shell electron excitation, Streitwolfs excitation function for conduction electron excitation, and Quinn's mean free path for plasmon excitation. For verification the energy loss spectra obtained from the Monte Carlo calculations were then compared with experiment done with commercial type Auger microprobes, JAMP-3, for angle of incidence 45° and JAMP-10 for normal incidence at primary electron energies of 1.5 and 3.0 keV, respectively. The results show satisfactory agreement between theory and experiment.