蒙特卡洛方法计算材料表面激发参数

电子非弹性散射平均自由程（IMFP）是用表面电子能谱进行表面化学定量分析时极为重要的一个参数,它可以用测量的弹性峰电子能谱分析以及蒙特卡洛模拟来确定.为了更加精确地确定电子非弹性散射平均自由程,必须对弹性峰电子能谱中的表面激发效应进行修正,通常使用介电响应理论方法计算得到的表面激发参数.然而,通过理论计算得到的表面激发参数不能包含电子在材料内部输运过程中弹性散射的影响,进而影响所测的电子非弹性散射平均自由程的准确度.在这个工作中,我们采用蒙特卡洛方法来确定包含弹性散射效应时的表面激发参数.所得到的表面激发参数在不同能量、角度情况下,尤其是在弹性散射效应显著的60°以上的大角度入射、出射情况下,都与实验测量值符合得非常好.基于这些新确定的表面激发参数,可以在弹性峰电子能谱测量中获得更为准确的电子非弹性散射平均自由程数据.     

Motion and acceleration of electrons in high-intensity laser standing waves

The motion and the energy of electrons driven by the ponderomotive force in linearly polarized high-intensity laser standing wave fields are considered. The results show that there exists a threshold laser intensity, above which the motion of electrons incident parallel to the electric field of the laser standing waves undergoes a transition from regulation to chaos. We propose that the huge energy exchange between the electrons and the strong laser standing waves is triggered by inelastic scattering, which is related to the chaos patterns. It is shown that an electron's energy gain of tens of MeV can be realized for a laser intensity of 10²⁰ W/cm².     

Electron IMFPs in bulk Cd0.88Mn0.12Te crystals determined by EPES

The inelastic mean free path (IMFP) of electrons is a basic parameter for surface-sensitive electron spectroscopies (AES, XPS, EELS) in quantitative analyses.Cd_1−xMn_xTe mixed crystals are currently of great interest due to their magnetic and magneto-optical properties. Since information on electron transport processes in these semimagnetic compounds is scarce, their systematic studies are highly desirable.In the present work, the IMFPs in Cd_0.88Mn_0.12Te (1 1 0) crystal samples were obtained from EPES with use of the Ni standard in the electron energy range 500-2000 eV. In addition, we also explored the effect of bulk Mn content in the determination of the IMFP. Relative EPES measurements were carried out using the MICROLAB 350 spectrometer. The sample surface was sputter cleaned and amorphized by Ar⁺ ions. Surface composition of the samples was monitored in situ by XPS and AES. The measured IMFPs were uncorrected for surface excitations and compared with those predicted from the TPP-2M and G-1 formulae. Also, the values of the IMFPs determined here were compared with those evaluated from the expression of Sekine et al. However, accuracy of this expression is rather poor except the case of pure CdTe (x = 0). In general, good agreement was found between the measured IMFPs in Cd_0.88Mn_0.12Te and the corresponding predicted IMFPs. The root-mean-square deviation from IMFP values predicted from the TPP-2M formula was 1.2 Å. The mean percentage deviation from the TPP-2M IMFPs was 9.3%.     

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.     

Electron impact ionisation cross sections for atomic and molecular allotropes of phosphorous and arsenic

We report electron impact total ionisation cross sections for phosphorous (P), arsenic (As), diphosphorous (P₂), diarsenic (As₂), tetra phosphorous (P₄) and tetra arsenic (As₄) from the threshold of the target to 2000 eV. We employed spherical complex optical potential to compute total inelastic cross sections (Q_inel). The total ionisation cross section is extracted from the total inelastic cross section using the complex scattering potential–ionisation contribution method. The results of most of the targets studied here compare well with the measurements and the theoretical data wherever available. The correlation between the peak of ionisation cross sections with the number of target electrons and polarisability is also reported. It is observed that the maximum ionisation cross sections depend linearly on the number of target electrons and polarisability of the target. This linear correlation is used to predict the maximum ionisation cross sections for the targets (I₂, HI and PF₃) where no experimental data are available.     

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.     

2S-excitation of atomic hydrogen by electrons at intermediate energies

The method of Das developed recently to analyse elastic scattering of electrons by atoms has been extended in the present paper to inelastic scatterings. The method has been applied for the computation of 2S-excitation, cross-sections at two typical intermediate energiesviz. 54.4 eV and 100 eV energies. Some of the results are compared with available experimental results and are found to be in satisfactory agreement with these.     

Universal quantification of elastic scattering effects in AES and XPS

Elastic scattering of photoelectrons in a solid can be accounted for in the common formalism of XPS by introducing two correction factors, β_eff and Q_x. In the case of AES, only one correction factor, Q_A, is required. As recently shown, relatively simple analytical expressions for the correction factors can be derived from the kinetic Boltzmann equation within the so-called “transport approximation”. The corrections are expressed here in terms of the ratio of the transport mean free path (TRMFP) to the inelastic mean free path (IMFP). Since the available data for the TRMFP are rather limited, it was decided to complete an extensive database of these values. They were calculated in the present work for the same elements and energies as in the IMFP tabulation published by Tanuma et al. An attempt has been made to derive a predictive formula providing the ratios of the TRMFP to the IMFP. Consequently, a very simple and accurate algorithm for calculating the correction factors β_eff, Q_x and Q_A has been developed. This algorithm can easily be generalized to multicomponent solids. The resulting values of the correction factors were found to compare very well with published values resulting from Monte Carlo calculations.     

Spin dependent mean-free path of low-energy electrons in ferromagnetic materials

Spin resolved attenuation measurements of electrons transmitted through overlayers of Fe and Co show that the electron inelastic mean free path (IMFP) in these materials is spin-dependent at low energies. The spin-up IMFP is larger than the spin-down IMFP. The values from different studies are in reasonable agreement. The data suggest that the origin of the spin dependence is mainly due to inelastic processes. Effects from spin dependent elastic scattering have not been identified directly in these experiments. The spin filter effect based on preferential attenuation of spin-down electrons can be used as a basis of spin polarization detectors.     

Simulations of inelastic electron tunneling spectroscopy of semifluorinated hexadecanethiol junctions

The inelastic electron tunneling spectroscopy (IETS) of semifluorinated hexadecanethiol junctions is theoretically studied. The numerical results show that the C-F vibration modes of semifluorinated alkanethiol series can not be detected, and the C-H stretching mode in IETS is related to the CH₂ vibration. It is demonstrated that the Raman modes are preferred over IR modes in IETS, which is in good agreement with the experimental measurements presented by Beebe et al. [Nano Lett., 2007, 7(5): 1364].      

A Numerical Calculation for Electron Impact Excitation of Copper

The scattering of electrons by atomic copper has been studied using Born approximation and the concept of the generalized oscillator strength (GOS). Differential and total cross-sections for the excitation of the 3d¹⁰ 4p² P state are calculated at incident energies of 100 eV and are compared with other available experimental and theoretical data. The agreement between our calculation for the differential cross-section and the available experimental results is fairly good at the forward angles, while the agreement at large angles is poor. The calculated total cross-sections are compared with the experimental data and those predicted by several theories. It is found that our calculation for the total cross-sections are in a good agreement with the close coupling calculation of Msezane and Henry (1986a, Physical Review A 33, 1631) for incident energies greater than 20 eV. The integrated cross-section measurements of Ismail and Teubner (1995, Journal of Physics B: Atomic, Molecular and Optical Physics 28, 4164) are in good agreement with the present calculation.     

Total cross-sections for positron scattering by a series of molecules

The additivity rule is employed to obtain the total (elastic+inelastic) cross-sections for positron scattering from molecules including a number of diatomic, polyatomic molecules (H₂, N₂, HCl, CO₂, NH₃, SF₆, CH₄, C₂H₄ and C₃H₈) over an incident energy range of 10-1000 eV. The total cross-sections (TCS) of the constituent atoms of molecules are obtained by employing a complex optical model potential (composed of static, polarization and absorption potential). The present results are compared with experimental data and other theoretical calculations, good agreement is obtained in intermediate- and high-energy region. Received: 11 November 1997 / Revised: 23 March 1998 / Accepted: 16 June 1998     

Chemical sputtering,a discussion of mechanisms

Sputtering can be defined as the process whereby particles leave the surface as a direct consequence of the presence of incident radiation. When particles leave the surface as a result of receiving momentum from the collision cascade induced by the incident radiation, the process is called “physical sputtering”. If the incoming radiation (ions, electrons, or photons) induces a chemical reaction which leads to the subsequent desorp-tion of particles, the process could be classified as “chemical sputtering”. There are a number of molecules such as CH₄, CF₄, CF₃H, CF₃CI, etc., whose binding energy to a large variety of surfaces is believed to be only a few kcal/mole. Therefore, these molecules will not remain absorbed at room temperature. Consequently, if they are generated from surface atoms by radiation-induced processes, they will almost immediately desorb into the gas phase. This process is one type of chemical sputtering. Recent data obtained in plasma environments suggest that this type of reaction is a widely occurring phenomena: however, few systematic quantitative investigations of the subject have been completed. In this paper we will review the evidence for chemical sputtering and discuss mechanisms based on experimental information obtained for the chemical sputtering of silicon and SiO₂ under argon ion bombardment in the presence of a molecular beam of XeF2. Under these conditions, 25 or more silicon atoms can leave the surface per incident argon ion. About 75% of the silicon is emitted as SiF₄ (gas) and the rest leaves as silicon atoms or SiFx radicals. The total yield (silicon plus fluorine) is greater than 100 atoms/ion. The measured yields are a strong function of XeF₂ flux and a much weaker function of ion energy in the range 500-5000 eV. The chemical-sputtering yield for SiO₂ is smaller than that of silicon by about an order of magnitude, but it is still larger than the physical-sputtering yield. Moreover, SiO₂ is also sputtered by electrons. These results indicate that the incident radiation induces a chemical reaction between silicon and adsorbed fluorine which produces SiF₄, and the SiF4 is subsequently desorbed into the gas phase. We define this process as chemical sputtering. The large yields are probably a consequence of weak binding between the surface and the SiF₄ molecule.     

Temperature dependence of the inelastic scattering in a GaAs/n-AlGaAs selectively doped heterojunction with InGaAs quantum dots

Inelastic scattering processes of two-dimensional electron gas (2DEG) have been investigated in a inverted GaAs/n-AlGaAs heterojunction with self-organized InGaAs quantum dots (QDs) embedded near the 2DEG channel where the electron population in the QDs is controllable by the gate voltage V_g. By analyzing magnetoresistance, the inelastic scattering time τ_ε have been evaluated as functions of V_g at 0.6, 0.8, 1.2, and 1.7 K. It is found that τ_ε increases with V_g below 0.8 K and decreases above 1.2 K, which suggests that the dominant scattering mechanisms below 0.8 K and above 1.2 K are different. To interpret this behavior, we have calculated the inelastic scattering time theoretically. It is found that the experimental data are well explained by a theoretical model where a 2D electron is considered to be inelastically scattered both by the other 2D electrons and by the trapped electrons in QDs. It is also found that the 2DEG–2DEG scattering is dominant at low temperature, while the 2DEG-QDs scattering becomes important as the temperature increases.     

强流电子束入射弯曲宏观石英管的导向效应研究

研究了强流（~129 nA）、 高能（1 500～1 900 eV）电子束在大角度(9°)弯曲宏观石英管中的导向效应。 实验分别测量了入射流强及能量对出射电子角分布值（FWHM）和传输效率的影响。 实验观察到出射电子角分布FWHM随着入射电子流强和入射电子能量增加变化均不明显; 发现电子传输效率随入射流强增加而增加, 但随入射能量增加而减小, 这与高电荷态离子导向中离子传输效率随入射能量增加而增加的现象相反。 分析发现, 与高电荷态离子导向机制不同, 电子束导向并非是由电子在石英管内壁的自组织充电过程引起的, 而是入射电子与管内壁弹性和非弹性散射碰撞共同作用的结果。 By using an incident electron beam with the high current and high energy, the guiding effect of the bended macroscopic quartz tube for the electron beam has been investigated. The angular distributions of outgoing electrons depending on the current and energy of incident electrons were measured. The dependences of electron transmitted fraction on energy and current of incident electrons are also shown. As the incident electron energy increasing, the electron transmitted fraction increases, but it decreases while the incident electron current increasing. The results have been compared with the present data. This work presents, the process of guiding electrons is essentially different from that of guiding highly charged ions, the guiding electron beam was caused by both elastic and inelastic collisions between electrons and inner walls of quartz tube, rather than self organized charging effect on the surface of inner wall of quartz tube.     

Backscattering of low-energy (0–8 eV) electrons by a silicon surface

An experimental apparatus and method for investigating elastic and inelastic backscattering (180°) of low-energy (0–8 eV) monoenergetic electrons by a solid surface are described and the first results are presented for the reflection of electrons by samples of pure single-crystalline silicon with a polished surface (Si), doped p-type single-crystalline silicon with a porous surface (Si-p) as well as H₂O and H₂O₂ passivated porous samples, Si-p + H₂O and Si-p + H₂O₂. A structure due to the excitation of surface plasmons has been observed for the first time in the loss spectra. Features corresponding to a resonance excited state of molecular nitrogen adsorbed on the surface of porous silicon have been observed in the constant residual energy spectra. Zh. Tekh. Fiz. 67, 103–108 (May 1997)     

Theoretical total ionization cross-sections of CH x,CF x,SiH x,SiF x ( ) and CCl 4 targets by electron impact

Total ionization cross-sections of electron impact are calculated for the molecular targets CH_x, CF_x, SiH_x, SiF_x (x = 1-4) and CCl₄ at incident energies 20-3 000 eV. The calculation is based on Complex Scattering Potential approach, as developed by us recently. This leads to total inelastic cross-sections, from which the total ionization cross-sections are extracted by reasonable physical arguments. Extensive comparisons are made here with the previous theoretical and experimental data. The present results are satisfactory except for the CF_x and SiF_x (x = 1-3) radicals, for which the experimental data are lower than most of the theories by more than 50%. Received 23 May 2002 / Received in final form 24 October 2002 Published online 21 January 2003 RID="a" ID="a"e-mail: knjoshipura@yahoo.com     

Monte Carlo calculation using a personal computer for electron probe microanalysis

Monte Carlo simulation for calculating x‐ray spectra under electron impact is carried out by a personal computer. Reduction of the CPU time by reducing the number of incident electrons for calculation is performed. One of the methods used to reduce the number of incident electrons to obtain the results within an acceptable error is provided. We limit the present report to five typical compounds: an oxide (Al₂O₃), a nitride (TiN), a carbide (Al₄C₃), a light–light element compound (BN), and a heavy–light element compound (PbS). We calculate the relative x‐ray intensity generated from the five compounds listed above with the change of the number of the incident electrons. The calculated x‐ray intensity is then converted into the mass % by the ZAF method. Comparisons are made between the calculation and experiment for Al₄C₃ and PbS. It is found that the present Monte Carlo method is useful for investigating a relationship between the number of incident electrons and the acceptable error. The results of 150 incident electrons can be applied in practical analysis, and the calculation time is less than 2 min. Copyright © 2004 John Wiley & Sons, Ltd.     

Kinetic parameters,collision rates,energy exchanges and transport coefficients of non-thermal electrons in premixed flames at sub-breakdown electric field strengths

The effects of an electric field on the collision rates, energy exchanges and transport properties of electrons in premixed flames are investigated via solutions to the Boltzmann kinetic equation. The case of high electric field strength, which results in high-energy, non-thermal electrons, is analysed in detail at sub-breakdown conditions. The rates of inelastic collisions and the energy exchange between electrons and neutrals in the reaction zone of the flame are characterised quantitatively. The analysis includes attachment, ionisation, impact dissociation, and vibrational and electronic excitation processes. Our results suggest that Townsend breakdown occurs for E/N = 140 Td. Vibrational excitation is the dominant process up to breakdown, despite important rates of electronic excitation of CO, CO₂ and N₂ as well as impact dissociation of O₂ being apparent from 50 Td onwards. Ohmic heating in the reaction zone is found to be negligible (less than 2% of peak heat release rate) up to breakdown field strengths for realistic electron densities equal to 10¹⁰ cm^?3. The observed trends are largely independent of equivalence ratio. In the non-thermal regime, electron transport coefficients are insensitive to mixture composition and approximately constant across the flame, but are highly dependent on the electric field strength. In the thermal limit, kinetic parameters and transport coefficients vary substantially across the flame due to the spatially inhomogeneous concentration of water vapour. A practical approach for identifying the plasma regime (thermal versus non-thermal) in studies of electric field effects on flames is proposed.     

Electron impact ionisation cross section for organoplatinum compounds

This article reports electron impact ionisation cross sections for platinum-based drugs viz., cisplatin (H₆N₂Cl₂Pt), carboplatin (C₆H₁₂N₂O₄Pt), oxaliplatin (C₈H₁₄N₂O₄Pt), nedaplatin (C₂H₈N₂O₃Pt) and satraplatin (C₁₀H₂₂ClN₂O₄Pt) complexes used in the cancer chemotherapy. The multi-scattering centre spherical complex optical potential formalism is used to obtain the inelastic cross section for these large molecules upon electron impact. The ionisation cross section is derived from the inelastic cross section employing complex scattering potential–ionisation contribution method. Comparison is made with previous results, where ever available and overall a reasonable agreement is observed. This is the first attempt to report total ionisation cross sections for nedaplatin and satraplatin complexes.