Low-energy electron backscattering spectroscopy of Mg/Mo films

Using a low-energy (0–5 eV) electron backscattering technique based on a hypocycloidal electron spectrometer with high energy (∼50 meV) and angular (∼1–5°) analyzer resolution, the specific features of the energy distribution of the density of electron states in magnesium films are investigated. The proposed technique enables not only the details of the distribution of the filled electron states density of as-prepared films, but also the evolution of surface electron states in process of film exposure to the residual-gas atmosphere to be studied.     

Effect of interference on field electron emission

Carbon nanotubes are studied with the help of an electron projector and dispersion field-emitted energy analyzer. Probe current-voltage characteristics of two samples exhibited wave-like modulation caused by periodic variations in field-emitted electron energy spectra observed with increasing emission voltage. Deviations of the current from the average Fowler-Nordheim line were up to ±35% within one period. The explanation relies on the model of Young’s interference from two coherent sources.     

Effects of adsorbates on electron spin polarization in low energy electron diffraction from tungsten (001): II. Ordered CO overlayer

Electron spin polarization and intensity profiles have been measured in low electron diffraction (LEED) for the (00) beam at θ = 13° and ø = 0° from a W(001) surface exposed to CO and annealed to obtain an ordered c(2 × 2) CO overlayer. The annealed surface with additional CO adsorbed was also studied. The polarization was found to be sensitive to the surface condition and the very distinct P?V profile corresponding to the c(2 × 2) overlayer is believed to be a very sensitive indicator of CO in the β₃ phase. The properties of the annealed surface exposed to further CO suggest the use of this surface as a low energy electron spin polarization analyzer.     

Reconstructing the energy spectra of backscattered electrons with allowance for the spectrometer’s response function

The inverse problem of reconstructing the true spectrum of electrons backscattered from massive and layered targets with allowance for the spread function of the toroidal sector energy analyzer and for the response function of the spectrometer??s electron detector is solved. We present the results from studying the energy spectra of a number of homogeneous samples and film-on-substrate systems obtained at different energies of the irradiating electron beam at normal incidence of the electrons on the surface, and at a 45° angle of backscattered electron detection.     

Au表面等离子体激元激发的扫描探针电子能谱

结合扫描隧道显微镜（STM）与电子能谱仪是实现表面微区元素分析的途径之一．我们将环形电子能量分析器和三维扫描探针系统相结合,建立了一台扫描探针电子能谱仪(SPEES)．通过测量针尖近场发射束流激发的Au表面能量损失谱,我们用研究了Au原子的等离子体激元激发现象．进一步通过改变针尖－样品距离,我们研究了Au等离子体激元峰与弹性散射峰的强度比随针尖－样品距离变化的关系．研究结果发现该强度比与针尖－样品距离的关系并不是单调变化,而是在一个特定位置存在极大．     

High efficiency and high energy-resolution spin-polarized photoemission spectrometer

We have developed a new spin-polarized photoemission spectrometer combined a commercial hemispherical electron energy analyzer with an electron spin-polarimeter employing the very low energy electron diffraction (VLEED) at a Fe(001)p(1×1)-O surface. The new polarimeter has realized the effective Sherman function of 0.40 and the figure of merit of 1.9×10^-2, which is approximately two orders of magnitude higher than that expected with a compact Mott scattering spin-polarimeter. The new system enabled us to measure spin- and angle-resolved photoemission spectra with high energy and momentum resolutions.     

Development of a new experimental setup for studying collisions of keV-electrons with thick and thin targets

Development of a new lectron-recoil ion/photon coincidence setup for investigating some of the electron induced collision processes, such as electron bremsstrahlung, electron backscattering, innershell excitation and multiple ionization of target atoms/molecules in bombardment of electrons having energies from 2.0 keV to 30.0 keV with solid and gaseous targets is described. The new features include the use of a compact multipurpose scattering chamber, a time-of-flight spectrometer for detection of multiply charged target ions, a 45°-parallel plate electrostatic analyzer for measuring energy and angle of the ejected electrons, a room temperature high resolution Si-PIN photo diode X-ray detector for counting the collisionally induced photons, a coincidence data acquisition system consisting of a 200 MHz Pentium based 8K-multichannel analyzer and a standard network of a fast/slow coincidence electronics. In particular, the details of design, fabrication and assembly of indigenous components employed in the setup are presented. Selected experiments planned with the setup are mentioned and briefly discussed. A report on performance, optimization, efficiency, time resolution etc. of the time-of-flight (TOF) spectrometer and that of the 45°-parallel plate electrostatic analyzer (PPEA) is presented. Test spectra of electron-recoil ion coincidences, energy distribution of ejected electrons and characteristic plus non-characteristic X-ray spectrum are illustrated to exhibit the satisfactory performance of the developed setup.     

A Surface Femtosecond Two-Photon Photoemission Spectrometer for Excited Electron Dynamics and Time-Dependent Photochemical Kinetics

搭建了一套研究金属和金属氧化物表面的超快激发态电子动力学和光化学动力学的飞秒双光子光电子能谱仪. 该装置将半球形电子能量分析仪和成像技术相结合,同时测量光电子的能量和角度分布.通过Mach-Zehnder干涉仪测量时间分辨的双光子光电子能谱获得超快激发电子态的动力学信息. 这一功能在Cu(111)上得到了证实. 另外还发展了一个通过实时测量双光子光电子能谱来研究表面光化学的方法,并成功应用到CH₃CH₂OH/TiO₂(110)体系. 研究表明,只有将两种方法结合起来才能正确地研究光诱导的表面激发共振的动力学.     

Design of an Auger Electron Mössbauer Spectrometer (AEMS) using a modified cylindrical mirror analyzer

We have designed and built an Auger Electron Mössbauer Spectrometer (AEMS) for the detection of resonant ⁵⁷Fe Auger electrons using a modified commercial cylindrical mirror analyzer (CMA). The CMA final aperture was modified intentionally in order to increase electron transmission at the expense of reducing its energy resolution, from an original value of 0.5 % to a value of 11 % after the modification. The Channeltron detector electronics and the pre-amplifier were also modified in order to increase the counting efficiency. The electron energy analyzer is selective in energy in the 30 eV–3000 eV range, so the spectrometer can be used to detect MNN (45 eV) and LMM (600–700 eV) Fe Auger signals, what gives it a high surface sensitivity for Fe containing samples. We have used it to acquire the Fe LMM Auger signals generated from the de-excitation process after γ-Ray resonant nuclear absorption. The spectrometer can be used to study samples non-enriched in ⁵⁷Fe, with acquisition times from 5 to 7 days, what is a big advantage. From electron trajectory Monte Carlo simulations in metallic iron, the mean-escape-depth of the detected Auger signals has been estimated in approximately 1 nm. Fe K conversion electrons and KLL Auger electrons with mean escape depths of 129 nm and 78 nm respectively also contribute to the detected signal although in a lesser proportion.     

Surface modification study of low energy electron beam irradiated polycarbonate film

The effect of low energy electron beam irradiation on polycarbonate (PC) film has been studied here. The PC film of thickness 20 μm was exposed by 10 keV electron beam with 100 nA/cm² current density. The irradiated film was characterized by mean of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and residual gas analyzer (RGA). Formation of unsaturated bonds and partial graphitization of the surface layer are measured by XPS. Results of the AFM imaging shows electron implantation induce changes in surface morphology of the polymer film. The residual gas analyzer (RGA) spectrum of PC is recorded in situ during irradiation. The results show the change in cross-linking density of the polymer at the top surface.     

Energy distribution of electrons field-emitted from carbon nanoemitters

The height of an extra low-energy maximum in the energy distribution of electrons tunneling from crystalline carbon fibers and carbon nanotubes is studied as a function of emitter heating and emitter rotation relative to the energy analyzer axis. The relationships found are related to emission from electron states on the surface of the reconstructed nanocrystals and nanotubes.     

Energy distribution of field emission electrons from carbon nanocrystals

Using a field electron microscope and a field electron dispersion energy analyzer, carbon nanocrystals contained in polyacrylonitrile (PAN) fibers are studied. Transition of the emitting nanocrystals into the second stable state has been discovered, corresponding to an emission current and field electron energy distribution of magnitudes lower by nearly an order of magnitude, with an additional low-energy peak. Heating the samples at 750° C restores the initial characteristics of the field electron energy distribution and current-voltage characteristics of the nanocrystals. The forbidden bandwidth of the nanocrystals has been determined.     

Temperature measurements in plasmas generated by using lasers at different intensities

The temperature of laser-generated pulsed plasmas is an important property that depends on many parameters, such as the particle species and the time elapsed from the laser interaction with the matter and the surface characteristics.

Laser-generated plasmas with low intensity (<10¹⁰ W/cm²) at INFN-LNS of Catania and with high intensity (>10¹⁴ W/cm²) in PALS laboratory in Prague have been investigated in terms of temperatures relative to ions, electrons, and neutral species. Time-of-flight (ToF) measurements have been performed with an electrostatic ion energy analyzer (IEA) and with different Faraday cups, in order to measure the ion and electron average velocities. The IEA was also used to measure the ion energy, the ion charge state, and the ion energy distribution.

The Maxwell–Boltzmann function permitted to fit the experimental data and to extrapolate the ion temperature of the plasma core.

The velocity of the neutrals was measured with a special mass quadrupole spectrometer. The Nd:Yag laser operating at low intensity produced an ion temperature core of the order of 400 eV and a neutral temperature of the order of 100 eV for many ablated materials. The ToF of electrons indicates the presence of hot electron emission with an energy of ～1 keV.     

Electron spectrograph with a hyperbolic electrostatic field

The electron-optical properties of an energy analyzer representing a parallel-plate capacitor with a linear distribution of the potential on the upper plate are studied. It is shown that first-order focusing conditions in this analyzer do not depend on the particle energy and the linear dispersion is proportional to the square root of the energy. Such an analyzer can simultaneously record an electron spectrum in a wide energy range, that is, operate as a spectrograph. Double focusing conditions in a box-type analyzer are found, which allows the relative aperture of the spectrograph to be raised several-fold.     

Study of the ion composition of an expanding magnesium plasma produced by a CO2 laser

The evolution of the ion composition of a laser plasma during its expansion over a large distance is studied. The plasma is produced by a TIR CO₂ laser with a pulse energy up to 100 J and duration of ～20 ns. X-ray diagnostics with the use of a spectrograph and X-ray PIN diodes was applied to study the plasma near the target surface. At large distances from the target surface, time-of-flight neutral-particle diagnostics with the use of an electrostatic analyzer and ion collector was applied. Calculations performed with the GIDRA-1 code agree well with experimental data.     

Work function measurements with a high resolution electron energy loss spectrometer: Application to the interaction of oxygen with Ag(110)

The monochromatized electron beam of a high resolution electron energy loss (HREEL) spectrometer is used for accurate (±5 meV) measurement of the work function changes during exposure of a Ag(110) single crystal surface to oxygen. Absolute calibration of the results is made by comparison with Kelvin probe data. The procedure allows the precise determination of the electron impact energy, which is an important parameter for quantitative HREELS analysis. Furthermore, in the case of oxygen adsorbed on Ag(110), the occurrence of several LEED (n×1) superstructures enables a calibration of the HREELS data with respect to surface coverage.     

Nonlinear photoelectron emission from metal surfaces induced by short laser pulses: the effects of field enhancement by surface plasmons

Nonlinear electron emission processes induced by surface plasmon oscillations have been studied both experimentally and theoretically. The measured above-threshold electron spectra extend up to high energies whose appearance cannot be explained solely by standard non-perturbative methods, which predict photon energy separated discrete energy line spectra with the known fast fall–plateau–cutoff envelope shape, even when taking the large field enhancement into account. The theoretical analysis of our data, based on the concept of plasmon-induced surface near-field effects, gives a reasonably good explanation and qualitative agreement in the whole intensity range.     

Generation of monoenergetic ultrashort electron pulses from a fs laser plasma

Ultrashort high-energy electron beams are generated by focusing fs Ti:sapphire laser pulses on a thin metal tape at normal incidence. At laser intensities above 10¹⁶ W/cm² , the fs laser plasma ejects copious amounts of electrons in a direction parallel to the target surface. These electrons are directly detected by means of a backside illuminated X-ray CCD, and their energy spectrum is determined with an electrostatic analyzer. The electrons were observed for two laser polarization directions, parallel and perpendicular to the observation direction. At the maximum applied intensity of 2×10¹⁷ W/cm², the energy distribution peaks at around 35 keV with a hot tail detectable up to about 300 keV. The number of electrons per shot at 35 keV is about 5×10⁸ per sterad per keV. Quasi-monoenergetic electron pulses with a relative energy spread of 1% were produced by using a 50-m slit in the beam path after the analyzer. This approach offers great potential for time-resolved studies of plasma, liquid, and surface structures with atomic-scale spatial resolution. PACS 41.75.Fr; 52.38.Kd; 52.70.Nc     

Calibration of electron-energy spectrometers

Relative photoionization cross-sections of strong peaks in the He(I) photoelectron spectra of N₂, CO, CO₂, and O₂, are tabulated. These data have been measured with an electron energy analyzer whose relative luminosity has been calibrated to an accuracy of ± 5 ‰ Thus, the tables will be useful for calibrating the transmission of other analyzers for electron energies below 9 eV. Correction for angular distribution effects is discussed.