Secondary Electron Background Research Articles

Direct evidence for low-energy electron emission following O LVV Auger transitions at oxide surfaces

Oxygen, the third most abundant element in the universe, plays a key role in the chemistry of condensed matter and biological systems. Here, we report evidence for a hitherto unexplored Auger transition in oxides, where a valence band electron fills a vacancy in the 2s state of oxygen, transferring sufficient energy to allow electron emission. We used a beam of positrons with kinetic energies of sim 1 eV to create O 2s holes via matter-antimatter annihilation. This made possible the elimination of the large secondary electron background that has precluded definitive measurements of the low-energy electrons emitted through this process. Our experiments indicate that low-energy electron emission following the Auger decay of O 2s holes from adsorbed oxygen and oxide surfaces are very efficient. Specifically, our results indicate that the low energy electron emission following the Auger decay of O 2s hole is nearly as efficient as electron emission following the relaxation of O 1s holes in hbox {TiO}_2. This has important implications for the understanding of Auger-stimulated ion desorption, Coulombic decay, photodynamic cancer therapies, and may yield important insights into the radiation-induced reactive sites for corrosion and catalysis.

Measurement of the Low-Energy Electron Inelastic Mean Free Path in Monolayer Graphene

Measuring the electron-transport properties of substrate-supported nanomaterials with the traditional two-point comparison method is difficult at electron energies below 50 eV, where core-level signals are too feeble to be detected against the strong secondary-electron background. Herein, a data-driven spectral analysis technique is used to study the low-energy electron-transport properties of substrate-supported target nanomaterials, while eliminating the influence of the substrate signal. Applying this technique, the electron-transport properties of the effective attenuation length and the inelastic mean free path (IMFP) can be determined with extremely high efficiency over the entire measured energy range of 6--600 eV. Further, these results show excellent agreement with other experimental and theoretical results. Significant differences are observed between monolayer graphene and the bulk graphite IMFP, which illustrates the importance of the nanometer effect in the electron-transport properties of the material. Furthermore, this technique is readily applicable to any ultrathin material that can be transferred onto a polycrystalline gold substrate.

Mitigation strategies against radiation-induced background for space astronomy missions

The Advanced Telescope for High ENergy Astrophysics (ATHENA) mission is a major upcoming space-based X-ray observatory due to be launched in 2028 by ESA, with the purpose of mapping the early universe and observing black holes. Background radiation is expected to constitute a large fraction of the total system noise in the Wide Field Imager (WFI) instrument on ATHENA, and designing an effective system to reduce the background radiation impacting the WFI will be crucial for maximising its sensitivity. Significant background sources are expected to include high energy protons, X-ray fluorescence lines, 'knock-on' electrons and Compton electrons. Due to the variety of the different background sources, multiple shielding methods may be required to achieve maximum sensitivity in the WFI. These techniques may also be of great interest for use in future space-based X-ray experiments. Simulations have been developed to model the effect of a graded-Z shield on the X-ray fluorescence background. In addition the effect of a 90nm optical blocking filter on the secondary electron background has been investigated and shown to modify the requirements of any secondary electron shielding that is to be used.

Time of flight spectrometer for background-free positron annihilation induced Auger electron spectroscopy.

We describe a novel spectrometer designed for positron annihilation induced Auger electron spectroscopy employing a time-of-flight spectrometer. The spectrometer's new configuration enables us to implant monoenergetic positrons with kinetic energies as low as 1.5 eV on the sample while simultaneously allowing for the detection of electrons emitted from the sample surface at kinetic energies ranging from ∼500 eV to 0 eV. The spectrometer's unique characteristics made it possible to perform (a) first experiments demonstrating the direct transition of a positron from an unbound scattering state to a bound surface state and (b) the first experiments demonstrating that Auger electron spectra can be obtained down to 0 eV without the beam induced secondary electron background obscuring the low energy part of the spectra. Data are presented which show alternative means of estimating positron surface state binding energy and background-free Auger spectra.

Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers.

Conduction and valence band energies (ECB, EVB) for CdSe nanorods (NRs) functionalized with Au nanoparticle (NP) tips are reported here, referenced to the vacuum scale. We use (a) UV photoemission spectroscopy (UPS) to measure EVB for NR films, utilizing advanced approaches to secondary electron background correction, satellite removal to enhance spectral contrast, and correction for shifts in local vacuum levels; and (b) waveguide-based spectroelectrochemistry to measure ECB from onset potentials for electron injection into NR films tethered to ITO. For untipped CdSe NRs, both approaches show EVB = 5.9-6.1 eV and ECB = 4.1-4.3 eV. Addition of Au tips alters the NR band edge energies and introduces midgap states, in ways that are predicted to influence the efficiency of these nanomaterials as photoelectrocatalysts. UPS results show that Au tipping shifts EVB closer to vacuum by up to 0.4 eV, shifts the apparent Fermi energy toward the middle of the band gap, and introduces additional states above EVB. Spectroelectrochemical results confirm these trends: Au tipping shifts ECB closer to vacuum, by 0.4-0.6 eV, and introduces midgap states below ECB, which are assigned as metal-semiconductor interface (MSI) states. Characterization of these band edge energies and understanding the origins of MSI states is needed to design energy conversion systems with proper band alignment between the light absorbing NR, the NP catalyst, and solution electron donors and acceptors. The complementary characterization protocols presented here should be applicable to a wide variety of thin films of heterogeneous photoactive nanomaterials, aiding in the identification of the most promising material combinations for photoelectrochemical energy conversion.

Improved Tougaard background calculation by introduction of fittable parameters for the inelastic electron scattering cross‐section in the peak fit of photoelectron spectra with UNIFIT 2011

The shape of the background in x‐ray photoemission spectra is strongly affected by scattered electrons from inelastic energy loss processes. A polynomial of low order has very often been applied to model the secondary‐electron background, giving satisfying results in some cases. An improved analysis employing the Tougaard background model has been successfully used to characterize the inelastic loss processes. However, the correct usage of the Tougaard background needs a well defined inelastic electron scattering cross‐section function λ(E) · K(E, T) (λ = inelastic mean free path, E = kinetic energy, T = energy loss). This paper presents a four‐parameter loss function λ(E) · K(E, T) = B · T/(C + C′ · T2)2 + D · T2 with the fitting parameters B, C, C′ and D implemented in the background function allowing the improved estimation of the λ(E) · K(E, T) function for homogenous materials. The fit of the background parameters is carried out parallel to the peak fit. The results will be compared with the parameters recommended by Tougaard. The calculation of inelastic electron scattering cross‐sections of clean surfaces from different materials using UNIFIT 2011 will be demonstrated. Copyright © 2011 John Wiley & Sons, Ltd.

The wire electrode of the KATRIN experiment

The KArlsruhe TRItium Neutrino experiment, KATRIN will determine the effective mass of the e with a sensitivity of 0.2 eV (90% C.L.) via a measurement of the T2 beta-spectrum near the endpoint at 18.6 keV. An ultra-low background of a few mHz and an energy resolution of 0.9 eV are necessary in order to reach this sensitivity. To suppress secondary electron background from cosmic muons and from environmental radioactivity we will equip KATRIN's main spectrometer with a 650 m2 large, two-layered and quasi-massless wire electrode. The electrode has to be non-radiactive, non-magnetic, bakeable at 350°C and compatible with a vacuum of 10−11 mbar. The 248 modules of the electrode have to be produced with a mechanical precision of O(0.1) mm. We present the design, as well as the mass production and automated quality assurance of the electrode modules. This project is funded by the German Ministry for Education and Research (BMBF).

Master curves for gas amplification in low vacuum and environmental scanning electron microscopy

The concept of universal amplification profiles for gas cascade amplification of signals in low vacuum and environmental scanning electron microscopes is demonstrated both experimentally and theoretically using water vapor. For a given gas, cascade amplification gain profiles can be plotted onto a single master curve where the independent reduced parameter is the ratio of pressure to amplification field strength. When plotted in this fashion, both desired secondary electron and spurious background signal components fall onto respective master curves, with the amplitude being a function of anode bias only. These master curves can be described by simple Townsend Gas Capacitor equations using only two gas-specific parameters. As long as single scattering conditions apply, this approach allows for simplified, direct comparison of the gain characteristics of different gases and allows more intelligent selection of imaging conditions. The utility of treating signal amplification in this manner is demonstrated through a series of images collected under a variety of conditions, but with the ratio of pressure to amplification field strength kept constant. In practice, the range of operational parameter space in which this description can be applied to imaging is limited, as images typically have a mixture of secondary and backscattered contributions.

Characterization of pulsed laser deposited a-C films by means of reflection electron energy loss spectroscopy

Amorphous carbon films have been deposited by ablating a high purity graphite target using a KrF excimer pulsed laser. Both the laser energy density (up to 15 J/cm 2) and the substrate temperature (up to 900 K) were varied. Reflection electron energy loss spectra have been carried out in an ultra high vacuum surface analysis apparatus. The loss spectra were processed by removing the elastic peak contribution and the secondary electron background. The optical functions were deduced using a procedure based on the assumption that surface effects are negligible with respect to the bulk ones. The loss spectra are dominated by the presence of the π and π+σ plasma resonances, whose energy positions and shapes are significantly affected by the different preparation conditions. In particular, it has been made possible to determine, on the basis of reflection electron energy loss (REEL) data only, the sp 2/sp 3 bonding ratio. Overall, these results are also relevant for the correlation between the details of the preparation method and the material electronic properties.

High-resolution positron-annihilation-induced Auger electron spectrometer

Positron-annihilation-induced Auger electron spectroscopy (PAES) uses a beam of low-energy positrons to excite Auger transitions via annihilation of core electrons. This mechanism imbues PAES with a high degree of surface specificity and the ability to eliminate the large collisionally induced secondary-electron background typically present in conventional Auger spectra. Here, we describe a high-resolution PAES system with an energy resolution ΔE/E=2.5%, approximately five times better than previous PAES spectrometers. The system consists of a compact low-energy (∼10 eV) electrostatically focused positron beam and a large cylindrical mirror analyzer. High-resolution PAES spectra from Cu(100) and Ge(100) surfaces are presented to illustrate the capabilities of the system. Energy spectra of secondary electrons and reemitted positrons resulting from bombardment of the surface by low-energy positron are also presented.

Background-free Auger line shape of Ag N 2,3VV measured with positron annihilation induced Auger electron spectroscopy

Auger spectra for the Ag(100) N 2,3VV transition obtained utilizing positron annihilation induced Auger electron spectroscopy (PAES) are presented. This low energy transition of Ag is difficult to separate from the large secondary background which is present using electron induced Auger electron spectroscopy (EAES) and has not been studied extensively. In our measurements, the secondary electron background is eliminated by utilizing a positron beam whose energy is below the energy range of the of the Auger electrons. Measurements of the temperature dependence of PAES intensity, I PAES, and positronium fraction, f Ps, indicate that I PAES is proportional to the quantity (1 − f Ps. Electron energy spectra taken at high temperature ( f Ps ≈ 1) clearly demonstrate that the prominent low energy tail (LET) present in the PAES spectra is associated with Auger transitions and is not due to secondary electrons induced by collisions with the primary positron beam. The Auger line shape of the N 2,3VV transition of Ag(100) is accounted for in terms of the convolution of the density of states of the N 2,3 core levels and the density of states of the correlated two-hole final states estimated from the N 2,3 XPS core level spectra and the Auger line shape of the M 4VV transition. In addition, possible mechanisms leading to the production of the LET including a N 2N 3V transition followed by a N 3V-VVV transition and/or electron energy loss during the escape from the topmost layer are discussed.

Application of positron annihilation induced Auger Electron Spectroscopy to the study of surface chemistry

Positron annihilation induced Auger Electron Spectroscopy (PAES), makes use a beam of low energy positrons to excite Auger transitions by annihilating core electrons. This novel mechanism provides PAES with a number of unique features which distinguishes it from other methods of surface analysis. In PAES the very large collisionally induced secondary electron background which is present under the low energy Auger peaks using conventional tecniques can be eliminated by using a positron beam whose energy is below the range of Auger electron energies. In addition, PAES is more surface selective than conventional Auger Spectroscopy because the PAES signal originates almost exclusively from the topmost atomic layer due to the fact that the positrons annihilating with the core electrons are trapped in an image correlation well just outside the surface. In this paper, recent applications of Positron Annihilation Induced Auger Electron Spectroscopy (PAES) to the study of surface structure and surface chemistry will be discussed including studies of the growth, alloying and inter-diffusion of ultrathin layers of metals, metals on semiconductors, and semiconductors on semiconductors. In addition, the possibilities for future application of PAES to the study of catalysis and surface chemistry will be outlined.

What are the right values of [formula omitted] and the heavy quark kinetic energy?

The values of two important parameters of the heavy quark effective theory, Λ and μ π 2 (the mean value of the heavy quark three momentum squared), have been determined recently by Gremm et al. [M. Gremm, A. Kapustin, Z. Ligeti, M. Wise, CALT-68-2043, hep-ph/9603314 (original and revised versions)] from the precise CLEO data on the shape of the electron spectrum in semileptonic B meson decays. The values obtained by Gremm et al.: Λ = (0.55 ± 0.05) GeV , μ π 2 = (0.35 ± 0.05) GeV 2, disagree with the result obtained earlier by Chernyak [Nucl. Phys. B 457 (1995) 961] from the D meson semileptonic width. The purpose of this note is to show that the main reason for a disagreement is the secondary electron background present in the data, which influences strongly the extracted values of Λ and μ π 2. We determine the amount of this background from the selfconsistency conditions, and subtract it out. As a result, the values of Λ and μ π 2 become a factor two smaller and agree with Chernyak (q.v.).

Auger photoelectron coincidence spectroscopy using synchrontron radiation

The technique of Auger-photoelectron coincidence spectroscopy (APECS) is described and illustrated with a case study of the Cu(100) 3 p and M 23VV spectra. APECS offers many advantages over the conventional singles spectroscopy such as isolating overlapping spectral features, reducing secondary electron background, and revealing new decay modes. In the coincidence Cu Auger spectra discussed here, the multiplet structure of the quasi-atomic 3 d 8 Auger final state is clearly observed, as well as different intensities for the multiplet components for the p 1 2 and p 3 2 transitions. Furthermore, the spectra reveal evidence for a Coster-Kronig decay channel for 3p 1 2 core holes, and illustrate that the sum of the Auger electron and photoelectron kinetic energies is conserved. Possible technical improvements that can increase the counting efficiency are also discussed.

Auger line shape measurements using positron annihilation induced Auger electron spectroscopy

Conventional methods of Auger electron spectroscopy (AES) make use of energetic electron or photon beams to create the core-hole excitations that lead to the Auger transition. The energetic beams result in a large secondary electron background in the Auger peak region. In positron annihilation induced Auger electron spectroscopy (PAES), the core holes are created by matter-antimatter annihilation and not through collisional ionization. Measurements are reviewed which indicate that PAES can eliminate the secondary electron background by the use of very low (∼10 eV) positron beam energies and that PAES has greatly increased surface selectivity due to the trapping of positrons in surface state prior to annihilation. A new PAES spectrometer has been developed in our laboratory with an energy resolution which is an order of magnitude better than previous PAES spectrometers. The high-resolution PAES system has been used to measure the Auger M 2,3 VV line shape from a clean polycrystalline Cu surface. The atomic-like Auger M 2 VV and M 3 VV features are clearly resolved. Differences observed between the PAES spectra and spectra obtained using electron induced Auger spectroscopy are discussed.

CF3I adsorption on InSb(001)

The adsorption of CF3I on the indium-rich InSb(001)-c(8 × 2) has been studied using primarily ultraviolet photoelectron spectroscopy (UPS) and the change in the work function, Δϕ, with some low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) studies. Adsorption at 110 K was molecular with precursor adsorption kinetics giving a constant sticking probability to saturation, which occurred at one monolayer. Multilayer adsorption was not expected, or observed, at this temperature. Δϕ, as a function of coverage was positive and showed a constant slope to about half coverage (Δϕ≈ 0.45 eV), where it changed to a smaller value, giving Δϕ≈ 0.7 eV at saturation. The molecule is thought to adsorb with the iodine end down. Desorption was found to be first order with an activation energy of ca. 38 kJ mol–1 for ν= 1012– 1013 s–1. The incident electron beams used for LEED and AES cracked the adsorbed molecule to chemisorbed C, I and F in a few seconds. The chemisorbed halogens so produced were desorbed by heating to 500 K, with a concomitant decrease in the surface indium concentration, indicating desorption of indium halides. However, the carbon concentration in the vicinity of the incident beam remained almost constant, even after heating to 500 K. The incident Hel, 21.2 eV, radiation affected the surface over 1000s of seconds by desorbing CF3I molecules intact. The process, which is thought to be due to electron capture from the secondary electron background into the threshold resonance of CF3I < 20 meV, stopped after ca. 1/3 of the monolayer had been desorbed. The reasons for molecular desorption under the action of secondary electrons from Hel radiation, but total cracking of the molecule under the 2 keV incident beam used for AES, are discussed.

Photoelectron Emission from the Cesiated Diamond (110) Surface

ABSTRACTCesiation of type IIB diamond (110) crystals was studied using a combination of ultraviolet photoemission spectroscopy, x-ray photoemission spectroscopy, and low energy electron diffraction. The diamond (110) crystal was hydrogen treated by exposure to a hydrogen microwave discharge. Although cesium was largely unreactive with the hydrogenated diamond surface, cesiation yielded a large enhancement in the secondary electron yield of the diamond surface and the negative electron affinity (NEA) condition. An increase in the downwards band bending of approximately 0.75-0.9 eV was inferred from the shift in the valence band edge following cesiation. In addition, (lx 1) LEED patterns were observed at all cesium coverages. Exposure of the cesiated diamond surface to molecular oxygen significantly reduced the NEA peak (relative to the secondary electron background); however, recovery of the NEA peak was observed when the molecular oxygen source was removed.

The ICEBall: a multiple element array for in-beam internal conversion electron spectroscopy

The construction and testing of a multiple-element mini-orange spectrometer array, the Pitt ICEBall, is described. The performance of the array in tests with sources and in in-beam studies is discussed, and the questions of gamma-ray, delta-ray and secondary electron background are addressed. The ICEBall has a maximum efficiency of 18% of 4π, in a compact package capable of inclusion within large gamma-detector arrays such as the Oak Ridge Spin Spectrometer.

UV inverse photoemission from low-d-occupancy transition metals

The authors present angle integrated isochromat spectra (11.1 eV<hv<22.5 eV) from six low-d-occupancy transition metals (Ti, V, Zr, Nb, Hf and Ta). In all cases the spectral shape basically reflects the energy distribution of the empty electron states: the agreement is, however, less satisfactory if compared with the same measurements for noble and near-noble metals and with X-ray inverse photoemission from the whole 3d and 4d series. The paper is focused on the observed discrepancies and on the possible role of secondary electron background cross section and many-body effects in determining the spectral shape.

Positron Annihilation Induced Auger Electron Spectroscopy

A review is given on the results of PAES (positron annihilation induced Auger Electron Spectroscopy) studies to data, with a concentration on those results obtained at the University of Texas at Arlington. Low energy positions, trapped in a surface localized state annihilate with core electrons resulting in the emission of Auger electrons. The advantages of PEAS include: (i) the elimination of the very large secondary electron background, and (ii) increased surface selectivity. (AIP)