Low-energy Electrons In Solids Research Articles

Low-energy electron transport in non-uniform media

We simulated the transport of low and medium energy electrons with energies between 1.26eV and 10keV in non-uniform carbon targets using the track structure Monte Carlo code TRAX which has several applications in biophysics and radiation physics. Cross sections for electrons incident on carbon have been critically assessed. Furthermore the code has been extended to handle non-uniform targets allowing a complex geometry description. Solid state targets, which are commonly used as targets in electron spectrometers and other devices can be non-uniform, e.g. have highly irregular surfaces or pinholes. The resulting electron spectra can be significantly affected by these non-uniformities. We reproduce experimental data obtained by GSI’s Toroid electron spectrometer using thin solid state foils as targets. This unique experiment was designed to gain further insight in the emission and transport of low energy electrons in solids to improve the description of microscopic energy deposition. The realistic implementation of non-uniform targets in TRAX was verified by comparison with available experimental data. The increased backscattering due to the roughness of an unpolished target in comparison with polished ones could be reproduced as well as secondary electron spectra from the Toroid.

Attenuation lengths of low-energy electrons in solids: The case of CoO

The effective attenuation length (EAL) of low-energy electrons in CoO is investigated by photoemission spectroscopy experiments $(5\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}19\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ by measuring the Ag Fermi-edge signal through a CoO overlayer of increasing thickness. The EAL is found to increase when lowering the electron energy, but the experimental values are much smaller than expected from the commonly used Seah--Dench formulas [Surf. Interface Anal. 1, 2 (1979)].

USING HIGH-CURRENT NANOSECOND LOW-ENERGY ELECTRON BEAMS FOR RADIATION STERILIZATION OF POWDERED DRUGS

Modern technological schemes of manufacturing tabletized drugs usually eliminate microbiological contamination of processed and intermediate products. Therefore, sterility of the final drugs is determined by microbiological cleanness of the initial powdered materials. Powdered drug components cannot always be sterilized by even the most effective thermal method, because the parent drugs frequently possess insufficient thermal stability. Chemical sterilization by volatile agents (ethylene oxide, ozone) is also problematic because of the risk of toxin accumulation. Under these conditions an alternative is offered by the radiation sterilization process, since ionizing radiation is a universal damaging factor for microbiological flora of any kind (fungi, bacteria, spores, viruses), while a required sterilization dose of 25 – 50 kGy (irrespective of the radiation type) usually produces no detectable physicochemical changes in the irradiated material [1]. Radiation sterilization is usually produced using -radiation from isotope sources or high-energy (above 5 MeV) electron beams possessing considerable penetration ability in condensed matter. Construction of such radiation sources involves significant expenditures for the radiation protection of personnel. As a rule, special sterilization centers are built around such sources, where the final products are sterilized in a ready-to-use packed form [2, 3]. There is certain experience in using low-energy (150 – 500 keV) electron beams for sterilization purposes. The creation and use of such radiation sources requires neither capital investment for special buildings nor large expenditures for radiation protection. These systems can be arranged and operated immediately on enterprises. A disadvantage of such radiation sources is the relatively small penetration depth of low-energy electrons in solids (not exceeding 1.5 mm in water-equivalent materials). For this reason, use of these systems is restricted to surface sterilization, mostly of films, foils, paper, and related packaging materials [2 – 7]. Nevertheless, low-energy electron beams can ensure not only surface sterilization but bulk treatment as well, provided that the objects processed admit leveling of the irradiation in depth by means of intense stirring [8] or aeration, whereby the average density of the target decreases and the effective electron penetration depth increases [9]. In resent years, nanosecond pulse low-energy electron accelerators capable of operating at a high repetition rate (50 – 400 Hz) have been developed [10, 11], which possess the advantages of long working life, small dimensions, and relatively low cost. Low-energy electron beams of nanosecond pulse duration, characterized by high absorbed radiation dose rates per pulse, can be used for the sterilization of radiation-unstable preparations (some vitamin solutions, enzymes, etc.), since the doses required for sterilization under such conditions are significantly (3 – 10 times) lower [12, 13]. One possible variant of drug powder sterilization by low-energy electron beams is implemented in an experimental setup developed at Pharmaceutical Chemistry Journal Vol. 36, No. 12, 2002

Calculations of electron inelastic mean free paths for carbides

Knowledge of the values of inelastic mean free paths (IMFPs) for low-energy electrons in solids is important for quantitative surface analysis by AES and XPS as well as for determining the surface sensitivity of other electron-spectroscopic methods of surface characterization. In previous papers, we have described the Penn algorithm for calculations of electron inelastic mean free paths in solids, and used this algorithm for calculations of IMFPs for 50 – 2000 eV electrons in a group of 27 elements, a group of 15 inorganic compounds, and a group of 14 organic compounds. We fitted the calculated IMFPs for the groups of elements and organic compounds to a modified form of the Bethe equation4 for inelastic electron scattering in matter and found that the four parameters in this equation could be related empirically to several material parameters. The resulting general formula TPP-2M for IMFPs in solids could be used to estimate IMFPs for other materials.

Monte Carlo simulation of inelastic background in direct and inverse photoemission

Abstract A simple model is described which allows the inelastic background in direct and inverse photoemission spectra to be simulated. The basic physical approximations of the model are described, and the flow diagrams of the Monte Carlo algorithms are presented. The input parameters of the simulation program are the electron inelastic mean free path and the loss function. In order to illustrate the possibilities of this approach we show simulations of direct and inverse photoemission spectra. A good agreement between simulation and experiment can be obtained if the loss function is known (high electron energies). For low electron energies such as those encountered in UV inverse photoemission the situation is more complex and it is necessary to use model loss function and fit the simulation with the experiment. In this way it is possible to obtain, at least in principle, information on the scattering mechanisms of low energy electrons in solids.

Inelastic Mean Free Paths of Low-Energy Electrons in Solids

We present a summary of recent calculations of the electron inelastic mean free paths (IMFPs) of 50-2000 eV electrons in 27 elements and 15 inorganic compounds. These calculations are based in part on experimental optical data to represent the dependence of the inelastic scattering probability on energy loss and the theoretical Lindhard dielectric function to represent the dependence of the scattering probability on momentum transfer. The calculated IMFPs for the elements were fitted to a modified form of the Bethe equation for inelastic electron scattering in matter and the four parameters in this equation were empirically related to other material parameters. The resulting formula, designated ΤPP-2, provides a convenient means for predicting IMFPs in other materials. We have used two powerful integral equations or sum rules to evaluate the optical data on which our IMFP calculations are based. While the optical data for the elements satisfied these sum rules to an acceptable degree, there were significant deviations in the data for the compounds. In addition, differences in IMFPs calculated from the optical data for the compounds and the values predicted by ΤPP-2 correlated with the average errors of the optical data as determined by the sum rules. IMFPs calculated from ΤPP-2 for these compounds are therefore believed to be more reliable than IMFPs obtained from the imperfect optical data. PACS numbers: 82.80.Pv, 79.20.Fv, 79.60.—i, 78.90.+t

Energy and material dependence of the inelastic mean free path of low-energy electrons in solids

Calculations have been made of the inelastic mean free paths (IMFP’s) of 100–2000-eV electrons in C, Mg, Al, Al2O3, Cu, Ag, Au, and Bi. These calculations have been based on experimental optical data and on theory. The optical data gives the dependence of the differential inelastic scattering cross section at zero momentum transfer on electron energy loss; the data used here satisfy optical sum rules closely. Theory is needed to specify the dependence of the differential inelastic scattering cross section on momentum transfer; results for free-electron-like solids were assumed to be applicable to the present materials. The calculated IMFP’s show significant deviations from the dependencies on electron energy and material expected from the formulas of Seah and Dench, Szajman et al., and Ashley.

Range of Low-Energy Electrons in Solids

The continuous-slowing-down-approximation range and range straggling for electrons with energies ≤ 10 keV in Al, Au, Ag, and Cu are calculated. The mean energy loss per unit pathlength, or the stopping power, is described by a statistical electron gas model. A correction to the stopping power due to the Z31 effect is included. Comparisons of the calculated CSDA ranges with experimental data are made.

Attenuation lengths of low-energy electrons in solids derived from the yield of proton-excited Auger electrons: beryllium and aluminum

Values are reported for the absolute yields of $\mathrm{KVV}$ Auger electrons from beryllium and ${L}_{23}\mathrm{VV}$ Auger electrons from aluminum excited by 60- to 220-keV proton bombardment. The measurements were made using semi-infinite evaporated samples, and the results were used to derive effective values of the inelastic attenuation lengths for low-energy Auger electrons in the surface regions of the samples. The attenuation lengths determined using this technique were 6.1 $\mathrm{A}$ for 100-eV electrons in Be and 1.9 $\mathrm{A}$ for 67-eV electrons in Al. These effective attenuation lengths are appropriate for use in Auger-electron spectroscopy and x-ray photoelectron spectroscopy.

Attenuation lengths of low-energy electrons in solids

Abstract A compilation is presented of measured attenuation lengths of low-energy electrons in solids in the energy range (40 to 2000 eV) normally employed in X-ray photoelectron and Auger-electron spectroscopy. The techniques used to obtain electron attenuation lengths are summarized, and it is pointed out that the accuracy of measurement needs both to be defined adequately and to be improved for more meaningful intercomparisons of data and theory. An approximate expression is derived to predict attenuation lengths using either dielectric data (derived from optical or electron-energy-loss data) or average excitation energies estimated from electron binding energies for given materials at electron energies greater than about 500 eV. Good agreement is found between the predictions of this formula and some measured attenuation lengths (e.g. for Al, C, Mo, W) but further work is required to validate the formula and to extend it to lower electron energies.

Low-energy Electron Diffraction

One of the most powerful techniques available for surface structural analysis is low energy electron diffraction (LEED). It is widely used in materials science research to study surface structure and bonding and the effects of structure on surface processes. However because it requires single crystals and ultrahigh vacuum conditions it has limited value for applied surface analysis, which is often concerned with polycrystalline or amorphous materials. LEED has many similarities to X-ray and neutron diffraction but it is preferred for surface studies because of the short mean free path of low energy electrons in solids.

New Method for Range Measurements of Low-Energy Electrons in Solids

A new method for determining the range of low-energy electrons in solids is described and compared with older techniques. In the new method, the material under study is deposited on a thick layer of greatly different electron backscattering characteristics. When the incident electrons have just sufficient energy to penetrate to the support and return to the surface, a marked change in the backscattered fraction occurs. In this manner, the need for very thin organic supporting films of unknown stopping power is avoided. Range-energy data from 1 to 15 kev for films of aluminum on a gold substrate obtained by this technique are found to agree with the data obtained by the use of cathode luminescence to detect electron penetration. The technique was applied to thin films of gold in the energy region from 2 to 15 kev. It was found that the practical ranges in mg/cm2 for gold and aluminum were the same below 10 kev, unlike the situation in the mev region where the effective penetration into Al is about twice that for Au due to the importance of nuclear scattering in a high Z element. Good agreement is found with the Bohr-Bethe theory for aluminum at all energies and for Au below ∼10 kev.