Interaction Of Low-energy Electrons Research Articles

Low-energy electron stimulated desorption of neutrals from multilayers of SiCl4 on Si(111)

The interaction of low-energy electrons with multilayers of SiCl(4) adsorbed on Si(111) leads to production and desorption of Cl((2)P(32)), Cl((2)P(12)), Si, and SiCl. Resonant structure in the yield versus incident electron energy (E(i)) between 6 and 12 eV was seen in all neutral channels and assigned to dissociative electron attachment (DEA), unimolecular decay of excited products produced via autodetachment and direct dissociation. These processes yield Cl((2)P(32)) and Cl((2)P(12)) with nonthermal kinetic energies of 425 and 608 meV, respectively. The Cl((2)P(12)) is produced solely at the vacuum surface interface, whereas the formation of Cl((2)P(32)) likely involves subsurface dissociation, off-normal trajectories, and collisions with neighbors. Structure in the Cl((2)P(32)) yield near 14 and 25 eV can originate from excitation of electrons in the 2e, 7t(2) and 6t(2), 6a(1) levels, respectively. Although the 14 eV feature was not present in the Cl((2)P(12)) yield, the broad 25 eV feature, which involves complex Auger filling of holes in the 6t(2) and 6a(1) levels of SiCl(4), is observed. Direct ionization, exciton decay, and DEA from secondary electron scattering all occur at E(i)>14 eV. Si and SiCl were detected via nonresonant ionization of SiCl(x) precursors that are produced via the same states and mechanisms that yield Cl. The Si retains the kinetic energy profile of the desorbed precursors.

Electron induced functionalization of diamond by small organic groups

In this work, we show that the interaction of low energy electrons with CH3CN molecules condensed on hydrogenated diamond substrates induces functionalization of the diamond. The process is discussed in terms of electron dissociation mechanisms of acetonitrile molecules and interaction of the subsequent fragments with the substrate. At 2 eV incident electron energy, the dissociative electron attachment reaction is alone operative, so that it is possible to demonstrate that these electrons induce exclusively covalent attachment of about 1 ML of H2CCN fragments on diamond through Cdiam–C and Cdiam–N linkages and to propose a description of the microscopic steps involved in the process.

Electron-driven processes in high-pressure plasmas

This review article summarizes results from selected recent studies of collisional and radiative processes initiated and driven by low-energy electron interactions with atoms and molecules in high-pressure plasmas. A special emphasis of the article is on spectroscopic studies of plasmas used as sources for non-coherent vacuum ultraviolet radiation such as rare excimer emissions and atomic and molecular emissions from plasmas in admixtures of rare gases and the molecular gases H2 and N2. An attempt is made to correlate the various observed emission features and their dependence on the plasma operating parameters (pressure, power, gas mixture, mode of excitation, etc.) to the underlying microscopic atomic and molecular processes.

Microarray technology for the study of DNA damage by low-energy electrons

The damage induced to a model DNA (dT25) immobilized on a gold surface by the interaction of low-energy (1 eV) electrons was studied by means of microarray technology. High quality single-stranded DNA arrays were hybridized with a dye-marked complementary strand after irradiation with electrons and the normalized fluorescence data were used to quantify the DNA damage. The data clearly show the sensitivity of the method. A significant loss of genetic information was already observed at dose as low as few hundred of electrons per immobilized oligonucleotide. The results imply that single stranded DNA and RNA are appreciably more sensitive to radiation and the attack of secondary electrons during replication, transcription or translation stages than the current radiation damage models envisage.

Positive and negative ion formation via slow electron collisions with 5-bromouridine

Cation and anion formation is studied experimentally upon inelastic low energy electron interactions with 5-bromouridine (BrUrd) using a crossed electron/neutral beams set-up combined with a quadrupole mass spectrometer. The BrUrd molecule belongs to the class of radiosensitizers that increase the sensitivity of DNA (or RNA) to ionizing radiation. In the case of positive ion formation the ionization efficiency curves are measured near the threshold and the corresponding appearance energies are determined using a non-linear least square fitting procedure. The anion yields are investigated in the electron energy range from about 0 to 14 eV. From the comparison of the present results for negative and positive ions information concerning the underlying mechanism of radiosensitizers is deduced.

Reactions in condensed formic acid (HCOOH) induced by low energy (<20 eV) electrons

The interaction of low energy (< 20 eV) electrons with a five monolayer (ML) film of formic acid (HCOOH) deposited on a cryogenically cooled monocrystalline Au substrate is studied by electron stimulated desorption (ESD) of negatively charged fragment ions. A comparison with results from gas phase experiments demonstrates the strong effect of the environment for negative ion formation via dissociative electron attachment (DEA). From condensed phase formic acid (FA) a strong H desorption signal from a resonant feature peaking at 9 eV is observed. In the gas phase, the dominant reaction is neutral hydrogen abstraction generating HCOO- within a low energy resonance, peaking at 1.25 eV. ESD studies on the isotopomers HCOOD and DCOOH indicate effective H/D exchange in the precursor ion at 9 eV prior to dissociation. The evolution of the desorption signals in the course of electron irradiation and the features in the thermal desorption spectra (TDS) of the electron irradiated film suggest the formation of CO2 at electron energies above 8 eV.

The Effect of Model Approximations on Single-Collision Distributions of Low-Energy Electrons in Liquid Water

The development of cross sections for the inelastic interaction of low-energy electrons with condensed tissue-like media is best accomplished within the framework of the dielectric theory. In this work we investigate the degree to which various model approximations, used in the above methodology, influence electron single-collision distributions. These distributions are of major importance to Monte Carlo track structure codes, namely, the energy-loss spectrum, the inelastic inverse mean free path, and the ionization efficiency. In particular, we make quantitative assessment of the influence of (1) the optical data set, (2) the dispersion algorithm, and (3) the perturbation and exchange Born corrections. It is shown that, although the shape and position of the energy-loss spectrum remains almost fixed, its peak height may vary by up to a factor of 1.5. Discrepancies in the calculated inelastic inverse mean free path are largely within 20-30% above 100 eV; they increase drastically, though, at lower energies. Exchange and perturbation Born corrections increase gradually below 1 keV leading to a approximately 30 to 40% reduction of the inverse mean free path at 100 eV. The perturbation effect contributes more than the exchange effect to this reduction. Similar to the dispersion situation, the effect of Born corrections at lower energies is also unclear since the models examined disagree strongly below 100 eV. In comparison, the vapor data are higher than the liquid calculations by 20 to 50% as the energy decreases from 1 to 0.1 keV, respectively. The excitation contribution is the main cause of this difference, since the ionization efficiency in the liquid levels off at approximately 90%, whereas the plateau value for the vapor is approximately 70%. It is concluded that electron inelastic distributions for liquid water, although in some respects distinctively different from the vapor phase, have associated uncertainties that are comparable in magnitude to the phase differences. The situation below 100 eV is uncertain.

Electron attachment to monomeric and dimeric forms of glycolaldehyde

Low energy electron interactions with gas phase monomeric and dimeric forms of glycolaldehyde are reported in the energy range from about 0 to 14 eV, with special emphasis on the formation and decay of transient negative parent ions. Differences between anion fragmentation obtained from monomeric and dimeric forms are observed and discussed. Dissociative electron attachment to the dimeric configuration is shown to have many energetically accessible reaction channels.

An approach to Monte Carlo simulation of low-energy electron and photon interactions in air

In this paper we present an approach to an energy deposition model for low energy (10 eV–10 keV) photons and electrons. The model is based on Monte Carlo simulations of the tracks and energy loss resulting from single electron scattering events with the constituent molecules of air, namely N 2 and O 2. The simulation code is based on the Geant4 toolkit. The original physics in Geant4 low-energy processes for photon interactions was used for the incoming photons and secondary X- and gamma-rays, from energies below 1–10 keV. Two new electron interaction processes were developed,and both of them were considered as discrete processes. The new implemented physics is based on experimental cross-sections for elastic and inelastic scattering, complemented with theoretical calculations. Ionization is treated as a subprocess of the inelastic process. The implementation of the new processes in the standard Geant4 frame is considered. Validation tests and comparison with other Monte Carlo results are discussed. The current limitations of this version of the code are also outlined. The code can be of interest in areas where energy deposition at microscopic level is crucial, such as microdosimetry.

Low Energy Electron Interactions with Uracil: The Energetics Predicted by Theory

Low energy electrons (LEEs) induce strand breaks and base damage in DNA and RNA via fragmentation of molecular bonding. This includes the formation of hydrogen atoms from N−H and C−H bond dissociations in the bases thymine, cytosine and uracil, respectively. To better understand the dissociation of uracil induced by LEEs, we theoretically characterized the potential energy surfaces (PESs) along the N−H and C−H bonds of the uracil anion, as well as the energetics involved. The PESs show that an activation barrier of less than 1 eV exists for the N1−H dissociation with rather flat PES beyond N−H = ∼1.5 Å. The PESs for C5−H and C6−H show larger barriers, which increase monotonically with bond stretching. All the N−H and C−H bond dissociations are endothermic; the adiabatic PESs suggest the energy threshold for formation of hydrogen from N−H and C−H bonds are in the order: 0.78 (N1−H) < 1.3 (N3−H) < 2.2 (C6−H) < 2.7 eV (C5−H). The H-deleted uracil radicals (U-yl radical family) are found to have exceptionally high adiabatic electron affinities, namely, 3.46 (N1), 3.8 (N3), 2.35 (C5), and 2.67 eV (C6). During the H bond breaking process of an uracil transient anion, these electron affinities compensate the extra energy needed to break the N−H or C−H bonds. This process may therefore explain the large hydrogen yield found experimentally from uracil upon attachment of LEEs. Potential applications of this process for the synthesis of uracil analogues using LEE irradiation are suggested.

Characteristics of multimode combined intense laser-induced electron acceleration and violent bunch compression

The interaction of low energy electrons with multimode combined intense laser beams in vacuum was studied by using three-dimensional test particle simulation. The process of interaction and the ponderomotive potential structures of some multimode combined intense laser beams are analyzed in detail. This article presents the detailed characteristics of the multimode combined intense laser-induced electron acceleration and violent bunch compression [Wang et al., Appl. Phys. Lett. 82, 2752 (2003)]. The properties of the output electron bunch are also investigated.

Electron Collisions with Complex Targets: Diffraction Effects in Stimulated Desorption

We describe a new electron-scattering technique involving incident beam diffraction in electron-stimulated desorption (ESD) to probe ground-state surface bonding configurations of adsorbates and excited state configurations which lead to cation ESD. We present a qualitative theoretical description of diffraction in electron-stimulated desorption (DESD) and then demonstrate this process by studying the ESD of Cl+ from Cl terminated Si(111) (1 × 1) surfaces. The fine structure in the Cl+ ESD yields as a function of azimuth is due to interference of the incident electron-beam with the elastically scattered fraction. This interference forms a surface standing wave in the initial state of the desorption process. Comparison of the experimental data to the qualitative theory indicates that desorption is primarily initiated by an excitation localized on the Si atom bonded to Cl. The ~17 eV threshold energy is consistent with an ESD mechanism involving ionization of the Si 3s level followed by Auger cascading from the σ-bonding surface state and shake-up of an electron in the non-bonding surface π-level. The 2-hole, 1-electron final state produced can lead to the ESD of Cl+ due to the hole-hole Coulomb repulsion. Direct ionization of the Si–Cl σ-bonding state can also contribute to Cl+ ESD. In general, DESD provides a useful probe of the geometry and dynamics governing low-energy electron interactions with ordered complex targets.

Low energy electron interaction with free and bound SF5CF3: Negative ion formation from single molecules, clusters and nanofilms

The interaction of free electrons with the potent greenhouse molecule SF5CF3 is studied under different degrees of aggregation: single molecules at collision free conditions, clusters within a supersonic molecular beam and condensed molecules. Electron collisions with single molecules are dominated by SF5− formation produced via dissociative electron attachment (DEA) within a resonance located below 2 eV. In clusters, undissociated parent anions SF5CF3− (and larger complexes containing undissociated anions) are observed in addition to the fragment ions. This indicates that (i) SF5CF3 possesses a positive adiabatic electron affinity and (b) low energy attachment is partly channeled into nondissociative processes when the molecule is coupled to an environment. Electron impact to condensed phase SF5CF3 exhibits a remarkably strong F− desorption signal appearing from a pronounced resonance located at 11 eV while in the gas phase at 11 eV only a weak DEA signal is observed. Electron induced desorption from sub-monolayers of SF5CF3 on an amorphous H2O ice surface is found to be more efficient compared to desorption of SF5CF3 from a Xe surface. The implications of these results for the heterogeneous photochemistry of SF5CF3 adsorbed on ice or dust particles in the Earth’s atmosphere are discussed.

Interactions of low-energy electrons with organic electroactive compounds

The electron energy loss spectra, the dependence of elastic and inelastic scattering on projectile electron energy, excitation functions and fluorescence spectra under scattering of monoenergetic electrons with tunable energies from 0 to 50 eV at an angle of 90°, for electroactive compounds – N, N ′-diphenyl- N, N ′-bis(3-methyl-phenyl)-1,1 ′-biphenyl-4,4 ′-diamine (TPD) and tris(8-hydroxyquinolinato)aluminum (Alq 3) – in the gas phase and in thin solid amorphous film are presented. The energy dependence of elastic and inelastic scattering cross-sections for Alq 3 is essentially different from Born-approximation data. The behaviour of both solid and gas phase EELS is similar. The nature of electron movement in both phases is the same.

Interaction of low-energy electrons with surface: the SEES and TCS study

The fine structure (FS) of the experimental true secondary electron emission spectra (SEES) along the normal to an Ir(1 1 1) surface and the target (or total) current spectra (TCS) along the normal to an Ir(1 0 0) surface are interpreted theoretically. It is shown that the FS of SEES and TCS is mainly due to the electronic structure of unoccupied high-level electronic states (above the vacuum level E vac) to which electrons are scattered or from where they are emitted. The predominant role of the bulk energy-band structure effects in the spectra formation is shown. In addition, a comparison to existing experimental data is given.

Synthesis of Cl2 induced by low energy (0–18 eV) electron impact to condensed 1,2-C2F4Cl2 molecules

The interaction of low energy electrons (0–18 eV) with C2F4Cl2 molecules condensed in multilayer amounts on a cryogenically cooled Au substrate generates Cl2, as can be seen from the energy and temperature dependence of the Cl− desorption signal. The cross section for Cl2 formation exhibits two pronounced resonant features with maxima near 0 and 10 eV, dropping to essentially zero in the energy range between the resonances (near 3 eV). The energy dependence of the reaction cross section qualitatively follows that of Cl− desorption, which itself can be correlated to dissociative electron attachment (DA) processes in the gas phase. While at 10 eV excited negative ion resonances and electronically excited states of neutral C2F4Cl2 may be involved in the process, in the low energy region (<2 eV) the reaction can only be initiated by dissociative electron attachment. Possible reaction pathways are discussed.

Electron interactions with plasma processing gases: present status and future needs

The work conducted at the National Institute of Standards and Technology over the last 7 years on the systematic synthesis and assessment of fundamental knowledge on low-energy electron interactions with plasma processing gases is briefly summarized and discussed. This work deals mostly with the primary electron interaction processes. Assessed data on electron collision cross sections and rate coefficients are discussed and their multiple value is indicated. Data needs are identified for electron interactions with ground-state, excited, and transient species, and for electron energy losses. It is indicated that the currently limited knowledge on electron reactions at surfaces and interfaces can be aided by related existing knowledge on electron interactions in dense gases, liquids, clusters, and molecular layers on cold surfaces.

On the Origin of the Iron K Line in the Spectrum of the Galactic X‐Ray Background

We propose a mechanism for the origin of the Galactic ridge X-ray background that naturally explains the properties of the Fe K line, specifically the detection of the centroid line energy below 6.7 keV and the apparent broadness of the line. Motivated by recent evidence of nonthermal components in the spectrum of the Galactic X-ray/γ-ray background, we consider a model that is a mixture of thermal plasma components of perhaps supernova origin and nonthermal emission from the interaction of low-energy cosmic-ray electrons (LECRe) with the interstellar medium. The LECRe may be accelerated in supernova explosions or by ambient interstellar plasma turbulence. Atomic collisions of fast electrons produce characteristic nonthermal, narrow X-ray emission lines that can explain the complex Galactic background spectrum. Using the ASCA GIS archival data from the Scutum arm region, we show that a two-temperature thermal plasma model with kT ~ 0.6 and ~2.8 keV, plus a LECRe component models the data satisfactorily. Our analysis rules out a purely nonthermal origin for the emission. It also rules out a significant contribution from low-energy cosmic-ray ions, because their nonthermal X-ray production would be accompanied by a nuclear γ-ray line diffuse emission exceeding the upper limits obtained using OSSE, as well as by an excessive Galaxy-wide Be production rate. The proposed model naturally explains the observed complex line features and removes the difficulties associated with previous interpretations of the data which evoked a very hot thermal component (kT ~ 7 keV).

Background discrimination capabilities of a heat and ionization germanium cryogenic detector

The discrimination capabilities of a 70 g heat and ionization Ge bolometer are studied. This first prototype has been used by the EDELWEISS dark matter experiment, installed in the Laboratoire Souterrain de Modane, for direct detection of WIMPs. Gamma and neutron calibrations demonstrate that this type of detector is able to reject more than 99.6% of the background while retaining 95% of the signal, provided that the background events distribution is not biased towards the surface of the Ge crystal. However, the 1.17 kg day of data taken in a relatively important radioactive environment show an extra population slightly overlapping the signal. This background is likely due to interactions of low energy photons or electrons near the surface of the crystal, and is somewhat reduced by applying a higher charge-collecting inverse bias voltage (−6 V instead of −2 V) to the Ge diode. Despite this contamination, more than 98% of the background can be rejected while retaining 50% of the signal. This yields a conservative upper limit of 0.7 event day −1 kg −1 keV recoil −1 at 90% confidence level in the 15–45 keV recoil energy interval; the present sensitivity appears to be limited by the fast ambient neutrons. Upgrades in progress on the installation are summarized.

HIGH RESOLUTION ELECTRON ENERGY LOSS SPECTROSCOPY APPLIED TO A GRAPHITE SURFACE MODIFIED BY ION BOMBARDMENT

This work presents the application of the technique known as high resolution electron energy loss spectroscopy (HREELS) to the study of a modern technological surface. First the physics of the interaction of low energy electrons with surfaces is briefly reviewed. The dielectric theory of inelastic electron scattering is outlined, with its application to surfaces and the excitation of phonons, polaritons and plasmons. Then a study of the modification of a graphite surface by Ar and H ion bombardment is presented, in relation with graphite surfaces modified by plasma wall interactions in fusion reactors. The observations of phonons and low energy plasmons are reported, with the C–H stretching vibrations as well following the H + bombardment. These observations are related to structural and chemical modifications induced by the ion bombardment.