Low-energy Secondary Electrons Research Articles

Low‐energy electron interactions with tungsten hexacarbonyl – W(CO)6

Low-energy secondary electrons are formed when energetic particles interact with matter. High-energy electrons or ions are used to form metallic structures from adsorbed organometallic molecules like W(CO)(6) on surfaces. We investigated low-energy electron attachment to W(CO)(6) in the gas phase to elucidate possible reactions during surface modification. Two crossed electron/molecular beam setups were utilised: (i) a high-resolution electron monochromator combined with a quadrupole mass spectrometer which was used for the measurement of relative cross sections as a function of the electron energy, and (ii) a double focusing mass spectrometer used for measurements of metastable decays of anions. The study was performed in the electron energy range between ~0 and 14 eV. W(CO)(6) efficiently decomposed upon attachment of a low-energy electron and no stable W(CO)(6)(-) anion was observed on mass spectrometric time scales. The transient negative ion formed lost instead sequentially CO ligands. The fragment anions W(CO)(5)(-), W(CO)(4)(-), W(CO)(3)(-), and W(CO)(2)(-) were observed. However, no W(-) was detectable. Dissociative electron attachment (DEA) to W(CO)(6) led to strong dissociation but a complete loss of all CO ligands was not observed in DEA. Deposit contaminations might be a direct result of DEA reactions close to the irradiation spot in beam deposition techniques.

Electron emission from amorphous solid water after proton impact: Benchmarking PTra and Geant4 track structure Monte Carlo simulations

Track structure Monte Carlo simulations of ionising radiation in water are often used to estimate radiation damage to DNA. For this purpose, an accurate simulation of the transport of densely ionising low-energy secondary electrons is particularly important, but is impaired by a high uncertainty of the required physical interaction cross section data of liquid water.A possible tool for the verification of the secondary electron transport in a track structure simulation has been suggested by Toburen et al. (2010), who have measured the angle-dependent energy spectra of electrons, emitted from a thin layer of amorphous solid water (ASW) upon a passage of 6MeV protons.In this work, simulations were performed for the setup of their experiment, using the PTB Track structure code (PTra) and Geant4-DNA. To enable electron transport below the ionisation threshold, additional excitation and dissociative attachment anion states were included in PTra and activated in Geant4. Additionally, a surface potential was considered in both simulations, such that the escape probability for an electron is dependent on its energy and impact angle at the ASW/vacuum interface.For vanishing surface potential, the simulated spectra are in good agreement with the measured spectra for energies above 50eV. Below, the simulations overestimate the yield of electrons by a factor up to 4 (PTra) or 7 (Geant4-DNA), which is still a better agreement than obtained in previous simulations of this experimental situation. The agreement of the simulations with experimental data was significantly improved by using a step-like increase of the potential energy at the ASW surface.

Laser-plasma accelerator based femtosecond high-energy radiation chemistry and biology

Regarding the different protocols used for external cancer radiotherapy (X or γ rays, electron, proton or ions beams) or radioimmunotherapy (Auger electron emitting radionuclides), the initial energy deposited in integrated biological systems (biomolecular and sub-cellular targets) represents a decisive parameter for the primary and more delayed radiation damage. A short-range energy distribution governs mainly i) the early survival probability of secondary electrons, ii) the spatio-temporal distribution of short-lived reactive radicals inside nascent tracks, iii) the primary biomolecular alterations triggered by low energy secondary electrons. The thorough understanding of these fundamental processes requires a real-time investigation of primary radiation events, typically in the temporal range 10−14- 10−11 s. Laser-plasma accelerators based High Energy Radiation Femtochemistry (HERF) represents a newly emerging interdisciplinary field which can be driven in strong synergy with the generation of ultrashort particle beams in the MeV energy domain. The innovating developments of HERF would favour the investigation of prethermal radiation processes in aqueous and biochemically relevant environments. In this way, the quantum character of a very-short lived low-energy electron state (p-like configuration) represents a promising sub-nanometric probe to explore early radiation processes in native tracks. The specific properties of ultra-short electron beams accelerated by TW laser are very useful for future developments of spatio-temporal radiation biophysics in complex biological systems such as living cells.

Geant4 physics processes for microdosimetry simulation: Very low energy electromagnetic models for protons and heavy ions in silicon

The Geant4-DNA extension of the Geant4 Monte Carlo simulation toolkit aims at modeling early biological damages induced by ionizing radiation at the DNA scale, and it can now track particles down to very low energies in liquid water. New models, called “MuElec”, have been implemented for microelectronic applications following the same initial theory, to track low energy electrons in silicon. This paper presents the extension of these MuElec models to incident protons and heavy ions in silicon. First, the theory of the model is presented. The resulting cross sections and stopping powers are compared with data from the literature. The model is then implemented in Geant4 and used to simulate proton tracks. Various physical quantities are extracted from the simulation, and compared with data from the literature and with results from simulation using other Geant4 models. It is shown that the generation of low-energy electrons results in more physically meaningful low-energy secondary electron tracks, which significantly modifies the proton and ion track core on the nanometer scale.

Excess Electron Interactions with Solvated DNA Nucleotides: Strand Breaks Possible at Room Temperature

When biological matter is subjected to ionizing radiation, a wealth of secondary low-energy (<20 eV) electrons are produced. These electrons propagate inelastically, losing energy to the medium until they reach energies low enough to localize in regions of high electron affinity. We have recently shown that in fully solvated DNA fragments, nucleobases are particularly attractive for such excess electrons. The next question is what is their longer-term effect on DNA. It has been advocated that they can lead to strand breaks by cleavage of the phosphodiester C(3')-O(3') bond. Here we present a first-principles study of free energy barriers for the cleavage of this bond in fully solvated nucleotides. We have found that except for dAMP, the barriers are on the order of 6 kcal/mol, suggesting that bond cleavage is a regular feature at 300 K. Such low barriers are possible only as a result of solvent and thermal fluctuations. These findings support the notion that low-energy electrons can indeed lead to strand breaks in DNA.

Simulation of Fast Electron Transport in Thin Fullerite C60 Films

Monte Carlo simulation of the C60-fullerite film irradiation by 5 keV electron beam at normal incidence has been performed. Average transverse coordinates of the primary and secondary electrons as well as the density of collisions of several types were drawn in dependence on the penetration depth. It was shown that a swarm of low energy secondary electrons provides efficient excitation of valence electrons and gives the main contribution to polymerization of C60-fullerite.

Energy-level alignment at the Alq3/Fe3O4(001) interface

We have used the technique of metastable de-excitation spectroscopy to probe the interfacial electronic structure of the organic semiconductor (OSC) Alq3 deposited onto clean Fe3O4(001) substrates. We have measured shifts in the low-energy secondary electron cutoff and energetic onset of the highest occupied molecular orbital (HOMO) of Alq3 as the coverage increases from the sub-ML range to multilayer formation. We find that the presence of an interfacial dipole induces a uniform decrease in the valence band electronic states by 1.2 eV with respect to the vacuum level and modifies the position of the HOMO energetic onset to 1.8 eV below the substrate Fermi level. The strong intrinsic dipole moment of Alq3 is suggested as the origin for these changes in accordance with previous studies of Alq3 deposited onto various substrates.

Highly Reproducible Secondary Electron Imaging under Electron Irradiation Using High-Pass Energy Filtering in Low-Voltage Scanning Electron Microscopy

The reproducibility of contrast in secondary electron (SE) imaging during continuous electron irradiation, which caused surface contamination, was investigated using SE high-pass energy filtering in low-voltage scanning electron microscopy (SEM). According to high-pass energy-filtered imaging, dopant contrast in an indium phosphide remained remarkably stable during continuous electron irradiation although the contrast in unfiltered SE images decreased rapidly as a contamination layer was formed. Charge neutralization and the SE energy distributions indicate that the contamination layer induces a positive charge. This results in a decrease of low-energy SE emissions and reduced dopant contrast in unfiltered SE images. The retention of contrast was also observed in high-pass energy-filtered images of a gold surface. These results suggest that this imaging method can be widely used when SE intensities decrease under continuous electron irradiation in unfiltered SE images. Thus, high-pass energy-filtered SE imaging will be of a great assistance for SEM users in the reproducibility of contrast such as a quantitative dopant mapping in semiconductors.

A comparison of X-ray and proton beam low energy secondary electron track structures using the low energy models of Geant4

Purpose: Lethal cell damage by ionising radiation is generally initiated by the formation of complex strand breaks, resulting from ionisation clusters in the DNA molecule. A better understanding of the effect of the distribution of ionisation clusters within the cell and particularly in regard to DNA segments could be beneficial to radiation therapy treatment planning. Low energy X-rays generate an abundance of low energy electrons similar to that associated with MeV protons. The study and comparison of the track structure of photon and proton beams could permit the substitution of photon microbeams for single cell ion irradiations at proton facilities used to predict the relative biological effectiveness (RBE) of charged particle fields. Materials and methods: The track structure of X-ray photons is compared with proton pencil beams in voxels of approximate DNA strand size (2 × 2 × 5 nm). The Very Low Energy extension models of the Monte Carlo simulation toolkit GEometry ANd Tracking 4 (Geant4) is used. Simulations were performed in a water phantom for an X-ray and proton beam of energies 100 keV and 20 MeV, respectively. Results: The track structure of the photon and proton beams are evaluated using the ionisation cluster size distribution as well as the radial dose deposition of the beam. Conclusions: A comparative analysis of the ionisation cluster distribution and radial dose deposition obtained is presented, which suggest that low energy X-rays could produce similar ionisation cluster distributions to MeV protons on the DNA scale of size at depths greater than ∼10 μm and at distances greater than ∼1 μm from the beam centre. Here the ionisation cluster size for each beam is less than ∼100. The radial dose deposition is also approximately equal at large depths and at distances greater than 10 μm from the beam centre.

Cover Picture: The Role of Dissociative Electron Attachment in Focused Electron Beam Induced Processing: A Case Study on Cobalt Tricarbonyl Nitrosyl (Angew. Chem. Int. Ed. 40/2011)

Absolute cross-sections for dissociative electron attachment (DEA) to the typical focused electron beam induced processing (FEBIP) precursor cobalt tricarbonyl nitrosyl are reported. Nanostructures written by FEBIP can potentially be smeared and contaminated by back-scattered and secondary low-energy electrons. In their Communication on page 9475 ff. Š. Matejčík, O. Ingólfsson, and co-workers discuss the role of DEA to metal–organic compounds in FEBIP.

Abstract 4903: Identification of damage products derived from C3′-deoxy-3′-thymidinyl radical, a proposed intermediate from DNA-LEEs interactions

Abstract Radiotherapy uses ionizing radiation (IR) to target deoxyribonucleic acids (DNA) among other biological molecules for cancer treatment. As a result genotoxic, mutagenic and recombinogenic lesions can be formed. Oxidative damage to DNA through IR is partially produced by the secondary particles created along the ionization track. Secondary low-energy electrons (LEEs, 1-20 eV) are the most abundant secondary species produced by primary IR. Although it has been established that LEEs can produce substantial yields of DNA single and double strand breaks, the mechanism of their formation has not been established. Elucidation of the mechanism of damage by LEEs is essential for the development of global models of cellular radiolysis and more efficient methods of cancer radiotherapy. It was proposed that LEE can add to DNA constituents resulting in the formation of transient molecular anion intermediates that eventually lead to bond dissociation. It is believed that carbon-centered radicals on the sugar moiety are a major reactive intermediate formed in this process. One of these is the C3′-deoxy-3′-thymidinyl radical. The synthesis of C3′-deoxy-3′-(selenophenyl)thymidine as a precursor of this radical has been completed. Initial photolysis experiments for photochemical generation of the radical of interest were performed and the formation of the radical intermediate of interest has been established. Identification of damage products derived from this intermediate in nucleosides is currently in progress. The modified nucleoside was incorporated into oligonucleotides using reverse DNA synthesis. Photochemical generation of the C3′-deoxy-3′-thymidinyl radical within DNA was achieved using ultraviolet light (≥ 320nm). Identification of the damage products obtained from the generation of the C3’ radical is currently in progress. Oligonucleotides sequences with different base context were synthesized in order to investigate the effect of sequence on the distribution of damage products. The effect of the concentration of GSH on the damage product distribution is also under investigation. Through these efforts a link will be established between the products of LEEs and the deleterious effects of radiation in cancer radiotherapy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4903. doi:10.1158/1538-7445.AM2011-4903

Far infrared for decreasing surface radiation dose of phantom with EBT film dosimetry

Abstract Purpose: The skin-sparing advantage of high-energy X-rays is degraded by secondary low energy electrons, which increase the surface dose and shift the position of the maximum dose towards the surface. Studies of mounting purging magnet and helium gas replacing air on the linear accelerator to sweep secondary electrons have been conducted and published. This study describes the amount of low energy electron contamination generated by a clinical photon beam from medical linear accelerator can be reduced by projecting specific wavelength (17,000 nm) of Far Infrared (FIR) to the irradiating area simultaneously. Specific wavelength of FIR can decrease the absorption of low energy contamination electrons in the build-up region.

Spreading of an SR beam spot (diameter 0.5 μm, 95 eV) photoelectron image on the surface of WO3 − x films

The 2D-photoemission image of the beam spot was obtained for the first time for the W5+ oxidation state on the preliminary irradiated WO3 − x thin film surface, created by scanning of the SR beam over the film surface. The W5+ beam profile intensity was found to spread up to a distance of 3.2 μm for an amorphous film and 5.5 μm for a polycrystalline film, it exceeds considerably the beam spot size. The image saturation dose was reached faster for a polycrystalline film. Among the possible mechanisms explaining this phenomenon, for the case of an almost unchangeable O2s state under irradiation, a choice was made in favor of a photon-generated charge diffusion due to low-energy secondary electrons from photoemission, which produce the “coloration” effect, e− + W6+ (W5+) W5+ → W5+(W4+). The O512-eV Auger peak was found to degrade at the distance of 1.5–2 mm outside the beam spot under long-time electron beam irradiation, which is attributed to electron-stimulated oxygen desorption and outdiffusion.

Synergy between low and high energy radical femtochemistry

The deleterious effects of ionizing radiation on integrated biological targets being dependent on the spatio-temporal distribution of short-lived radical processes, a thorough knowledge of these early events requires a real-time probing in the range 10−15 – 10−10 s. This manuscript review is focused on the synergy that exists between low (1–10 eV) and high (MeV) energy radiation femtochemistry (LERF, HERF respectively). The synergy remains crucial for the investigation of primary radical processes that take place within the prethermal regime of low energy secondary electrons. The quantum character of very-short lived electron in a prehydrated configuration provides a unique sub-nanometric probe to spatially explore some early radiation-induced biomolecular damage. This approach would foreshadow the development of innovative applications for spatio-temporal radiation biology such as, i) a highly-selective pro-drug activation using well-defined quantum states of short-lived radicals, ii) the real-time nanodosimetry in biologically relevant environments, and iii) the ultrashort irradiation of living cells.

Electron emission from condensed phase material induced by fast protons.

Monte Carlo track simulation has become an important tool in radiobiology. Monte Carlo transport codes commonly rely on elastic and inelastic electron scattering cross sections determined using theoretical methods supplemented with gas-phase data; experimental condensed phase data are often unavailable or infeasible. The largest uncertainties in the theoretical methods exist for low-energy electrons, which are important for simulating electron track ends. To test the reliability of these codes to deal with low-energy electron transport, yields of low-energy secondary electrons ejected from thin foils have been measured following passage of fast protons. Fast ions, where interaction cross sections are well known, provide the initial spectrum of low-energy electrons that subsequently undergo elastic and inelastic scattering in the material before exiting the foil surface and being detected. These data, measured as a function of the energy and angle of the emerging electrons, can provide tests of the physics of electron transport. Initial measurements from amorphous solid water frozen to a copper substrate indicated substantial disagreement with MC simulation, although questions remained because of target charging. More recent studies, using different freezing techniques, do not exhibit charging, but confirm the disagreement seen earlier between theory and experiment. One now has additional data on the absolute differential electron yields from copper, aluminum and gold, as well as for thin films of frozen hydrocarbons. Representative data are presented.

Quantifying reaction spread and x-ray exposure sensitivity in hydrogen silsesquioxane latent resist patterns with x-ray spectromicroscopy

Direct-write soft x-ray lithography with an ∼50 nm diameter beam is used to pattern features in hydrogen silsesquioxane (HSQ) thin films. Scanning transmission x-ray microscopy of the undeveloped patterns (latent patterns) at the oxygen K-edge reveals a two-stage cross-linking mechanism. Oxygen and silicon near edge x-ray absorption fine structure spectra of latent patterns show an increase in oxygen content and no change in silicon content within exposed regions. A dose and thickness dependent spatial spread of the cross-linking reaction beyond the exposure boundaries is observed and quantified in detail. Strong area-dependent exposure sensitivity (attributed to cross-linking beyond the exposed region) is observed in latent patterns. A lateral spread in the cross-linking of &amp;gt;70 nm (full width at half maximum) is observed on both sides of the lines created with 580 eV x-rays (λ=2.14 nm) in 330±50 nm thick HSQ films at low dose (0.6±0.3 MGy, 27±12 mJ/cm2) (1 MGy=106 J/kg absorbed energy). At a higher dose (111±29 MGy, 5143±1027 mJ/cm2), this spread increased to 150 nm. Preliminary results indicate that latent line widths increased with increasing delay between film spin-coating and exposure. Sharper lines are observed after room temperature development of the latent HSQ patterns in NaOH/NaCl solution (onset dose of 3.9±1.0 MGy, 181±36 mJ/cm2) due to the removal of material below a critical degree of cross-linking. Given the short range of low energy secondary electrons in condensed media (&amp;lt;10 nm at ≤580 eV), the observed spread is likely due to the propagation of reactive ions or radicals beyond the exposed regions.

Energy Loss of Hydrogen- and Helium-Ion Beams in DNA: Calculations Based on a Realistic Energy-Loss Function of the Target

We have calculated the electronic energy loss of proton and α-particle beams in dry DNA using the dielectric formalism. The electronic response of DNA is described by the MELF-GOS model, in which the outer electron excitations of the target are accounted for by a linear combination of Mermin-type energy-loss functions that accurately matches the available experimental data for DNA obtained from optical measurements, whereas the inner-shell electron excitations are modeled by the generalized oscillator strengths of the constituent atoms. Using this procedure we have calculated the stopping power and the energy-loss straggling of DNA for hydrogen- and helium-ion beams at incident energies ranging from 10 keV/nucleon to 10 MeV/nucleon. The mean excitation energy of dry DNA is found to be I = 81.5 eV. Our present results are compared with available calculations for liquid water showing noticeable differences between these important biological materials. We have also evaluated the electron excitation probability of DNA as a function of the transferred energy by the swift projectile as well as the average energy of the target electronic excitations as a function of the projectile energy. Our results show that projectiles with energy ≲100 keV/nucleon (i.e., around the stopping-power maximum) are more suitable for producing low-energy secondary electrons in DNA, which could be very effective for the biological damage of malignant cells.

Oxygen plasma activation of Cr(CO) 6 on α-Fe 2O 3(0001)

The chemistry of Cr(CO) 6 on the Fe 3O 4(111) surface termination of α-Fe 2O 3(0001) was explored using temperature programmed desorption (TPD), Auger electron spectroscopy (AES), static secondary ion mass spectrometry (SSIMS) and low energy electron diffraction (LEED) both with and without activation from an oxygen plasma source. No thermal decomposition of Cr(CO) 6 was detected on the surface in the absence of O 2 plasma treatment, with first layer molecules desorbing in TPD at 215 K from a close-packed overlayer. The interaction of first layer Cr(CO) 6 with the Fe 3O 4(111)-termination was weak, desorbing only ∼ 30 K above the leading edge of the multilayer state. Activation of multilayer coverages of Cr(CO) 6 with the O 2 plasma source at 100 K resulted in complete conversion of the outer Cr(CO) 6 layers, presumably to a disordered Cr oxide film, with Cr(CO) 6 molecules near the surface left unaffected. Absence of CO or CO 2 desorption states suggests that all carbonyl ligands are liberated for each Cr(CO) 6 molecule activated by the plasma. AES and SSIMS both show that O 2 plasma activation of Cr(CO) 6 results in a carbon-free surface (after desorption of unreacted Cr(CO) 6). LEED, however, shows that the Cr oxide film was disordered at 600 K and likely O-terminated based on subsequent water TPD. Attempts to order the film at temperatures above 650 K resulted in dissolution of Cr into the α-Fe 2O 3(0001) crystal based on SSIMS, an observation linked to the Fe 3O 4(111) termination of the surface and not to the properties of α-Cr 2O 3/α-Fe 2O 3 corundum interface. Nevertheless, this study shows that O 2 plasma activation of Cr(CO) 6 is an effective means of depositing Cr oxide films on surfaces without accompanying carbon contamination.

Effect of a magnetic field on the track structure of low-energy electrons: a Monte Carlo study

The increasing use of MRI-guided radiation therapy evokes the necessity to investigate the potential impact of a magnetic field on the biological effectiveness of therapeutic radiation beams. While it is known that a magnetic field, applied during irradiation, can improve the macroscopic absorbed dose distribution of electrons in the tumor region, effects on the microscopic distribution of energy depositions and ionizations have not yet been investigated. An effect on the number of ionizations in a DNA segment, which is related to initial DNA damage in form of complex strand breaks, could be beneficial in radiation therapy. In this work we studied the effects of a magnetic field on the pattern of ionizations at nanometric level by means of Monte Carlo simulations using the Geant4-DNA toolkit. The track structure of low-energy electrons in the presence of a uniform static magnetic field of strength up to 14 T was calculated for a simplified DNA segment model in form of a water cylinder. In the case that no magnetic field is applied, nanodosimetric results obtained with Geant4-DNA were compared with those from the PTB track structure code. The obtained results suggest that any potential enhancement of complexity of DNA strand breaks induced by irradiation in a magnetic field is not related to modifications of the low-energy secondary electrons track structure.

Electron-induced interaction of selected hydrocarbons withTiO2 surfaces: the relevance to extreme ultraviolet lithography

The aim of this work is to characterize desorption inducedby electronic transition processes that affect the reflectivity ofTiO2-capped multilayer mirrors used in extreme ultraviolet (EUV) lithography. A low energyelectron beam is employed to mimic excitations initiated by EUV radiation. Temperatureprogrammed desorption, x-ray photoelectron spectroscopy, and low energy ionscattering are used to analyze the surface reactions. Carbon film growth on theTiO2(011) crystalline surface is measured during 10–100 eV electron bombardment inbenzene or methyl methacrylate vapor over a wide range of pressures andtemperatures near 300 K. Low energy secondary electrons excited by EUVphotons contribute substantially to the carbon accumulation on cleanTiO2 cap layers. Forbenzene on clean TiO2, secondary electron effects dominate in the initial stages of carbon accumulation, whereas for C-coveredTiO2, direct excitations appear to dominate. We report on the adsorption energy, thesteady-state coverage of the molecules on the surface and the cross sections forelectron-stimulated dissociation: all key parameters for understanding and modeling theprocesses relating to the EUV lithography mirrors.