Decreasing Electron Energy Research Articles

Cage opening and fragmentation of C60 fullerene induced by an ultrashort laser pulse

Response of C60 fullerene to a 40 fs full-width at half-maximum laser pulse with a photon energy of 2.0 eV and different laser intensities is studied by semiclassical dynamics simulation technique. The simulation results show that soon after the irradiation with a strong laser pulse, many C–C bonds abruptly break but no fragments are produced. The breaking of multiple C–C bonds induces a quick increase in the kinetic energy and potential energy and a decrease in electronic energy. These results suggest that the opening of the C60 cage is an effective channel for the conversion of electronic energy to kinetic energy for the electronically excited C60 fullerene.

Sub-Ångstrom Low-Voltage Performance of a Monochromated, Aberration-Corrected Transmission Electron Microscope

Lowering the electron energy in the transmission electron microscope allows for a significant improvement in contrast of light elements and reduces knock-on damage for most materials. If low-voltage electron microscopes are defined as those with accelerating voltages below 100 kV, the introduction of aberration correctors and monochromators to the electron microscope column enables Angstrom-level resolution, which was previously reserved for higher voltage instruments. Decreasing electron energy has three important advantages: (1) knock-on damage is lower, which is critically important for sensitive materials such as graphene and carbon nanotubes; (2) cross sections for electron-energy-loss spectroscopy increase, improving signal-to-noise for chemical analysis; (3) elastic scattering cross sections increase, improving contrast in high-resolution, zero-loss images. The results presented indicate that decreasing the acceleration voltage from 200 kV to 80 kV in a monochromated, aberration-corrected microscope enhances the contrast while retaining sub-Angstrom resolution. These improvements in low-voltage performance are expected to produce many new results and enable a wealth of new experiments in materials science.

The attenuation length of low energy electrons in Yb

Photoelectron emission spectra in a photon energy range between 7.5 and 21 eV aremeasured for in situ grown polycrystalline Yb films. By comparing bulk and surface corelevel shifted 4f components we give an estimation of the effective attenuation length (EAL)for low energy (6–20 eV) electrons in Yb, establishing a moderate increase of the EALupon electron energy decrease. The experimental EAL data are found to be afactor of four smaller than those predicted from the so-called ‘universal curve’.

Fluid modelling of an atmospheric pressure dielectric barrier discharge in cylindrical geometry

A numerical parameter study has been performed for a cylindrical atmospheric pressure dielectric barrier discharge (DBD) in helium with nitrogen impurities using a two-dimensional time-dependent fluid model. The calculated electric currents and gap voltages as a function of time for a given applied potential are presented, as well as the number densities of the various plasma species. This study shows that for the geometry under consideration the applied voltage parameters have a large impact on the electric current profiles and that the discharge current is always determined by the electron and ion conduction currents while the displacement current is nearly negligible. A relative broadening of the current profiles (compared with the duration of the half cycle of the applied voltage) with an increase in the applied frequency is obtained. Nearly sinusoidal current wave forms, usually typical for radio frequency DBDs, are observed while still operating at the frequencies of tens of kilohertz. For the setup under investigation, the Townsend mode of the DBD is observed in the entire range of applied voltage amplitudes and frequencies. It is shown that the average power density dissipated in the discharge increases with rising applied voltage and frequency. An increase in applied voltage frequency leads to an increase in the electron density and a decrease in electron energy, while increasing the voltage amplitude has the opposite effect.

Measured and modeled backscatter of ionospheric photoelectron fluxes

This paper examines the backscatter of ionospheric photoelectrons from the thermosphere using photoelectron measurements made on closed field lines in the magnetosphere from 1997 to 2004. The satellite made simultaneous measurements of both the precipitating photoelectrons from the sunlit conjugate hemisphere and the backscattered electrons from the dark thermosphere. The backscatter ratios of the observed fluxes are in very good agreement with photoelectron model calculations for energies ranging from a few eV to several hundred eV and for different levels of solar activity. The backscattered electron flux consists of a direct elastic backscatter component that is a weak function of energy and a much stronger cascade component that greatly increases with decreasing electron energy so that the backscatter ratio ranges from 0.2 at high energies to greater than one at low energies. Although the photoelectron fluxes can vary by up to an order of magnitude over the solar cycle, the backscatter ratios show only a small variation above 20 eV. There can be a factor of 2 or more variability in the backscatter ratios for energies below 20 eV due to thermal electron Coulomb interactions. The calculations show that 55–60% of the precipitating flux energy is backscattered from the thermosphere back along the field line to the conjugate hemisphere. The model–data comparisons show that photoelectrons are able to travel the long journey from the sunlit hemisphere to the satellite without significant degradation indicating that pitch angle scattering and trapping of photoelectrons in the magnetosphere may be small.

A study of errors in trajectory simulation with relevance for 0.2–50 eV electrons in liquid water

A highly simplified model of the elastic scattering of electrons in a nm-size volume of liquid water, where the water molecules are regarded as point scatterers and inelastic scattering is neglected, is studied for electron energies 0.2–50 eV. This model allows an exact quantum mechanical solution of the multiple elastic scattering problem. The exact solutions are compared with the corresponding trajectory simulations. Two different data sets for the elastic scattering mean free path are discussed and used, one based on cross sections for water molecules in the gas phase and the other on cross sections derived from scattering in amorphous ice. In each case, the comparison provides a detailed insight into the character and magnitude of the error in the trajectory simulation, and gives a preliminary indication of an upper bound to the limits of validity of trajectory simulation in the real case of electron scattering in liquid water. Main results: with a fully random distribution of scatterers, the trajectory simulation is found to be a surprisingly good approximation down to quite low electron energies ( ∼ 10 eV ). Below a few eV, the error increases rapidly with decreasing electron energy. A substantial increase of the error in trajectory simulation results when short-range order (minimum distance between scatterers) is introduced; the observed effect may partly be understood from the theory of kinematic diffraction in liquids. At very low electron energies (a few eV) one may note a quantum effect in the form of a standing wave pattern in the average distribution of scattering events.

Oxide Thickness Measurement by Scanning Electron Microscopy with Controlling Ultra-Low Voltage

The secondary electron (SE) images were recorded for Si wafers with and without SiO2 thin (10 nm-50 nm) films by a scanning electron microscope (SEM) with changing primary electron energy from 2.5 keV down to 0.2 keV during one frame scan. Brightness of the SE image increased up to critical values and then decreased with decreasing primary electron energy for the specimens with SiO2 films. The critical voltage is higher for specimen with thicker film. These results are discussed by means of the oxide film thickness, the penetration depth of incident electrons, and the surface potential caused by charging. [DOI: 10.1380/ejssnt.2008.35]

Driving force for the WO3(001) surface relaxation

Abstract The optimized structure of the WO 3 (0 0 1) surface with various types of termination ((1 × 1)O, (1 × 1)WO 2 , and c(2 × 2)O) has been simulated using density functional theory with the Perdew–Wang 91 gradient corrected exchange–correlation functional. While the energy of bulk WO 3 depends weakly on the distortions and tilting of the WO 6 octahedra, relaxation of the (0 0 1) surface results in a significant decrease of surface energy (from 10.2 × 10 −2 eV/A 2 for the cubic ReO 3 -like, c(2 × 2)O-terminated surface to 2.2 × 10 −2 eV/A 2 for the relaxed surface). This feature illustrates a potential role of surface relaxation in formation of crystalline nano-size clusters of WO 3 . The surface relaxation is accompanied by a dramatic redistribution of the density of states near the Fermi level, in particular a transformation of surface electronic states. This redistribution is responsible for the decrease of electronic energy and therefore is suggested to be the driving force for surface relaxation of the WO 3 (0 0 1) surface and, presumably, similar surfaces of other transition metal oxides.

Capacitively Coupled RF Discharge Breakdown in Gas Mixtures

The RF (13.56 MHz) gas breakdown in Ne-N2, Ne-Ar and He-N2 mixture systems are investigated experimentally in a parallel plate capacitively coupled device with varying the pressure, the mixing ratio and the electrode separation, specifically focussing on the breakdown at the small values of the mixing ratio. The breakdown voltage curves obtained have the V-shaped or L-shaped characteristics. The breakdown voltage decreases with the addition of a small amount of N2 or Ar in each mixture system, and the ratios of decrease in breakdown voltage to rare gas breakdown voltage in these mixture sysytems are about 22, 32 and 30 % at the maximum, respectively. The decrease of breakdown voltage is observed in a certain restricted pressure range, which is narrower in Ne-N2 system than in Ne-Ar one. This feature in Ne-N2 system is probably due to the decrease of electron energy attributed to the vibrational excitation of N2 by electron impact. Compared with the DC breakdown and the microwave one, the decrease ratio in the RF breakdown is relatively small, suggesting that there exist some competitive ionization processes accompanied by excited particles including the Penning ionization.

Quantum study of slow electron collisions with Rydberg atoms

Slow collisions of electrons with Rydberg sodium atoms have been quantum mechanically investigated by using the time-dependent close-coupling (TDCC) method. To make such large-scale calculations possible, it is essential to implement the TDCC method with the powerful finite-element discrete variable representation (FEDVR). Besides visualizing the slow collision dynamics, our quantum calculations show interesting features in the slow electron impact ionization of Rydberg atoms. The energy sharing between two continuum electrons changes dramatically as the incoming electron energy decreases. Predominant equal-energy sharing is observed in such a cold (e,2e) process, even though the impact electron energy is still twice the binding energy of the Rydberg electron.

An experimental study of recombination and polarity effect in a set of customized plane parallel ionization chambers

Plane parallel ionization chambers are an important tool for dosimetry and absolute calibration of electron beams used for radiotherapy. Most dosimetric protocols require corrections for recombination and polarity effects, which are to be determined experimentally as they depend on chamber design and radiation quality. Both effects were investigated in electron beams from a linear accelerator (Varian 21CD) for a set of four tissue equivalent plane parallel ionization chambers customized for the present research by Standard Imaging (Madison WI). All four chambers share the same design and air cavity dimensions, differing only in the diameter of their collecting electrode and the corresponding width of the guard ring. The diameters of the collecting electrodes were 2 mm, 4 mm, 10 mm and 20 mm. Measurements were taken using electron beams of nominal energy 6 to 20 MeV in a 10 cm x 10 cm field size with a SSD of 100 cm at various depths in a Solid Water slab phantom. No significant variation of recombination effect was found with radiation quality, depth of measurement or chamber design. However, the polarity effect exceeded 5% for the chambers with small collecting electrode for an effective electron energy below 4 MeV at the point of measurement. The magnitude of the effect increased with decreasing electron energy in the phantom. The polarity correction factor calculated following AAPM protocol TG51 ranged from approximately 1.00 for the 20.0 mm chamber to less than 0.95 for the 2 mm chamber at 4.1 cm depth in a electron beam of nominally 12 MeV. By inverting the chamber it could be shown that the polarity effect did not depend on the polarity of the electrode first traversed by the electron beam. Similarly, the introduction of an air gap between the overlying phantom layer and the chambers demonstrated that the angular distribution of the electrons at the point of measurement had a lesser effect on the polarity correction than the electron energy itself. The magnitude of the absolute difference between charge collected at positive and negative polarity was found to correlate with the area of the collecting electrode which is consistent with the explanation that differences in thickness of the collecting electrodes and the number of electrons stopped in them contribute significantly to the polarity effect. Overall, the polarity effects found in the present study would have a negligible effect on electron beam calibration at a measurement depth recommended by most calibration protocols. However, the present work tested the corrections under extreme conditions thereby aiming at greater understanding of the mechanism underlying the correction factors for these chambers. This may lead to better chamber design for absolute dosimetry and electron beam characterization with less reliance on empirical corrections.

Nucleation and growth of defect clusters in CeO2 irradiated with electrons

Nucleation and growth process of defect clusters in cerium dioxide (CeO2) with fluorite-type crystal structure has been investigated in situ under electron irradiation by using high voltage transmission electron microscopy. Planar defect clusters were formed with electron irradiation ranging from 200 to 1000keV at temperatures below 450K. The defect clusters were determined to be faulted-interstitial type dislocation loops lying on {111} planes. The growth rate of dislocation loops was found to increase with decreasing electron energy. An analysis of the fluence dependence of the growth process of dislocation loops suggests an increase in the vacancy mobility with decreasing electron energy. The rate of the electronic excitation is discussed in terms of the radiation-induced diffusion of oxygen-ion vacancies.

Electron-beam induced dissociation of dislocation loops in magnesia–alumina ceramics

Kinetics of interstitial-type dislocation loops in α-Al 2O 3 and MgO·Al 2O 3, which were induced by 6 keV Ar + ions at 300 K, has been investigated under electron irradiation at 300–870 K by transmission electron microscopy. Dislocation loops disappeared during electron irradiation at energies of 100, 150 and 200 keV, where displacement damage is hardly induced in α-Al 2O 3 and MgO·Al 2O 3 with those electron energies. The disappearing rate of dislocation loops increases with decreasing electron energy and with decreasing irradiation temperature. Further, a portion of dislocation loops in α-Al 2O 3 has been found to grow under the subthreshold-energy electron irradiation, and the average size of the surviving dislocation loops decreases with irradiation at 300 K but increases at 370, 420 and 570 K. We have reached a conclusion that dislocation loops in α-Al 2O 3 and MgO·Al 2O 3 dissociate into isolated interstitials through ionizing radiation. The generated interstitial has been found to mostly recombine with radiation-induced vacancies at lower irradiation temperature (300 K) but to be absorbed to the surviving dislocation loops at higher temperature (370–570 K). The disappearing rate of loops is discussed in terms of a balance between the dissociation process by ionizing radiation and the diffusion process of the generated interstitials.

Electronic structure of doped trans-polyacetylene

The behavior of electronic structures of doped trans-polyacetylene is revealed by a simple method. (C24H26)+n is used to simulate p-type doped trans-polyacetylene at various doping concentrations. The electronic structure is calculated, by CNDO/2 method. These calculations show that at low doping level, the decrease of electronic energy compensates the increase of elastic energy, thus the bond alternation exists, and the charge carriers are solitons. When doping level is high, the increase of elastic energy is larger than the decrease, of electronic energy, the bond alternation disappears, solitons no longer exist, and polyacetylene is in a metalic state.

Approximate Expression of Ellipse Polarization Degree for Gyrosynchrotron Radiation

We study the elliptical polarization degree (EPD) of gyrosynchrotronradiation for the case of transverse propagation. The EPD decreaseswith increasing ellipticity R (i.e. ratio of the major and minoraxial lengths) and with decreasing electron energy spectral indexδ. The EPD value varies from 0.55 to 0.15 in the whole rangeof 3⩽R⩽10 and 2⩽δ⩽7. An approximateexpression for the EPD is proposed, in which we assume that theemitting electrons have an isotropic pitch angle distribution and apower-law distribution in energy. The expression is designed for theelectron energy spectral index δ from 2 to 7, as well as theharmonic numbers s from 2 to 70 with a statistical error of ±6%. The expression can be applied to the polarization analysis andmagnetic field strength diagnostic for solar limb events.

Nanometer and high aspect ratio patterning by electron beam lithography using a simple DUV negative tone resist

The micro resist technology’s ma-N 2400 series DUV negative tone resist is evaluated for electron beam lithography using the Gaussian beam machine LION LV1. We could demonstrate the high resolution capability of this resist and the possibility of delineating dense patterns with high aspect ratios. Lines and spaces with dimensions down to 150 nm in an 800 nm resist layer and down to 50 nm in a 180 nm resist layer can be resolved. The patterns show steep sidewalls and demonstrate the possibility of generating resist features with high aspect ratios using a simple single-layer resist technology. The aspect ratio has a value of, at least, about 5. The exposure doses for the resist layers in these experiments range from 120 to 200 μC/cm 2 using 20 keV electrons. Exposures with electron energies less than 20 keV show that the resist sensitivity increases with decreasing electron energy. For 2.5 keV electron energy a dose of only 10 μC/cm 2 is sufficient. Due to the small penetration depth of low energy electrons only resist layers of less than 100 nm thickness can be used in the low energy range.

Very low energy electron scattering in nitromethane, nitroethane, and nitrobenzene

Absolute total integral and total backward scattering cross sections are reported for CH 3 NO 2 , C 2 H 5 NO 2 , and C 6 H 5 NO 2 at electron-impact energies from 30 meV to several eV, with some additional data at higher energies. These experimental data extend the range of species studied in low-energy electron scattering to include target molecules with dipole moments large enough to support dipole-bound states. Data provide a further test of the validity of the Born point-dipole approximation for the calculation of rotationally inelastic scattering cross sections for polar molecules, which are of importance in modeling the chemical and physical characteristics of industrial and natural plasmas. For CH 3 NO 2 and C 2 H 5 NO 2 , large cross sections are found, monotonically increasing with decreasing electron energy in qualitative and to some degree quantitative agreement with theory based on the Born model. As in earlier studies other polar species, the Born model is found to underestimate significantly backward-scattering cross sections. C 6 H 5 NO 2 shows a powerful dip in the total integral scattering cross sections <200 meV, a feature not found in the backward-scattering cross section and totally at variance with any simple theory. The suggestion is made that this behavior, in common with ClO 2 and Cl 2 O, may arise through interference between direct Born-type rotational excitation and an indirect channel involving temporary negative ion states in the continuum.

Influence of Heat on Impurity States in an Artificial Semiconductor Atom

semiconductor as the potential barrier material of the artificial atom. Different heat dependent effective masses and different heat dependent dielectric constants are used for the two semiconductors constitute the ASA. The lowest energy and the binding energy of the ground state are calculated. The calculations have shown that the lowest electron energy decreases by increasing the ASA radius. For very large radii the lowest energies corresponding to different aluminum contents approach a certain value which is the bulk limit. At a constant ASA radius the lowest energy increases as a function of temperature. A pronounced deviation is obtained when the calculations of the present work are compared with those of Montenegro and Merchancano [18] in which the effects of heat and mismatches for both effective masses and dielectric constants were neglected. It is found that decreasing the temperature and shrinking the radius of the ASA lead to more binding of the donor electron. Increasing the aluminum concentration also enhances the donor electron binding energy. Therefore the electron ground state energy and its associated binding energy are both mainly functions of temperature, ASA radius, and aluminum content.

Regrowth of heavy-ion implantation damage by electron beams

Electron transparent Si, Ge and GaP samples were implanted with 50 keV Xe + ions to a dose around 10 11 ions/cm 2. At this implantation condition, each heavy ion created a small, spatially isolated amorphous zone. Following the ion implantation, the samples were irradiated with electrons having energies from 25 to 300 keV at temperatures 90 and 300 K. At each electron energy, transmission electron microscopy (TEM) images of a selected sample area were taken in regular time intervals to record the evolution of the implant damage. At all electron irradiation conditions it was observed that the amorphous zones shrunk with increasing electron dose and some of them eventually disappeared. The most interesting result is the observation that regrowth occurs below the displacement threshold and, moreover, regrowth rate increases when decreasing electron energy below ∼100, ∼125 and ∼200 keV, for Si, GaP and Ge, respectively. These results are discussed in terms of possible mechanisms stimulating regrowth.

Spatial distributions of elastically backscattered electrons from copper and silver

We investigated the spatial distributions regarding the pathlength, the penetration depth, and the lateral displacement of 200-2000 eV electrons elastically backscattered from copper and silver. We calculated these distributions by the Monte Carlo method using elastic scattering cross sections and inelastic inverse mean free paths for volume and surface excitations. In our approach, we applied the partial wave expansion method and the finite difference technique to calculate electron elastic cross sections by the Hartree-Fock-Winger-Seitz scattering potential for solid atoms. We employed the extended Drude dielectric function to estimate electron inelastic mean free paths inside the solid and electron surface excitation parameters outside the solid. Our study was focused on the energy dependence of the pathlength distribution, the maximum depth distribution and the radial distribution. We found that both the radial displacement and the maximum depth of backscattered electrons were on the order of a few angstroms. The maximum depth and the pathlength distributions obeyed the exponential attenuation form. The ratio of the attenuation lengths for the pathlength and the maximum depth distributions saturated to a value somewhat greater than two. Considering the back and forth trajectories of backscattered electrons, it revealed that most electrons were backscattered from the solid through a single elastic scattering or a few elastic scatterings. As electron energy decreases, this ratio became larger, indicating that the probability for smaller scattering angles increases.