Low Incident Electron Energies Research Articles

Electron-impact scattering by arsine

We present elastic cross sections for electron interactions with arsine (${\mathrm{AsH}}_{3}$) in gas phase over an extensive energy range from 0.5 to 5000 eV by combining two computational methods. The ab initio R-matrix method is employed for low-energy computations up to 15 eV and the intermediate to high-energy calculations were performed using the spherical complex optical potential (SCOP) method. The elastic cross section computed through the $R$ matrix and SCOP formalism shows consistency at crossover energy (11--12 eV) and gives reasonable accord with the available data. Besides elastic cross sections, the differential cross section at low and intermediate incident electron energies is reported. The electronic excitation cross section is also reported at low energies. Shape resonance is observed at around 2.4 eV due to the formation of a transient negative ion state.

Work function effects on the positive charging of supported insulating samples exposed to X-rays (as in XPS) and other irradiations

Some aspects of the time dependence of positive charging of insulating films are evaluated when they are set on conductive substrates and are exposed to X-rays. This evaluation is based on the influence of the electrostatic surface potential due to the positive charges left by the effective secondary electron emission into the vacuum, δx, combined with some work function effects. Valid up to a surface potential of ∼3–5V, an expression for the nearly exponential decay of δx, time constant τ, is established as a function of the fluence of the irradiation and the geometry and characteristic of the sample and of its environment. The results being a function of the spectral and angular distributions of the emitted SEs, the order of magnitude of the corresponding factors are evaluated numerically for two extreme situations, that of inorganic insulators and that of floating conductors in order to be extended easily to other types of materials. The decay may be preceded by a dwell time, τs, particularly when an external collector is negatively biased. When flood guns are used, the work function effects limit the efficiency of the neutralization effects by the increasing reflection coefficient of the very low energy incident electrons. Finally, the mechanism of the degradation effects of electrons that are issued from the sample holder/sample interface is also developed. This approach may also be adapted other projectiles such as UV photons, ions or electrons.

Molecular rearrangement reactions in the gas phase triggered by electron attachment

Bond formation and rearrangement reactions in gas phase electron attachment were studied through dissociative electron attachment (DEA) to pentafluorotoluene (PFT), pentafluoroaniline (PFA) and pentafluorophenol (PFP) in the energy range 0-14 eV. In the case of PFA and PFP, the dominant processes involve formation of [M - HF](-) through the loss of neutral HF. This fragmentation channel is most efficient at low incident electron energy and for PFP it is accompanied by a substantial conformational change of the anionic fragment. At higher energy, HF loss is also observed as well as a number of other fragmentation processes. Thermochemical threshold energies have been computed for all the observed fragments and classical trajectories of the electron attachment process were calculated to elucidate the fragmentation mechanisms. For the dominant reaction channel leading to the loss of HF from PFP, the minimum energy path was calculated using the nudged elastic band method.

Analytic three-channel model for resonance-averaged direct and indirect dissociative recombination processes of molecular ions

An analytic three-channel model is developed for the description of simultaneous direct and indirect dissociative recombination (DR) cross sections of molecular ions with electrons. The model is formulated in terms of three parameters describing the interactions between the electron-ion entrance channel $e$, the dissociation channel $d$, and an ``active'' quasibound molecular Rydberg channel $a$ and yields resonance-averaged cross sections for low incident electron energies. The relative magnitudes of the parameters determine (i) the height of the downward steps at the ion vibrational thresholds and (ii) the relative contributions of the direct and indirect processes. The model is intended to serve for the empirical analysis of observed DR cross sections, and in particular it allows the assessment of the extent to which reionization of the capture complex competes with dissociation. The model is applied to the DR of H${}_{3}$${}^{+}$ and HCO${}^{+}$.

High-resolution electron energy loss spectroscopy of anions chemisorbed on electrode surfaces: The effect of counter cations

In the course of experiments that included Auger electron spectroscopy (AES) and high-resolution electron energy loss spectroscopy (HREELS) on cation exchange at benzoquinone sulfonate chemisorbed on a Pd(111) electrode, it was found that, whereas the AES spectra remained invariant as the counter cation was varied from H+ to K+ to Cs+, profound changes occurred in the HREELS spectra. Specifically, the intensity of the spectral features decreased noticeably when H+ was replaced with K+. And, when the K+ ions were exchanged with Cs+, nothing but a flat-line (dead) spectrum was observed; even the elastic peak was completely attenuated. When the Cs+ ions were displaced by protons, the initial undiminished spectrum was fully restored. This outcome, while unrelated to cation-exchange selectivity, is of exceptional significance in surface electron spectroscopy. It appears that the positive ions on the surface attracted the low-energy incident electrons such that backscattering towards the energy analyzer was hindered; partially by K+ but totally by the larger Cs+ ion. The use of HREELS to examine the molecular integrity of chemisorbed anionic species must thus take cognizance of the possibility that the counter cation chosen to preserve interfacial-layer electroneutrality can have a profound effect. To circumvent such complication, low-valent and small-radii cations will have to be employed. In addition, although subject to instrument limitations, higher incident-electron energies could be adopted. AES, with incident-electron energies in the kV range, is impervious to the presence of counter cations.

Free-free transitions in the presence of laser fields and Debye potential at very low incident electron energies

Free - free transition is studied for electron – Hydrogen atom in ground state inside a plasma in presence of an external homogeneous, monochromatic and linearly polarized laser field. The effect of plasma screening is considered in the frame work of Debye Shielding approximation. The incident electron is considered to be dressed by the laser in a non perturbative manner by choosing the Volkov solutions in both the channels. The major effect of plasma screeing on the laser assisted cross scetions is to enhance the latter significantly both for the singlet and triplet transitions.

Differential cross sections for the electron impact excitation of pyrimidine

We report on differential cross section (DCS) measurements for the electron-impact excitation of the electronic states of pyrimidine. The energy range of the present measurements was 15-50 eV with the angular range of the measurements being 10°-90°. All measured DCSs displayed forward-peaked angular distributions, consistent with the relatively large magnitudes for the dipole moment and dipole polarizability of pyrimidine. Excitations to triplet states were found to be particularly important in some energy loss features at the lower incident electron energies. To the best of our knowledge there are no other experimental data or theoretical computations against which we can compare the present results.

Valley polarized electronic transmission through a line defect superlattice of graphene

The band structure of a one-dimensional superlattice formed by patterning graphene lattice with a periodic line defect array is calculated. The lowest conduction subband of such a superlattice connects two inequivalent Dirac points with flat dispersion, which is reminiscent of the flat-bottomed subband of a zigzag-edged graphene nanoribbon (ZEGNR). According to such a subband characteristic, a finite length line defect superlattice in graphene, instead of a ZEGNR, can be utilized to realize valley filtering function. The calculated electronic transmission spectrum of such a structure shows the nearly perfect valley polarization for low energy incident electrons.

Electron-impact dissociation and ionization of NH+: formation of N+ and N2+

Absolute cross sections for electron-impact dissociation and ionization of NH+ leading to the formation of N+ and N2+ products have been measured by applying the animated electron–ion beam method, in the energy range from the respective thresholds up to 2.5 keV. The maximum total cross sections are observed to be (15.7 ± 0.7) × 10−17 cm2 and (11.1 ± 0.2) × 10−18 cm2 for N+ and N2+, respectively. Absolute cross sections are determined separately for dissociative excitation and for dissociative ionization processes. The measurements for slow N+ ions show a noticeable contribution in the low incident electron energy range; these data are attributed to excitation processes. Dissociative excitation is assumed to play a significant role in the collision energy region close to the vertical excitation energies for the lowest electronic transitions in the Franck–Condon region. The yields of fast N+ ions have also been measured; these energetic dissociations are generally ascribed to ionization processes. Kinetic energy release distributions are seen to extend from 0 eV to 15 eV for the N+ fragments and up to 20 eV for the N2+ ones. The present energy thresholds and kinetic energy release results are compared with available published data, allowing in some cases identification of fragmentation patterns and of molecular states contributing to observed processes. The possibility of dissociative excitation of the molecular ion which could occur via a mechanism involving autoionizing resonances is discussed.

Theoretical Study on Inner Shell Electron Impact Excitation of Lithium

Cross sections for electron impact excitation of lithium from the ground state 1s22s to the excited states 1s2s2, 1s2p2, 1s2snp (n = 2–5), 1s2sns (n = 3–5), 1s2pns (n = 3–5), and 1s2pnp (n = 3–5) are calculated by using a full relativistic distorted wave method. The latest experimental electron energy loss spectra for inner-shell electron excitations of lithium at a given incident electron energy of 2500 eV [Chin. Phys. Lett. 25(2008)3649] have been reproduced by the present theoretical investigation excellently. At the same time, the structures of electron energy loss spectra of lithium at low incident electron energy are also predicted theoretically, it is found that the electron energy loss spectra in the energy region of 55–57eV show two-peak structures.

Analysis of band broadening in vibrational high-resolution electron-energy-loss spectra of condensed methane

High-resolution vibrational electron-energy-loss spectra of multilayer condensed films of methane recorded at 20 K show a strong tailing of the vibrational bands that clearly exceeds the instrumental resolution. At low incident electron energy, this tailing is remarkably less important for the dipole-allowed bending vibration (nu(4)) than for other bands. Also, the tailing becomes less pronounced with increasing size of the molecule as demonstrated by spectra of ethane and heptane recorded under the same conditions. Dipole coupling, rotational broadening, and multiple inelastic scattering have been considered as origins of this band broadening. While the first two effects can be excluded, multiple scattering involving a low-frequency phonon band provides a reasonable explanation as demonstrated by simulations of the spectrum of methane using a classical two-stream model. A lower phonon frequency in the cases of the larger molecules is held responsible for the better resolved vibrational signals in the spectra of ethane and heptane.

Bright-field TEM imaging of single molecules: Dream or near future?

We examine the suitability of spherical aberration ( C S)-corrected (CS) and uncorrected (UC) transmission electron microscopes (TEM) for conventional bright-field imaging of radiation-sensitive materials. We have chosen an individual molecule suspended in vacuum as a hypothetical example of a well-defined radiation-sensitive sample. We find that for this particular sample, CS instruments provide about 30% improvement over an UC instrument in terms of signal/noise ratio per unit electron dose at 300 kV. The lowest imaging doses can be achieved in CS instruments equipped with high-brightness electron source operated at low incident electron energies. Our calculations suggest that it may be possible to image individual, iodine- or bromine-substituted organic molecules in bright-field mode, at doses lower than the accepted values for radiation damage of aromatic molecules.

Review of structures in low-energy bremsstrahlung: Classical and quantum descriptions

A brief review of recent advances in studying structures in energy dependence of the bremsstrahlung cross sections for low incident electron energies is presented. Examples of structures are given in both classical and quantum formalisms. It is shown that the origin of the structures can be formulated as a lack of contribution to the radiation from electrons with certain angular momenta at certain energies. In quantum mechanics the lack of contribution to the total cross section from certain electron angular momenta is due to zeroes in corresponding dipole matrix elements. In classical mechanics summation over angular momentum is replaced by integration and structures are due to suppressed or enhanced contribution from certain intervals of angular momentum. A survey of the known properties of the matrix elements’ zeroes is given.

Polarization of radiation emitted from He-like ions following electron impact

We have performed semi-relativistic R-matrix calculations with the lowest 17 target levels and non-relativistic R-matrix calculations with the lowest 11 target terms for electron-impact excitation of the 1s2p 1P1 state of He-like ions (Be2+, C4+, O6+, Cl15+, Fe24+, Kr34+ and Xe52+) to obtain polarization fraction at low incident electron energies. Results are compared with those available in previous calculations and experiment.

Electron-impact excitation of the3s3pPo1state of Mg-like ions:S4+,Ar6+, andCa8+

Electron-impact excitation of the $3s3p\phantom{\rule{0.2em}{0ex}}^{1}P^{o}$ state of Mg-like ions (${\mathrm{S}}^{4+},\phantom{\rule{0.3em}{0ex}}{\mathrm{Ar}}^{6+}$, and ${\mathrm{Ca}}^{8+}$) from their ground state is studied theoretically using the $R$-matrix method with 31 target states. Results are reported in the range of low incident electron energies for the total and differential cross sections of this excitation process as well as for the polarization fraction and the Stokes parameters ${P}_{1},\phantom{\rule{0.3em}{0ex}}{P}_{2}$, and ${P}_{3}$ of the photons emitted on the decay of the excited $3s3p\phantom{\rule{0.2em}{0ex}}^{1}P^{o}$ state to the ground state. The agreement between experiment and the present results is good. We obtain a polarization fraction of about 35% at low incident electron energies in these ions.

Electron-beam-induced amorphization in SiC

We performed electron irradiation into silicon carbide (SiC) using a transmission electron microscope equipped with a field-emission gun. It was found that a crystalline-to-amorphous phase transformation takes place with 200-keV electrons at room temperature. The incident electron energies are much larger than the threshold displacement energy of carbon atoms, while it is almost the same as or smaller than that of silicon atoms. An amorphized area undergoes a transformation from amorphous SiC to amorphous silicon because of the exclusive displacements of carbon atoms rather than silicon atoms by the low incident electron energy of $<300\mathrm{keV}.$ We also discuss amorphization mechanisms in electron-irradiated SiC within the context of our results as well as previous observations.

Crystallinity effects on primary beam and Auger electron signal intensities observed for graphite

This paper shows the experimental outcomes of a study of crystallinity dependent diffraction effects on both elastically-scattered and Auger electrons produced from highly-oriented pyrolytic graphite (HOPG) using a commercial scanning Auger electron microscope (SAM). This system is equipped with a coaxial cylindrical mirror analyser (CMA). Scanning electron images (SEM), backscattered electron images (BSE), Auger maps and the Auger spectra, using a range of relatively low incident electron energies, are obtained and analysed in detail. It is shown that the regional contrast shown in the SEM and BSE images and in the Auger maps, in the electron energy range used, is formed predominantly by forward focusing along the close-packed 〈2 1 ̄ 1 ̄ 2〉-type atomic chains of the graphite lattice. The intensity of the exiting electrons received by the CMA can be interpreted as a convolution of the diffraction of both incident and exiting electrons, e.g. that of the exiting elastically scattered electrons can vary by up to 50% on changing the incident direction of the primary beam and the orientation of the specimen. Increasing the energy of incident beam enhances forward focusing and increases the contribution from diffraction along the secondary close-packed atomic chains, e.g. the 〈3 3 04〉-type atomic chains present. The carbon (KLL) Auger electrons produced are found to behave in a way similar to the primary electrons in regard to the diffraction process but with less marked diffraction evident.

Electron stimulated desorption of negative ions: A time-of-flight experiment

We present a setup for stimulated desorption experiments of negative ions using low energy incident electrons and time-of-flight identification of the desorbed ions. It consists of a pulsed electron gun, an electrostatic focusing system, and a channel plate detector setup. Electron beams down to sub-eV energies can be used due to electrostatic shielding and the compensation of the earth’s magnetic field by a set of Helmholtz coils. The main advantage is the large acceptance for ions of all masses, energies, and desorption angles at the same time, which keeps measuring time reasonably short and allows us to gain information before degeneration of the irradiated sample occurs, even if weak desorption channels are investigated. We demonstrate the power of our setup with first results from a boron doped, (100)-oriented diamond sample, which is partly oxidized and partly hydrogenated with some water contaminations on it. Different binding states of oxygen are disclosed clearly by different desorption thresholds. The C–O binding on oxidized diamond forms a carbonyl group. The 1b2 orbital of water can be seen in a O desorption threshold. The yield of negative hydrogen desorption shows a linear increase for incident electron energies higher than 13 eV. It results from a dipolar dissociation as has been published previously.

Extension of the binary-encounter-dipole model to relativistic incident electrons

Formulas for the total ionization cross section by electron impact based on the binary-encounter-dipole (BED) model and its simpler version, the binary-encounter-Bethe (BEB) model are extended to relativistic incident electron energies. Total ionization cross sections for the hydrogen and helium atoms from the new relativistic formulas are compared to experimental data. Relativistic effects double the total ionization cross section of H and He at incident electron energy $\ensuremath{\approx}300 \mathrm{keV}$ and dominate the cross section thereafter. A simple modification of the original BED-BEB formulas is proposed for applications to ion targets and inner-shell electrons of neutral atoms and molecules. The relativistic and nonrelativistic BEB cross sections are compared to the K-shell ionization cross sections by electron impact for the carbon, argon, nickel, niobium, and silver atoms. For carbon and argon, the relativistic effects are small, and both forms of the BEB cross sections agree well with available experimental data. For the nickel and heavier atoms, the relativistic increase of cross sections becomes noticeable from about 100 keV and higher in the incident electron energy. The empirical formula by Casnati et al. [J. Phys. B 15, 155 (1982)] after correcting for relativistic effects as shown by Quarles [Phys. Rev. A 13, 1278 (1976)] agrees well with the BEB cross sections for light atoms. However, the peak values of the Casnati cross sections become higher than the relativistic BEB peak cross sections as the atomic number increases. The BEB model is also applied to the total ionization cross section of the xenon atom, and the theory agrees well with experiments at low incident electron energies, but disagrees with experiment at relativistic incident energies.

Electron–hole pair creation at simple metal surfaces

Cross-sections for dipole excitation of low energy (0.1–0.3 eV) electron–hole pairs at the (111) and (100) surfaces of aluminium have been measured with use of electron energy loss spectroscopy. We observe a substantial crystal-face effect at low incident electron energies (<6 eV); the Al(111) data agree very well with predictions from the current jellium theory, while the Al(100) data are substantially larger. At electron energies larger than 6 eV, experiment and theory show qualitatively different behaviours; the measured cross-sections decrease with increasing electron energy for both surfaces, while the cross-section calculated from the jellium theory increases.