Incident Electron Energy Research Articles

The brightness of the CO Cameron bands in the martian discrete aurora: A study based on revised cross sections

We analyze the consequences of the new laboratory measurements of the emissions cross sections of the Cameron bands excited by electron impact on CO2 and CO in the Mars aurora. We use Monte Carlo simulations to investigate the importance of these changes and their dependence on the initial electron energy. The Mars Climate Database (MCD) is used as an input for the temperature and neutral density profiles. Auroral density profiles of O2+, CO2+ and electrons in the aurora are calculated for a range of precipitating electron energies. The total nadir brightness of the Cameron bands is reduced by a factor of 4.5 at 150 eV and 5.1 at 700 eV relative to calculations made with the previously recommended cross section. The relative contributions to the production of the Cameron bands by electron collisions with CO2 and CO are examined. It varies with the CO/CO2 density ratio near the emission peak that depends on the neutral model and season. Dissociative recombination of CO2+ ions provides a contribution between 10% and 45% at the altitude of maximum production of the a 3Π state. The efficiency of the nadir brightness of the Cameron bands is 2.5–3.6 kilorayleighs per incident electron energy flux of 1 mW m−2, with a small dependence on the initial electron energy. Model simulations predict that the intensity ratio of the Cameron bands to the CO2+ UV doublet (UVD) depends on the electron initial energy. It varies from 2.0 at 500 eV to 5.3 at 50 eV, for both Ls = 90° and 225°.

Electron impact ionization cross-sections for CH3 and SiH3 radical targets

Single differential cross sections as a function of secondary electron energy at fixed incident electron energies 100 and 200 eV for the targeted molecule CH3 and SiH3 have been evaluated by using a modified Jain-Khare semi-empirical approach. The integral or total electron-impact ionization cross-sections and the averaged energy loss from differential ionization cross-sections are calculated for incident electron energy from the ionization threshold to the 5 keV regimes. The temperature-dependent ionization rate coefficient is also calculated from total ionization cross-sections and Maxwell-Boltzmann electron energy distribution.The calculated values of total ionization cross sections corresponding to (e, 2e) processes for CH3 and SiH3 molecules are in good accord with available theoretical and experimental results.

Modulation of Cathodoluminescence by Surface Plasmons in Silver Nanowires.

The waveguide modes in chemically-grown silver nanowires on silicon nitride substrates are observed using spectrally- and spatially-resolved cathodoluminescence (CL) excited by high-energy electrons in a scanning electron microscope. The presence of a long-range, travelling surface plasmon mode modulates the coupling efficiency of the incident electron energy into the nanowires, which is observed as oscillations in the measured CL with the point of excitation by the focused electron beam. The experimental data are modeled using the theory of surface plasmon polariton modes in cylindrical metal waveguides, enabling the complex mode wavenumbers and excitation strength of the long-range surface plasmon mode to be extracted. The experiments yield insight into the energy transfer mechanisms between fast electrons and coherent oscillations in surface charge density in metal nanowires and the relative amplitudes of the radiative processes excited in the wire by theelectron.

Resonantly enhanced interatomic Coulombic electron capture in a system of three atoms

In interatomic Coulombic electron capture, the capture of a free electron at an atomic center is accompanied by the radiationless transfer of the excess energy to a neighboring atom of different species, leading to ionization of the latter. We show that this interatomic process can be strongly enhanced by the presence of an additional third atom, provided the energy of the free-bound capture transition in the first atom is resonant to a dipole-allowed excitation energy in this assisting atom. The relation of the resonantly enhanced three-center electron capture with other processes is discussed, and its dependencies on the incident electron energy and the spatial geometry of the triatomic system are illustrated.

Electron impact ionization cross sections of highly charged open L-shell tungsten ions

L-shell electron-impact ionization (EII) cross sections for highly charged tungsten ions were measured at incident electron energies of 29.10 and 38.92 keV using the Shanghai-EBIT. Resolved x-rays from radiative recombination (RR) processes were recorded with a high-purity Ge detector in a static electron energy scanning mode. Absolute EII cross sections were obtained by normalizing to the theoretical RR cross sections. The experimental results were compared with the calculated results using the relativistic distorted-wave method implemented in the flexible atomic code and the relativistic Lotz semi-empirical equation. The measurements showed general agreement with the calculated results by two theoretical methods for Li- to N-like W ions. The experimental uncertainties are not sufficiently small to discern the two theoretical results. Furthermore, the influence of Breit interaction on the EII cross sections of open L-shell tungsten ions was studied, and the effect is small but non-negligible. The measured EII cross sections of open L-shell tungsten ions would contribute to fusion plasma studies.

Measurements of continuous spectra of photons from 4/6 MeV dual energy e-LINAC

The continuous spectra of photons originated due to bombardment of 4 MeV, 6 MeV and dual energy (4 MeV ＋ 6 MeV) electrons on a thick Tungsten target have been measured for a recently developed 4/6 MeV dual energy electron-LINAC. The energy of incident electrons was deduced from the end-point energy of the measured bremsstrahlung spectrum. The experimental value of the incident electrons energy was found to be 4.10 ± 0.15 MeV and 6.15 ± 0.17 MeV, which are in good agreement with the expected value 4.00 MeV and 6.00 MeV, respectively, from the tuned operational parameters of the e-LINAC. The fractional intensity contribution due to 4 MeV and 6 MeV electrons in the continuous spectrum of photons from dual energy mode has been extracted to be 0.43 ± 0.11 (4 MeV) ＋ 0.57 ± 0.11 (6 MeV).

Electron scattering with ethane adsorbed on rare gas multilayers: Hole transfer, coulomb decay, and ion dissociation.

Positive ion desorption following electron impact dissociative ionization of ethane adsorbed on Ar, Kr, and Xe multilayers has been studied as a function of incident electron energy from threshold to 100eV. Based on the dependence of ion yields on the identity of the rare gas, it is likely that the majority of ethane molecules undergo indirect ionization following hole transfer from the ionized underlying rare gas. This has also been corroborated by density of states calculations showing the energetic alignment of the outer valence states of ethane and the condensed rare gas ionization energies. Due to the near-resonant nature of charge transfer for single-hole states, the ethane molecular ion is excited to different final ionic states on different rare gases, which leads to differences in ion desorption yields and branching ratios. The quantitative yields increase with increasing ionization energy gap between the rare gas and ethane, in the order Ar > Kr > Xe. The large increase in yields from 25eV onwards for all rare gases is likely due to the formation and decay of two-hole states on neighboring rare gas and ethane molecules due to interatomic and intermolecular Coulomb decay (ICD) and not electron transfer mediated decay (ETMD). The ICD and ETMD pathways become accessible when the incoming electron has sufficient energy to excite the inner valence ns level of the rare gas to a Rydberg state or ionize it. The experimental findings are supported by calculations of thresholds, density of states for the final configurations of these processes, and coupling strengths for hole transfer between ethane and rare gases. The fragment ion branching ratios vary with energy from threshold to about 35eV, showing the fragmentation pattern changes with the mode of hole transfer and availability of excess energy. Sigma C-C bonds are more likely to break than C-H bonds in the mid-20eV range, and this effect is most pronounced for Xe, followed by Kr, and then Ar.

Cold-target electron-ion-coincidence momentum-spectroscopy study of electron-impact single and double ionization of N2 and O2 molecules

We report experimental studies of electron-impact ionization of nitrogen (${\mathrm{N}}_{2}$) and oxygen (${\mathrm{O}}_{2}$) molecules using a cold target recoil ion momentum spectrometer (COLTRIMS, reaction microscope). The recoil ion is detected in coincidence with one outgoing electron such that the momentum vectors and consequently the kinetic energies for these final-state particles are determined. The ionization cross sections for producing the doubly and singly charged parent ions were measured as a function of the incident electron energy ranging from 50 to 600 eV. For molecular dications, e.g., ${\mathrm{O}}_{2}^{2+}$, the cross sections are obtained by measuring both the ion time of flight and two-dimensional position spectra, which is demonstrated to be an efficient way to suppress the contribution of the dissociation channels with the ${\mathrm{O}}^{+}$ products as both ions have the identical mass-to-charge ratios. The projectile energy-loss spectra correlated to the final-state ions are obtained, which provide direct evidence of the ionization mechanisms of ${\mathrm{N}}_{2}$ and ${\mathrm{O}}_{2}$ molecules. It is found that the ${\mathrm{O}}_{2}^{2+}$ dication is produced mainly by Auger process after single ionization in $2{\ensuremath{\sigma}}_{g}$ inner-valence shell of ${\mathrm{O}}_{2}$ molecule, while the ${\mathrm{N}}_{2}^{2+}$ dication is generated by the direct removal of two electrons from the outermost $3{\ensuremath{\sigma}}_{g}$ orbital of ${\mathrm{N}}_{2}$ molecule.

Oscillator strengths and cross sections of the valence-shell excitations of CH[formula omitted]I studied by fast electron scattering

Oscillator strengths and cross sections of the valence-shell excitations in CH3I are important for studying molecular photodissociation and monitoring the destruction of ozone in the Earth’s atmosphere. Based on the relative flow technique, absolute generalized oscillator strengths of the valence-shell excitations in CH3I have been determined by using an angle-resolved electron energy loss spectrometer. The measurement was carried out at an incident electron energy of 1.5 keV and an energy resolution of 80 meV. Optical oscillator strengths for the dipole-allowed transitions have been obtained at the zero limit of the squared momentum transfer, providing an independent cross check with the previous experimental results. The integral cross sections from the excitation threshold to 5000 eV have also been obtained by means of the BE-scaling method. The present experimental oscillator strengths and cross sections can supplement the fundamental spectroscopic data of CH3I and will play an important role in the studies of molecular photodissociation dynamics.

Chemical and electrical modifications of few-layer graphene films via sub-10 keV electron beam irradiation

Graphene has attracted special attention due to its mechanical and electrical properties. In this work, we describe the effects of sub-10 keV electron beam irradiation on the electrical conductivity of few-layer graphene films deposited on a glass substrate. The irradiation process was performed in vacuum at 10–6 Torr for 30 min per sample. The superficial chemical structure and optical properties of the samples were evaluated before and after electron irradiation using spectroscopic techniques (UV-Vis, Raman, and XPS), and the Van der Pauw method was used to determine the sheet resistance. It was found that the sheet resistance and the defect density decrease as the energy of incident electrons increases. For instance, the sheet resistance has been reduced by 17.3% after the sample was irradiated with a 10 keV electron beam. This could be explained by the reduction of defect density on the irradiated samples caused by the removal of oxygen content on graphene flakes, estimated by Raman and XPS, respectively. Hence, electron beam irradiation could be used to modify the electrical conductivity of graphene films based on defect engineering.

Experimental determination of the Ag L -subshell x-ray-production cross sections by electron impact through two bulk target methods

The experimental determination of x-ray-production cross sections by electron impact on thick samples is faced in this work, especially focusing on the $L$ subshells of Ag. The integrodifferential method used by An et al. is compared with a recently implemented approach involving the ionization distribution function. The first method has shown good results under suitable conditions, namely, when multiple scattering, bremsstrahlung enhancement, and other secondary effects are negligible. Monte Carlo simulations were performed in order to analyze the influence of these conditions and therefore to extend the validity range of the method by adding a correction factor. The second approach uses the ionization distribution function $\ensuremath{\phi}(\ensuremath{\rho}z)$ to predict the intensity of the emitted photons; this magnitude is assessed by the poema software, previously developed, and it is used to determine correction factors to the cross sections included in the software database. Both approaches were tested by fitting several $L$-shell spectra measured in a wide interval of incident electron energies, ranging from 4.0 to 25 keV, with an electron microprobe equipped with a wavelength-dispersive spectrometer. The x-ray-production cross sections obtained for each $L$ subshell and the total $L$ x-ray-production cross section were compared with theoretical predictions based on the distorted-wave approach theory and with experimental determinations when available.

Inelastic squared form factors of the vibronic states of B1Σu+, C1Πu , and EF1Σg+ for molecular D2 studied by high-energy electron scattering

The inelastic squared form factors of the valence-shell excitations of ${\mathrm{D}}_{2}$ were measured by high-energy electron scattering and calculated by the multireference single- and double-excitation configuration interaction method in the present work. It is found that for the ${B}^{1}{\mathrm{\ensuremath{\Sigma}}}_{u}{}^{+}$ state of ${\mathrm{D}}_{2}$, the theoretical calculation cannot satisfactorily reproduce the inelastic squared form factors with a higher vibrational quantum number. For the vibronic states of ${C}^{1}{\mathrm{\ensuremath{\Pi}}}_{u}$ and $E{F}^{1}{\mathrm{\ensuremath{\Sigma}}}_{g}{}^{+}$ of ${\mathrm{D}}_{2}$, obvious discrepancies between our electron-scattering results and theoretical calculations are found. Similar phenomena observed for ${\mathrm{H}}_{2}$, HD, and ${\mathrm{D}}_{2}$ may be attributed to electronic-vibrational coupling and the failure of the first Born approximation at an incident electron energy of 1500 eV. Furthermore, the present inelastic squared form factors of ${\mathrm{D}}_{2}$ are in good agreement with the ones of ${\mathrm{H}}_{2}$ and HD, which indicates that there is no isotope effect for the electronic matrix elements of ${\mathrm{H}}_{2}$, HD, and ${\mathrm{D}}_{2}$ in the momentum space.

Secondary electron emission characteristics of alumina coating on metallic substrate prepared by atomic layer deposition

The main objective of this study is to research the secondary electron emission characteristic of different thickness alumina coatings which are prepared by atomic layer deposition (ALD) technique on several metallic secondary-emitting materials. The experimental results reveal that secondary electron yield (SEY) property concerned with the metal work function and the surface topography of alumina coatings when coating thickness is in the nanometer level. In addition, after utilizing SEY measurement setup and pulsed electron gun, the SEY curves of metallic substrates with different thickness alumina coatings and primary energy (PE) of incident electron could be measured. From an analysis of the SEY curves, some key parameters related to secondary electron emission characteristics such as the maximum SEY value and its corresponding PE of incident electron, and the penetration depth of incident electron can be obtained. Furthermore, basing on the ALD technique and on the SEY results of stainless steel substrate coated with 80-cycle alumina thin film, a novel 18-stage discrete dynode electron multiplier (DDEM) was fabricated and tested. The gain of DDEM 1.6× 106 was obtained.

Experimental and Theoretical Study on Electron Momentum Spectroscopy of SF6: Distorted-Wave and Vibrational Motion.

The vibrational and distorted-wave effects are usually invoked to explain the measured electron momentum profiles for molecular orbitals. The vibrational effect can be accounted for quantitatively by a harmonic analytical quantum mechanical approach within the plane-wave impulse approximation (PWIA). On the other hand, quantitative calculation considering the distorted-wave effect was available only recently by a multicenter-three-distorted-wave (MCTDW) method (Phys. Rev. A2022, 105, 042805). Here, we report a joint experimental and theoretical investigation on electron momentum spectroscopy of SF6. The experiments were performed using a high-sensitivity (e, 2e) spectrometer employing non-coplanar symmetric geometry with incident electron energy equal to 1200 eV + binding energy. The experimental electron momentum profiles are compared with theoretical calculations by the MCTDW method at equilibrium geometry and by the PWIA method both at equilibrium geometry and considering vibrational motions. For all the measured orbitals, large discrepancies were observed between the experiments and the PWIA calculations at equilibrium geometry. For the highest occupied molecular orbital 1t1g, the vibrational effect can partly explain the high intensity of the experimental momentum profile at low momenta. For the other orbitals, the influence of the vibrational effect is negligible. On the other hand, the MCTDW calculations improve the agreement with the experiments for all the observed orbitals, indicating that the distorted-wave effect plays an important role in reproducing the measured momentum profiles of SF6.

Spin-transport across a two-dimensional metal-semiconductor interface with infinite potential in presence of spin-orbit interactions: Double refraction and spin-filtering effect

The spin-transport across a two-dimensional metal-semiconductor junction with a Dirac-delta function potential at the interface and the Rashba and Dresselhaus spin-orbit interactions in the semiconductor region is studied exactly using discontinuous boundary conditions and the spin-polarized reflected and refracted current density and differential conductance are calculated. It is shown that in the presence of an infinite interface potential, an increase in the incident electron's energy reduces the spin splitting. It is also shown that the reflected and refracted coefficients, the spin-polarized currents and the corresponding differential conductance depend strongly on the spin-orbit interactions. The reflected spin polarization, however, becomes zero due to the infinite potential. The Rashba coupling enhances the refracted spin polarization while the Dresselhaus coupling reduces it. Thus, the maximum in polarization occurs at small values of Dresselhaus coupling and large values Rashba coupling. Interestingly, though the presence of delta-potential at the interface does not change the splitting angle between the spin-up and spin-down electrons, it causes a constant shift in the spin polarization.

Electron-induced ionization and cationic fragmentations of the isolated molecule of 2,4-imidazolidinedione (hydantoin): a study of the relaxing path thresholds.

In this work, our new experimental setup has been used to study the ionization and fragmentation of a prebiotic molecule, hydantoin, by electron impact. Scanning of the incident electron energy allows the determination of the appearance thresholds of the cations. The vertical ionization potential was found to be in good agreement with previous data. Dissociation thresholds for the main fragmentation patterns were also measured. In parallel, thanks to quantum chemical calculations, reaction schemes compatible with the experimental results are given.

Cross sections for elastic electron scattering by benzene at low and intermediate energies

We present experimental and theoretical differential cross sections for elastic electron scattering from benzene. The present experimental results are obtained at incident electron energies ranging from 1 to 50 eV and for scattering angles from ${10}^{\ensuremath{\circ}}$ to ${130}^{\ensuremath{\circ}}$. The experimental measurements are compared to available results from 1 to 10 eV and to theoretical results from 10 to 50 eV, both based on the Schwinger multichannel (SMC) method. Different electron scattering calculations are carried out by employing varied basis sets and multichannel coupling schemes. This allows us to (i) obtain relatively converged cross sections with respect to the inclusion of Rydberg excited states, (ii) observe significant variations in the forward scattering as a function of the multichannel coupling scheme, and (iii) explore possible effects stemming from states lying above the ionization threshold. Overall, the agreement between experiments and models is found to be very good to excellent. The remaining discrepancies point out areas in which the SMC method should be improved.

Joint theoretical and experimental study on elastic electron scattering from a methane molecule

We report on a combined experimental and theoretical study of elastic differential cross section DCS (E, θ) from methane molecule in the intermediate energy range (50-300eV), with experimental results limited to an angle interval of 25 to 125 degrees. Theoretical results are acquired using two approximations: a simple sum of individual atomic cross sections and the other with molecular effect taken into the account. The experimental setup based on a crossed beam technique comprising of an electron gun, a single capillary gas needle, and a detection system with a channeltron was used to measure differential cross sections. The absolute scale for the cross sections is obtained by a relative-flow method using argon gas as a reference. The dependence of DCS on the energy of incident electrons has been considered in detail. The results are analysed and compared with available theoretical and experimental data.

Arc Formation on Solar Array Surfaces Under Energetic Electron Irradiation and Vacuum Exposure

The harsh space environment induces local discharges (arcs) on satellite solar panels, which can cause reduced optical transmission through solar array coverglasses, thus negatively affecting long-term satellite missions. This article presents the experimental results of low-energy (4–5 keV) electron irradiation of three common types of space solar array coverglass, CMX, CMG, and fused silica at various bias voltages. The dependence of the discharge rate on the bias voltage, coverglass composition, energy of the incident electrons, vacuum exposure, and surface conductivity was investigated. It is shown that the arc frequency is inversely related to the incident electron energy. Physicochemical models of the mechanisms occurring in different types of space array coverglasses under low-energy electron irradiation are proposed to explain the experimental findings.

Local inverse inelastic mean free path at interface

We calculated a local inverse inelastic mean free path (local‐IIMFP) for electrons crossing a medium–medium interface, considering various incident electron energies, crossing angles and combinations of materials. We used an extension of a classical dielectric model developed by Li and co‐workers for an electron crossing a surface (interface vacuum‐medium). Moreover, the integration over the distance of the local‐IIMFP allows to obtain the interface excitation parameter (or IEP) characterizing the change in excitation probability for an electron crossing an interface once caused by the presence of the interface in comparison with an electron for which only volume excitations are considered. We perform these calculations for angles between 0° and 80°, for electron energies between 500 and 2500 eV and for various pairs of materials, as Al/In for its academic interest or Au/Si and SiO2/Si for their technological importance. Small but not negligible variations of the local‐IIMFP and the IEP were observed for metal–metal or metal–semiconductor interfaces, while quite significant variations are obtained when one of the materials is a insulator.