Energy Electron Impact Research Articles

Atomic data and spectral line intensities for Ni XIV

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Ni XIV. We include in the calculations all the configurations belonging to the n = 3 complex, and provide data for the lowest 143 fine-structure levels, belonging to the configurations 3s 23p 3, 3s3p 4, 3s 23p 23d, 3p 5, 3s3p 33d, and 3s 23p3d 2. Collision strengths are calculated at six incident energies for all transitions: 0.112, 8.07, 21.3, 43.4, 80.3, and 141.8 Ry above the threshold of each transition. Calculations have been carried out using the Flexible Atomic Code. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates of the present work, statistical equilibrium equations for level populations are solved at electron densities covering the range of 10 8–10 14 cm −3 and at an electron temperature of log T e ( K ) = 6.3 , corresponding to the maximum abundance of Ni XIV. Spectral line intensities are calculated, and their diagnostic relevance is discussed. This data set is available in version 6.0 of the CHIANTI database.

Microwave window breakdown experiments and simulations on the UM/L-3 relativistic magnetron

Experiments have been performed on the UM/L-3 (6-vane, L-band) relativistic magnetron to test a new microwave window configuration designed to limit vacuum side breakdown. In the baseline case, acrylic microwave windows were mounted between three of the waveguide coupling cavities in the anode block vacuum housing and the output waveguides. Each of the six 3 cm deep coupling cavities is separated from its corresponding anode cavity by a 1.75 cm wide aperture. In the baseline case, vacuum side window breakdown was observed to initiate at single waveguide output powers close to 20 MW. In the new window configuration, three Air Force Research Laboratory-designed, vacuum-rated directional coupler waveguide segments were mounted between the coupling cavities and the microwave windows. The inclusion of the vacuum side power couplers moved the microwave windows an additional 30 cm away from the anode apertures. Additionally, the Lucite microwave windows were replaced with polycarbonate windows and the microwave window mounts were redesigned to better maintain waveguide continuity in the region around the microwave windows. No vacuum side window breakdown was observed in the new window configuration at single waveguide output powers of 120+MW (a factor of 3 increase in measured microwave pulse duration and factor of 3 increase in measured peak power over the baseline case). Simulations were performed to investigate likely causes for the window breakdown in the original configuration. Results from these simulations have shown that in the original configuration, at typical operating voltage and magnetic field ranges, electrons emitted from the anode block microwave apertures strike the windows with a mean kinetic energy of 33 keV with a standard deviation of 14 keV. Calculations performed using electron impact angle and energy data predict a first generation secondary electron yield of 65% of the primary electron population. The effects of the primary aperture electron impacts, combined with multiplication of the secondary populations, were determined to be the likely causes of the poor microwave window performance in the original configuration.

Suppressing high-power microwave dielectric multipactor by the sawtooth surface

This paper theoretically and experimentally researches the effect of a periodic sawtooth surface on vacuum multipactor. Dynamic calculation and particle-in-cell simulation are applied to analyze the electron impact energy and multipactor scenario on the periodic isosceles triangular surface with different slope angles and heights. It has been discovered that, with the slope angle increasing, the impact energy significantly decreases to be lower than the first crossover energy of the secondary yield curve, leading to suppressed multipactor. S-band high power microwave dielectric breakdown experiment, with microsecond pulse length, was conducted. It is confirmed by experiment that the periodic sawtooth surface with sufficiently large slope angle (such as 45°) can effectively increase the breakdown threshold of about 2 compared to the flat surface.

The effect of grooved surface on dielectric multipactor

The effect of periodic rectangular grooves on vacuum multipactor has been theoretically and experimentally investigated. Dynamic calculation is applied to research the electron trajectory and impact energy under groove surface. Two-dimensional electromagnetic particle-in-cell simulation is used to analyze and compare multipactor scenario, statistic energy, and secondary emission yield on the flat surface with that on the corrugated surface. It has been found by computational and simulative analysis that grooved surface can explicitly suppress multipactor in the developmental stage of multipactor. S-band high power microwave (HPM) dielectric breakdown experiment under vacuum, with microsecond pulse length was conducted. It was confirmed by experiment that periodic grooves perpendicular to the major electric field can effectively increase transmitted power.

Differential cross sections for low-energy electron elastic scattering by lanthanide atoms: La, Ce, Pr, Nd, Eu, Gd, Dy, and Tm

Elastic differential cross sections (DCSs) in angle of electron scattering by the representative lanthanide atoms La, Ce, Pr, Nd, Eu, Gd, Dy, and Tm have been calculated in the electron-impact energy range $0\ensuremath{\le}E\ensuremath{\le}1\text{ }\text{eV}$. Additionally, the DCSs in electron-impact energy are also presented at scattering angles $\ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{}$, $90\ifmmode^\circ\else\textdegree\fi{}$, and $180\ifmmode^\circ\else\textdegree\fi{}$ for unambiguous identification of the binding energies (BEs) of the negative ions formed during the collisions as resonances. The shape resonances and the DCSs critical minima are identified as well. A Thomas-Fermi-type potential incorporating the vital core-polarization interaction is used for the calculations. Dramatically sharp resonances are found to characterize the near-threshold electron elastic DCSs, whose energy positions are identified with the BEs of the resultant negative ions. A procedure is suggested for measuring reliably the BEs of tenuously bound $(\text{BE}<0.1\text{ }\text{eV})$, weakly bound $(\text{BE}<1\text{ }\text{eV})$, and complicated open $d$- and $f$-subshell negative ions through the elastic DCSs both in scattering angle and electron-impact energy.

Dissociative excitation ofH2Sby electron impact

Vacuum-ultraviolet emissions following the dissociative excitation of ${\text{H}}_{2}\text{S}$ by electron impact at 100 eV incident energy have been measured. The absolute photoemission cross sections are presented over the wavelength range from 94 to 170 nm and the H, SI, or SII transitions producing the features identified. The measured cross section of the Lyman-$\ensuremath{\alpha}$ emission at 121.6 nm for an electron-impact energy of 100 eV is $(9.79\ifmmode\pm\else\textpm\fi{}0.67)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}\text{ }{\text{cm}}^{2}$. Excitation functions of the dominant H and SI emission lines are shown for electron-impact energies from threshold to 300 eV. We study the near-threshold region of the excitation functions and use the measured threshold energies to identify the dominant fragmentation channels.

Atomic data and spectral line intensities for Ni XXV

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Ni XXV. The configurations used are 2 s 2 , 2 s 2 p, 2 p 2 , 2 l 3 l ′ , 2 l 4 l ′ , and 2 s 5 l ′ , with l = s,p and l ′ = s,p, and d giving rise to 92 fine-structure levels in intermediate coupling. Collision strengths are calculated at seven incident energies (50, 100, 150, 225, 300, 375, and 450 Ry) for the transitions within the three lowest configurations corresponding to the 10 lowest energy levels, and at five incident energies (150, 225, 300, 375, and 450 Ry) for transitions between the lowest five levels and the n = 3 , 4 , and 5 configurations. The calculations are carried out using the distorted wave approximation. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates of the present work, statistical equilibrium equations for level populations are solved at electron densities covering the 10 8 – 10 14 cm - 3 range at an electron temperature of log T e ( K ) = 7.3 , corresponding to the maximum abundance of Ni XXV. Spectral line intensities are calculated, and their diagnostic relevance is discussed. This dataset will be made available in the next version of the CHIANTI database.

Resonances in low-energy electron elastic cross sections for lanthanide atoms

Total and Mulholland partial cross sections for the elastic scattering of electrons from the lanthanide atoms lanthanum to lutetium are calculated for the electron impact energy range $0\ensuremath{\leqslant}E\ensuremath{\leqslant}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The recently developed Regge-pole methodology, which naturally embodies the crucial electron correlation effects together with a Thomas-Fermi-type potential incorporating the vital core-polarization interaction are used for the calculations. Dramatically sharp resonances are found to characterize the near-threshold electron elastic scattering total and Mulholland partial cross sections, whose energy positions are identified with the electron affinities (EA's) of these atoms through a close scrutiny of the imaginary part of the complex angular momentum. The unambiguous extracted EA values of the lanthanide atoms vary from a low value of $0.016\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the Tm atom to a high value of $0.631\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the Pr atom; none is predicted to have a lower EA value than the former. All the negative ions of the lanthanide atoms can be classified through their binding energies (BE's) as weakly bound negative ions (BE's $<1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$), while only three qualify to be classified as tenuously bound (BE'$<0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$). Ramsauer-Townsend minima, shape resonances, and the Wigner threshold behavior for these lanthanides are also determined. Comparisons of the present calculated EA's with those from various experimental measurements and other theoretical calculations are presented and discussed. In particular, our extracted EA value for the complicated open $d$- and $f$-subshell Ce atom agrees excellently with the most recently measured [Walter et al., Phys. Rev. A 76, 052702 (2007)] and calculated values, while for Nd and Eu the agreement with the latest calculated values of O'Malley and Beck [Phys. Rev. A 77, 012505 (2008); Phys. Rev. A 78, 012510 (2008)] is outstanding. These agreements give great credence to the already demonstrated predictive power of the Regge-pole methodology to extract unambiguous and reliable binding energies for tenuously bound and complicated open-shell negative ionic systems, requiring no a priori knowledge of the EA values whatsoever. This new perspective to the EA determination of atoms from low-energy electron elastic scattering resonances promises far-reaching implications for future accurate and reliable theoretical EA values, even for small molecules and clusters.

Electron attachment to trans-azobenzene

The temporary anion states of gas-phase trans-azobenzene are characterised by means of electron transmission spectroscopy (ETS) in the 0-6 eV range. The measured energies of vertical electron attachment are compared with the energies of the pi* virtual orbitals of the neutral molecule supplied by HF (at MP2 optimized geometries) and B3LYP calculations. The calculated energies, scaled with empirical equations, reproduce quantitatively the energies of the corresponding spectral features and predict a positive vertical electron affinity of 0.83 eV. The total anion current at the walls of the collision chamber and the mass-selected molecular anion current are also reported as a function of the impact electron energy. In agreement with previous data, long-lived (>1 mus) parent molecular anions are detected at zero eV and near 1 eV. The close similarity of the electron transmission spectrum with the derivatives with respect to energy of the anion currents suggests strongly that shape resonances produced by electron capture into empty pi* orbitals are the initial step in formation of the long lived molecular anions. This appears to rule out mechanisms in which direct formation of core-excited anion states are invoked. However, according to DFT calculations, conversion of the shape resonances around 1 eV to longer-lived sigma-pi* core-excited doublet anion states is possible on energetic grounds.

Atomic data and spectral line intensities for Ca XVII

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Ca XVII. The configurations used are 2 s 2 , 2 s 2 p , 2 p 2 , 2 l 3 l ′ , 2 l 4 l ′ and 2 s 5 l ′ , with l = s,p , and l ′ = s,p,d giving rise to 92 fine-structure levels in intermediate coupling. Collision strengths are calculated at seven incident energies (15, 30, 75, 112.5, 150, 187.5 and 225 Ry) for the transitions within the three lowest configurations corresponding to the 10 lowest energy levels, and at five incident energies (75, 112.5, 150, 187.5 and 225 Ry) for transitions between the lowest five levels and the n = 3 , 4 , 5 configurations. Calculations have been carried out using the distorted wave approximation. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates of the present work, and R-Matrix results for the 2 s 2 , 2 s 2 p , 2 p 2 configurations available in the literature, statistical equilibrium equations for level populations are solved at electron densities covering the range of 10 8 – 10 14 cm - 3 at an electron temperature of log T e ( K ) = 6.7 , corresponding to the maximum abundance of Ca XVII. Spectral line intensities are calculated, and their diagnostic relevance is discussed. This dataset will be made available in the next version of the CHIANTI database.

Elastic electron scattering by silver atom

The experimental and theoretical studies of elastic electron scattering by silver atom have been carried out. The experimental investigation was based on crossed beam technique with effusive atomic beam being perpendicularly crossed by electron beam. The measurements were performed at electron-impact energies (E0) of 10, 20, 40, 60, 80 and 100eV and for a range of scattering angles (θ) from 10° up to 150°. The absolute differential cross sections (DCSs) have been obtained from the elastic-to-inelastic (the unresolved silver resonant lines 4d105p2P1/2, 3/2) intensity ratio at θ=10° at each E0. Calculations have been performed using the parameter-free complex optical potential (OP) with the inclusion of spin-orbit interaction for the same E0. Comparison between present experiment and theory has been made.

Resonant Auger decay of 2p hole in argon induced by electron impact

We present an experimental study of L-MM resonant Auger spectra of argon after electron impact excitation. The electron spectra were measured at ten different electron impact energies between 442.6eV and 461.7eV. During (e,2e) measurement the energy of the second electron was kept fixed at 209.6eV, corresponding to the energy of one of the strongest resonant Auger transitions from the [2p3/23d] state. Except for the monopole excitations, the recorded spectral structures are explained on the basis of photon impact data.

The influence of desorption gas to high power microwave window multipactor

Gas desorbed by electrons plays a key role in multipactor saturation and final plasma breakdown at the vacuum side of the window in high power microwave systems. A multipactor model involving electron-neutral collision and ionization is established. When desorption gas pressure reaches 1Torr, the electron impact energy apparently decreases, multipactor saturates at a lower surface charging field, and multipactor saturation meets easier, compared to vacuum. Experiments for different material windows of high power microwave, with the power of 1GW, frequency of 9.4GHz, and 20ns single and multiple pulses was conducted. It was discovered that gas pressure rose prominently in the horn, when breakdown occurred. The increase in local gas pressure during the 20ns pulse is estimated from the experiment to reach several Torr, a pressure that is consistent with the presented dynamic model.

Benchmark Dyson Orbital Study of the Ionization Spectrum and Electron Momentum Distributions of Ethanol in Conformational Equilibrium

An extensive study, throughout the valence region, of the electronic structure, ionization spectrum, and electron momentum distributions of ethanol is presented, on the ground of a model that focuses on a mixture of the gauche and anti conformers in their energy minimum form, using weight coefficients obtained from thermostatistical calculations that account for the influence of hindered rotations. The analysis is based on accurate calculations of valence one-electron and shakeup ionization energies and of the related Dyson orbitals, using one-particle Green's Function (1p-GF) theory in conjunction with the so-called third-order Algebraic Diagrammatic Construction scheme [ADC(3)]. The confrontation against available UPS (HeI) measurements indicates the presence in the spectral bands of significant conformational fingerprints at outer-valence ionization energies ranging from approximately 14 to approximately 18 eV. The shakeup onset is located at approximately 24 eV, and a shoulder at approximately 14.5 eV in the He I spectrum can be specifically ascribed to the minor anti (C(s)) conformer fraction. Thermally and spherically averaged Dyson orbital momentum distributions are computed for seven resolvable bands in model (e, 2e) ionization spectra at an electron impact energy of 1.2 keV. A comparison is made with results obtained from standard (B3LYP) Kohn-Sham orbitals and EMS measurements employing a high-resolution spectrometer of the third generation. The analysis is qualitatively in line with experiment and reveals a tremendously strong influence of the molecular conformation on the outermost electron momentum distributions. Quantitatively significant discrepancies with experiment can nonetheless be tentatively ascribed to strong dynamical disorder in the gas phase molecular structure.

Investigation of the molecular conformations of ethanol using electron momentum spectroscopy

The valence electronic structure and momentum-space electron density distributions of ethanol have been investigated with our newly constructed high-resolution electron momentum spectrometer. The measurements are compared to thermally averaged simulations based on Kohn–Sham (B3LYP) orbital densities as well as one-particle Green's function calculations of ionization spectra and Dyson orbital densities, assuming Boltzmann's statistical distribution of the molecular structure over the two energy minima defining the anti and gauche conformers. One-electron ionization energies and momentum distributions in the outer-valence region were found to be highly dependent upon the molecular conformation. Calculated momentum distributions indeed very sensitively reflect the distortions and topological changes that molecular orbitals undergo due to the internal rotation of the hydroxyl group, and thereby exhibit variations which can be traced experimentally. The B3LYP model Kohn–Sham orbital densities are overall in good agreement with the experimental distributions, and closely resemble benchmark ADC(3) Dyson orbital densities. Both approaches fail to quantitatively reproduce the experimental momentum distributions characterizing the highest occupied molecular orbital. Since electron momentum spectroscopy measurements at various electron impact energies indicate that the plane wave impulse approximation is valid, this discrepancy between theory and experiment is tentatively ascribed to thermal disorder, i.e. large-amplitude and thermally induced dynamical distortions of the molecular structure in the gas phase.

Electron-impact excitation of theB Σ1u+andC Π1uelectronic states ofH2

Differential and integral cross sections for electron-impact excitation of the dipole-allowed $B\text{ }{^{1}\ensuremath{\Sigma}}_{\mathrm{u}}^{+}$ and $C\text{ }{^{1}\ensuremath{\Pi}}_{\mathrm{u}}$ electronic states of molecular hydrogen have been measured. The differential cross sections were determined by analysis of normalized energy-loss spectra obtained using a crossed-beam apparatus at the electron-impact energies of 40, 100, and 200 eV. Integral cross sections were subsequently derived from these data. The present work was undertaken in order to investigate some ambiguities between earlier experimental data and recent BEf-scaled cross sections as defined and calculated by Kim [J. Chem. Phys. 126, 064305 (2007)] and also to extend the energy range of the available data. Optical oscillator strengths, also determined as a part of the present investigation, were found to be in fair accordance with previous measurements and some calculations.

Electron impact calculations of total and ionization cross-sections for Germanium Hydrides (GeH X; X = 1–4) and Digermane, Ge 2H 6

In this article we report calculations of the total elastic Q el, total ionization, Q ion, and total (complete), Q T, cross-sections for the germanium hydrides GeH X ( X = 1–4) and digermane (Ge 2H 6) upon electron impact for energies from circa threshold to 2000 eV. Spherical Complex Optical Potential (SCOP) formalism is employed to evaluate Q el, Q inel and Q T. Total ionization cross-sections, Q ion, are derived from the total inelastic cross-sections, Q inel, using a semi-empirical formulation developed by us called ‘Complex Spherical Potential-ionization contribution’ (CSP-ic) method. A comparison of the various cross-sections is provided to show their relative contribution to the total cross-section Q T. Present results are compared with available experimental and theoretical data wherever possible and overall agreement is good. However, no experimental data on the Q ion are available for the germanium hydrides, GeH X , X = 1–3 or digermane.

Electron collisions with ethylene oxide molecules

The absolute total cross section (TCS) for electron–ethylene oxide (c-C2H4O) collisions was measured at electron-impact energies extending from 0.7 to 400 eV using the linear electron-transmission technique. In the present TCS energy function a distinct peak is visible near 4.6 eV which may be attributed to formation of a short-lived negative ion. Moreover, at intermediate and high energies the integral elastic (ECS) and ionization (ICS) cross sections were determined numerically. The calculated sum, ECS + ICS, is in good agreement with the experimental TCS in the overlapping energy range. Finally, the present experimental TCS for electron scattering from c-C2H4O is compared with that for its cyclic isoelectronic counterpart c-C3H6, and similarities and differences are pointed out and discussed.

Atomic data and spectral line intensities for Ar XV

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Ar XV. The configurations used are 2 s 2 , 2 s 2 p , 2 p 2 , 2 l 3 l ′ , 2 l 4 l ′ and 2 s 5 l ′ , with l = s,p and l ′ = s,p,d giving rise to 92 fine-structure levels in intermediate coupling. Collision strengths are calculated at eight incident energies (10, 20, 30, 60, 120, 180, 240, and 300 Ry) for transitions within the three lowest configurations, and five incident energies (60, 120, 180, 240, and 300 Ry) for transitions between the lowest five levels and the n = 3, 4, 5 configurations, using the distorted wave approximation. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates of the present work, and R-matrix results for the 2s 2, 2s2p, 2p 2 configurations available in the literature, statistical equilibrium equations for level populations are solved at electron densities covering the range of 10 8–10 14 cm −3 at an electron temperature of log T e ( K ) = 6.6 , corresponding to the maximum abundance of Ar XV. Spectral line intensities are calculated, and their diagnostic relevance is discussed. Observed line ratios indicate electron temperatures of the relevant emitting plasma close to log T e ( K ) = 6.6 . This dataset will be made available in the next version of the CHIANTI database.

Study of the photoelectron and electron momentum spectra of cyclopentene using benchmark Dyson orbital theories

A complete study of the valence electronic structure and related electronic excitation properties of cyclopentene in its C(s) ground state geometry is presented. Ionization spectra obtained from this compound by means of photoelectron spectroscopy (He I and He II) and electron momentum spectroscopy have been analyzed in details up to electron binding energies of 30 eV using one-particle Green's function (1p-GF) theory along with the outer-valence (OVGF) and the third-order algebraic diagrammatic construction [ADC(3)] schemes. The employed geometries derive from DFT/B3LYP calculations in conjunction with the aug-cc-pVTZ basis set, and closely approach the structures inferred from experiments employing microwave spectroscopy or electron diffraction in the gas phase. The 1p-GF/ADC(3) calculations indicate that the orbital picture of ionization breaks down at electron binding energies larger than approximately 17 eV in the inner-valence region, and that the outer-valence 7a' orbital is also subject to a significant dispersion of the ionization intensity over shake-up states. This study confirms further the rule that OVGF pole strengths smaller than 0.85 foretell a breakdown of the orbital picture of ionization at the ADC(3) level. Spherically averaged (e, 2e) electron momentum distributions at an electron impact energy of 1200 eV that were experimentally inferred from an angular analysis of EMS intensities have been interpreted by comparison with accurate simulations employing ADC(3) Dyson orbitals. Very significant discrepancies were observed with momentum distributions obtained from several outer-valence ionization bands using standard Kohn-Sham orbitals.