Energy Electron Impact Research Articles

Elastic scattering of slow electrons from Y, Ru, Pd, Ag and Pt atoms: search for nanocatalysts

Electron elastic scattering total cross sections (TCSs) for Y, Ru, Pd, Ag and Pt atoms are investigated in the electron impact energy range 0 ⩽ E ⩽ 7.0 eV. The complex angular momentum method that incorporates the electron–electron correlations and core-polarization interactions, crucial for the existence and stability of most negative ions, is used for the calculations. The TCSs for these atoms are found to be characterized by shape resonances (short-lived resonances), Ramsauer–Townsend minima and very sharp resonances (long-lived resonances) that correspond to the existence of stable bound states of the negative ions formed during the collision as Regge resonances. Using the configuration of the negative-ion resonances and Ramsauer–Townsend minima in the Au TCS as the benchmark for nanocatalysts, we conclude that the Y, Ru, Pd, Ag and Pt atoms represent excellent candidates for nanocatalysts individually or in various combinations. Calculated electron elastic TCSs for Y, Ru, Pd, Ag and Pt atoms are presented as illustrations.

Dissociative excitation and fragmentation of S8 by electron impact

The vacuum-ultraviolet emission spectrum from 136 nm to 168 nm following the dissociative excitation of a predominantly S(8) target by electron impact at 100 eV incident energy was measured. The relative cross sections for the dominant multiplets at 138.9, 142.9, 147.9, and 166.7 nm are presented. Excitation functions are shown for electron-impact energies from below threshold to 360 eV for the two most prominent emissions at 142.5 nm and 147.4 nm. Five thresholds are clearly apparent in both excitation functions. For the four highest energy channels, the energy separation between the adjacent thresholds is approximately constant and the cross sections reduce regularly as the threshold energies increase. We suggest possible fragmentation pathways of the dissociating S(8) molecule that reproduce the energies of our observed thresholds.

Atomic data and spectral line intensities for Fe XV

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for the n = 4 and 5 complexes in Fe XV. We include in the calculations all the configurations in the n = 3 , 4, and 5 complexes, corresponding to 283 fine-structure levels. We also provide accurate A values for the n = 3 complex. Collision strengths are calculated at five incident energies for all transitions: 12.1, 28.2, 50.4, 80.9 and 123.0 Ry above the threshold of each transition. An additional energy has been added, whose value is between 0.00075 and 0.40 Ry, depending on the transition. Calculations have been carried out using the Flexible Atomic Code and the distorted-wave approximation. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. We combined the excitation rate coefficients and the radiative transition rates calculated in the present work with R -Matrix results available in the literature for the n = 3 complex, to solve the statistical equilibrium equations for level populations at electron densities in the 10 8–10 14 cm −3 range and at an electron temperature of log T e ( K ) = 6.3 , corresponding to the maximum abundance of Fe XV. Spectral line intensities are calculated and compared with available observations.

Low-energy electron elastic scattering from Mn, Cu, Zn, Ni, Ag, and Cd atoms

Electron elastic total cross sections (TCSs) for ground and excited Mn, Cu, Zn, Ni, Ag, and Cd atoms have been investigated in the electron-impact energy range 0 \ensuremath{\leqslant} $E$ \ensuremath{\leqslant} 1 eV. The near-threshold TCSs for both the ground and excited states of these atoms are found to be characterized by Ramsauer-Townsend minima, shape resonances, and extremely sharp resonances corresponding to the formation of stable bound negative ions. The recently developed Regge-pole methodology where the crucial electron-electron correlations are embedded is employed for the calculations. From close scrutiny of the imaginary parts of the complex angular momenta, we conclude that these atoms form stable weakly bound ground and excited negative ions as Regge resonances through slow electron collisions. The extracted electron binding energies from the elastic TCSs of these atoms are contrasted with the available experimental and theoretical values.

Differential Positron and Electron Ionization Studies: How a Plus or Minus Sign Makes a Difference

Doubly and triply differential data for single ionization of argon, neon and molecular nitrogen have been measured for positron and electron impact energies of 200, 250, and 500 eV. Ratios of triply to doubly differential ionization yields as functions of the projectile scattering angle and energy loss are compared in order to investigate differences associated with the sign of the projectile charge. For all three systems, systematic differences are observed in the relative contributions of binary interactions. These differences are shown to increase with increasing scattering angle. Nearly identical recoil intensities are found for positron and electron impact ionization of the atomic targets but not for the molecular target.

Atomic data and spectral line intensities for Ni XVII

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Ni XVII. We include in the calculations the 23 lowest configurations, corresponding to 159 fine-structure levels: 3 l3 l′, 3 l4 l″, and 3s5 l‴, with l, l′ = s, p, d, l″ = s, p, d, f, and l‴ = s, p, d. Collision strengths are calculated at five incident energies for all transitions at varying energies above the threshold of each transition. One additional energy, very close to the threshold of each transition, has also been included. Calculations have been carried out using the Flexible Atomic Code in the distorted wave approximation. Additional calculations have been performed with the University College London suite of codes for comparison. 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) = 6.5, corresponding to the maximum abundance of Ni 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.

Fe XVII X-RAY LINE RATIOS FOR ACCURATE ASTROPHYSICAL PLASMA DIAGNOSTICS

New laboratory measurements using an Electron Beam Ion Trap (EBIT) and an x-ray microcalorimeter are presented for the n=3 to n=2 Fe XVII emission lines in the 15 {\AA} to 17 {\AA} range, along with new theoretical predictions for a variety of electron energy distributions. This work improves upon our earlier work on these lines by providing measurements at more electron impact energies (seven values from 846 to 1185 eV), performing an in situ determination of the x-ray window transmission, taking steps to minimize the ion impurity concentrations, correcting the electron energies for space charge shifts, and estimating the residual electron energy uncertainties. The results for the 3C/3D and 3s/3C line ratios are generally in agreement with the closest theory to within 10%, and in agreement with previous measurements from an independent group to within 20%. Better consistency between the two experimental groups is obtained at the lowest electron energies by using theory to interpolate, taking into account the significantly different electron energy distributions. Evidence for resonance collision effects in the spectra is discussed. Renormalized values for the absolute cross sections of the 3C and 3D lines are obtained by combining previously published results, and shown to be in agreement with the predictions of converged R-matrix theory. This work establishes consistency between results from independent laboratories and improves the reliability of these lines for astrophysical diagnostics. Factors that should be taken into account for accurate diagnostics are discussed, including electron energy distribution, polarization, absorption/scattering, and line blends.

VUV fluorescence following electron-impact dissociative excitation of CS2

Electron-impact dissociation of CS${}_{2}$ has been studied by observation of the atomic spectral emission features in the range 115--170 nm. Absolute photoemission cross sections are presented over the complete wavelength range for an incident electron energy of 100 eV. As an example, the measured cross section of the strong C i emission at 165.7 nm, which is a prominent feature in many solar and other extraterrestrial spectra, is ($1.45\ifmmode\pm\else\textpm\fi{}0.19)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$ cm${}^{2}$. Comparison with earlier cross-sectional measurements suggest that these were too high by a factor of more than three. Excitation functions of the dominant C i (156.1 nm) and S i (147.4 nm) emission lines have been measured for electron-impact energies from threshold to 360 eV. From appearance energy measurements in the near-threshold region, likely fragmentation channels are identified which involve both two-fragment breakup and total fragmentation of the parent CS${}_{2}$.

Atomic data and spectral line intensities for Ni XI

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Ni XI. We include in the calculations the 12 lowest configurations, corresponding to 180 fine-structure levels: 3s 23p 6, s 23p 53d, 3s 23p 43d 2, 3s3p 63d, 3s 23p 54 l, and 3s3p 64 l with l = s, p, d, f. Collision strengths are calculated at five incident energies for all transitions: 7.45, 17.6, 31.4, 50.1, and 75.2 Ry above the threshold of each transition. An additional energy, very close to the transition threshold, has been added, whose value is between 0.0007 Ry and 0.25 Ry depending on the levels involved. Calculations have been carried out using the Flexible Atomic Code. The scattering problem is solved in 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, combined with close coupling collision excitation rate coefficients available in the literature for the lowest 17 levels, 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 = 6.1 , corresponding to the maximum abundance of Ni XI. 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.

Resonant Auger decay of Ar2p3/2−14sand2p3/2−14pstates excited by electron impact

Auger spectra of resonantly excited $2{p}_{3/2}^{\ensuremath{-}1}4s$ and $2{p}_{3/2}^{\ensuremath{-}1}4p$ states in argon were measured by $(e,2e)$ technique. The 99.2-eV scattered electrons were detected in coincidence with ${L}_{3}--{M}_{23}{M}_{23}$ Auger electrons, and the experiment was performed at 343.6- and 344.9-eV electron impact to tune the energy loss to the energy of the dipole-allowed and the dipole-forbidden excitations, respectively. The resonant Auger spectra are obtained upon subtraction of the overlapping signal due to the outer-shell ionization, which was recorded at 340-eV electron-impact energy. The most intense groups of Auger transitions from $2{p}_{3/2}^{\ensuremath{-}1}4s (J=1,2)$ and $2{p}_{3/2}^{\ensuremath{-}1}4p (J=0,1,2,3)$ states are identified by comparison with the results of the two-step model, based on distorted-wave Born approximation with exchange and multiconfiguration descriptions of the relaxed states. The $4 s$ spectrum displays a substantially larger shake-up contribution than the one observed in photoexcitation experiments, which may be explained by the interference of the resonant decay path with the direct ionization excitation of the Ar $3p$ subshell. The majority of the observed $4p$ signal is assigned to the monopole and quadrupole excitations of the ground state.

Excitation dynamics and post-collision interaction for lowest autoionizing states in Li and Li2 excited by electron impact

The ejected-electron spectra in the region of the lowest autoionizing states in Li and have been studied at an observation angle of 54.7° over the electron-impact energy range 0–6 eV above the excitation threshold at 56.4 eV. The strong near-threshold resonance excitation has been revealed for the and (1s → 3σg)1Σ+g molecular transitions. For the atomic , and molecular states the post-collision interaction (PCI) energy shift of corresponding lines has been measured and compared with electron-impact excitation functions for these states. The general appearance of the PCI data for the and states reveals a close agreement with the classical and semi-classical predictions, showing a minimal correlation between resonance excitation of the subshell and the PCI. Meanwhile, in the energy region of 0–1 eV the energy shift of the line reveals an abrupt decrease to nearly zero values, similar to that observed earlier for the (np5(n+1)s2)2P3/2 lines in the ejected-electron spectra of Na (n = 2) and K (n = 3) atoms. The calculations show the possibility for extrema of the PCI shift to appear in those energy regions where the negative-ion resonances are present in the excitation function of the state.

Excitation of the 3p 55p levels of argon from the 3p 54s metastables

Measurements have been reported recently of cross sections for electron-impact excitation of the argon 3p 55p manifold of levels from the 3p 54s metastable levels. We report results of calculations of the direct excitation cross sections at electron-impact energies from threshold to 500 eV for these transitions. The collision cross sections are calculated in the scaled plane-wave Born approximation (scaled PWB) developed by Kim. This scaling transforms the Born cross sections for dipole-allowed and spin-allowed excitations into reliable cross sections that compare as well with accurate measurements as does the sophisticated and more complex convergent close coupling method. We have used jj coupling in a single LS configuration Dirac-Fock calculation to describe the target wavefunctions. The optical emission cross section measurements, by Jung et al., include estimated contributions of 25% or more from cascading from higher excited levels and so are larger than direct excitation cross sections. Nevertheless, our cross sections agree reasonably well with the measurements for transitions where the core j value remains unchanged, except for the 1s 5–3p 8 excitation. The results do not agree well with the measurements when the core j value changes, as similarly seen in the results of recent relativistic distorted wave calculations by Sharma et al.

Calculations of total and ionization cross-sections on electron impact for alkali metals (Li, Na, K) from threshold to 2 keV

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 targets Li, Na, K having low ionization thresholds and high polarizability upon electron impact for energies from circa threshold to 2000 eV. We have employed the well-known spherical complex optical potential (SCOP) formalism, which provides total elastic cross-section ( Q el ) and its inelastic counterpart ( Q inel ). The sum of Q el and Q inel gives the total (complete) cross-section, Q T , which is found to be very high at the lower energies (around 5 eV). This is attributed to high polarizabilities and low ionization thresholds of these atoms. Q inel includes Q ion and we have developed a semi-empirical method, called complex scattering potential-ionization contribution (CSP-ic) to extract ionization cross-sections from calculated total inelastic cross-section. Present calculations also provide information on the total excitation processes of these targets. The calculated cross-sections are examined as functions of incident electron energy along with available comparison and overall good agreement is observed.

Complex angular momentum analysis of low-energy electron elastic scattering from lanthanide atoms

Electron attachment to the lanthanide and Hf atoms resulting in the formation of stable excited lanthanide and Hf anions as Regge resonances is explored in the near-threshold electron impact energy region, $E<1.0$ eV. The investigation uses the recent Regge-pole methodology wherein is embedded the electron-electron correlations together with a Thomas-Fermi--type model potential incorporating the crucial core-polarization interaction. The near-threshold electron elastic total cross sections (TCSs) for the lanthanide and Hf atoms are found to be characterized by extremely narrow resonances whose energy positions are identified with the binding energies (BEs) of the resultant anions formed during the collision as Regge resonances. The extracted BEs for excited lanthanide anions are contrasted with those of the most recently calculated electron affinities (ground state BEs). We conclude that the BEs for the ${\mathrm{Pr}}^{\ensuremath{-}}$, ${\mathrm{Tb}}^{\ensuremath{-}}$, ${\mathrm{Dy}}^{\ensuremath{-}}$, ${\mathrm{Ho}}^{\ensuremath{-}}$, ${\mathrm{Er}}^{\ensuremath{-}}$, and ${\mathrm{Tm}}^{\ensuremath{-}}$ anions of O'Malley and Beck [Phys. Rev. A 79, 012511 (2009)] are not identifiable with the electron affinities as claimed. Formation of bound excited anions is identified in the elastic TCSs of all the lanthanide atoms including Hf, except Eu and Gd. The imaginary part of the complex angular momentum $L$, $\mathrm{Im}L$ is used to distinguish between the shape resonances and the bound excited negative ions. These results challenge both experimentalists and theoreticians alike since the excited anions are very weakly bound, but mostly tenuously bound ($\mathrm{BEs}<0.1$ eV). Shape resonances and Ramsauer-Townsend minima are also presented.

Scattering of electrons from acetaldehyde and acetone

Absolute total cross sections (TCSs) for electron scattering from acetaldehyde [(CH3)CHO] and acetone [(CH3)2CO] molecules have been measured at electron-impact energy extending from 0.7 to 400 eV, using a linear electron-transmission technique. The shape of obtained TCS energy dependences appears typical for targets of high electric dipole moment; both TCSs decrease with the energy increase; only between 4 and 12 eV is a weak enhancement in TCSs is clearly perceptible. The experimental TCSs for (CH3)CHO and for (CH3)2CO are also compared with those for their cyclic isomeric counterparts: c-(CH2)2O and c-(CH2)3O, respectively.

Influence of the Injection Speeds on the Discharge Characteristics and Minimization of Delay Time in a Pseudospark Discharge

The study put forth demonstrates that the seed electrons average kinetic energy influences the discharge characteristics making it possible to maximize the rate of development of the virtual anode in a pseudospark, with a suitable choice of the neutral gas pressure as determined by the seed's average injection speed. This investigation also brings to light two distinct operating regimes; (1) mid-energy, where electron-impact ionization energy losses result in a decrease in the cross-section as the electrons travel downstream and (2) high-energy, where, in contrast, the ionization cross-section increases. In the latter case, both the fastest delay time and the neutral gas pressure producing this value have linear dependencies on the seed electrons energy resulting in a constant value of their product over the different injection speeds. The discharge is seeded by injecting a current pulse for a period of one nanosecond along the axis from the hollow cathode cavity back wall over a range of mean speeds corresponding to 100 to 900 V accelerations; the initial electric field is insufficient to enhance ionization throughout most of the hollow cathode backspace. Data is obtained through computer simulation using the two-dimensional kinetic plasma code OOPIC Pro. 51.50.+v, 52.75.Kq, 52.80.Tn.

Low-energy electron elastic scattering cross sections for excited Au and Pt atoms

Abstract Electron elastic total cross sections (TCSs) and differential cross sections (DCSs) in both impact energy and scattering angle for the excited Au and Pt atoms are calculated in the electron impact energy range 0 ⩽ E ⩽ 4.0 eV. The cross sections are found to be characterized by very sharp long-lived resonances whose positions are identified with the binding energies of the excited anions formed during the collisions. The recent novel Regge-pole methodology wherein is embedded through the Mulholland formula the electron–electron correlations is used together with a Thomas–Fermi type potential incorporating the crucial core-polarization interaction for the calculations of the TCSs. The DCSs are evaluated using a partial wave expansion. The Ramsauer–Townsend minima, the shape resonances and the binding energies of the excited Au− and Pt− anions are extracted from the cross sections, while the critical minima are determined from the DCSs.

Electron impact calculations of total ionization cross sections for environmentally sensitive diatomic and triatomic molecules from threshold to 5 keV

In this paper we report calculations of the total ionization cross sections, Qion, for environmentally sensitive molecules, namely CO, CO2, CS, OCS, CS2 and S2, upon electron impact for energies from circa threshold to 5000 eV. Spherical complex optical potential (SCOP) formalism is employed to evaluate total inelastic cross sections, Qinel. Total ionization cross sections, Qion, are extracted from the total inelastic cross sections, using a semi-empirical formalism developed by us, called the ‘complex spherical potential-ionization contribution’ (CSP-ic) method. The present results are compared with both theoretical and experimental data available in the literature and overall good agreement is observed for all the reported molecules.

Integral cross sections for electron-impact excitation of the C 3Πu, E 3Σ+g and a″ 1Σ+g states of N2

Absolute integral cross sections (ICSs) for electron-impact excitation of the C 3Πu, E 3Σ+g and a″ 1Σ+g states of N2 from the X 1Σ+g (v″ = 0) ground-state level are presented for electron-impact energies of 13, 15, 17.5, 20, 25, 30, 50 and 100 eV. ICSs are also presented for the individual vibrational levels of the C 3Πu state (v′ = 0, 1, 2, 3 and 4). The ICSs were derived from recent differential cross sections determined via energy-loss spectroscopy (Malone et al 2009 Phys. Rev. A 79 032704; Malone et al 2009 Phys. Rev. A 79 032705). The results are compared with existing measurements in the literature.

Energy shift of the (2p53s2)2P3/2 line in ejected-electron spectra of sodium atoms excited by low-energy electron impact

The energy shift of the line resulting from the decay of the (2p53s2)2P3/2 lowest autoionizing state in Na atoms has been measured in the ejected-electron spectra obtained at near-threshold electron impact energies. Contrary to the smooth asymptotic behavior predicted by the known models of the PCI effect, the data reveal pronounced oscillatory character of the energy dependence of the shift in the energy regions where the strong negative-ion resonances are present in the excitation function of the 2P3/2 state.