Electron-impact Excitation Cross Sections Research Articles

Electron-impact excitation of neon: a pseudo-state convergence study

A number of convergent close-coupling and R-matrix with pseudo-state (RMPS) calculations for H-like, He-like, Li-like and Be-like ions have demonstrated that coupling to the target continuum can have large effects on the electron-impact excitation cross sections of neutral and low-charge species. However, no one has yet attempted such advanced calculations on a system as complex as neutral neon. We report on a series of RMPS calculations of electron-impact excitation of Ne using recently developed parallel Breit–Pauli R-matrix programs. Our largest calculation included 235 spectroscopic and pseudo-state levels in the close-coupling expansion of the target. Although the results clearly reveal the importance of coupling to the target continuum in this atom, the pseudo-state expansion is not yet sufficiently complete to provide reliable cross sections for energies above the ionization limit. However, this is the largest intermediate-coupling calculation that can be performed with present computer resources. Thus, we have also carried out a series of RMPS calculations in LS coupling with different pseudo-state expansions. Comparisons of these results have allowed us to determine the approximate size of the pseudo-state expansion required to achieve convergence in future intermediate-coupling calculations for neon.

Integral cross section for electron-impact excitation of the 3p53d states of argon

Theoretical close-coupling R-matrix and two different first-order distorted-wave Born approximation (DWBA) calculations are presented for excitation of argon from its ground state to the twelve states in the (3p53d) manifold. The results are found to be very sensitive to the choice of atomic wavefunctions. The low-energy R-matrix results in general and, in several cases, the unitarized DWBA as well as the high-energy DWBA results are in fair agreement with the experimental data of Chilton and Lin (1999 Phys. Rev. A 60 3712).

Low-energy electron scattering from atomic hydrogen. II. Elastic and inelastic scattering

We present measurements of differential cross sections for elastic electron scattering from atomic hydrogen at 20 eV and 40 eV incident electron energies and ratios of differential cross sections for electron-impact excitation of atomic hydrogen to the $n=2,$ 3, and 4 levels at incident electron energies of 14.6 eV, 15.6 eV, 17.6 eV, 20 eV, 25 eV, and 40 eV with scattering angles ranging from $10\ifmmode^\circ\else\textdegree\fi{}$ to $130\ifmmode^\circ\else\textdegree\fi{}.$ We compare our results to available experimental measurements and recent convergent close-coupling calculations. Our results resolve significant discrepancies that existed between theory and past experiments.

Angular momentum coupling and electron-impact excitation cross sections of rare-gas atoms

Electron-impact excitation cross sections for the heavy rare gases are examined in the context of the angular momentum coupling of the final states. Measurements for excitation into n′p5np levels with J=0 are found to vary in a common way for excitation of Ne, Ar, Kr, and Xe.

Measurements of Differential and Doubly-Differential Cross-Sections for Electron Impact Elastic Scattering, Excitation, and Ionization of Atomic Hydrogen

Accurate, normalized doubly-differential cross-sections, differential cross-section ratios and relative differential cross-sections for electron impact ionization, excitation to the H(n = 3,4) levels and elastic scattering of atomic hydrogen have been measured at incident electron energies ranging from 14.6 to 40 eV for scattering angles of 10° to 130°. The measurements are normalized to the accurate differential cross-section for electron impact excitation of H(n = 2) from the convergent close-coupling theory, which has recently been shown to be accurate within 10%. Our measurements show very good agreement in comparison with the most recent theoretical models viz. the Convergent Close-Coupling (CCC) model and the Exterior Complex Scaling (ECS) model.

Ion Collisions in the Highly Charged Universe

The phenomenal output of high-resolution infrared, visible, EUV, and x-ray spectra from NASA and ESA satellites, and the marked improvement in ground-based observations, has led to a parallel need for accurate atomic data. This need is in large part being addressed, thanks to the equally remarkable development of laboratory facilities. The astrophysics observations include measurements at the Sun, stars, the interstellar medium, and the recent surprising discovery of x-ray emissions from comets. The required database includes cross sections for electron-impact excitation of highly charged ions (HCIs), direct and indirect electron-ion recombination, photoionization, single and multiple charge exchange of HCIs with neutral gases, and x-ray emission. In addition, the observed photon emissions result from a balance between collisional excitation and radiative decay. Hence accurate lifetimes, branching fractions, and Einstein A and B coefficients are required. Some recent spacecraft observations will be reviewed, the underlying atomic physics summarized, and the need for relevant atomic data in HCIs discussed.

Electron-impact excitation of beryllium and its ions

Inelastic electron scattering from light atomic species is of fundamental importance and has significant applications in fusion-plasma modeling. Therefore, it is of interest to apply advanced nonperturbative, close-coupling methods to the determination of electron-impact excitation for these atoms. Here we present the results of R matrix with pseudostate (RMPS) calculations of electron-impact excitation cross sections through the $n=4$ terms in Be, ${\mathrm{Be}}^{+},$ ${\mathrm{Be}}^{2+},$ and ${\mathrm{Be}}^{3+}.$ In order to determine the effects of coupling of the bound states to the target continuum in these species, we compare the RMPS results with those from standard R-matrix calculations. In addition, we have performed time-dependent close-coupling calculations for excitation from the ground and the metastable terms of ${\mathrm{Be}}^{+}$ and the metastable term of ${\mathrm{Be}}^{3+}.$ In general, these results are found to agree with those from our RMPS calculations. The full set of data resulting from this work is now available on the Oak Ridge National Laboratory Controlled Fusion Atomic Data Center web site, and will be employed for collisional-radiative modeling of Be in magnetically confined plasmas.

Electron-impact ionization of atomic hydrogen at incident electron energies of 15.6, 17.6, 25, and 40 eV

Absolute doubly differential cross sections for the electron-impact ionization of atomic hydrogen have been measured from near threshold to intermediate energies. The measurements are calibrated to the well-established, accurate differential cross section for electron-impact excitation of the atomic hydrogen transition H(12S22S+22P). In these experiments background secondary electrons are suppressed by moving the atomic hydrogen target source to and from the collision region. Measurements cover the incident electron energy range of 14.6–40 eV, for scattering angles of 10°–120° and are found to be in very good agreement with the results of the most advanced theoretical models—the convergent close-coupling model and the exterior complex scaling model.

Electron-impact excitation cross sections of the higher argon3p5np(n=5,6,7)levels

We have measured the electron-impact excitation cross sections for argon into the ten levels of ${3p}^{5}5p$ configuration, as well as numerous levels of the ${3p}^{5}6p$ and ${3p}^{5}7p$ configurations. Fluorescence from the decay of the excited atoms to the levels of ${3p}^{5}4s$ configuration was used to determine the optical-emission cross sections. These results were combined with transition probabilities to find apparent cross sections into the ${3p}^{5}5p$ and ${3p}^{5}6p$ levels. These new cross sections of the $\mathrm{Ar}{(3p}^{5}np)$ levels along with the available $\mathrm{Ne}{(2p}^{5}3p),$ $\mathrm{Kr}{(4p}^{5}5p),$ and $\mathrm{Xe}{(5p}^{5}6p)$ data help provide a global view of the excitation behaviors of the ${\mathrm{np}}^{5}{n}^{\ensuremath{'}}p$ levels of the rare-gas series.

Electron-impact excitation of Li to high principal quantum numbers

The time-dependent close-coupling method is used to calculate electron-impact excitation cross sections for the $\mathrm{Li}(2s)\ensuremath{\rightarrow}\mathrm{Li}(n\mathcal{l})$ and $\mathrm{Li}(2p)\ensuremath{\rightarrow}\mathrm{Li}(n\mathcal{l})$ transitions at incident energies just above the ionization threshold. The implementation of the time-dependent close-coupling method on a nonuniform lattice allows the study of continuum-coupling effects in excitations to high principal quantum number, i.e., $n<~10.$ Good agreement is found with R-matrix with pseudostates calculations, which also include continuum-coupling effects, for excitations to low principal quantum number, i.e., $n<~4.$ Poor agreement is found with standard distorted-wave calculations for excitations to all principal quantum numbers, with differences still at the 50% level for $n=10.$ We are able to give guidance as to the accuracy expected in the ${n}^{3}$ extrapolation of nonperturbative close-coupling calculations of low n cross sections and rate coefficients.

Resonances in electron-impact integral excitation cross sections of the magnesium atom

The resonance structure of the integral cross sections of excitation of the magnesium atom by low-energy electrons is analyzed in terms of the R-matrix method. The collision strengths are calculated in terms of the close-coupling approximation with consideration of 25 atomic states, including 13 physical target states and 12 pseudostates. The latter simulate the remaining bound and continuum states of the Mg atom that were not included in the close-coupling expansion explicitly. The positions and widths of the resonances found are determined, and the resonances are classified with respect to the mechanism of formation of short-lived states of the negative Mg ion. The results of calculations are compared with available experimental data.

Low-energy electron scattering by methylsilane

We report calculated elastic and inelastic cross sections for low-energy electron collisions with methylsilane, CH3SiH3, obtained using the Schwinger multichannel method. The elastic cross sections, obtained within the static-exchange approximation, are compared with elastic results for C2H6 and Si2H6. Electron-impact excitation cross sections were computed for sixteen electronic states arising from excitation out of the two highest-lying valence orbitals. The dissociation of the lowest few states was examined through limited electronic-structure calculations, which indicated that the 2 1,3A1 states dissociate to CH3SiH+H2 while the 1 1,3E states dissociate to CH3+SiH3.

Model of a He–Xe low-pressure dc positive column plasma

The positive column plasma of dc glow discharges at low pressure in a mixture of helium and 2% xenon is studied. Such glow discharges are favored candidates for the design of mercury-free light sources. A self-consistent model of the column plasma is presented based on strict radially resolved treatment of the nonlocal non-equilibrium kinetics of the electron component, the space-charge potential and the densities of ions and excited atom states. A detailed reaction kinetic scheme of the low-lying xenon excited states has been developed and is included in the model. The model is validated by a comparison with measurements of the axial electric field and the densities of the lowest metastable and resonant xenon levels. The latter was obtained by tunable diode laser absorption and probe diagnostics. The initial results of the model show a pronounced radial structure of the rare-gas column plasma and nonlocal properties of the electron power budget caused by space-charge confinement. The impact of the uncertainty of atomic data used in the model on the quantitative results is investigated in detail. A large impact of the cross sections of electron-impact excitation of the xenon ground state and the excitation and ionization of the xenon excited states, which are known with insufficient reliability only, has been observed.

Absolute cross section forSi2+(3s3p3Po→3s3p1Po)electron-impact excitation

We have measured the absolute energy-averaged cross section for electron-impact excitation of Si{sup 2+}(3s3p {sup 3}P{sup o}{yields}3s3p {sup 1}P{sup o}) from energies below threshold to the turn-on of the 3s3p {sup 3}P{sup o}{yields}3p{sup 2} {sup 3}P transition. A beams modulation technique with inclined electron and ion beams was used. Radiation at 120.65 nm from the decay of the excited ions to the 3s{sup 2} {sup 1}S ground state was detected using an absolutely calibrated optical system. The fractional population of metastable Si{sup 2+}(3s3p {sup 3}P{sup o}) in the incident ion beam was determined to be 0.256{+-}0.035(1.65{sigma}). The experimental energy spread ranged from 0.85 eV (full width at half maximum) at the lowest energies to 0.56 eV at the highest. Resonance features consistent with 12-state close-coupling R-matrix calculations are seen.

Electron collisions with ethylene

We have measured absolute elastic scattering and vibrational excitation crosssections for electron impact on ethylene. The experimental data have beenobtained on two different crossed-beam electron spectrometers and they cover theenergy range from 1 to 100 eV and scattering angles between 10° and130°.Both differential (in angle) and energy-dependent cross sections have beenmeasured. The differential cross sections have also been analysed using amolecular phase shift analysis technique in order to derive the integral elastic andelastic momentum transfer cross sections. Comparison is made with earlier data,where available, and also with a number of recent theoretical calculations.

Excitation Cross Sections for Li-like Ions of Beryllium and Boron

We report on calculation of electron-impact excitation cross sections for Li-like ions of boron and beryllium. The data were produced with a number of modern methods in atomic collision theory, such as convergent close-coupling, K-matrix and Coulomb–Born-exchange. The results obtained are compared with other calculations and available expermental data, and the recommended cross sections for all transitions between atomic terms with principal quantum numbers n ≤ 4 are presented as tables of fitting parameters.

Absolute measurements of cross-sections for near-threshold electron-impact excitation of Na-like and Mg-like multiply-charged ions

The JILA-ORNL merged electron–ion beam energy-loss apparatus has been used to measure absolute cross-sections for excitation of multiply-charged ions over the past several years. Recent measurements for the Na-like ions Al2+ and Cl6+ and the Mg-like ion Cl5+ are presented and are compared with earlier measurements on the isoelectronic ions and with theoretical results.

Determination of the parameters of phosphor activators in thin-film electroluminescent capacitor structures

A method for determining the concentration and electron-impact excitation cross section of activated emission centers in the phosphor layer of a thin-film electroluminescent capacitor structure, based on measurements of the brightness as a function of the applied alternating voltage amplitude and frequency, is analyzed. The error of determination of the parameters of electroluminescence excited by alternating-sign ramp voltage is evaluated. The parameters of electroluminescent structures based on the ZnS:Mn, ZnS:TbF3, and ZnS:SmF3 films are presented.

Absolute emission cross sections for electron-impact excitation of the3p−3stransition inAl2+

Crossed beams of electrons and ${\mathrm{Al}}^{2+}$ ions combined with fluorescence detection were used to measure the absolute cross section for electron-impact excitation of the ions to yield 186.3 nm and 185.5 nm photons from the $3s\ensuremath{-}3p$ transition. The measured cross section near the 6.7 eV threshold energy is about $16\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{2},$ and some resonance structure near threshold is indicated. Total relative uncertainties $(1\ensuremath{\sigma})$ are typically about 6%, to be combined with an 11% absolute calibration uncertainty. Total uncertainties at high confidence level (90%) are around 20%, using a coverage factor of 1.7. Comparisons made with various theoretical calculations, specifically distorted wave and nine-state close coupling, show better agreement of experimental values with distorted wave calculations, with the measured values being higher near threshold than the most recent close-coupling calculations.

Merged-beam measurements of electron-impact excitation ofAl2+(3s2S→3p2P)

Absolute cross sections for electron-impact excitation of the ${3s}^{2}\stackrel{\ensuremath{\rightarrow}}{S}{3p}^{2}P$ transition in ${\mathrm{Al}}^{2+}$ were measured near threshold using the merged electron-ion beams energy-loss technique. Although the present results are lower than the previous crossed-beams fluorescence measurements of Dunn et al. [Phys. Rev. A 66, 032706 (2002)] by about 30%, these two experimental excitation cross sections at threshold are in agreement when the energy resolutions and total expanded uncertainties are considered. The present results are in excellent agreement with the published close-coupling calculations, but lie about 30% lower than the distorted-wave predictions.