Direct Excitation Cross Sections Research Articles

Calculations of electron-impact excitation and dielectronic recombination rate coefficients of highly charged silicon ions

The rate coefficients for dielectronic recombination and electron-impact excitation processes with H-like to Be-like silicon ions (${\mathrm{Si}}^{13+}\text{\ensuremath{-}}{\mathrm{Si}}^{10+}$) are calculated using relativistic distorted-wave approach. The prominent $▵n=1$ and the weaker $▵n=2$ and 3 dielectronic recombination (DR) resonances with $K$-shell excitations are presented, and compared with the existing DR experimental rate coefficients of ${\mathrm{Si}}^{13+}, {\mathrm{Si}}^{12+}$, and ${\mathrm{Si}}^{11+}$ ions, it is found that the theoretical DR rate coefficients are in very good agreement with the experimental results. With the same approach, the direct and resonant electron-impact excitation (EIE) cross sections associated with $1s\ensuremath{-}{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ core excitations are calculated for the ground states of ${\mathrm{Si}}^{13+}\text{\ensuremath{-}}{\mathrm{Si}}^{10+}$ ions and found in excellent agreement with the available experiment. Finally, we present the synthesized DR and EIE rate coefficients for the sum of all detected $(13+\ensuremath{\sim}10+)$ charge states of Si and these agree well with the experimental results.

Convergent close-coupling approach to proton collisions with helium

SynopsisWe develop a two-center wave-packet convergent close-coupling approach to proton collisions with helium including electron exchange. The convergence of the final results is studied in terms of the included projectile- and target-centered pseudostates. We present electron-capture and single- and double-ionisation cross sections in the projectile energy range from 15 keV to 1 MeV. We also provide partial cross sections for electron capture and direct excitation into the n = 2 shell states of hydrogen and helium, where n is the principal quantum number of the final state. Our results are in good agreement with available experimental data and other calculations.

Wave-packet continuum-discretization approach to proton collisions with helium

We extend the two-center wave-packet convergent close-coupling method to proton collisions with helium. The target is treated as a three-body system, where correlations between the electrons are taken into account. We apply a frozen-core approximation, where one of the electrons is described by the He+1s orbital, and obtain the helium singlet wave functions as well as the energy levels using a numerical approach. The wave-packet approach is used to discretize the continuum of the target and the hydrogen atom formed after electron capture by the projectile. Convergence of the results is studied in terms of the included projectile- and target-centered states. We present electron-capture and single- and double-ionization cross sections for protons incident on He in the ground state in the energy range from 15 keV to 1 MeV. We also provide partial cross sections for electron capture and direct excitation into the n=2 shell states of hydrogen and helium, respectively. Our results are in good agreement with available experimental data and other calculations, where available.

Saturation of coupling of collective levels in optical model calculations of even-even actinides

Saturation of the coupling-scheme is studied in dispersive optical model calculations of nucleon induced reactions on 238 U target. Recently derived potential that is based on a soft-rotator-model (SRM) description of the collective levels of the target nucleus with volume conservation is used. It is show that calculated direct excitation cross sections of all considered non-GS bands levels are larger than the 4+ GS band level excitation, and can't be ignored in coupled-channel calculations. SRM couplings of 21 levels constitute a saturated coupling scheme that allows a precise calculation of the compound-nucleus (CN) formation cross sections up to several MeV.

A case study of energy transfer mechanism from uranium to europium in ZnAl2O4 spinel host by photoluminescence spectroscopy

Zinc aluminate (ZAO), a member of spinel class of inorganic compounds has been of much interest of late due to its wide range of use in catalysis, optical, electronic and ceramic industries. When doped with several lanthanides, this material has proved to be a potential host matrix for phosphors. As lanthanides suffer from poor (direct) excitation and emission cross sections, the use of a co-dopant ion can help to circumvent this and extract better emission from a lanthanide doped ZAO system. In this connection, energy transfer mechanism from uranium to europium in the ZAO host was investigated by photoluminescence spectroscopic technique. It was seen that uranium gets stabilized in the hexavalent state as UO66− (octahedral uranate) where as the lanthanide ion, Eu is stabilized in its trivalent state in the ZAO host. In the co-doped system, an efficient energy transfer pathway from the uranate to europium ion was observed. Based upon emission and life time data a suitable mechanism was proposed for the energy transfer (quenching) process. It was proposed that after excitation by photons, the uranate ions transfer their energy to nearby 5D1 level of Eu3+ ions which non-radiatively de-excites to the corresponding lower levels of 5D0. Further this 5D0 level decays in a radiative mode to the 7F manifold giving the characteristic emission profile of trivalent Eu. It was proposed that both static and dynamic types of energy transfer mechanism were responsible for this process.

Dissociative Excitation of Nickel Atom Spectral Lines in e–NiBr2 Collisions (Transitions from Even Levels Belonging to 3d84s5s and 3d84s4d Configurations)

Dissociative excitation of nickel atoms in e‐NiBr2 collisions studied by the method of extended crossing beams with registration of optical radiation of excited atoms. In this article the results are presented, related to dissociative excitation cross-sections of even parity states belonging to 3d 8 4s5s and 3d 8 4s4d configurations. Obtained data compared with direct excitation crosssections of the same levels in electron-atom collisions.

Dissociative excitation of even levels of the singly charged yttrium ion in collisions of slow electrons with yttrium tribromide molecules

The method of extended crossing beams in conjunction with optical spectroscopy is used to study the dissociative excitation of even levels of YII in e-YBr3 collisions. At the exciting electron energy of 100 eV, 38 dissociative excitation cross sections are measured. Comparison of these cross sections with the cross sections for direct excitation in e-Y collisions shows that, for all the investigated even levels of YII, dissociative excitation is more efficient than direct one.

Electron-impact excitation of the 3 P o and 3 D o levels of a nickel atom

The excitation cross sections of transitions from the 3Po and 3Do levels of a nickel atom in electron-atom collisions at an exciting-electron energy of 50 eV have been analyzed by the method of extended crossing beams with detection of the optical signal from excited atoms. Eleven optical excitation functions have been recorded in the electron-energy range of 0–200 eV. The direct excitation cross sections of the energy levels and the contribution of the cascade transitions were calculated using the data obtained.

The dissociative excitation of odd doublet cobalt atom levels in collisions of electrons with cobalt dichloride molecules

The extended crossed beam method was used to measure the dissociative excitation cross sections in e-CoCl2 collisions for 91 transitions from odd doublet levels of cobalt atoms. The dissociative and direct CoI excitation cross sections were compared. Possible reaction channels at low electron energies were considered. The results were used to calculate the total dissociative excitation cross section of CoI levels at an exciting electron energy of 100 eV. For the z 2 F°, z 2 G°, and y 2 G° levels, cascade population contributions were also determined.

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.

Optically active Er3+ ions in SiO2 codoped with Si nanoclusters

Optical properties of directly excited erbium (Er3+) ions have been studied in silicon rich silicon oxide materials codoped with Er3+. The spectral dependence of the direct excitation cross section (σdir) of the Er3+ atomic I415/2→I411/2 transition (around 0.98 μm) has been measured by time resolved μ-photoluminescence measurements. We have determined that σdir is 9.0±1.5×10−21 cm2 at 983 nm, at least twice larger than the value determined on a stoichiometric SiO2 matrix. This result, in combination with a measurement of the population of excited Er3+ as a function of the pumping flux, has allowed quantifying accurately the amount of optically active Er3+. This concentration is, in the best of the cases, 26% of the total Er population measured by secondary ion mass spectrometry, which means that only this percentage could provide optical gain in an eventual optical amplifier based on this material.

The dissociative excitation of cobalt atom even sextet levels in collisions of electrons with CoCl2 molecules

The cross sections of dissociative excitation of 46 transitions from cobalt atom even sextet levels in collisions of electrons with CoCl2 molecules were measured at an exciting electron energy of 100 eV. The dissociative and direct CoI excitation cross sections were compared. The results were used to calculate the total cross sections of dissociative excitation of CoI levels at an electron energy of 100 eV. Possible reaction channels at low electron energies are discussed.

Resonance electron-impact excitation and polarization of the magnetic quadrupole line of neonlikeBa46+ions

Detailed calculations have been carried out for the direct electron-impact excitation and resonant excitation cross sections from the ground state to the individual magnetic sublevels of ${(2{p}_{3∕2}^{\ensuremath{-}1}3{s}_{1∕2})}_{2}$ of highly charged ions of neonlike barium by using a fully relativistic distorted-wave method. The contributions of resonant excitations to the linear polarization of the magnetic quadrupole $(M2)$ line $[{(2{p}_{3∕2}^{\ensuremath{-}1}3{s}_{1∕2})}_{2}\text{\ensuremath{-}}2{p}^{6}\phantom{\rule{0.2em}{0ex}}^{1}S_{0}]$ have been investigated systematically. It is found that the $4l5{l}^{\ensuremath{'}}$, $4l6{l}^{\ensuremath{'}}$, and $5l5{l}^{\ensuremath{'}}$ resonances have significant depolarizing or enhancing effects on the linear polarization. These results agree very well with the experimental measurements by Tak\'acs et al. [Phys. Rev. A 54, 1342 (1996)].

The Electron‐Excited Mid‐Ultraviolet to Near‐Infrared Spectrum of H2: Cross Sections and Transition Probabilities

At high resolving power -->(?/? ? ? 10,000) , we have measured the electron impact-induced fluorescence spectrum of H2 from 3300-10000 ? at 20 and 100 eV. We have analyzed the visible-optical-near-IR (VOIR) emission spectrum under optically thin, single-scattering experimental conditions. The high-resolution spectrum contains transitions from the gerade Rydberg series of singlet states to the ungerade series of Rydberg states ( -->EF1?g+, -->GK1?g+, --> H1?g+, -->I1?g, -->J1?g... ? B1?u+, -->C1?u, -->B'1?u+, -->D1?u) and the Rydberg series of triplet states dominated by -->d3?u, -->k3?u, -->j3?g? a3?g+ band systems. A model VOIR spectrum of H2 from 7500 to 10000 ?, based on newly calculated transition probabilities and line positions including rovibrational coupling for singlet gerade states, is in excellent agreement with the observed intensities. The rotational line emission cross sections for direct excitation of the singlet states at 100 eV and triplet states at 20 eV were measured. The absolute cross section values for excitation to the singlet gerade states at 100 eV was measured to be -->(4.58 ? 1.37) ? 10?18 cm2; the excitation cross section to the triplet states at 20 eV was found to be -->(1.38 ? 0.41) ? 10?18 cm2. The singlet gerade emission cross sections are the cascading cross sections to the UV spectrum (including the Lyman and Werner band systems) and the triplet state emission cross sections are the cascade dissociation cross sections of the H2 -->(a3?g+?b3?u+) continuum for the production of fast hydrogen atoms. Electrons are also effective in exciting high vibrational levels belonging to ungerade symmetry (B, B', C, D) which subsequently emit toward gerade symmetry states in the VOIR domain. Strong emission from the ungerade-gerade transitions are observed from 7300 to 7400 ?.

Laboratory Measurements of 3 → 2 X‐Ray Line Ratios of F‐like Fexviiiand Nixx

The intensity ratios of 3 {yields} 2 emission lines of Fe XVIII and Ni XX were measured on the Livermore electron beam ion trap (EBIT-I) with a flat-field grating spectrometer. The results were compared with distorted-wave (DW) calculations obtained with the Flexible Atomic Code and recent close-coupling calculations using the R-matrix code. The measured 3s {yields} 2p/3d {yields} 2p ratios are about 20-40% higher than the theoretical values. When more extended configuration interaction is included in the DW theory, the agreement with the measurements improved slightly. At the beam energies of these measurements, no significant resonance contribution is expected to be present, and the discrepancies represent the uncertainties in the direct excitation cross sections.

B-spline R-matrix with pseudostates approach for electron impact excitation of atomic nitrogen

The B-spline R-matrix with pseudostates approach has been used to calculate excitation cross sections for the forbidden 2s22p34So–2Do, 2Po, 2s22p32Do–2Po and resonance 2s22p34So–2s22p23s4P, 2s2p44P, 2s22p24s4P and 2s22p23d4P transitions in atomic nitrogen for incident electron energies from threshold to 120 eV. The excitation of these transitions gives rise to prominent lines in the spectra of solar and planetary atmospheres. The 24 spectroscopic bound and autoionizing states together with 15 pseudostates are included in the close-coupling expansion. The pseudostates are chosen to approximate the loss of flux into the infinite number of bound and continuum states that are dipole coupled with ground configuration terms. The contribution of the ionization continuum is significant for resonance transitions. An accurate description of target wavefunctions has been obtained on the basis of non-orthogonal spectroscopic and pseudo-orbitals to adequately account for the correlation corrections and interactions. A comparison of calculated cross sections with the measured absolute direct excitation cross sections is presented. A good agreement with measured integral cross sections is noted except at 5 eV for the forbidden transition and 30 eV for the resonance transitions.

Angular momentum partitioning and the subshell multipole moments in impulsively excited argon ions

˛ s 1 Dd4p 2 F5/2 s463.7 nmd, s 1 Pd3d 2 D5/2˛ s 1 Dd4p 2 D5/2 s448.2 nmd, and s 1 Dd4s 2 D3/2˛ s 1 Dd4p 2 P3/2 s423.7 nmd. We develop the angular momentum algebra necessary to extract from these data, starting from the overall atomic J multipoles, the partitioning of orbital angular momentum into the 1 D core electric quadrupole and hexadecapole moments, and the outer 4p electric quadrupole moment. The magnetic dipole of the outer electron is also determined. This procedure requires the assumption of good LS coupling for these states, which is justified. We recouple these individual core- and outer-electron moments to calculate the initial electric quadrupoles, hexadecapoles, and hexacontatetrapoles of the initial excited-state manifold. The detailed time structure of the electron-atom collision is considered, as well as the time evolution of the excited ionic state. The Rubin-Bederson hypothesis is thus shown to hold for the initial ionic L and S terms. The consequences of the breakdown of LS coupling are considered. From the circular polarization data, estimates of the relative importance of direct and exchange excitation cross section are made. We discuss experimental issues related to background contributions, Hanle depolarization of the fluorescence signal, and cascade contributions. Nonlinearity of the equations relating the Stokes parameters to the subshell multipole moments complicates the data analysis. Details of the Monte Carlo terrain-search algorithm used to extract multipole data is discussed, and the implications of correlation between the various subshell multipole moments is analyzed. The physical significance of the higher-order multipole moments is discussed, and graphical representations of the effects of these multipoles on the excited ionic charge clouds is presented.

Theoretical study of cross sections of excitation and ionization for electron-ion impact in average-atomic model

By using a distorted-wave Born exchange approximation method in averaged-atomic model, we calculated the direct excitation and direct ionization cross sections of several ions in plasmas. The ionization cross sections of ions at different temperatures or different plasma densities are systematically studied. It is found that due to the shift of the energy levels produced by the effects of the temperature and the density of plasma, the ionization cross sections of ions in plasmas are rather different from those of free atoms.

Accurate cross sections for excitation of resonance transitions in atomic oxygen

Electron collision excitation cross sections for the resonance 2p43P–2p33s3So, 2p43P–2p33d3Do, 2p43P–2p33s3Do, 2p43P–2p33s3Po, and 2p43P–2s2p53Po transitions have been calculated by using the R matrix with a pseudostates approach for incident electron energies from near threshold to 100 eV. The excitation of these transitions gives rise to strong atomic oxygen emission features at 1304, 1027, 989, 878, and 792 Å in the spectra of several planetary atmospheres. We included 22 spectroscopic bound and autoionizing states and 30 pseudostates in the close‐coupling expansion. The target wave functions are chosen to properly account for the important correlation and relaxation effects. The effect of coupling to the continuum is included through the use of pseudostates. The contribution of the ionization continuum is significant for resonance transitions. Measured absolute direct excitation cross sections of O I are reported by experimental groups from the Jet Propulsion Laboratory and Johns Hopkins University. Good agreement is noted for the 2p43P–2p33s3So transition (λ1304 Å) with measured cross sections from both groups that agree well with each other. There is disagreement between experiments for other transitions. Our results support the measured cross sections from the Johns Hopkins University for the 2p43P–2p33d3Do and 2p43P–2p33s3Do transitions, while for the 2p43P–2p33s3Po transition the agreement is switched to the measured cross sections from the Jet Propulsion Laboratory.

Excitation of a singly charged Sr Ion by e-Sr collisions

Excitation of spectral lines of SrII, including laser lines, is studied experimentally in collisions of slow electrons with strontium atoms. Twenty-one excitation cross sections are measured at an energy of electrons of 30 eV. Five optical excitation functions are recorded at an energy ranging from 0 to 200 eV. Direct-excitation cross sections for the 5p2P°1/2, 3/2 levels and the contribution to their population by cascade transitions are calculated. Excitation cross sections of two laser transitions in the IR spectral region are determined using known branching ratios. The results obtained are compared with data of previous experiments.