2s2 2p6 Research Articles

Near- K -Edge Double and Triple Detachment of the F− Negative Ion: Observation of Direct Two-Electron Ejection by a Single Photon

Double and triple detachment of the F^{-}(1s^{2}2s^{2}2p^{6}) negative ion by a single photon have been investigated in the photon energy range 660 to 1000eV. The experimental data provide unambiguous evidence for the dominant role of direct photodouble detachment with a subsequent single-Auger process in the reaction channel leading to F^{2+} product ions. Absolute cross sections were determined for the direct removal of a (1s+2p) pair of electrons from F^{-} by the absorption of a single photon.

Theoretical Investigation of Metallic Heterofullerenes of Silicon and Germanium Mixed with Phosphorus and Arsenic Atoms M-A8E6, A = Si, Ge; E = P, As; and M = Cr, Mo, W.

Recently, metallic heterofullerenes were experimentally prepared from mixed Ge-As clusters and heavier elements of groups 14 and 15. We found that the shape of these heterofullerenes doped by transition metals appears to be a general structural motif for both silicon and germanium clusters when mixing with phosphorus and arsenic atoms. Structural identifications for MSi8P6, MSi8As6, MGe8P6, and MGe8As6 clusters, with M being a transition metal of group 6 (Cr, Mo and W), showed that most MA8E6 clusters, except for Cr-doped derivatives CrSi8As6, CrGe8P6, and CrGe8As6, exhibit a high-symmetry fullerene shape in which metal dopant is centered in a D3h A8E6 heterocage consisting of six A3E2 pentagonal faces and three A2E2 rhombus faces. The stability of the MA8E6 metallic heterofullerene is significantly enhanced by formation an electron configuration of [1S2 1P6 1D10 1F14 1G18 2S2 2P6 2D10] enclosing 68 electrons. The A8E6 heterocages give a great charge transfer (∼4 electrons) to centered dopant, establishing subsequently a d10 configuration for metal, and as a consequence, it induces an additional stabilization of the resulting ME8P6 fullerene in a high-symmetry D3h shape and completely quenches the high spin of the metal atom, finally yielding a singlet spin ground state.

Mn@B3N3Si8 +: a stable singlet manganese-doped hetero-atom-mixed silicon fullerene

A B3N3Si8 cage is formed upon substitution of Si sites of rhombus faces of the pure Si14 cluster by B and N atoms. Doping by the ion Mn+ leads to the hetero-silicon fullerene B3N3Si8Mn+ which comprises three rhombi (BNBN, Si3B and Si3N) and four pentagons (two Si2B2N and two Si2BN2). Hetero-atoms form polarized Si-N and Si-B bonds as indicated by electron localization function (ELF) maps and NBO charges. The Mn center connects the B3N3Si8 cage by ionic interactions. Valence electrons of B3N3Si8Mn+ occupy a shell configuration of [1S2 1P6 1D10 1F14 1G12 2S2 2P6 2D10] and induce a certain thermodynamic stability. The high spin of the Mn+ metal cation is completely quenched within the hetero-Si fullerene.

Prominent role of multielectron processes inK-shell double and triple photodetachment of oxygen anions

The photon-ion merged-beams technique was used at a synchrotron light source for measuring absolute cross sections of double and triple photodetachment of O$^{-}$ ions. The experimental photon energy range of 524-543 eV comprised the threshold for K-shell ionization. Using resolving powers of up to 13,000, the position, strength and width of the below-threshold 1s 2s2 2p6 2S resonance as well as the positions of the 1s 2s2 2p5 3P and 1s 2s2 2p5 1P thresholds for K-shell ionization were determined with high precision. In addition, systematically enlarged multi-configuration Dirac-Fock calculations have been performed for the resonant detachment cross sections. Results from these ab-initio computations agree very well with the measurements for the widths and branching fractions for double and triple detachment, if double shake-up (and -down) of the valence electrons and the rearrangement of the electron density is taken into account. For the absolute cross sections, however, a previously found discrepancy between measurements and theory is confirmed.

Atomic Data and Laser Transitions in as - Like Gallium

Fine structure calculations of the energy levels, the wavelengths, the oscillator strengths, log gf and the transition probabilities for transitions among the terms belonging to 1s2 2s2 2p6 3s2 3p6 3d104s2ns, n=5-6, 1s2 2s2 2p6 3s2 3p6 3d104s2np, n=4-6, 1s2 2s2 2p6 3s2 3p6 3d104s2nd, n=4-6 and1s2 2s2 2p6 3s2 3p6 3d104s2nf, n=4-6 configurations of As (III) have been calculated using configurations interaction Cowan atomic structure code. Our calculated values for the above mentioned quantities have been compared with the corresponding experimental data and other theoretical calculations where a satisfactory agreement is found. We also report on some unpublished values for energy levels, oscillator strengths and transition probabilities for As like gallium. These atomic data are taken as the basis for studying laser transitions between levels of As(III). Excitation rate coefficients of As like gallium are calculated using the analytical formulas of Vriens and Smeets (1980) and with considering using the collisional radiative model code CRMO of Allam (2006). A simple modification to these formulas has been included by introducing effective quantum numbers. The energy values, the radiative data and rate coefficients are then used to calculate the population densities by solving the coupled rate equations. Among these calculations positive gain coefficients are found at three selected values of electron temperature, namely 7.087 eV, 14.147 eV and 21.261 eV which are displayed as a function of the electron impact density.

Energy Levels, Oscillator Strengths, and Transition Probabilities of Ni XIX and Cu XX

Energy levels, oscillator strengths, and transition probabilities, for the 1s2 2s2 2p6, 2p53l (l = 0, 1, 2), 2p54l (l = 0, 1, 2, 3) states in Ni XIX and Cu XX are calculated using COWAN code. The Correlation and relativistic effects are considered. The calculations are compared with other results in the literature. A good agreement is found. We also report on some unpublished energy values.

High Gain Predictions for Ni-Like Ta Ions

Atomic structure data and effective collision strengths for 1s2 2s2 2p6 3s2 3p6 3d10 and 54 fine-structure levels are contained in the configurations 1s2 2s2 2p6 3s2 3p6 3d9 4l (l = s, p, d, f) for the nickel-like Ta ion. These data are used in the determination of the reduced population for the 55 fine structure levels over a wide range of electron densities (from 1021 to 1023) and at various electron plasma temperatures. The gain coefficients for those transitions with a positive population inversion factor are determined and plotted against the electron density.

Energy Levels, Transition Probabilities, Collision Strength, and Reduced Population Calculations for High Gain Predictions in Neon-Like Krypton

We calculate the atomic structure, energy levels, oscillator strengths, transition probabilities, and collision strengths for Kr XXVII. The data refer to the 157 fine-structure levels belonging to the configurations (1s2) 2s2 2p6, 2s22p53l, 2s12p63l, 2s22p54l, 2s12p64l, 2s22p55l, and 2s12p65l, where l = s, p, d, f, and the calculations are performed using the fully relativistic atomic structure program FAC. We use the obtained data to calculate the level populations and gain coefficients employing the MATLAB R2012a computer program for solving simultaneously the coupled rate equations. Finally, we determine the 157 fine-structure population levels and gain coefficients for those transitions with a positive inversion factor and plot the electron density in wide range from 1019 to 1023.

Unusual under-threshold ionization of neon clusters studied by ion spectroscopy

We carried out time-of-flight mass spectrometry for neon clusters that were exposed to intense free electron laser pulses with the wavelength of 62 nm, which induce optical transition from the ground state (2s2 2p6) to an excited state (2s2 2p5 nl ) in the Ne atoms. In contrast to Ne+ ions produced by two-photon absorption from isolated Ne atoms, the Ne+ ion yield from Ne clusters shows a linear dependence on the laser intensity (I). We discuss the ionization mechanisms which give the linear behaviour with respect to I and expected features in the electron emission spectrum.

Theoretical study of the photoionization process of Ne-like Ar, Fe, Kr and Xe ions

Photoionization (PI) cross sections of the ground state 1s2 2s2 2p6 1 S0 of Ne-like Ar8+, Fe16+, Kr26+ and Xe44+ ions are calculated by using the Dirac atomic R-matrix code based on a fully relativistic R-matrix method. To analyze the detailed resonant structure, systematical calculations based on the multiconfiguration Dirac–Fock method are performed for the dominant 2s → np resonant transitions, and the resonant energies, strengths and total natural widths are presented. The resonant structures and characteristics of the PI cross sections and resonance strengths along the Ne-like sequence are discussed with emphasis on the influence of relativistic effects. For the Ar8+ ion, good agreement is found between the present results and available theoretical calculations and experimental data.

Extreme ultraviolet and soft x-ray laser emission from Ni-like Nd ions

We use the energy levels, transition probabilities, and effective collision strengths for the states 1s2 2s2 2p6 3s2 3p6 3d10 and 1s2 2s2 2p6 3s2 3p6 3d9 4 l (l = s, p, d, and f) of a nickel-like Nd ion to determine reduced population for 55 fine-structure levels over a wide range of electron density values (from 1020 to 4 · 1022 cm−3) at various electron plasma temperatures in the range of 1–2 keV. We find the gain coefficients for those transitions with the positive population inversion factor and show their dependences on the electron density.

Search for Laser Lines in Sodium like Fe Plasmas

Energy levels, transition probabilities and effective collision strength for 1s2 2s2 2p6 3l, 4l, 5l (l = 0, 1, 2, 3, 4) states of sodium like Fe are used in the determination of the reduced populations for 21 fine structure levels over a wide rang of electron density values (1018 to 1020 cm–3) and at against electron plasma temperatures. Gain coefficients are evaluated and plotted against the electron density.

XUV and Soft X-Ray Laser Radiation from Ni-Like Au

Atomic structure data and effective collision strengths from literature for 1s2 2s2 2p6 3s2 3p63d10 and 34 fine-structure levels contained in the configurations 1s2 2s2 2p6 3s2 3p63d9 4l (l = s, p, d) for the nickel-like Au ion are used in the determination of the reduced population for these levels over a wide range of electron densities and at various electron plasma temperatures. The gain coefficient for those transitions with positive population inversion factor are determined and plotted against the electron density.

Experimental rate coefficient for dielectronic recombination of neonlike iron forming sodiumlike iron

The rate coefficient for dielectronic recombination (DR) of Ne-like Fe16+ forming Na-like Fe15+ was measured employing the merged electron-ion beams technique at the heavy-ion storage-ring TSR of the Max-Planck-Institut für Kernphysik in Heidelberg, Germany. In the electron-ion collision energy range of 240–840 eV the merged-beams recombination rate coefficient is dominated by DR associated with 2s2 2p6 1S0 → 2s2 2p5 3d 1P1 core excitation. The experimental Fe16+ DR plasma rate coefficient is derived from the measured merged-beams rate coefficient. It is in good agreement with recent theoretical results.

Electron impact excitation of Ne-like Ni XIX

In this paper we report atomic data, namely energy levels, radiative rates, lifetimes, collision strengths, and excitation rate coefficients, for transitions among 89 levels of the (1s2) 2s2 2p6, 2s2 2p53l, 2s2p6 3l, 2s2 2p54l, and 2s2p64l configurations of Ni XIX. For the calculations of energy levels and radiative rates the grasp code has been adopted, whereas for the calculations of collision strengths darc is employed.

Photoionization of ground 1s2 2s2 2p6 1Se0 and excited 1s2 2S22p53S 3Po0,1,2 states of Si V using relativistic Breit–Pauli R-matrix method

The close-coupling R-matrix method has been used to calculate photoionization cross sections for the ground and the lowest three excited states using the relativistic Breit–Pauli approximation. Configuration interaction wave-functions are used to represent eleven lowest LSJ target states of Si VI retained in the R-matrix expansion. Relativistic effects are clearly revealed from resonance converging on the excited states of Si VI.PACS No.: 32.80.Fb

Electron-Ion Recombination Measurements Motivated by AGN X-Ray Absorption Features: Fe xiv Forming Fe xiii

Recent spectroscopic models of active galactic nuclei (AGN) have indicated that the recommended electron-ion recombination rate coefficients for iron ions with partially filled M-shells are incorrect in the temperature range where these ions form in photoionized plasmas. We have investigated this experimentally for Fe XIV forming Fe XIII. The recombination rate coefficient was measured employing the electron-ion merged beams method at the Heidelberg heavy-ion storage-ring TSR. The measured energy range of 0-260 eV encompassed all dielectronic recombination (DR) 1s2 2s2 2p6 3l 3l' 3l'' nl''' resonances associated with the 3p1/2 -> 3p3/2, 3s -> 3p, 3p -> 3d and 3s -> 3d core excitations within the M-shell of the Fe XIV 1s2 2s2 2p6 3s2 3p parent ion. This range also includes the 1s2 2s2 2p6 3l 3l' 4l'' nl''' resonances associated with 3s -> 4l'' and 3p -> 4l'' core excitations. We find that in the temperature range 2--14 eV, where Fe XIV is expected to form in a photoionized plasma, the Fe XIV recombination rate coefficient is orders of magnitude larger than previously calculated values.

Level energies and lifetimes in the 3p4 3d configuration of chlorine-like ions

Large-scale multiconfiguration Dirac-Fock wavefunctions have been used to study excitation energies and lifetimes of the 3s2 3p4 3d configuration in the chlorine isoelectronic sequence. We present an ab-initio calculation of all dipole allowed transitions from 3s2 3p4 3d to the 3s2 3p5 (J = 1/2, 3/2) ground state levels for 6 ions in the atomic range 18 ≤ Z ≤ 41. To generate the wavefunctions we used the active-space method outside a closed 1s2 2s2 2p6 core. Our study incorporates the major correlation effects by including all virtual single, double and triple excitations into the 3l subshells as well as all singles and doubles into 4l. Excitations into the 4l and higher subshells do not contribute so much to the transition energies, but they clearly affect the transition probabilities and lifetimes. Additionally, the rearrangement of the electron density during the spontaneous emission is taken into account by a separate optimization of the wavefunctions in the 3p4 3d and 3p5 configurations. This often reduces the deviations between the lifetimes in different gauge forms, in particular at the neutral end of the isoelectronic sequence. Comparison is made with previous calculations and available experimental data. We conclude that the accuracy of the presented excitation energies (apart from Ar II) has been improved to about 5000 cm−1 for higher charged ions, i.e. by about a factor of 2 or so with respect to recent computations.

Inelastic electron–ion scattering in hot and dense plasmas

An atomic model to describe the density and temperature effects on inelastic electron-ion collisions in dense plasmas is presented. This model has been used for Na-like iron to compute the optically allowed and forbidden collision rates from the ground configuration 1s2 2s2 2p6 3s to the 1s2 2s2 2p6 41 excited levels with l = 0,1,2. The focus is on temperature and density ranges 100–300 eV and 1022–1024 cm-3.

Doubly autoionizing capture resonances ine + Mg+ collisions

Fine details in the cross section for electron impact ionization of Mg+ ions (configuration 1s2 2s2 2p6 3s) have been measured with an energy resolution of 0.3 eV. Structure on top of a smooth direct-ionization “background” reflects the presence of indirect ionization mechanisms. Such contributions in a Na-like ion involvenon-resonant excitation of a 2p electron to singly autoionizing intermediate states subsequently decaying into the channel of net single ionization of the parent ion. We observe even stronger indirect contributions fromresonant excitation of a 2p electron with simultaneous dielectronic capture of the projectile electron into doubly autoionizing states which decay by successive emission of two electrons.