Search Results (34)

Simulated (or measured) photoabsorption spectra often provide the first indication of how matter interacts with light when irradiated by some radiation source. In addition to the direct, often slowly varying photoabsorption cross-section as a function of the incident photon frequency, such spectra typically exhibit numerous resonances and edges arising from the interaction of the radiation field with the subvalence or even inner-shell electrons. Broadly speaking, these resonances reflect photoexcitation, with its subsequent fluorescence, or the autoionization of bound electrons. Here, a (relativistic) cascade model is developed for estimating the photoabsorption of (many) atoms and multiply charged ions with a complex shell structure across the periodic table. This model helps distinguish between level- and shell-resolved, as well as total photoabsorption, cross-sections, starting from admixtures of selected initial-level populations. Examples are shown for the photoabsorption of C${}^{+}$ ions near the $1s-2p$ excitation threshold and for Xe${}^{2+}$ ions in the photon energy range from 10 to 200 eV. While the accuracy and resolution of the predicted photoabsortion spectra remain limited due to the additive treatment of resonances and because of missing electronic correlations in the representation of the levels involved, the present implementation is suitable for ions with quite different open-shell structures and may support smart surveys of resonances along different isoelectronic sequences. Full article

The line ratios in X-ray emission resulting from charge exchange between highly charged ions (HCIs) and neutral atoms are not only crucial for accurately modeling astrophysical X-ray emissions but also offer a unique perspective on the charge exchange processes happening during collisions. The K X-ray spectra following charge exchange between Mg¹¹⁺ and He are presented for a collision velocity of 1489 km/s (11.5 keV/amu). The spectra were measured by two Silicon Drift Detectors capable of resolving the Mg¹⁰⁺ Kα, Kβ, Kγ, and Kδ+ lines. The line intensity ratios of Kβ, Kγ, and Kδ+ relative to the Kα line, as well as the hardness ratio, were obtained. The experimental results were compared with the theoretical results from a cascade model that utilizes the state cross-sections produced by multichannel Landau–Zener (MCLZ) calculation. It was discovered that the K X-ray spectrum features can be reproduced well by MCLZ theory when the contributions of both single electron capture (SEC) and autoionizing double capture (ADC) processes are included. This finding implies that the ADC feeding mechanism is significant and should be taken into account for the X-ray emission during charge exchange between highly charged ions and multielectron atoms. Full article

Earlier, a significant enhancement of the nondipole parameters ${\gamma }_{2p}$, ${\delta }_{2p}$, and ${\zeta }_{2p}={\gamma }_{2p}+3{\delta }_{2p}$ in the photoelectron angular distribution for Ne $2p$ photoionization was predicted, owing to resonance interference between dipole ($E1$) and quadrupole ($E2$) transitions. This enhancement manifests as narrow resonance spikes in the parameters due to the low-energy $2s\to 3p$ and $2s\to 4p$ dipole, as well as the $2s\to 3d$ quadrupole autoionizing resonances. Given the unique nature of this predicted enhancement, it requires further validation, specifically regarding whether these narrow spikes in ${\gamma }_{2p}$, ${\delta }_{2p}$ and ${\zeta }_{2p}$ will or will not retain their values for experimental observation if one accounts for a typical finite frequency spread in the ionizing radiation. To address this, we revisit the previous study, now incorporating the effect of frequency spread in the ionizing radiation, assuming a spread as large as 5 meV at the half-maximum of the radiation’s intensity. In the present paper we demonstrate that while the frequency spread does affect the resonance enhancement of ${\gamma }_{2p}$, ${\delta }_{2p}$ and ${\zeta }_{2p}$, these parameters still retain quantitatively significant values to be observed experimentally. The corresponding calculations were performed using the random phase approximation with exchange, which accounts for interchannel coupling in both dipole and quadrupole photoionization amplitudes. Full article

CO is one of the important molecules in dense molecular clouds, and its dissociation induced by cosmic ray heavy ions is a fundamental process for molecular breaking up and rearrangement in astronomical networks. Extensive laboratory simulations are required to understand molecular evolution in astrophysical contexts. Here, we investigate the CO dissociation induced by 1 keV/u ${\mathrm{Ar}}^{2+}$ using cold target recoil ion momentum spectroscopy. Kinetic energy release for double electron capture ${\mathrm{Ar}}^{2+}+\mathrm{CO}\to {\mathrm{Ar}}^0+{\mathrm{C}}^{+}+{\mathrm{O}}^{+}$ and transfer ionization ${\mathrm{Ar}}^{2+}+\mathrm{CO}\to {\mathrm{Ar}}^{+}+{\mathrm{C}}^{+}+{\mathrm{O}}^{+}+{\mathrm{e}}^{-}$ was obtained. The dissociation mechanisms are attributed to different KER distributions. The autoionization process is identified below the ${\mathrm{CO}}^{2+}$ double ionization threshold. Full article

Based on the detailed term accounting approach, the relationship between extreme ultraviolet conversion efficiency and plasma conditions, which range from 5 to 200 eV for plasma temperature and from 4.63 × 10¹⁷ to 4.63 × 10²² cm⁻³ for plasma density, is studied for lithium plasmas through spectral simulations involving very extended atomic configurations, including a benchmark set of autoionizing states. The theoretical limit of the EUV conversion efficiency and its dependence on sustained plasma time are given for different plasma densities. The present study provides the necessary understanding of EUV formation from the perspective of atomic physics and also provides useful knowledge for improving EUV conversion efficiency with different technologies. Full article

Electron-impact ionization (EII) processes are essential for modelling high-temperature plasma in quite different research areas, from astrophysics to material science to plasma and fusion research and in several places elsewhere. In most, if not all, of these fields, partial and total EII cross sections are required, and often for a good range of electron energies, in order to determine, for instance, the level population of ions and spectral line intensities in plasma under both local and non-local thermodynamic equilibrium conditions. To obey these needs, various kinds of semi-empirical EII cross sections have been applied in practice, often simply because of the large computational demands in dealing explicitly with two free electrons within the continuum. Here, we expand Jac, the Jena Atomic Calculator, to provide such empirical EII cross sections for (most) atoms and ions across the periodic table. Five empirical models from the recent literature have been implemented to support a simple and rapid access to the partial EII cross sections for electrons from a (partly filled) shell ${(n\ell )}^q$ as well as the total ionization cross sections. We here restrict ourselves to the direct part of the EII cross section, whereas the impact excitation of electrons with subsequent autoionization and the resonant electron capture with double autoionization have been left aside in this first implementation. Rapid access to the (direct) EII cross sections will help already to better understand the role of electron-impact processes in the diagnostics of fusion plasma or the interpretation of astrophysical spectra. Full article

The lowest single and doubly excited autoionizing states of neon have been studied using a non-monochromatic electron beam and a high-resolution electrostatic analyzer at incident electron energies from 43.37 to 202 (±0.4) eV at three ejection angles, 40°, 90° and 130°. The 2s2p⁶3s(^3,1S) and 2s2p⁶3p(^3,1P) as well as the 2p⁴3s3p doubly excited states have been observed and their energy determined. The influence of the PCI effect in the energy region of the 2s2p⁶3s(^3,1S) states has been investigated. New features in the ejected electron spectra in the low kinetic energy region 3–20 eV at 202 eV incident energy have been observed and assigned. Full article

The generally accepted pathway to Local Thermodynamic Equilibrium (LTE) in atomic physics, where collision rates need to be much larger than radiative decay rates, is extended to complex autoionizing states. It is demonstrated that the inclusion of the non-radiative decay (autoionization rate) on the same footing, like radiative decay, i.e., the LTE criterion $n_{e,crit}\times C\gg A+\Gamma$ ($n_{e,crit}$ is the critical electron density above which LTE holds, $C$ is the collisional rate coefficient, and $A$ is the radiative decay rate) is inappropriate for estimating the related critical density. An analysis invoking simultaneously different atomic ionization stages identifies the LTE criteria as a theoretical limiting case, which provides orders of magnitude too high critical densities for almost all practical applications. We introduced a new criterion, where the critical densities are estimated from the non-autoionizing capture states rather than from the autoionizing states. The new criterion is more appropriate for complex autoionizing manifolds and provides order of magnitude reduced critical densities. Detailed numerical calculations are carried out for Na-like states of aluminum, where autoionization to the Ne-like ground and excited state occurrences are in excellent agreement with the new criterion. In addition, a complex multi-electron atomic-level structure and electron–electron correlation are identified as simplifying features rather than aggravating ones for the concept of thermalization. Full article

We measured spectra of low energy electrons emitted in the interaction of singly charged Ne⁺ ions with the Mg surface at incident ion energies ranging from 50 eV to 4 keV. The study examines issues related to the excitation of both the surface and the bulk plasmons of the target. We will also focus on the dynamics of the production of the singlet Ne2p⁴(¹D)3s² and triplet Ne2p⁴(³P)3s² autoionizing states of projectiles scattered in a vacuum. The threshold behavior of the autoionization lines show that double excitation occurs simultaneously in a single scattering. The predominant excitation of the triplet state indicates the importance of charge rearrangement and the electron correlation effects during the collisional excitation. Full article

Opacity determines radiation transport through material media. In a plasma source, the primary contributors to atomic opacity are bound–bound line transitions and bound-free photoionization into the continuum. We review the theoretical methodology for state-of-the-art photoionization calculations based on the R-matrix method as employed in the Opacity Project, the Iron Project, and solution of the heretofore unsolved problem of plasma broadening of autoionizing resonances due to electron impact, Stark (electric microfields), Doppler (thermal), and core-excitations. R-matrix opacity calculations entail huge amount of atomic data and calculations of unprecedented complexity. It is shown that in high-energy-density (HED) plasmas, photoionization cross sections become 3-D energy–temperature–density-dependent owing to considerable attenuation of autoionizing resonance profiles. Hence, differential oscillator strengths and monochromatic opacities are redistributed in energy. Consequently, Rosseland and Planck mean opacities are affected significantly. Full article

This work discusses studies of electron emissions during the interaction of low energy (in the keV energy range and below) singly charged ions with Aluminum surfaces. Analysis of the spectra provides insight into the electronic excitation processes and the dynamics of the interaction of the projectiles with the surface excitation. The work is primarily focused on the clarification of the role of electron promotion in charge exchange processes that occur during the cascade of atomic collisions. The work highlights the importance of the solid environment and of electron correlation in the understanding of charge exchange and energy deposition in ion-solids interactions. Full article

In this paper, we investigate the applicability of different theories on the intensity-dependent ionization rate for C²⁺ atomic targets at different laser wavelengths (frequency) and at linear polarization. We use the analytical formulas and draw conclusions, from numerical comparison with the results from ab initio ‘two-state model’ R-matrix Floquet calculation, on their correct predictions of the ionization rate. The single-photon ionization has been studied in the vicinity of the 1s² (²P^o)2pns (¹P^o), n = 5–12 autoionizing resonances at non-perturbative laser intensity. The results obtained from Perelomov–Popov–Terent’ev and Ammosov–Delone–Krainov models are compared in a region away from resonance where the two-state model description is not as good. To quantify the deviation between theoretical models, we analyze the ratio between different data sets as functions of the Keldysh parameter. We conclude that the results obtained with the model of Perelemov–Popov–Terent’ev are the closest to the ab initio R-matrix Floquet calculation. Full article

The results of experimental and theoretical studies on electron-impact excitation of the $6s6p P_1^{°}{}_{ }{}^3\to 6s^2 S_0^{ }{}_1{}^{}$ (λ190.8 nm) and $6s7s S_0^{ }{}_1{}^{}\to 6s6p P_1^{°}{}_{ }{}^3$ (λ179.9 nm) intercombination transitions in the single-charged thallium ion are presented. The crossed-beams technique was used in combination with a spectroscopic method in the experiment. A distinct structure revealed in the cross-sections of both lines results from electron decay of atomic autoionizing states and radiative transitions from upper ionic levels. The dominant mechanism of the structure formation was the Coster–Kronig process. Relativistic distorted wave calculations were performed to obtain emission cross-sections for the above transitions. The absolute values of the cross-sections were found to be (0.25 ± 0.08) × 10⁻¹⁶ cm² (λ190.8 nm) and (0.10 ± 0.04) × 10⁻¹⁶ cm² (λ179.9 nm) at the electron energy of 100 eV. Full article

An extensive study of photoionization from neon excited states was performed. The R-matrix approach was applied to calculate a photoionization cross-section from the metastable $2p^5{(}^2{P}_{J_f})3s{\left[K\right]}_{0,2}$ and dipole-allowed $2p^5{(}^2{P}_{J_f})3s{\left[K\right]}_1$ states. The resonance structures and Cooper minimum accessible in photoionization from the excited states by the photons with energy below 30 eV were analyzed. The parameters of the lowest autoionizing states (AISs) of even parity were extracted by fitting of the photoionization cross-section. For the dipole-allowed states, calculations are presented for unpolarized, linearly and circularly polarized radiation. Full article

The atomic structure of californium is probed by two-step resonance ionization spectroscopy. Using samples with a total amount of about $2\times {10}^{10}$ Cf atoms (ca. 8.3 pg), ground-state transitions as well as transitions to high-lying Rydberg states and auto-ionizing states above the ionization potential are investigated and the lifetimes of various atomic levels are measured. These investigations lead to the identification of efficient ionization schemes, important for trace analysis and nuclear structure investigations. Most of the measurements are conducted on ${}^{250}$Cf. In addition, the isotope shift of the isotopic chain ${}^{249-252}$Cf is measured for one transition. The identification and analysis of Rydberg series enables the determination of the first ionization potential of californium to $E_{\mathrm{IP}}=50,666.76\left(5\right)$${\mathrm{cm}}^{-1}$. This is about a factor of 20 more precise than the current literature value. Full article