Excitation Autoionization Research Articles

Absolute cross sections for electron-impact single ionization of Kr4+, Kr5+, and Kr7+ ions.

Absolute total cross sections for electron-impact single ionization of Kr[sup 4+], Kr[sup 5+], and Kr[sup 7+] ions have been measured using a crossed-beams technique from below threshold to 500, 400, and 500 eV, respectively, with absolute uncertainties of 12% or less. The measured cross sections are in good agreement with distorted-wave calculations and show significant contributions from excitation autoionization. Nonzero cross sections below threshold for the Kr[sup 4+] and Kr[sup 5+] ions suggest the presence of metastable ions in those two species that were extracted from an electron-cyclotron-resonance ion source. Such evidence of metastable ions was not found in the case of the Kr[sup 7+] ion measurment.

Excitation-autoionization contributions to the electron-impact ionization of Kr4+-Kr7+

We report on calculations for the electron-impact ionization of the krypton ions Kr[sup 4+]--Kr[sup 7+]. Results are first obtained from a configuration-average distorted-wave calculation. Multiplet-resolution and channel-coupling effects are investigated using an [ital LS]-coupled nonrelativistic [ital R]-matrix method for the excitation autoionization of Kr[sup 6+] and Kr[sup 7+], and the inclusion of resonance effects is discussed. Term dependence and relaxation effects, which are found to be significant, are incorporated through the use of pseudo-orbitals. These [ital R]-matrix results are found to be in good agreement with [ital LS]-coupled distorted-wave calculations. Final results for the excitation-autoionization contributions for Kr[sup 6+] and Kr[sup 7+] are obtained in a semirelativistic intermediate-coupled distorted-wave approximation, including both term dependence and relaxation effects. Comparison to experiment is made where available.

Exafs like oscillations in X-ray excited autoionization spectra assisted by compton process

Oscillating features have been observed in the background of the inelastic electrons excited by high energy monochromatic X-ray photons at kinetic energy above the Cr M 2,3 VV Auger transition. These structures are absent when X-ray photons of h ̵ ω ⩽ 1.5 keV are used. We suggest that these extended features are due to an autoionization process experienced by the 3 p core electron assisted by a Compton scattering with the X-ray photons. Through the inelastic Compton process the incoming photons excite the core electrons towards empty states above the Fermi level in a continuous manner up to a maximum energy, which strongly depends on the incident photon energy. These excited states are modulated in energy by the crystalline structure as it has been shown by means of the EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy. These oscillating structures are compared with those obtained through electron excitation for the same Auger transition in the Extended Fine Auger Structure (EXFAS) spectroscopy.

Calculation of doubly excited even strontium states

The perturbation theory with zero order model potential has been successfully used for calculating the spectral characteristics (energies, radiative lifetimes, autoionization widths) of highly excited Rydberg and autoionization states of rare earths. Here, the same method is applied for investigating the 4dns, 4dnd and 4dng doubly excited Rydberg states of Sr. The obtained energies of the states are in good agreement with the experimental data of other authors. The calculation aims to give a more complete and reliable information about the experimentally observed spectra, as well as to predict the location of more highly excited states.

Resonant width and energy determined by photoionization from excited divalent atoms.

Explicit examples are presented to illustrate the presence of a more symmetric resonant structure in spectra from bound excited states of a divalent atom. Comparison between the theoretical ground-state and bound excited-state photoelectron spectra shows that both the resonant energy ${\mathit{E}}_{\mathit{r}}$ and the resonant width \ensuremath{\Gamma} of a doubly excited autoionization state of a divalent atom (e.g., alkaline-earth-metal atom) can be determined experimentally from photoionization originated from a bound excited state more effectively.

Resonance contributions to the electron-impact ionization of few-electron highly charged ions

We present results for the electron-impact ionization of Li-like, Be-like, and B-like Fe, Kr, and Xe ions. We have included the contributions from direct ionization, excitation autoionization, and dielectronic-capture double autoionization in our calculations. We discuss isonuclear and isoelectronic trends for the various contributions, focusing on the resonance contributions in particular.

Widths of the doubly excited resonances of two-electron atoms below the n=2 threshold.

We present an alternative theoretical procedure for the positions and widths of the doubly excited resonances of a two-electron atom based on a configuration-interaction procedure for continuum by using a B-spline-based finite basis set. More specifically, by examining the energy variation of the probability density corresponding to the singly excited ionization channel, the resonant energy ${\mathit{E}}_{\mathit{r}}$ of the doubly excited autoionization state can be directly linked to the energy at the minimum probability density. The width of the resonance can be identified with the energy separation between two energies, which are located on the opposite side of the center of the resonance and each corresponds to a probability density twice the minimum value at ${\mathit{E}}_{\mathit{r}}$. The effectiveness of this procedure is demonstrated by the excellent agreement between our calculated widths of the Feshbach resonance for ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ and He below the n=2 threshold and the ones from the most accurate existing theoretical calculations.

Double processes ine ? ? He collisions

Different electron-helium collision processes which can lead to a joint change of state of both target electrons are reviewed. They are examined with the help of a deliberately simplified plane wave Born series model. Certain terms which are second order in the interaction potentials are shown to correspond to contributions obtainable in first order models using distorted wave descriptions of the projetile and/or wavefunctions for two-electron subsystems which take account of the Coulomb interaction between the electrons. The remaining terms are related to double-hit mechanisms where the projectile interacts first with one and then with the other target electron. Applications to double excitation (autoionization), ionization with excitation and double ionization are discussed.

Atomic-structure effects in multiphoton ionization of magnesium.

We present an application of a simple configuration-interaction approach to multiphoton ionization of magnesium, using a finite ${\mathit{L}}^{2}$-basis set constructed from a nearly complete set of one-particle frozen-core Hartree-Fock orbitals. The effects due to multielectron interactions to two- and three-photon processes at energies both near and away from the resonant structures dominated by singly excited intermediate states and doubly excited autoionization states are examined in detail. Our calculation has shown that the multiphoton spectra are mostly dominated by the resonant structures due to intermediate singly excited bound states. Such spectra are qualitatively different from the single photoionization spectrum, which often exhibits a strong presence of doubly excited autoionization structures. Our calculated results are in close agreement with other existing experimental and theoretical results for single- and two-photon ionizations.

Photodissociation dynamics of doubly excited Rydberg states of molecular hydrogen

We have applied photofragment ion imaging to investigate the dissociation dynamics of low-lying, doubly excited states of molecular hydrogen. A doubly excited electronic state is one in which both of the hydrogen electrons reside in excited molecular orbitals. Two-step, two-color multiphoton excitation of H2, first via 201.8 nm, two-photon excitation into the E, F 1Σ+g(vE=0, J=1) state, followed by ∼563 nm, 1+m (m=1, 2) excitation through the B″ 1Σ+u(v=0, J=0, 2), D 1Πu(v=2, J=1, 2), and B′ 1Σ+u(v=4, J=0, 2) states provides a ready means of populating several low-lying doubly excited states of H2 at increasing internuclear separations. From these doubly excited repulsive states, both dissociation and autoionization processes are possible. Because the excitation energy remains relatively constant as each intermediate state is accessed, differences in the photodissociation dynamics via each state can be ascribed directly to the effects of changing internuclear separation and electronic symmetry of the intermediate and dissociative states. H+ fragments detected from each photodissociation pathway are distinguished by their differing velocities, determined from an ion image.

Distorted-wave calculations of cross sections for electron-impact ionization of lithiumlike ions

We used distorted-wave and multiconfigurational Dirac-Fock methods to calculate direct and indirect contributions to cross sections for electron-impact ionization of ${\mathrm{C}}^{3+}$, ${\mathrm{N}}^{4+}$, ${\mathrm{O}}^{5+}$, and ${\mathrm{F}}^{6+}$. The features due to excitation autoionization and resonant excitation double autoionization observed in recent cross-beam measurements [G. Hofmann et al., Z. Phys. D 16, 113 (1990)] are well reproduced in our results. Our cross sections are also in good agreement with reported results of nonrelativistic close-coupling calculations [S. S. Tayal and R. J. W. Henry, Phys, Rev. A 42, 1831 (1990); 44, 2955 (1991)] over most of the energy range studied. We examine the effects of radiative decay on the intermediate states, and we show that neglect of certain channels in the close-coupling calculation results in overestimation of the cross sections in the region of the 1s2s2pnl resonances.

Total cross sections of electron-impact ionization ofAr7+

The electron-impact single-ionization cross section of ${\mathrm{Ar}}^{7+}$ has been measured from 18 to 1186 eV using a crossed-beams technique. The present results are higher than previous measurements at the higher energies and are in good agreement with recent distorted-wave calculations that include contributions from excitation autoionization. Below the threshold for direct ionization at 144 eV, the cross section contains contributions due to space-charge modulation and excitation of metastable ions in the target beam into autoionizing states. These transitions are predominantly spin allowed and dipole forbidden. Scaling laws for sodiumlike ions are discussed.

Measurement of electron-impact single-ionization cross sections of Ar8+

A crossed-electron-ion-beam experiment was used to measure the total ionization cross section for ${\mathrm{Ar}}^{8+}$ in the energy range from 50 to 600 eV. The present results are roughly in agreement with the previous measurements of Defrance et al. [Nucl. Instrum. Methods Phys. Res. B 23, 256 (1987)] above 450 eV. However, the present measurements show a significant contribution below the threshold for direct ionization. The present measurements agree with the distorted-wave calculations of Pindzola et al. [Phys. Rev. A 41, 1375 (1990)] when 3% of the incident ion beam is assumed to be in metastable states and excitation autoionization from metastable states is included.

Experimental and theoretical cross sections for electron-impact ionization of Ti5+

Absolute cross sections for electron-impact ionization of Ti{sup 5+} have been measured from below threshold to 1500-eV collision energies. Distorted-wave calculations including direct ionization and excitation autoionization from the 3{ital s}{sup 2}3p{sup 5} ground state and from the 3{ital s}{sup 2}3p{sup 4}3d excited configurations have been performed. Comparison of experiment and theory indicates that about 33% of the incident ions were in metastable configurations. Ionization rate coefficients and fitting parameters derived from the experimental results are also presented.

4d-shell photoionization of Xe, Cs, Ba, Sm, Eu and Yb atoms studied by photoion-yield spectra

A comparative study of multiple photoionization in the 4d ionization region (78-270eV) of M = Xe, Cs, Ba, Sm, Eu and Yb atoms is reported. Partial photion-yield spectra for Mn+ (n = 1-4) ions have been measured by using synchrotron radiation as a light source. Main products in Xe, Cs and Ba are Mn2+ and M3+ ions due to the creation of a 4d vacancy. In Sm and Eu, a large fraction of M+ ions are populated via the 4d-4f resonant excitation and subsequent autoionization to the 4f ionization continuum, which is squeezing the population of multiply-charged ions to about 50%. In Yb, a large amount of M+ ions are also populated, but are due largely to the direct 4f ionization. In all of the atoms studied, an appreciable or considerable amount of M4+ ions are observed, indicating the existence of photoelectron and/or Auger shake-off processes.

LINEAR STARK EFFECT OF THE n′dnl AUTOIONIZATION STATES OF Ca AND Sr ATOMS IN THE LOW-FIELD REGION

The electric field effect on the 3dnl and 4dnl doubly excited autoionization states of Ca and Sr atoms have been observed using a multisiep laser excitation technique. The Stark spectra show that splittings of the Stark levels are linearly dependent on the applied electric field in the lowfield region of F5. We have observed also that the autoionization resonance broadens with increasing electric field because of field-induced mixing of nearly degenerate 1 substates, which can play important role for increase of dielectronic recombination rate in plasmas.

Optical studies of multiply excited states

Optical studies of multiply-excited states are reviewed with emphasis on emission spectroscopy. From optical measurements, properties such as excitation energies, lifetimes and autoionization widths can be determined with high accuracy, which constitutes a challenge for modern computational methods. This article mainly covers work on two-, three- and four-electron systems, but also sodium-like quartet systems. Furthermore, some comments are given on bound multiply-excited states in negative ions. Fine structure effects on transition wavelengths and lifetimes (autoionization) are discussed. In particular, the most recent experimental and theoretical studies of multiply-excited states are covered. Some remaining problems, which require further attention, are discussed in more detail.

Tm autoionization rydberg states in the vicinity of the third and fourth atomic ionization limit

The energies of about 200 autoionization Rydberg states of the thulium atom have been measured in the 57380–58960 cm−1 energy region by laser multistep excitation and subsequent autoionization with an accuracy of ±0.4 cm−1. More than a half of the states have been grouped in series. New spectroscopic information has been derived. Three-step two-colour effective excitation schemes have been proposed for thulium atom detection.

Experimental cross sections for electron-impact ionization of chromium ions: Cr6+, Cr7+, Cr8+, and Cr10+

Absolute cross sections have been measured from below threshold to 1500 eV for electron-impact single ionization of chromium ions in initial charge states 6+, 7+, 8+, and 10+. The measurements utilized the Oak Ridge National Laboratory Electron Cyclotron Resonance ion source and electron-ion crossed-beams apparatus. The lowest three charge states, which have 3${s}^{2}$3${p}^{n}$ ground-state electron configurations, all exhibit ionization onsets at lower energies than expected for ground-state ions, indicating the presence of ions in metastable configurations in the incident beams. Significant contributions to the total cross sections by excitation autoionization are observed for each ion. The present measurements are compared to calculations of direct ionization from the Lotz formula, and the ${\mathrm{Cr}}^{10+}$ measurements are compared to previous data for the isoelectronic ion ${\mathrm{Ni}}^{14+}$.

HIGH-RESOLUTION MEASUREMENTS OF ELECTRON-IMPACT IONIZATION CROSS SECTIONS FOR LI-LIKE IONS

For crossed electron and ion beams we have developed an experimental technique which allows to measure cross sections for electron impact ionization of ions with relative uncertainties less than 0.1 %. At this level of accuracy it is possible to do ion spectroscopy of intermediate excited states contributing to net ionization via single and multiple autoionization processes. For ions B2+, C3+, N4+ and O5+ along the Li isoelectronic sequence we have performed quantitative measurements of state-resolved contributions from excitation-autoionization (EA), resonant-excitation-double autoionization (REDA) and resonant-excitation auto-double-ionization (READI).