Autoionization Probabilities Research Articles

Halogen loss induced by electron collisions in halouracils at low energies

SynopsisWe employed both eletronic scattering techniques and ab initio quantum dynamics to study electron-driven reactions in halouracils at low energies. Particularly, it was addressed halogen dissociation and H-loss probabilities in anionic 5-BrU and 5-IU. Besides, we also propose a model of how to include auto-ionization probabilities during the dynamics.

The energy structure and decay channels of the 4p6-shell excited states in Sr

The ejected-electron spectra arising from the decay of the 4p autoionizing states in Sr atoms have been studied precisely at the incident-electron energies close to excitation and ionization thresholds of the subshell. The excitation behaviors for 58 lines observed between 12 and 21 eV ejected-electron kinetic energy have been investigated. Also, the ab initio calculations of excitation energies, autoionization probabilities and electron-impact excitation cross sections of the states 4p (nl = 4d, 5s, 5p; = 4d, 5s, 5p; = 5s, 6s, 7s, 8s, 9s, 5p, 6p, 5d, 6d, 7d, 8d, 4f, 5g) have been performed by employing the large-scale configuration-interaction method in the basis of the solutions of Dirac–Fock–Slater equations. The obtained experimental and theoretical data have been used for the accurate identification of the 60 lines in ejected-electron spectra and the 68 lines observed earlier in photoabsorption spectra. The excitation and decay processes for 105 classified states in the 4p55s, 4p54d and 4p55s configurations have been considered in detail. In particular, most of the states lying below the ionization threshold of the 4p6 subshell at 26.92 eV possess up to four decay channels with formation of Sr+ in 5s, 4d and 5p states. Two-step autoionization and two-electron Auger transitions with formation of Sr2+ in the 4p6 ground state are the main decay paths for high-lying autoionizing states. The excitation threshold of the subshell in Sr has been established at 20.98 ± 0.05 eV.

Classification of the 5p5nln'l' LSJ energy levels of Cs excited by 30 eV electrons

Theoretical investigation of the 5p5 nl(L1S1)n'l' LSJ autoionizing states of Cs was performed by using large scale configuration interaction calculations of energy levels, autoionization probabilities and excitation cross sections obtained in the Dirac–Fock–Slater approximation. Classification of calculated energy levels in the LSJ coupling scheme of angular momenta and simulation of the intensities of ejected Auger electron spectrum were performed. The classified energy levels in the region from the excitation threshold up to 17.365 eV and simulated intensity spectrum were used for identification of the experimental ejected-electron spectrum of Cs excited by 30 eV electrons.

Classification of the 5p5n1l1n2l2 LSJ autoionizing states of Cs excited by 30 eV electron-impact

SynopsisClassification of the 5p5n1l1 (L'S')n2l2 LSJ autoionizing states of Cs excited by 30 eV electron-impact was performed by using more accurate calculations of energy levels, excitation cross sections and autoionization probabilities obtained in the Dirac-Fock-Slater approximation. Calculated parameters were used for simulation of the ejected Auger electron intensity spectrum and identification of the experimental ejected-electron spectrum of Cs excited by 30 eV electrons taking into account the asymmetry of the angular distribution of Auger electrons.

On spectroscopic classification of autoionizing levels in Rb atoms

Large scale configuration interaction calculations of energy levels, autoionization probabilities and electron-impact excitation cross sections of the autoionizing states 4p5nln'l' (nl = 5s, 4d, 4f,n'l'=4d, 5s, 5p, 5d, 5f, 5g, 6s, 6p, 6d, 7s, 7p) of Rb up to 19.6 eV were performed for the first time. The relativistic effects were taken into account in the Dirac-Fock-Slater approximation. The calculated data were used for the novel classification of the lines in an experimental ejected-electron spectrum.

Theoretical study of the 4p5nln′l′ autoionizing states of Rb excited by electron impact

Large scale configuration interaction calculations of energy levels, autoionization probabilities and electron-impact excitation cross sections of the autoionizing states 4p5nln′l′ (nl = 5s,4d,5p,4f; n′l′ = 4d,5s,5p,5d,5f,5g,6s,6p,6d,7s,7p) of Rb up to 19.6 eV were performed for the first time. The relativistic effects were taken into account in the Dirac–Fock–Slater approximation. The asymmetry parameters of the angular distribution of Auger electrons for a number of electron-impact excited states were calculated in a single configuration intermediate coupling approximation. Strong dependence on the energy of exciting electrons was found for the angular distribution of the emitted Auger electrons from the states with J > 3/2 at the polar ‘magic angle’ of 54.73°. The calculated data were used for the novel classification of lines in an experimental ejected-electron spectrum.

Dielectronic recombination lines of C+

The present paper presents atomic data generated to investigate the recombination lines of C ii in the spectra of planetary nebulae. These data include energies of bound and autoionizing states, oscillator strengths and radiative transition probabilities, autoionization probabilities, and recombination coefficients. The R-matrix method of electron scattering theory was used to describe the C2+ plus electron system.

Theoretical study of the 5p6-core autoionizing states of Cs excited by electron impact

Large scale configuration interaction calculations of energy levels, autoionization probabilities and electron-impact excitation cross sections of the autoionizing states 5p5nln'l'LSJ (nl=6s,5d, n'l'=5d,6s,6p,6d,7s,7p,8s) of Cs were made for the first time.

The 4p6 autoionization cross section of Rb atoms excited by low-energy electron impact

The autoionization cross section of rubidium atoms was obtained by measuring the total normalized intensities of ejected-electron spectra arising from the decay of the 4p5n1l1n2l2 autoionizing levels. The electron impact energy range from the 4p6 excitation threshold at 15.31 up to 50 eV was investigated. The cross section reaches the maximum value of (2.9 ± 0.6) × 10−16 cm2 at 21.8 eV impact energy. The general behaviours of the cross section and the role of particular autoionizing configurations in its formation were considered on the basis of large-scale configuration interaction calculations of energies, cross sections, autoionization probabilities in 5snl(n ⩽ 7; l ⩽ 4) and 4d nl(n ⩽ 5; l ⩽ 2) configurations as well as the measured excitation functions for the lowest levels in 5s2 and 4d5s configurations. The resonance behaviour of the cross section between 15.3 and 18.5 eV impact energy is caused exclusively by the negative-ion resonances present close to the excitation thresholds of the (5s2)2P and (4d5s)4P autoionizing levels. At higher impact energies, the autoionization cross section is composed of contributions from the high-lying quartet and doublet levels in 4d5s, 5p and 5s5p, 5d, 6s, 6p configurations. From the comparison of the present data with available experimental and calculated ionization cross sections, the 5s + 4p6 direct ionization cross section of rubidium atoms was determined with the maximum value of (7.2 ± 2.2) × 10−16 cm2 at 36 eV. It was also found that the 4p6 excitation–autoionization is the dominant indirect ionization process contributing over 30% of the total single ionization of rubidium atoms by electron impact in the 15.3–50 eV energy range.

Theoretical study of the 5p5nln′l′ autoionizing states of Cs

Large-scale configuration interaction calculations of energy levels, autoionization probabilities and electron-impact excitation cross-sections of the autoionizing states 5p5nln′l′ (nl=6 s, 5d; n′l′ =5d, 6s, 6p, 6d, 7s, 7p, 8s) of Cs were performed for the first time. The relativistic effects were taken into account in the Dirac–Fock–Slater approximation. The calculated data were used for the novel classification of lines in an experimental ejected-electron spectrum.

The role of the Jahn-Teller coupling in dissociative recombination of H3O+ and H ions

We discuss the importance of the non Born-Oppenheimer Jahn-Teller coupling in the dissociative recombination of H3O+ and H ions with low energy electrons. The coupling is known to be important for dissociative recombination of H, for which detailed and simplified theoretical models have been previously developed. In this study we discuss how the detailed theoretical model reproduces individual resonances in the experimental spectrum of H. We present a simplified theoretical model of the dissociative recombination of H3O+. Similarly to H, we consider the Jahn-Teller coupling as a key factor responsible for the capture of the electron into a Rydberg state associated with excited vibrational levels of the ion. We use a simplified Jahn-Teller model for highly symmetrical ions (not only for H3O+ and H) and adopt the normal mode approximation for the vibrational states of H3O+. Using the multi-channel quantum defect formalism and accurate ab initio calculations, we derive the cross sections for electron capture by the ion that is initially in the ground vibrational level. In our approximation, once the electron is captured by the ion, the autoionization probability is neglected compared to the predissociation probability. This allows us to evaluate the cross section for the dissociative recombination of H3O+ using just a few parameters obtained from ab initio calculation. The total cross section obtained is in good agreement with data from storage ring experiments.

Stochastic autoionization of a relativistic two-electron atom

Stochastization of a relativistic two-electron atom, which leads to diffusion autoionization, is studied. Using the Chirikov criterion, the condition of stochastization of the outer relativistic electron under the action of the inner electron is obtained. The diffusion coefficient, the autoionization time, and the autoionization probability are calculated.

The WC10 central stars CPD-56 8032 and He 2-113 -III. Wind electron temperatures and abundances

We present a direct spectroscopic measurement of the wind electron temperatures and a determination of the stellar wind abundances of the WC10 central stars of planetary nebulae CPD−56° 8032 and He 2–113, for which high-resolution (0.15-A) UCLES echelle spectra have been obtained using the 3.9-m Anglo-Australian Telescope. The intensities of dielectronic recombination lines, originating from autoionizing resonance states situated in the C2++e− continuum, are sensitive to the electron temperature through the populations of these states, which are close to their LTE values. The high-resolution spectra allow the intensities of fine-structure components of the dielectronic multiplets to be measured. New atomic data for the autoionization and radiative transition probabilities of the resonance states are presented, and used to derive wind electron temperatures in the two stars of 21 300 K for CPD−56°8032 and 16 400 K for He 2–113. One of the dielectronic lines is shown to have an autoionization width in agreement with the theoretical predictions. Wind abundances of carbon with respect to helium are determined from bound–bound recombination lines, and are found to be C/He=0.44 for CPD−56° 8032 and C/He=0.29 for He 2–113 (by number). The oxygen abundances are determined to be O/He=0.24 for CPD−56° 8032 and 0.26 for He 2–113. The effect of optical depth on the temperature and abundance determinations is investigated by means of a Sobolev escape-probability model. We conclude that the optically thicker recombination lines can still be used for abundance determinations, provided that their upper levels are far from LTE.

Analysis of correlation effects in autoionizing doubly excited states of barium using Coulomb Green's function

Though one would expect that large-angular momentum doubly excited states exhibit weak electronic correlations, it is shown in this paper that a first-order perturbation theory ignoring such correlations may completely fail in predicting correct autoionization probabilities: quadrupolar transitions are poorly described by lowest-order perturbation theory, except for very large angular momenta. Inclusion of second-order dipole-dipole term considerably improves the accuracy of the method. This effect is computed using Coulomb Green’s function in its analytical form, probably applied here for the first time to autoionization processes. Examples are given in barium for 5dj 5g [k] states (j = 3/2, 5/2) and for 5d5/2nl [k] states with l > 4.

Spectrum of plasma containing Ne- and Na-like ions: consistent account for Rydberg and autoionizing Rydberg series in balance equations

This is a theoretical study of argon plasma under conditions, where Ne- and Na-like ionization stages are dominant. Balance equations are generalized to treat 37 3l states of Ne-like ion and 37 adjacent series of Rydberg states of Na-like ion simultaneously. This allows us to include in the kinetics a diffusion-like motion of the state of the system "Ne-like ion plus one electron" through the multitude of excited levels spread over an energy region of 50 eV. The populations Ni - i (i = (1, ..., 37)) of the Ne-like ion states are introduced explicitly; those of adjacent 37 Rydberg series are accounted for through continuous functions Ni(ε). These functions describe the population distribution within each Rydberg series dependent on the Rydberg electron enegy ε. The elementary processes of the collisionalradiative model connecting all Ne- and Na-like states, as well as processes of redistribution of populations inside each Rydberg series, are accounted for. The rate coefficients for all processes within the Ne-like residue have been calculated previously, using a detailed many-body relativistic theory. The dielectric capture cross sections and autoionization probabilities are presented as analytical continuation of the collisional excitation cross sections. The excited-excited states transitions are included. The rest of the processes are treated in a simple semiclassical approximation. The Lotz formula is generalized by unambiguous analytical continuation to cover the case of bound-bound transitions between Rydberg states of Na-like ions. The radiation reabsorption in a long plasma cylinder is included through Biberman-Holstein coefficients for all transitions. The inclusion of Na-like states, accounting for diffusion-like processes, increases the population inversion for the "lasing candidates" by at least a factor of two for a wide range of plasma conditions. This is important for the ionization equilibrium too. Besides, the functions Ni(ε) bear diagnostic information. Detailed calculations have been done for the homogeneous steady-state Maxwellian plasma. The role of transient processes in the population inversion creation is under discussion.

Exchange and fine-structure effects in autoionization of large-angular-momentum doubly excited Rydberg states.

Autoionization probabilities of doubly excited atoms with one electron in a large-l Rydberg state are investigated through first-order perturbation theory in a single-configuration approach. In the framework of the jk-coupling scheme, the formalism takes into account multipoles higher than dipoles in the electronic interaction and exchange effects. Exchange effects are computed using the Froese method for Slater integrals and are demonstrated to be significant as long as l5. The essential importance of fine-structure transitions when the outer electron is in a large-l state is emphasized. Comparison with available experimental data shows that Slater integrals are reliably computed and that fair agreement is generally obtained for g states, while even strongly autoionizing f states are at least qualitatively represented. Correlation effects may explain some remaining disagreement with the measured autoionization widths.

Resonant dissociative recombination of H3+

A recently reported peak near 9.5 eV in the cross section for electron impact dissociative recombination of vibrationally cold H[sub 3][sup +] is due to four H[sub 3] resonance states. Using a complex Kohn, electron scattering variational method to obtain resonance energies and widths and a time-dependent wave packet calculation of the dissociation dynamics in the presence of autoionization, we obtain excellent agreement with the position, shape, and magnitude of the reported feature. Vibrational excitation of H[sub 3][sup +] is a dominant competing channel due to a high probability of autoionization during dissociation.

Investigation of double capture in Ne8++He, H2, by electron spectroscopy at 80 keV. II. Experimental results

For pt.I see ibid., vol.24, no.7, p.1695-72 (1991). A thorough analysis of electron spectra measured at 80 keV in collisions between Ne8+(1s2) ions and helium and molecular hydrogen targets is presented. The 1s23lnl' and 1s24lnl' lines are identified up to n=9. With helium, a fitting procedure for 1s23lnl' lines (all the lines for n=3, 4, and the first 3s5l ones), observed for the first time at different angles, make it possible to compare the authors' experimental results with two different theoretical data sets: their own theoretical values for positions, autoionization and radiative probabilities obtained by the AUTOLSJ method, which are given in a companion paper, and those recently published by Bachau et al. using a Feshbach method with a model potential. This allows the authors to test both theoretical positions and lifetimes of (3,n) states. The relative weight of the n=3, 4 and the first n=5 states is often angle dependent. The apparent, angle dependent, branching ratio for autoionization into the 2p and 2s continua of the (3,3) and the first (3,4) states is determined for several cases and discussed. Finally, differential and total cross sections are given and compared with other available experimental results.

Investigation of double capture in Ne8++He, H2, by electron spectroscopy at 80 keV. I. Theoretical considerations

Positions, autoionization and radiative probabilities of Ne8+ (1s23l3l'), Ne8+ (1s23l4l') and some Ne8+ (1s24l4l') states have been calculated with the AUTOLSJ method and compared with other recent theoretical data. These results will be compared with the authors' experimental electron spectra in a companion paper. The theoretical basis of the electron spectra analysis discussed in the companion paper is also established. Various kinds of perturbations of the electron lineshapes and angular distributions (autoionization from a Stark state, post-collision effects and interferences) are presented in a unified fashion and discussed.

Dielectronic recombination process in laser-produced tantalum plasmas.

The 3d-4f and 3d-5f emission patterns occurring between 4.5 and 6.5 A\r{} in the spectra of laser-produced plasmas of highly ionized tantalum are studied, and the pseudocontinuum which underlies the former is interpreted. For the transition subarrays suspected to be responsible for the pseudocontinuum, the average wavelengths and widths are computed ab initio in the formalism of spin-orbit-split arrays; these subarrays are proven to cover a continuous band of wavelengths. For their intensities, the dielectronic recombination (DR) process populates efficiently the upper configurations, which, in most cases, lie above the ionization levels. The autoionization and radiative-transition probabilities, and the DR coefficients, are computed ab initio, in the form of averages for whole configurations or subconfigurations of Cu-like tantalum. The compact formulas for these averages are valid in intermediate coupling, avoiding any diagonalization. All the needed radial integrals are computed by means of a relativistic parametric potential central-field code.