The authors attempt to give a comprehensive discussion of observations of atomic negative-ion resonances throughout the periodic table. A review of experimental and theoretical approaches to the study of negative-ion resonances is given together with a consideration of the various schemes that are used for their classification. In addition to providing, where possible, tabulated data for the energies, widths, and symmetries of these states, the authors also attempt to highlight regularities in their behavior both within groups of the periodic table and along isoionic sequences.

Atomic negative-ion resonances

Publisher Summary This chapter discusses a method of calculating the position and width of each doubly excited state of atoms and the approximate selection rules for different excitation processes. There are many theoretical approaches which are capable of predicting an accurate position and width of each doubly excited state. These methods, such as the configuration interaction method, the Feshbach projection technique, the close-coupling method, and the complex coordinate rotation technique have provide significant data required for sorting out the systematics of doubly excited states. A classification scheme of doubly excited states using the correlation quantum numbers K, T , and A is presented in the chapter. The adoption of quantum numbers in the asymptotic region for the description of doubly excited states seems unsatisfactory because important correlations occur in the region where the two electrons are close to each other.

Doubly excited states, including new classification schemes

Energy Levels and Classifications of Doubly-Excited States in Two-Electron Systems with Nuclear Charge. Z = 1,2,3,4,5, below the N = 2 and N = 3 Thresholds

The ejected-electron spectra of highly excited autoionising levels of Li, Be, B and C have been studied by using the projectile electron spectroscopy method. Singly and doubly core-excited states are strongly populated in 100 to 500 keV Li+, Be+, B+ and C+ single collisions with He and CH4. High resolution is obtained by selecting a small observation angle (6.4 degrees ) with respect to the beam axis. Numerous states, previously not seen, have been resolved and identified by comparison with existing theoretical data. For a large number of core-excited states, absolute Auger transition and excitation energies are given. In particular, isoelectronic sequences for singly core-excited lithium-like (Z=2-16) and doubly core-excited helium-like (Z=1-14) autoionising states are studied.

High-resolution projectile Auger spectroscopy for Li, Be, B and C excited in single gas collisions. I. Line energies for prompt decays

We investigate whether the intrashell portion of the double-excitation spectrum of the helium atom may be interpreted in terms of collective rotational and bending vibrational motion, analogous to the ro-vibrational motion of molecules. Earlier work by us had identified a few levels in each atomic shell as having a rotorlike spectrum cutoff at higher energies, analogous to the cutoff rotorlike spectra of some nuclei. In this paper we use the I and d supermultiplets of the preceding paper of this journal, to interpret the entire intrashell manifold for each principal quantum number N, as resembling cutoff rotational and bending vibrational levels of a highly nonrigid linear symmetric XYX ''molecular'' structure. We obtain a good qualitative fit of each intrashell spectrum using leading-order ro-vibrational energy formulas from molecular spectroscopy including anharmonicity, centrifugal distortion, and rotation-vibration coupling. We interpret these effects for low-lying intrashell states of two-electron atoms, and find them to be consistent with an average equilibrium configuration theta/sub 12/=180/sup 0/ for the interelectronic angle. Analysis of the time scales of the proposed rotational, bending, and other degrees of freedom suggests approximate separability. We also identify parts of the spectrum analogous to molecular l doubling, and indicate a possible link betweenmore » the related Coriolis force and a recently proposed mechanism for decay by autoionization. Certain aspects of our collective interpretation of the atomic supermultiplets are discussed which have close parallels in nuclear physics, including the Elliott model and the SU(6) interacting boson model for nuclear collective motion.« less

Ro-vibrational collective interpretation of supermultiplet classifications of intrashell levels of two-electron atoms

A procedure is used which combines the truncated diagnoalization method with the open-channel close-coupling approximation in the calculation of widths of autoionization states in two-electron systems. The method is capable of systematically and efficiently calculating the widths for all two-electron systems. Calculations are made for $^{1}P^{0}$ and $^{3}P^{0}$ helium, and $^{1}P^{0}$ hydrogen below the $N=2$ threshold. In particular, it is found that the lowest member of the third series [classified as $(2,3c)$)] has a width of 4.61 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}7}$ eV, four times smaller than any previous calculation. It is also shown that the (+)-(-) classification used by many authors is inadequate to describe these states. Detailed comparison is made with the three- and six-state close-coupling and other calculations.

Calculation of autoionization widths for two-electron systems

ENERGY LEVELS AND CLASSIFICATIONS OF TRIPLY EXCITED STATES OF Li, Be+, B2+, AND C3+

The energies, effective quantum numbers, classifications, and configuration mixings of the triply excited states of lithium in the energy range of 140eV to 160eV above the ground state are described in detail based on calculations using the T.D.M. method. A total of 30 Rydberg series with symmetries, S, S, P , P , and D, are investigated. The perturbation of a Rydberg series by an isolated state, or by another series, is examined and discussed. Differences in the literature on the designation of certain states is pointed out, and our results are compared with recent theoretical and experimental data. We observe that the configuration interactions previously described for two-electron systems (i.e., the mixing of the (2s) and (2p) S states), remain when a third

Hollow states of lithium

A computational method is described for obtaining inner-shell-vacancy states of three-electron atoms which combines a block-diagonalization procedure with generalized Feshbach projection operators applicable to systems with three or more electrons. Typically, the accuracy is about 1.5 parts per thousand (\ensuremath{\delta}E/E\ensuremath{\approxeq}1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$). The strength of the method is that it provides many energy levels for each Rydberg series. A quantum-defect analysis is then applied that identifies the members of each series and yields reliable quantum defects and series limits. The method is particularly important in symmetries for which multiple Rydberg series exist. The present work reports on $^{4}$${\mathit{P}}^{\mathit{e}}$ states of three-electron systems with 3\ensuremath{\le}Z\ensuremath{\le}10, which are compared with other calculations. The energies of $^{2}$${\mathit{S}}^{\mathit{e}}$ states of ${\mathrm{C}}^{3+}$ are also presented as an example requiring both the multielectron Feshbach projection operators and the quantum-defect analysis developed here. Four distinct Rydberg series are found for this case and their series limits obtained.

M. J. Conneely

Papers

Energy Levels and Classifications of Doubly-Excited States in Two-Electron Systems with Nuclear Charge. Z = 1,2,3,4,5, below the N = 2 and N = 3 Thresholds

Calculation of autoionization widths for two-electron systems

ENERGY LEVELS AND CLASSIFICATIONS OF TRIPLY EXCITED STATES OF Li, Be+, B2+, AND C3+

Hollow states of lithium

Three-electron systems with inner-shell vacancies