Quantum defect theory (QDT) is concerned with the properties of an electron in the field of a positive ion and, in particular, with expressing those properties in terms of analytical functions of the energy. It provides a unified theory of bound states, including series perturbations, autoionisation and electron-ion scattering, both elastic and inelastic. The main emphasis of the review is on the foundations of the theory. Properties of Coulomb functions are discussed in some detail and outline sketches are given of relevant topics in collision theory and radiative theory. One-channel and many-channel QDT are discussed separately. Applications to the following problems are considered: resonances, atomic collision calculations, systems with two energy levels of the ion core, helium, other rare gases, alkaline earths and other atomic systems, molecular hydrogen, dielectronic recombination.

https://iopscience.iop.org/article/10.1088/0034-4885/46/2/002/pdf

Quantum defect theory

The XUV spectra of free metal atoms obtained by absorption, photoion, photoelectron and two-photon spectroscopy are reviewed. The experimental results are discussed in comparison with the predictions of the different theoretical approaches. Emphasis is put on the basic excitation and de-excitation processes, the importance of electron correlation, on common features of groups and on trends along series.

https://iopscience.iop.org/article/10.1088/0034-4885/55/7/002/pdf

XUV spectroscopy of metal atoms

The purpose of this review is to describe some spectacular effects of many-electron interactions in certain core levels in heavy atoms, as well as to give an overview of closely related phenomena in molecules, solids and adsorbates. The central concept will be what we shall call giant Coster-Kronig (gCK) fluctuation and decay of a hole level, involving ineraction of a single hole with configurations with primarily two holes and one excited electron. In systems with an open “valence shell” structure. i.e. with empty levels spacially as compact as the occupied ones, the interaction process can become extremely strong and lead to a breakdown of the one-electron picture. In some cases, there is even a complete breakdown of the quasi-particle picture, in which case the spectral strength has no prominent discret features and rather shows a continuum-like distribution.

Breakdown of one-electron pictures in photoelectron spectra

An apparatus is described in which merged beams of ground-state Ba+ ions and energy-resolved radiation from the Daresbury Synchrotron Radiation Source were used to obtain absolute cross sections for the photoionisation of Ba+ in the wavelength range 420-775 AA (29.5-16.0 eV). Approximately 75 peaks were resolved, the largest of which corresponded to cross sections greater than 10-15 cm2; this indicated very large contributions from inner-shell excitation-autoionisation. Systematic errors were estimated to be less than +or-15% and the energy resolution ranged between 20 and 4 meV. The energies of some of the peaks can be correlated closely with series limits of inner-shell transitions of neutral barium measured by Connerade et al. (1979) and with energies of autoionising states of Ba+ calculated by Hansen (1974-5) and by Griffin et al. (1984) in Hartree-Fock approximations. The energies of some transitions also correspond quite closely with absorption lines observed by Roig (1976) by flash pyrolysis of barium vapour.

http://iopscience.iop.org/article/10.1088/0022-3700/19/24/015/pdf

Measurements of absolute cross sections for the photoionisation of Ba+ ions

Publisher Summary The chapter presents a description of the elementary collision processes associated with the production and capture of free charged particles in gases at thermal energies, up to a few electron volts, apart from photoionization that only commences with photons of several volts. The chapter explores the unresolved problems, inconsistencies, and discrepancies in the current literature. Photodissociation is treated only as a competitor for photoionization or photodetachment on the basis that are concerned with charged systems principally. The chapter focuses on photodetachment, because of the great increase in activity in the review and also on multiphoton ionization. The chapter discusses the specific processes: dissociative recombination and attachment, collisional detachment and ionization, ion-pair formation, ion-ion neutralization, photoionization, dissociative ionization, multiphoton ionization, and photodetachment. The chapter also presents the currently held physical pictures of the most important processes to give a sense of the orders of magnitude of their rates and cross sections, and to provide an entry into the enormous literature in this field.

Elementary attachment and detachment processes. I

New observations of the 5p spectra of Cs I and Ba I are reported. The extreme complexity of the structure does not permit precise configuration labels to be attached to all the excited levels. Nevertheless, more than 160 transitions in Ba I have been ordered into 14 series converging on experimentally known levels of the parent ion. An attempt has been made to analyse the limit structure by comparison with the data for Cs I and Hartree-Fock atomic structure calculations. The results obtained are consistent with previous interpretations of the double ionization anomaly in Ba I. Further comments are made on the comparison between experiment and the predictions of the r.p.a.e. theory for 5p excitation in Ba I. It is shown that the 5p$^{6}$ 6s $^{2}$S$\_{\frac{1}{2}}\rightarrow $ 5p$^{5}$ 6s$^{2}$ $^{2}$P$\_{\frac{1}{2}}$ and $^{2}$P$_{\frac{3}{2}}$ transitions of Cs I have been incorrectly identified and new assignments are proposed.

D. H. Tracy

Papers

A Study of 5p Excitation in Atomic Barium I. The 5p Absorption Spectra of Ba I, Cs I and Related Elements

Collective Excitation in the Ba I Absorption Spectrum