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The quark model of the atomic f shell, in which the angular properties of the states of the electrons are represented by the quark configuration (s+f)⁴, is generalized by assigning spins I=2 and 1 to the s and f quarks. This provides ⁵s+³f as a basis for the 26-dimensional irreducible representation (IR) of F₄ and corresponds to the inclusions F₄ contains/implies SO_I(3)*G₂ contains/implies SO_I(3)*SO_L(3). The shift operators I_+or- make it possible to connect different IR's of SO(8) such as the three IR's of dimension 8, any one of which can be taken to span the 8 states of s+f in virtue of the automorphisms of that group. The additional symmetries associated with null triality are thereby subsumed. Application of the Wigner-Eckart theorem to SO_I(3) provides a new perspective on relations between matrix elements of atomic operators within the f shell.

The accuracy of transition probabilities calculated from orthogonal operators is increased by including valence correlations in the transition matrix. In this way, the interaction with infinitely many configurations can be included perturbatively without the need to deal with huge matrices. The effectiveness of the method is exemplified by a comparison of the transition array 5d⁹ to 5d⁸6p in Hg IV, Tl V, Pb VI and Bi VII calculated with the strongly interacting 5d⁸6s taken into account both explicitly and using the above perturbative procedure.

We show that for configurations of the type 3p^M3d^N the influence of core excitations, in particular the excitation 3p² to 3d², is important both for energies and oscillator strengths. The latter is particularly true for transitions of the type 3p⁶3d^N to 3p⁵3d^N+1 due to the fact that the core excitation in the ground configuration leads to a configuration which has an allowed dipole transition to the excited state. Differences and similarities between 3p⁵3d^N and the more extensively studied 3p⁶3d^N configurations are pointed out and illustrated with several examples concerning highly charged ions. The conclusion is that, while core excitations are important for the description of transitions out of the open 3d shell, they are absolutely essential for transitions out of the closed 3p shell, gf values are given for astrophysically important transitions in Ni X.

A new recoil correction to the Lamb shift of order ( mu ³/M²)(Z alpha )⁴ has been found. This correction depends on the nuclear spin, and is associated with the absence of a zitterbewegung term in the Breit Hamiltonian for spin 0 and 1 nuclei.

We report on the first high-resolution (35-40 meV at 25 eV) and high-intensity electron spectra following L-shell ionization or excitation of laser-excited Na atoms by electron impact. The laser excitation was either 3s to 3p₃2/ or 3s to 3p₃2/ to 4d₅2/. Many Auger/autoionization states which are not accessible from Na atoms in the ground state are observed for the first time. For example, the energies of Auger states 2s2p⁶nl^1,3L, with nl=3p, 4p, 5p, 5d, 6d have been measured with high accuracy; they agree very well with theoretical values obtained by Zatsarinny (1993, 1994).

The neon 2p⁴¹S^e triple differential cross section (TDCS) of photoionization has been measured under equal energy sharing and displays two additional lobes compared with the related TDCS of helium. A model for the TDCS is derived which parameterizes the relative magnitudes and phases for the epsilon S epsilon p and epsilon p epsilon d continuum channels in addition to an electron correlation factor and reproduces the experimental data very well. This model traces the appearance of these additional lobes to the existence and interference of two continuum channels in the final state wavefunction. The present measurement serves as a test case for the more complicated treatment of energy- and angle-resolved double photoionization in rare gases other than helium,.

The contributions of all higher-order phaseshifts (virtually up to infinity), as given by Ali and Fraser (1977), to the total momentum-transfer and viscosity cross sections for elastic electron-atom (e-a) collisions are evaluated analytically. The magnitude and significance of these contributions are discussed in terms of the dependence on the truncation value of the electron angular momentum l* at low and intermediate electron energies, and on the polarization characteristics of the rare-gas atom. The importance of the quadrupole polarization potential in determining the respective truncation errors is pointed out.

We present a systematic study of electron-impact direct total ionization cross sections and associated spin asymmetries of lithium-like targets, Li, ..., O⁵⁺, calculated using the convergent close-coupling (CCC) method. For these targets we believe that the CCC method yields very accurate total ionization cross sections below the autoionization threshold. Agreement with experiment is found to be quite varied; and is satisfactory only for the Li, N⁴⁺ and O⁵⁺ targets. The ionization spin asymmetries become more substantial at high energies as the ionic charge increases, and are still significant at 1 keV for the e-O⁵⁺ ionization system.

The Auger decay rates of the 1s_1/2^(e)ns_1/2^(μ) state in the exotic mesic helium atoms is calculated. The smallness of the ratio β = m_e/m_μ ≈ 1/200 enables us to take into account the electron-meson interaction in the second-order perturbation theory and present the probability of the process in a closed analytic form.

An atom or a molecule is constituted by a set of bound electric charges with dynamics governed by the laws of quantum mechanics. These charges are sources of the quantized electromagnetic field which also binds them together, and thus the effects of their interaction with the field cannot be disregarded in principle. Consequently even overall neutral atoms, on which we focus our attention, are driven by a dynamics which is inextricably related to the dynamics of the quantized electromagnetic field. One of the most prominent aspects of the atom-held interaction is the existence of a cloud of virtual photons which dresses the atom even in the lowest possible energy state of the system. In this paper we review the static as well as the dynamic aspects of the theory of the virtual cloud around the neutral atoms. We begin by reviewing various forms of the atom-field coupling as well as various models of simplified atoms which will be used in the rest of the paper. The question then arises as how to characterize quantitatively the shape of the virtual cloud, and we show that the energy density of the electromagnetic field is a physical quantity suitable for this purpose. First a perturbative approach to calculating this shape is developed and applied to several physical models of a ground-state dressed atom. The next step is to consider virtual clouds which are out of equilibrium and examine their time development. This leads to the concept of half-dressed sources, which are discussed in a different physical context both in the absence and in the presence of real photons. In particular the role of the virtual cloud in ensuring causality of the field propagating in dressing and undressing processes is emphasized. Finally, the nature of the virtual cloud is further discussed in the light of theories concerning the dynamics of an atomic pair, the quantum theory of measurement and the effects of a driving electromagnetic field.

We calculate the energy spectrum of doubly excited ^1,3S^e equivalent-electron states, within a semiclassical planar model. The latter is based on the so-called asynchronous-mode configuration, proposed previously by Grujic and Simonovic (1991). Making use of the propagation method with regularization classical configurations with very complex structure and surprisingly rich kinematics have been computed. Semiclassical energies of singlet and triplet intra-shell S states are evaluated. Numerical results show that the model improves values for the ¹S^e states, obtained by other authors within the asymmetric-stretch collinear kinematics, but overestimates triplet-state energies. The general features of the model are discussed and compared with other proposed classical configurations.

A simple semiclassical model for fast estimations of outer- and inner-subshell ionization energies of atoms and ions is presented. The energies are calculated from the quantization condition with analytical potential. The quantum defect approximation for Rydberg states is developed and the relativistic correction is derived. Comparisons with experimental data and more accurate numerical calculations are made. The accuracy of the model is found to be comparable with that of the Hartree-Fock-Slater approximation.

L-shell X-ray production cross sections for ₂₉Cu, ₃₁Ga, ₃₂Ge, ₃₅Br, ₃₉Y, ₄₂Mo, ₄₇Ag, ₅₀Sn, ₅₇La, ₆₀Nd, ₆₄Gd, ₆₇Ho, ₇₀Yb, ₇₉Au and ₈₂Pb were measured for carbon ions in the 2-12 MeV energy range. The copper to yttrium data were previously measured using a Si(Li) detector with a beryllium window, while the molybdenum to lead X-rays were counted with a windowless Si(Li) detector, whose efficiency was determined by various normalization techniques. The measured X-ray cross sections are compared to predicted X-ray cross sections from the first Born and the ECPSSR theories. Using single-hole fluorescence yields, at low projectile velocities the first Born approximation overpredicts the data by as much as one order of magnitude while the ECPSSR theory at worse underestimates the data by about a factor of four. For the highest projectile velocities relative to target L-shell electrons, both theories converge toward the data to almost within experimental uncertainties. After modifying the fluorescence yield to account for multiple outer-shell ionization, the ECPSSR theory is brought into better agreement with the data for light targets but still overestimates the data for heavier targets. Also, the 2 MeV measurements using doubly ionized carbon ions are still significantly underestimated by the ECPSSR theory even after accounting for multiple ionization. Our multiple ionization correction assumes that the target foil thickness is thin enough to insure single collision conditions. However, in order to obtain acceptable signal-to-noise ratios, the target foils used in this experiment were too thick to provide single collision conditions. For thicker foils, ions are stripped inside the target resulting in an appreciable fraction of higher charge states that enhance the ionization of outer shells and the effective fluorescence yields. This effect could account for the remaining discrepancy between the ECPSSR theory and the data.

Theoretical oscillator strengths for transitions between the even 4s²4p² and the three odd (4s4p³, 4s²4p⁴d, 4s²4p5s) configurations of As⁺ are reported. Both ab initio, multiconfiguration Hartree-Fock and semiempirical configuration interaction techniques are used. The uncertainty of the results are estimated, by considering the reproduced energy structure, the agreement of results from the two methods and cancellation effects in the transition elements. For the strongest transitions the estimated uncertainty is about 10%.

The double circular 4f5g J=3 levels of barium are investigated using the jj-coupled eigenchannel R-matrix method together with the multichannel quantum defect theory. The calculation reproduces well the excitation spectra of 4f5g J=3 levels observed recently from the 5d5g J=2 levels and confirms that the 4f5g levels have low autoionization rates. For the first rime, excitation spectra calculated using the isolated-core excitation approximation are compared with those determined using dipole matrix elements calculated explicitly within the reaction volume. The influence of channel mixing on the characteristics of 4f5g J=3 levels is carefully analysed. Correlation effects are found to be responsible for the narrow autoionization widths of 4f5g J=3 levels. The behaviour of 4f5g levels is compared to that of higher-lying 4fng J=3 levels.

The angular distributions of photoelectrons ejected from polarized atoms are derived for different coupling cases and presented in vector form convenient for different applications. It is shown that the differences between two angular distributions corresponding to two orthogonal linear or circular polarizations (which are called linear or circular dichroism in the angular distribution, respectively), or to two directions of the initial atomic polarizations (opposite or mutually perpendicular), enables one to extract from experimental data more specific information on atoms. In particular, one can extract from these differences the state multipoles characterizing the initial atomic polarization. The general theory is illustrated by calculations for the 6p subshell of Tl atoms in the region of the 6s6p²(²D₃2/) autoionization resonance. The results can be applied to photoionization of any np¹ subshell in autoionization resonances with J=³/₂ or ⁵/₂.

The dual laser produced plasma (DLP) technique, has been used to study the EUV absorption spectra of germanium plasmas in the 50-400 AA region. The absorbing plasmas were produced by focusing the output of a Q-switched ruby laser onto a germanium target while the background continuum was produced by focusing the output of a Nd:YAG oscillator/amplifier system onto a samarium target. At a time delay of 50 ns and an absorbing plasma column length of 2 mm the spectrum recorded was predominantly due to Ge²⁺. We have investigated 3d and 3p sub-valence transitions. In the former, 3d¹⁰4s² to 3d⁹4s²nf and 3d⁹4s²np series converging on the ²D_5/2 and ²D_3/2 limits of Ge³⁺ were identified by isoelectronic comparison and Hartree-Fock calculations. In the 3p case no strong line transitions were observed which was shown by many body calculations to result from line broadening caused primarily by super Coster-Kronig decay of the 3p hole.

The channel-distorted wave method is combined with a two-centre description of the ejected electron state to embrace target ionization as well as capture-to-continuum aspects of the ionization process in energetic ion-atom collisions. The target-ionization peak, the electron-capture-to-continuum cusp and the binary ridge appear in the appropriate regions of momentum space of the ejected electron but each one of these features is perturbed by the presence of the other collision partner. In the cusp and in the binary-ridge regions, the essential effect of the residual target ion is represented by a modified Compton profile in which the momentum distribution of the initial electron state has been replaced by a corresponding form factor. Similarly, in the region of low electron energies with respect to the target nucleus, the standard expression for target ionization is modulated by a suitable off-energy-shell factor. The present theory agrees well with experimental data.

A systematic search was performed for the manifestation of quantum interference effects in the shape and angular distribution of the binary-encounter electron peak in collisions of partially stripped, or structured, heavy ions with noble gases and molecular hydrogen. The ionic species investigated were Cu^5+,19+, I^7+,23+, Au^11+,29+ and U¹³⁺, all at the same nominal velocity equivalent to 0.6 MeV amu^-1. Experimental double-differential cross sections for secondary electron emission in the binary encounter energy region are compared with a simple model based on the elastic scattering of quasi-free target electrons in the projectile field as well as with results of impulse approximation (IA) calculations. While these calculations provide a good qualitative overall description of the observed quantum effects for noble gas targets, this is not the case for H₂ targets. An attempt was made to incorporate target molecular structure into the impulse approximation code by allowing the binary electron amplitudes from each of the hydrogen atoms, assumed to constitute the H₂ molecule, to interfere. This approach, while demonstrating the strong influence of molecular orientation upon the intermediate energy region of the cross sections, did not meet with success, thereby indicating the necessity to consider the final-state interaction of the binary electron with the two protons of the residual H₂⁺ or H₂²⁺ target.

Vibrationally elastic, rotationally-summed cross sections for electron collisions with CH₄ are calculated with ab initio static+exchange (SE) interactions and using a symmetry-adapted, single-centre expansion (SCE) representation for the close-coupled (CC) equations. The dynamical correlation forces are included through a local density-functional theory (DFT) approach. Both integral and differential cross sections are calculated at the Ramsauer-Townsend (RT) minimum and at energies close to it. Comparisons with experiments and with previous calculations show that the present approach exhibits one of the best overall accords with measurements while still keeping the computational effort within modest limits.

Absolute total cross sections for electron scattering on GeH₄ molecules have been measured between 75 and 4000 eV. The dependence of the total cross section on energy is revealed to be different than for the lighter hydrides, CH₄ and SiH₄. It has been noticed that in the high energy limit the total cross sections for hydrides tend to the values of the corresponding isoelectronic noble gases. Self-consistency between different experimental partial cross sections and the absolute total cross section is checked for CH₄, SiH₄ and GeH₄. Upper limits for dissociation into neutral fragments are set for these gases. By comparison of the partial cross sections selected in this way, a conclusion on different partitioning schemes for noble gases and noble-like molecules (CH₄, SiH₄, GeH₄) is drawn. At intermediate energies the inelastic processes dominate the electron scattering on molecules. At 100 eV the elastic cross sections are almost equal for the pairs CH₄-Ne, SiH₄-Ar, GeH₄-Kr.

Triple differential cross sections (TDCS) have been calculated for ionization of helium-like lithium ions (Li⁺) by fast electron impact in the asymmetric geometry. The correlated final state wavefunction in the present model satisfies the asymptotic three-body boundary condition. The initial channel wavefunction involves a Coulomb wavefunction due to the long range Coulomb attraction between the incident electron and the screened ionic nucleus. TDCS results have been computed For three different incident energies: 250 eV, 500 eV and 1000 eV for fixed values of the ejected energies (5 eV and 10 eV) and scattering angles (4 degrees and 10 degrees ). An intense recoil peak (even larger than the binary one for low momentum transfer) is noted in the TDCS structure at lower incident energies. The binary to recoil ratio (b/r) is studied against the momentum transfer (q).

A pulsed crossed beam technique incorporating time-of-flight spectroscopy (previously developed in this laboratory) has been used to study the single and double ionization of ground-state oxygen atoms produced by an iridium tube furnace source. Relative cross sections sigma ₁ and sigma ₂ for single and double ionization have been determined in the respective energy ranges 14.1-2000 eV and 90-2000 eV which are wider than any previous measurements. These values have been normalized by reference to the absolute values of sigma ₁ previously measured by Brook et al at energies (50-997 eV) where the possible influence of an unknown admixture of long-lived excited atoms in their experiment should be small. However at energies below about 25 eV our values of al become much smaller than the values of Brook at al. Above 40 eV, the present ratios sigma ₂/ sigma ₁ are in satisfactory agreement with other measurements which extend to 400 eV. Our values of al have been compared with selected theoretical predictions. Some evidence of structure in the energy dependence of sigma ₁ in the region of the cross section maximum is consistent with predicted contributions from inner-shell 2s ionization and from autoionization in addition to outer 2p electron removal.

Total ionization cross sections for positron impact on He, Ar and H₂ targets from threshold to around 300 eV are presented and compared with values obtained by summing available partial cross section results. Reasonable accord is found with these data. The experimental method and apparatus is described in detail together with the systematic reliability checks.

The dipole polarizabilities of the 1, 2¹S and 2³S states of helium-like ions (2<or=Z<or=10) are calculated through a variation-perturbation approach. The polarized wavefunctions are constructed by using selecting B-spline basis functions in a CI scheme. The values of alpha _d are converged to an accuracy of at least 0.01% and are in excellent agreement with those obtained from explicitly correlated wavefunctions and with experiment (Leonard and Barker, 1975).

Various properties of the muonic ( mu ^-) and antihydrogen nuclei (p^-, d^- or t^-) helium atoms (³He or ⁴He) in their ground bound states with L=0 are presented. The properties of the helium-antihydrogen systems have been computed with high accuracy for the first time. It is shown that all such systems have the sharp two-body cluster structure. The problems of hyperfine splitting in the helium-muonic atoms and antiproton (antihydrogen nuclei) conservation are considered. The symmetry properties for the bound state spectra are also discussed.

The two dominant lasing lines in the Ne-like yttrium laser exhibit an apparently anomalous behaviour. The line at 157.14 AA demonstrates a marginally higher gain than that at 154.97 AA despite the fact that the latter is observed to be up to 100 times brighter. We describe a convincing explanation for this behaviour in terms of the 155 AA line being composed of two partially overlapping transitions-the existence of such an overlap (between a J=0-1 and a J=2-1 transition) has long been the suspected cause. We demonstrate that the intrinsic line shapes (in contrast to the gain narrowing of the spectral line shape) narrow along the path of the beam due to variations in electron density and ion temperature which arise as the beam refracts through the expanding medium. These variations are shown to be sufficient to partially decouple the lines at long plasma lengths. The resulting effect is that the beam undergoes large amplification initially, producing the observed intensity ratio between the 155 AA and 157 AA emission, but that the gain at 155 AA is reduced at the longer lengths examined experimentally.

Models for calculating time-dependent ionization are compared in simulations of a germanium plasma for conditions which are relevant to collisionally pumped X-ray lasers. A simple two-level model, which treats each ion stage with a ground state and single excited state, is compared with the collisional radiative model in the hydrogenic approximation and the time-dependent average atom model in simplified single Lagrangian test cell problems. A method for determining ionic populations from average atom level populations (Djaoui and Rose 1992) is investigated. The results show good overall agreement between the three models, but indicate that the two-level model underestimates stepwise ionization via excited states. The importance of this limitation is considered in more detail in full simulations of a collisionally pumped system with a 1.5D hydrodynamic code. The two-level model is tested against the hydrogenic collisional radiative approach and the importance of detailed modelling of the sodium- and neon-like ion stages is also considered. The simulations indicate that the discrepancies between the two-level and hydrogenic models are small. A detailed description of the neon-like ion stage, considering both enhanced ionization from the metastable levels and dielectronic recombination, is found to be more significant when evaluating the neon-like population.