Table of contents

The level of accuracy reached in the recent Lamb shift measurements for the multicharged ions requires the introduction of the non-resonant corrections which take into account the deviation of the natural line profiles from the simple Lorentz form. The frequency omega _max, corresponding to the maximum of the frequency distribution, differs from the energy difference Delta E: omega _max,= Delta E+ delta , where delta is the non-resonant correction. The value of delta , calculated for the process 2s+ gamma to 2_P3/2 to 1s+ gamma ' used recently for the Lamb shift measurement in hydrogenic phosphorus, is delta =-4.47*10^-5 eV, while the measured transition energy is: Delta E=EZ_P3/2-E_2s=2.23133(12) eV. This means that the correction delta becomes noticeable. However, the 2s-2_P1/2 energy shift measured in Li-like uranium is not affected by this correction.

A compilation of accurate energies at the numerical Hartree-Fock (NHF) level is reported for the anions from H^- to I^-. New values are presented for the transition metal and heavier anions. Excellent agreement is obtained for the rest of the anions with previous NHF results.

The paper is aimed at an extension of the global theory of spectral lineshapes which has been recently applied to the Paschen alpha line and which is known to yield lineshapes of spectral lines broadened by one type of charged perturber. This extension has been attempted in order to describe the profiles of the lines broadened by both electrons and ions. Numerical calculations are made for Paschen alpha , Paschen beta and Brackett alpha , and comparison with quasistatic theory and also with calculations from the program of Vidal, Cooper and Smith (1971) for Paschen a gives good agreement, mainly in the wings.

Laser optical pumping of caesium atoms in a high magnetic field of 1.514 T is reported. Steady absorption spectra from hyperfine Zeeman sublevels are obtained with two semiconductor lasers. The populations in different hyperfine Zeeman sublevels show unusual changes when the power of the pumping laser is changed. The optical pumping and the spin flip by hyperfine interaction bring about the matching of atoms from the lowest to the highest M_I sublevels of the ground state.

Two different forms of the spectroscopic factor for the shake-up satellites, one of the general many-body theory and another of a simple rearrangement model, proved to be equivalent when a particular form of one-electron orbitals was employed. This choice of the orbitals is illustrated by calculating the helium atom ionization with excitation to the n>1 ion states.

We study the redistribution of Rydberg states subject to half-cycle electromagnetic pulses as a function of the field strength and the duration of the pulses. It is found that the survival probability exhibits oscillations which are associated with the almost recurrent propagation of the electronic wavepacket in angular momentum space. The classical and quantal origin of the different Stark beat frequencies is discussed. The dependence of the beats on the shape of the pulse and effects due to a non-hydrogenic core are analysed. The momentum transferred by the field is identified as a fundamental variable for the excitation function.

Ion translational energy spectrometry has been used to determine the lifetime against spontaneous (unimolecular) dissociation of doubly charged molecular ions by studying the kinetic energies of fragment ions. Analysis of the shapes of the fragment ion kinetic energy distributions yields information on the lifetime of the precursor molecular ion. A value of 0.75+or-0.25 mu s is deduced for the lifetime of SH²⁺ ions. Correlation of this value with the shapes of calculated potential energy curves of low-lying electronic states of SH²⁺ enables deductions to be made of the importance of spin-orbit induced coupling between the lowest ⁴ Sigma ^- and ² Pi states and provides information on the vibrational level population in the doubly charged molecular ion.

The importance of ionization as an intermediate channel in the capture process H⁺+H to H+H⁺ is estimated by use of a simple model. It is found that for energies greater than about 40 keV the model predicts that capture via ionization contributes significantly to the total capture cross section.

An experimental investigation has been carried out to probe the scaling of single ionization cross sections of molecular targets with the projectile charge q. The cross sections scale like a sigma ₀q^alpha power law, similar to the case of atoms. In all cases the fitted sigma ₀ is in agreement with the measured proton impact cross sections, as predicted by perturbation theory within the independent electron approximation. The exponent alpha is found to be slightly smaller than the theoretically predicted value of 2. The same scaling law was found to hold for bare and dressed projectiles indicating that screening is approximately complete for these soft collisions.

Antihydrogen formation cross sections are calculated for low-energy collisions of antiproton with positronium by the hyperspherical coupled-channel method. The scattering wavefunctions are expanded in terms of a large number of sector-adiabatic states associated with the n=1-4 states of antihydrogen and n=1-3 states of positronium in the asymptotic separated-atom region. The present formation cross sections differ considerably from those of the unitarized Born approximation reported by Mitroy and Stelbovics (1994). The disagreement becomes larger with decreasing collision energy owing to the different energy dependence of the cross sections. The authors have confirmed that the formation cross sections from the 2s and 2p states are larger than that from the ground state by one order of magnitude.

Relativistic multiconfiguration Dirac-Fock transition energies and oscillator strengths are studied for the 3s²3p-3s²3d transition along the aluminium isoelectronic sequence. For certain systems, Al, Si⁺, Rh³²⁺, Ta⁶⁰⁺ and U⁷⁹⁺, transition energies, as well as oscillator strengths obtained in different schemes of calculation, namely the extended average level (EAL), average level (AL) and optimal level (OL) schemes, are compared and discussed. The influence of nuclear charge distribution upon energy levels and transition energies obtained are also investigated.

The authors present extensive configuration interaction (CI) calculations of oscillator strengths, transition probabilities and lifetimes of upper levels of all transitions in Ar II between the levels of 3s²3p⁵, 3s3p⁶, 3p⁴3d, 3p⁴4s and 3p⁴4p states. The calculations incorporate the major correlation effects and in addition allow for the orbital functions (particularly 3p) to be effectively different for different states. Some discrepancies with experimental energies remain, particularly those of levels with a 3p⁴¹S core. Prior to the calculation of the oscillator strengths, the authors 'fine-tune' the CI coefficients using experimental energies. Specifically, the final CI coefficients are determined following small adjustments to the diagonal Hamiltonian matrix elements so that the calculated eigenvalue differences agree with the corresponding experimental energy separations. The work describes in detail previous calculations of quartet levels and extends the calculations to other symmetries. Comparison with recent, highly accurate cascade-free experiments shows close accord in all but a small number of transitions.

Motion of a neutral two-body system transverse to a uniform magnetic field breaks the axial symmetry and causes a shift of the wavefunction outward from the Coulomb well. Therefore the preferable position of the centre of the transverse basis for the wavefunction expansion depends on the motion. In strong magnetic fields, the most appropriate is a set of Landau functions which describe the states of free charged particles. The authors employed this basis shifted with respect to the Coulomb well and considered radiative transitions between eigenstates with different shifts. Transverse parts of the dipole matrix elements are calculated analytically. The expressions obtained enable one to apply the adiabatic approximation to the photoionization of a moving hydrogen atom with the choice of the transverse (Landau) parts of wavefunctions being localized in the Coulomb and magnetic wells for the initial (ground) and final (continuum) states, respectively. Contrary to the usual adiabatic approach, this method gives good agreement with more precise non-adiabatic calculations.

The angular distribution of photoelectrons produced by excitation of polarized atoms to autoionizing states is analysed. In particular, core-excited autoionizing states produced in absorption of synchrotron radiation by laser-pumped excited atoms are considered. A general expression for the angular distribution of arbitrarily oriented laser and synchrotron beams is obtained and further reduced to a simpler form for a particular geometry used in experiments. A case of an isolated autoionizing state (resonance) interacting with several continua is considered and parametrization of the observable parameters in the region of the resonance is discussed. Peculiarities of the circular dichroism in the region of autoionizing resonances are analysed. Formulae suitable for particular resonances with J<or=5/2 are presented and all parameters necessary for analysis of experimental angular distributions are tabulated. As examples, excitations of autoionizing 2p⁵3s3p and 2s3p² states in Na and 3p⁵3d4s4p states in Ca are considered. Theoretical angular distributions are compared with recent experimental data.

The soft X-ray photoabsorption and total photoion yield spectra of atomic Cu have been determined in the range of the L-edges. At the L₂ and L₃ thresholds there are clearly visible jumps in the absorption cross section. Superimposed on this step-like increase are weak but in part surprisingly sharp structures. The atomic data are compared both to Hartree-Fock calculations and the soft X-ray absorption of copper metal.

The authors present the carbon and fluorine X-ray emission spectra of methane and the fluoromethanes (CH_4-xF_x, x=0, 1, 2, 3 and 4), measured in high resolution. Relative X-ray emission intensities in dipole velocity and length gauges have been derived from the independent particle approximation using Hartree-Fock ground state molecular orbitals. The results of the calculations are compared with the experimental spectra. In the experimental carbon (K-V) emission spectrum of CF₄ the authors observe dipole-forbidden electronic transitions. They suggest that these transitions arise due to vibronic coupling, i.e. intensity is borrowed from the dipole-allowed transitions.

High resolution threshold photoelectron and photoion spectra of the oxygen molecule have been measured in the photon energy range 12-50 eV using a penetrating field electrostatic analyser and a synchrotron radiation source. Besides the well known X ² Pi _g, a ⁴ Pi _u, A ² Pi _u, b ⁴ Sigma _g^-, B ² Sigma _g^-, c ⁴ Sigma _u^- O₂⁺ states, several other series of vibrational structure are observed in the 21-24 eV range. One series is interpreted to result, not from an ion state, but from an autoionization process involving a neutral state and the nuclear continuum of an ion state. Above 26 eV, three relatively strong features are observed at approximately 27.3 eV, 32.5 eV and 39.5 eV, interspersed with several weaker ones. Dissociation processes from some O₂⁺ states have been studied using a threshold photoelectron-photoion coincidence technique (TPEPICO). These observations identify the weak, broad feature at 24.96 eV as the nu =2 level of the c ⁴ Sigma _u^- state.

The intensity distribution in the focal region of a laser beam is calculated for a Gaussian profile truncated by a circular aperture. The results give the theoretical ionization volume as a function of the laser intensity and this allows calculation of the multiple ionization of atoms by an intense laser. As an example, the yields of multiply charged ions of xenon are calculated for focused intensities in the range 10¹³ to 10¹⁶ W cm^-2. Oscillations in the yield curves are produced, with an implication that this behaviour may explain some experimental observations.

Collisional excitation of ammonia from the ground ¹A₁ electronic state to various optically-allowed and dipole-forbidden states has been studied in 1.8 keV collisions with H₂⁺, N₂⁺, O₂⁺ and CO₂⁺ projectiles by energy loss spectrometry using a high-resolution ion translational energy spectrometer. The collisional excitation cross section is found to be sensitively dependent on whether the projectile ion is of Sigma or Pi symmetry. This dependence is interpreted as a manifestation of wavefunction overlap effects involving the highest occupied molecular orbitals of the target and projectile ions. Results of concomitant theoretical calculations of overlap integrals for different orientations of each projectile-target system provide support for the wavefunction overlap picture.

A new relativistic continuum distorted wave model is presented. A first-order scattering amplitude for inner-shell electron capture is calculated, and the complete analytic form of the transition amplitude is obtained. Results for differential and total cross sections are presented for transitions with and without change of spin. Comparison with experimental data and other theoretical models is made. Agreement with experiment for total cross sections is good. Inconsistencies in the matrix distorted wave model are highlighted, and the correct form of the residual interaction is given for the first time. The utility of approximate relativistic wavefunctions are discussed, and a number of misunderstandings concerning their use in collision theory addressed.

Electron excitation and transfer in He²⁺-He collisions have been studied in an energy interval of 4-65 keV u^-1. The general features of these processes are discussed on the basis of two-electron molecular energies. These suggest a common mechanism for single-electron transfer and transfer-excitation transitions in slow collisions, and similarly for excitation and double-transfer transitions. Cross sections are determined within the semiclassical close-coupling method. At low energies, the strongest channel is resonant two-electron transfer. For the other, weaker channels the author finds that the population of any given state in the projectile is about equally strong as the equivalent state in the target. The results from this work compare well with data from a companion experimental study. The author also assesses a few one-electron transitions in collisions with metastable helium.

By means of photon emission spectroscopy the authors have studied state selective one-electron capture and target-ion excitation in collisions of He²⁺ with He. The collision energy has been varied from 1 to 75 keV amu^-1. Four-body classical trajectory Monte Carlo calculations have been performed in the energy range of 50-300 keV amu^-1. In the energy range where experiment and theory overlap there is in general fair agreement, although the l-distributions within a principal quantum shell exhibit differences. At energies below 50 keV amu^-1 the experiments confirm the results of atomic orbital calculations, presented in an accompanying paper by Fritsch (1994). Combining all the experimental and theoretical data the authors have determined a reliable cross section data base for use in neutral helium beam based plasma diagnostics on large tokamaks.

Following slow collisions between O⁸⁺ and Ar, double-electron capture into 4l4l' and 4l5l' states of O⁶⁺ has been reported. The experimental results show that in 25% of the events the two electrons are retained on the oxygen presumably due to radiative stabilization. In order to understand this high rate of stabilization, we have calculated for the first time the decay properties of all the 4l4l' and 4l5l' terms in the isolated atom using a truncated diagonalization method with B-spline basis sets. The 4l4l' states are lying in the same range of energy as the higher members of the 3lnl' Rydberg series and we have extended our B-spline approach to represent accurately the 3lnl' states with n up to 50. In this static approach, we obtain 30.8% radiative stabilization for the 4l4l' manifold and 2.6% for the 4l5l' manifold assuming in both cases a statistical population of the states. We conclude that these values, when combined with the experimentally determined initial populations of the two configurations, are too small to explain the experimental features assuming a statistical population distribution and we propose that in order to clarify the situation an experimental and/or theoretical study of the population distribution over the two configurations is necessary.

We simulated the population dynamics of target and projectile levels during the interaction of slow, highly charged ions and C₆₀. Our investigation is based on a semiclassical over-barrier description of long-range charge exchange and a spherical model for the C₆₀ cluster. It includes the transient formation of hollow atoms and relaxation of the projectile due to Auger cascades. As a first application, we consider slow Ar⁸⁺ projectiles and give estimates for the occupation changes of all coupled target and projectile levels, charge-state evolutions and energy-resolved spectra of emitted Auger electrons. Our results are in satisfactory agreement with recent experimental data.

We present results for the scattering of low energy electrons (<15 eV) by H₂O and H₂S. We use a single-centre formalism and include exchange exactly and polarization through a parameter-free model potential. The exchange equations are solved using a linear algebraic method. We have calculated rotationally summed differential cross sections for both H₂O and H₂S; comparison with experiment is good. We also give results for rotational state-to-state transitions in H₂O, calculated within the adiabatic approximation.

The differential cross section of the ionization of hydrogen in its n=2 degenerate state by electron impact is investigated theoretically by applying the correlated double continuum wavefunction satisfying the exact asymptotic conditions. The levels 2p₀, 2p_+or-1, being energetically undistinguishable from the 2s in a coincidence detection, contribute equally to the differential cross section and show some interesting interference structures. The comparison of the results with those obtained by simpler procedures like Coulomb projected first Born calculations shows the importance of having the correct asymptotic behaviour in the final state, specially for small scattering angles. This work constitutes a necessary step for future calculations concerning complex atoms, whose wavefunctions could be constructed by 2p₀, 2p_+or-1 orbitals.

Double ionization of helium and H^- is investigated theoretically by using correlated wavefunctions for the target and the double continuum. The results show that the differential cross section for H^- is bigger than for helium and reveal some different angular distributions. Typical experimental situations are proposed.

A modulated crossed beam technique has been used to measure the differential cross sections for the excitation of the 3³P state in magnesium by electrons at impact energies of 10 and 20 eV. The cross sections are measured over a range of scattering angles from 10 to 130 degrees . Our results agree with the previous experimental cross sections of Williams and Trajmar (1978) within the combined uncertainties, however the present data reduce the uncertainty by almost an order of magnitude thereby allowing a meaningful comparison to be made with the predictions of the various theories. At this time no theory accurately predicts the behaviour of the cross sections over the whole angular range.

By unequal-frequency hybrid resonance followed by hybrid resonance four-wave mixing in Na₂-Na mixture, broadly tunable UV coherent radiation in the wavelength region 319.2-354.2 nm has been generated. The experimental and theoretical analysis have been focused on the particular mechanism and the features of the UV coherent radiation.

Calculations have been performed to model the state populations of argon plasmas at electron densities at and above those required for the validity of coronal equilibrium. Both effective and explicit models have been used, and both are based on the same set of atomic cross sections. The effective model includes ground and singly excited states explicitly, while the effect of autoionizing states is accounted for by branching factors which describe their depopulation into the various non-autoionizing states. The explicit model considers both autoionizing and non-autoionizing states explicitly. The effective model requires a significantly reduced amount of computer time and memory. Good agreement between the two models can be obtained through moderate densities if the branching factors include electron density dependent terms which describe the collisional stabilization of each autoionizing state. The effective model breaks down as density is increased because the population of individual autoionizing states become significant. Results for both ionization balance and radiated power loss are presented.