Table of contents

Relativistic coupled-cluster calculations of hyperfine structure and field isotope shift constants for the 6s and 6p states in Ba⁺ have been performed. Comparison with experimental hyperfine structures shows agreement within a few per cent and the authors assign a similar uncertainty to the electronic factor for the field shift: F(6s to 6p₃2/)=-4.20 (13) GHz fm^-2. This value, which is about 18% smaller than semi-empirical estimates used in earlier works, has been used to extract revised values for the changes delta (r²) in nuclear charge radii using optical isotope shifts for ^122-148Ba and muonic results for the stable isotopes ^134-138Ba. Also the factor B/Q, relating the observed quadrupole hyper-fine structure to the nuclear quadrupole moment, was calculated, giving B/Q=374 (4) MHz b^-1.

The 5s²5p ²P-5s5p²⁴P intercombination lines in indium-like ions have been identified from Sb III to La IX. For Sb III and Te IV previously reported values are confirmed and for I V, the 5s5p²⁴P₃₂ level have been revised while all other 5s5p²⁴P levels for all spectra have been reported for the first time. Thirty-three lines are classified, of which 20 are new.

Absolute photodetachment cross sections of Cu^- have been measured at 400, 532 and 650 nm using a Q-switched Nd:YAG laser system. The cross sections were obtained by studying the saturation of neutral atom production versus laser intensity. Data analysis includes the spatial and temporal properties of the laser pulse. The results are compared with existing theoretical predictions.

Binary-encounter electrons (BEe) produced in energetic ion-atom collisions are studied as a function of emission angle. The double differential cross sections for impact of projectiles carrying one and two electrons are compared with the corresponding bare-ion case for emission angles ranging from 3 to 60 degrees in the laboratory frame. The enhancement effect due to the electron core, previously observed for zero degrees by several authors, is found to prevail at forward angles up to about 40 degrees . Beyond this angle, regular screening begins to dominate, and the cross section approaches the value predicted for point-like projectiles. The data are compared with calculations based on the distorted-wave approximation for one-electron incident ions.

Coplanar helium (e,2e) measurements are presented for simultaneous ionization and excitation to the He II 2s, 2p manifold at an incident electron energy of 145.3 eV. The electrons resulting from the collision were detected doubly-symmetric in both energy and angle with respect to the direction of the incident electron beam. These results are compared with the analogous measurements in which the ion is left in the ground state.

Measurements of that part of the total positron-helium cross section not associated with positronium formation have been made for positron energies between 14 and 28 eV. The experimental results do not support the existence of a cusp in the elastic scattering cross section at the positronium formation threshold energy (17.8 eV) as significant as that previously deduced by Campeanu et al. (1987).

The utility of an approximation method for dealing with low symmetry, confined systems using basis functions that do not satisfy the boundary conditions is demonstrated by applying it to the problem of off-axis hydrogenic impurities in cylindrical quantum well wires. Results of calculations of binding energies are presented as a function of the wire radius and the displacement of the impurity from the axis. A previously unexpected behaviour of the binding energy as the location of the impurity and the radius of the well are varied is displayed.

Lower ^1.3S and ^1.3P states of the (N,n) manifold of two-electron atoms are studied. Analytical wavefunctions of these states diagonalizing the r^-1₁₂ operator at N>>1 are found. These As-N-functions provide the excellent approximations of exact wavefunctions even at small and moderate N (for lower states the overlap integral is greater than 0.999 at N=4 and n<or=10). Using As-N functions permits the specific features of the electron distribution over l to be elucidated. Variations of (cos theta ) along the series with fixed N are investigated and it is proved that the angle correlation becomes stronger as n increases. The analytical dependence of (r^-1₁₂) on quantum numbers is obtained. It is shown that there exists two systems of states ( nu =1 and nu =-1), energies of each system corresponding to that of the harmonic oscillator with different zero-point energies and vibrational quanta.

The differential Stark shift of the 1s2s2p ⁴P⁰ states of He^- and Li is studied. The scalar and tensor polarizabilities are calculated using a multiconfiguration interaction wavefunction. Polarization to the ⁴S, ⁴P and ⁴D configurations is considered. For He^- the scalar polarizability is calculated to be 8169.13 a³₀ and the tensor polarizability is -4463.5 a³₀. They are 45.64 and 6.86 a³₀, respectively, for lithium. The dynamic polarizability is calculated for the frequency range from zero to near the ionization threshold for He^- and to the first excitation threshold for lithium.

The authors discuss the radiation force in a one-dimensional sigma ⁺- sigma ^- laser standing wave, for transitions J to J+ Delta J, for any J. and Delta J=+or-1.0. in the limit of low saturation of the atomic transition. The interaction between the radiation field and the atom produces a motion-induced orientation of the atomic ground state which grows rapidly with J. The radiative force on the atom is sensitive to this orientation. For J to J+1 the dissipative part of the total force is much larger than the reactive part, at low velocities, while for the other cases the two contributions are roughly equal and opposite. By using a simple method of calculation, the authors gain much physical insight into the mechanism of induced orientation, and give quantitative results for the size of the radiation force, including both the low-velocity friction coefficient and the velocity capture range.

Measurements of the He I 587.6 nm (2³P-3³D) line-shift-to-width ratio in a dense (1.3-3.5*10²⁴ m^-3), cool 2.5-4.0*10⁴ K z-pinch plasma are reported. Despite difficulties imposed by the radial gradients present in this plasma, a change in the sign of the lineshift and asymmetry dependent on the density and temperature conditions was unambiguously observed. This is in qualitative agreement with recent calculations.

The angular distribution parameters beta of the N₅-0_2,3O_2,3 Auger lines has been measured between the 4d_5/2 and the 4d_3/2 thresholds of Xe using the CIS technique. The radiationless decay parameter alpha ₂ for each of the N₅-O₂,₃O_2.3 Auger lines and the alignment parameter A₂₀ for the intermediate ion were also determined. Comparisons with previous values of beta and alpha ₂ for the N₅-O_2.3O_2.3 Auger lines have been made. In addition, shakeoff to the various states of the Xe²⁺ 5p⁴ ion has been measured following 4d photoexcitation and found to be small. These results support the contention that near-zero kinetic energy electrons observed following similar photoexcitation in the rare gases result primarily from a two-step process rather than the direct process of shakeoff.

The present most sophisticated calculations for 2p photoionization in magnesium (RRPA and MBPT) still neglect important initial and final ionic state configuration interactions whose net effect can be described approximately by a spectroscopic factor and leads to an intensity borrowing model for the photo-process including main and satellite transitions. The validity of this model is checked by deriving from the spectrum of ejected electrons a numerical value for the spectroscopic factor and studying the angular distribution parameters for 2p photoelectrons and L₃-M₁M₁ Auger electrons. As expected from the given model, the angular distribution parameters are found to be in excellent agreement with the binding-energy-adapted theoretical predictions from RRPA and MBPT.

The authors present a kinetic model which describes the evolution of charged particles produced by high-intensity, ultrashort laser pulses, in non-resonant multiphoton ionization (MPI) processes. The model is based on the Vlasov-Maxwell equation and on the approximation of a linear profile for the space charge field. It has been used for predicting waveforms of the ion detector signal obtained in non-resonant MPI of Xe atoms with 30 ps, 10¹³ W cm^-2, 1.064 mu m laser pulses. The comparison of computed and experimental results is satisfactory, thus underlying the relevance space charge effects in typical non-resonant MPI conditions, and showing the effectiveness of the model in accounting for them.

An inhomogeneous differential equation method is used to calculate the Rayleigh and Raman scattering cross sections for atomic hydrogen and atomic caesium as functions of wavelengths below and above the ionization limits. The profile of the scattering cross section near the 6s-6p resonance of atomic caesium is determined by including an energy-independent radiative width in the scattering formula. The effect of replacing the electric dipole operation by a modified form that takes account of the polarization of the core by the valence electron is explored.

The energies and absolute transition probabilities of X-ray emission diagram lines associated with the ethanol molecule have been determined using an ab initio molecular orbital method with a relaxed Hartree-Fock multicentre model. The contribution to the observed carbon emission spectrum from both the central and terminal carbon atoms has been determined. Agreement between theory and experiment for the C K spectrum is qualitatively acceptable, but it is probable that the experimental spectral profile contains satellite contributions and the theoretical studies require further refinement to include correlation effects. The O K spectrum requires further study since the agreement between theory and experiment is poor. The origin of the discrepancy has not been conclusively identified.

The absolute photoabsorption cross section and the photoionization quantum efficiency of sulphur hexafluoride have been measured using a double ion chamber and monochromated synchrotron radiation. The absorption spectrum is unusual because all the prominent features can be associated with transitions into valence states. This behaviour is caused by a barrier in the molecular potential which leads to a suppression of Rydberg structure and an enhancement of intervalence features. The photoionization quantum efficiency reaches a value close to unity for wavelengths shorter than 600 AA, thus resolving an earlier anomaly. Although the Rydberg structure is weak, and superimposed upon a strong background, excitation into these states produces a noticeable effect on the photoionization quantum efficiency. A sum rule analysis has been carried out by combining the present absolute photoabsorption cross section with similar data covering the remaining wavelength regions.

In a beam experiment the authors have studied the energy pooling reaction between two laser excited Na* (3p) atoms, where in the final state one of the sodium atoms is in the 5s or 4d state and the other in the ground state. They have determined all eight independent partial cross sections and coherence terms, specified according to the various magnetic substate combinations of the colliding Na* (3p) atoms, as a function of the collision velocity. This is the maximum information that can be extracted from the experiment given the symmetry of the detection scheme. By applying a weak magnetic field, to orient the Na* (3p) atoms, the authors were able to distinguish between the two partial cross sections sigma ₁₁ and sigma _1-1, where the subscripts 11 and 1-1 refer to the initial magnetic substate combinations of the two collision partners. By using an electro-optical modulator the authors have furthermore improved the signal-to-noise ratio to an extent where it was possible to measure all coherence terms.

The authors have studies in this paper the competition between (projectile) core-conserving and core-varying single-electron capture processes in Ne⁶⁺-He collisions for impact energies between 0.07 and 1.2 keV amu^-1. By deconvoluting the spectra of the VUV radiation emitted by the decaying product ions, absolute state-selective cross sections for these processes were determined. The authors also obtained capture cross sections for metastable Ne⁶⁺ ions colliding on helium. Core-conserving capture channels dominate the charge transfer process at the higher impact energies investigated; with decreasing impact energies, however, the core-varying channels gain strength at the expense of the core-conserving channels. The results show that the capture cross sections for the core-conserving processes involving metastable primary ions are comparable with those for the ground-state primary ions. At lower impact energies, however, the capture cross sections for the metastables are almost three times larger than the cross sections for ground-state primary ions.

Electron drift velocities in sodium, potassium, rubidium and caesium vapours are calculated using theoretical momentum-transfer cross sections for electron-atom scattering. The results strongly disagree with experimental data for saturated vapours at low values of E/N (ratio of electric field to vapour number density). The author shows using the case of sodium as an example, that the disagreement could be explained by the presence of alkali dimers in saturated vapours which lead to the processes of electron-impact vibrational excitation and deexcitation and dissociative attachment. These processes strongly affect the electron energy distribution function and reduce drift velocities. Results of model calculations show that inclusion of inelastic processes involving the dimers leads to a better agreement with the experiment.

The cross section asymmetry for ionization of hydrogen atoms by electron impact is analysed in the eikonal approach. A new formulation is given for the evaluation of the exchange amplitude up to higher partial Coulomb waves. It is concluded that the cross section asymmetry gives an important criterion or interesting test of validity of approximation methods with the exchange effect.

Differential and integral electron impact excitation cross sections are reported for the n³S, n¹S and n³P (n=2,3) levels in He. Differential cross sections were measured at 30, 50 and 100 eV impact energies in the 10' to 135' angular range, extrapolated to 0 and 180', and then integrated over all angles to obtain integral cross sections at these three incident electron energies. In addition, differential cross sections were measured by excitation of the n³S (n=2,3,4,5), n¹S and n³P (n=2,3,4) levels at 60 eV impact energy and 30' scattering angle. Theoretical cross sections based on first-order many-body theory (FOMBT) are also reported for these three final state symmetries for the 30-500 eV impact energy range. These new experimental and theoretical results are compared with other data.

The authors present measurements of relative differential cross sections for the electron impact excitation of the 3¹D, 3³D and 3³P states of helium at energies of 40, 45, 60 and 80 eV and for scattering angles ranging from 30' to 120'. The results are extracted from existing data of electron-photon coincidence experiments. The results are in agreement with distorted wave calculations. The authors point out the consistency of the results with the angular behaviour of the coherence parameter rho ₀₀ also obtained from these coincidence experiments.

Line widths and shifts are calculated in the electron-impact approximation using close-coupling theory for the 17 optically allowed transitions between the lowest nine states in C IV. Approximate formulae must be used to estimate line widths for some 10⁶ transitions which are of importance for the calculation of stellar envelope opacities. The present results are used to test and confirm empirical parameters in the formula obtained by a Seaton (1988) in paper VIII in this series. Effective collision strengths are also calculated, taking full account of resonances, for all transitions between the lowest nine states. These are important in the solar corona and also in laboratory plasmas.

Electron-photon coincidence measurements are reported of the excitation of the 4s(3/2)^o₁ state of Ar at incident energies of 20 and 30 eV and electron scattering angles up to 80'. Isolation of the 4s(3/2) degrees ₁ state was accomplished in the photon channel using a LiF filter. Significant non-zero values of the spin-flip parameter, rho ₀₀, are reported at both energies as predicted by theory but the level of agreement with available perturbative theories becomes poorer (as might be expected) as the incident electron energy is reduced.

An analytic model potential for the e^--Rb⁺ system is generated from experimental data, using an iteration method. The potential obtained can reproduce rather accurately the energy levels of rubidium. The authors employed it in the calculation of elastic differential cross sections for electron (and positron) scatterings from rubidium in the conventional Glauber approximation. The differential cross sections calculated in the model potential approach are compared to those in the frozen-core approach, employing either the Clementi-Roetti or the Szasz-McGinn wavefunctions. The core correlation and polarization effects are found to significantly affect the cross section results.

A study is presented of the optical emissions, lifetimes, emission thresholds and relative emission cross sections, in the wavelength region ( lambda =200-600 nm) for molecular fragments derived from electron impact dissociation of CCl₂F₂. In addition to observations for previously reported molecular fragments including CCl, CCl⁺, CF, etc. some others have been identified here, including CCIF(A) and CF₂(A(0, n, O))(n=5,6). In contrast to 11-35 eV photodissociation processes, the partners in the electron dissociation of these carbenes are highly excited atoms, including atomic ions. In some cases comparisons between appearance potentials and spectroscopic transitions combined with published thermochemical data led to unique assignments of dissociation into specific channels.

The formation of ground-state positronium in collisions of positrons with hydrogen and helium atoms is considered within the distorted-wave Born approximation. In particular the authors analyse the eikonal and continuum-distorted-wave models in both their post and prior forms. Differential and total cross sections are calculated over a broad range of intermediate energies extending from 40 to 500 eV. The results confirm that the total cross section decreases rapidly with increasing energy, as predicted by the simple first Born approximation, and do not agree with experimental evidence of a slower decay. The computed total cross sections for helium targets agree with those of the close-coupling model for energies above 100 eV. Below 100 eV the authors find a large post-prior discrepancy. The calculated differential cross sections show evidence of a Thomas peak at high energies for the CDW models but not for the eikonal models.

The dynamical tensor polarizability in the ground state of hydrogen is calculated. It arises in the third order of perturbation theory with respect to operators of hyperfine and electric dipole interaction. The obtained analytical expression is a sum of linear and squared combinations of Gauss hypergeometric functions ₂F₁. It is pointed out that the double refraction effect can be observed in polarized atomic hydrogen.

The agreement between experimental results and theoretical predictions is improved for the temporal dynamics of the gain coefficient on the 3 to 2 transition of the C VI ion at 182 AA. It is shown that nj kinetics improves the description of radiative cleaning of the n=2 level in a partially reabsorbed, expanding plasma, surprisingly increasing the degree of frozen ionization. A detailed description of radiative trapping on the n=2,3 levels is included in the model in the form of the net radiative bracket, calculated through an exact solution of the radiative transfer problem. The accurate treatment of radiative transport is found to convincingly verify the Sobolev formulation of the escape factor.