Table of contents

We present a new approach to the renormalization procedure of a self-mass correction (SMC) to the one-particle energy level in an arbitrary local stationary field. A scheme proposed is based on the analytical properties of the SMC and does not use the cut-off parameters, usually employed in the standard technique. It is shown that the renormalized part of a SMC may be expressed by a convergent one-fold dispersion-like integral, which, together with a discrete spectrum contribution, gives the Lamb shift of the level considered.

Helium photoelectron satellites He⁺ nl (n=2-7) have been studied with monochromatized synchrotron radiation. The angular distributions were determined in the photon energy range from 80 eV to 603 eV showing, for all of the observed satellites, increasing angular distribution parameters beta which converge to beta =2 at high photon energies. The overall behaviour shows a clear tendency of reduced fractional intensity of s-like orbitals for states with higher principal quantum numbers n. In addition, at lower photon energies the satellites with higher n tend to have lower beta -values reflecting the influence of higher angular momenta near threshold.

A crossed beam technique has been used to investigate the angular distribution of electrons photodetached from the weakly-bound excited (2p³)²D state of the C^- ion via the (2p²) ³P epsilon s,d channels. At a photon energy of 2.076 eV the asymmetry parameter was measured to be 0.38+or-0.05. No theoretical predictions are currently available.

Ion translational energy spectrometry has been applied to study the kinetic energies of fragment ions resulting from spontaneous (unimolecular) dissociation of doubly charged molecules. Analysis of the shapes of the kinetic energy distributions yields information on the lifetime of the precursor molecular ion. A value of 1.0+or-0.25 mu s is deduced for the lifetime of CO²⁺ ions.

The close-coupling approximation using five basis sets: (a) H(1s)+Ps(1s), (b) H(1s,2s)+Ps(1s), (c) H(1s,2s)+Ps(1s,2s), (d) H(1s,2s,2p)+Ps(1s,2s) and (e) H(1s,2s,2p)+Ps(1s,2s,2p) has been employed to investigate positron-hydrogen scattering below the first excitation threshold. The present basis sets (d) and (e) have confirmed the existence of two consecutive resonances just above the Ps-formation threshold, as obtained by Basu et al. (1989) using four-state (H(1s,2s,2p)+Ps(1s)) CCA, in the elastic channel.

Bounds to different quantities of atomic systems, related in a direct way to the single-particle electron density of the system (such as the kinetic energy, T, the exchange energy in the Dirac-Slater form, E_ex, the average radial density, < rho >, and the electron density at the nucleus, rho (0)), are given in terms of two radial expectation values of the form <r^beta > and <r^beta lnr>). The quality of the bounds is numerically studied within the Hartree-Fock framework.

The accuracy of the spline collocation method for solving molecular orbital equations is tested for the hydrogen atom. Two different types of grids are used that concentrate points near the nucleus. A 40 point grid defined by a polynomial transformation, which is suitable for molecular calculations, is found to produce an accuracy within a mHartree of the exact value. For many electron systems, the authors have previously employed an algorithm for solving Poisson's equation which uses the eigenvectors of the kinetic energy matrices as a basis. In order to ensure the accuracy of the eigenvalues and eigenvectors of the kinetic energy, a method is implemented using scaled splines that leads to kinetic energy matrices having a simple block structure. The eigenvalues of the kinetic energy matrices in this scheme are quite accurate.

Energy-dispersive L X-ray spectra of uranium and gadolinium were recorded in both singles and coincidence mode and fitted with attention to both detector lineshape and Lorentzian broadening. The relative line intensities in the L, L₁, L₂ and L₃ X-ray series were deduced. These results favour the predictions of the Dirac-Fock model over those of the Dirac-Hartree-Slater model. This conclusion has implications for measurements of quantities such as L subshell ionization cross sections by charged particles, because the interpretation of such measurements relies in part upon a data-base of relative L X-ray line intensities.

The authors investigate the contribution of the unseparated Rydberg states which are usually neglected, in atomic K-edge X-ray spectra. They suppose that the core hole broadening is large, so that the global contribution of all unresolved Rydberg levels introduces an apparent threshold shift with a value nearly equal to the separation of the first unresolvable Rydberg line from the series limit. The apparent threshold has the same dependence on the photon energy as the true one. The region of unresolvable Rydberg lines can be described by means of a quasi-particle approach, by introducing a pseudo-wave vector formalism. The authors perform an analysis of Ne and Ar K-edge absorption spectra. Using the present theory, they derive a very simple procedure for the evaluation of the instrumental and the core hole broadening in X-ray spectra.

Two-dimensional quantum defect graphs are discussed in relation to series of interacting autoionizing resonances. Under appropriate conditions, 2D plots obtained for two autoionizing series possess the same form as for interacting series of bound states, but their interpretation is different, especially as regards the estimation of inter-series coupling. The effects of the asymmetry of autoionizing lines on the appearance of the 2D graphs is investigated.

New experimental results on quenching of A-coefficients by photons in a short discharge tube are presented. Experiments with the short discharge tube have eliminated questions related to the effects of absorption in a long laser tube, and have provided good confirmation of earlier results.

The self-broadening of four neon atomic lines (1s₅-2p₂, 1s₄-2p₂, 1s₃-2p₂, and 1s₂-2p₁) belonging to the 3s-3p transition group has been measured for the first time at high pressures (1-6 bar) and a low gas temperature (330 K), through a laser absorption spectroscopy technique. Pulsed X-ray photoswitched discharges have been used for the experiment. In our experimental conditions, the line profiles are found to be Lorentzian in shape, with a symmetric core with respect to a shifted line centre. Linear variations with the pressure are reported for the broadenings and the shifts. Broadening and shift constants relevant for the density range studied are deduced. These constants are in overall agreement with the low density results previously published, but new features are reported: extrapolation to zero pressure of the shifts measured above 1 bar does not give a zero value for the 1s₅-2p₂, and 1s₄-2p₂ lines, and a blue shift is measured for the 1s₂-2p₁ line. Assuming approximate interaction potentials, the impact theory gives broadening constants close to the measured ones. Nevertheless, the interpretation of the shift behaviours requires the use of unified theories including many-body collision effects.

Excitation energies and oscillator strengths from the ¹S₀ ground state to the first ³P₁⁰ and ¹P₁⁰ excited states of Mg-like ions are calculated using the multiconfiguration relativistic random-phase approximation including excitation channels from core electrons. The discrepancies between the relativistic random-phase approximation calculation and experimental results for the intercombination excitation energies and oscillator strengths are much reduced. The excitation energies and oscillator strengths for the resonance transition from the multiconfiguration relativistic random-phase approximation are in excellent agreement with experiment. A substantial part of the discrepancy in the ³P₁⁰-¹P₁⁰ separations is resolved when core excitation channels are taken into account.

Using synchrotron radiation in combination with atomic beam and time-of-flight technique, the partial ion yield spectra sigma (Xⁿ⁺) of Ho, Er and Tm in the region between 130-200 eV were measured. The signals are dominated by Fano-type resonances of singly charged photoions sigma (X⁺) which can be attributed to discrete transitions X4d¹⁰4fⁿ to X4d⁹4fⁿ⁺¹ followed by the autoionization process X4d⁹4fⁿ⁺¹ to X⁺4d¹⁰4f^n-1+ epsilon d,g. This interpretation is corroborated by ab initio Hartree-Fock calculations of the resonant photoabsorption cross section.

The density functional approach to local field is extended to a spin-polarized finite electronic system. The time-varying local field, which is dependent on the spin components of electrons, incorporates the polarization-type many-body effects of electronic correlations into the photoionization cross section. To examine the spin-dependent local field effects, the photoionization cross sections of the potassium and the bromine atom are calculated by using this time-dependent local spin density approximation. The results obtained are compared with available information from other theoretical methods and experiments.

A simple, versatile and effective scheme is presented for intensifying atomic beams by transverse laser cooling with two separated, intense standing waves. The first standing wave is negatively detuned while the second is positively detuned with respect to the cooling transition. A realization of the scheme, involving only one laser and simple optics, is proposed. Semiclassical calculations suggest that with a laser power of 0.1-1 W and an interaction length of a few cm, a flux enhancement of approximately 10²-10³ should be obtainable by a two-dimensional set-up. Specific results are presented for the case of metastable argon.

Differential cross sections for coherent scattering of 145.4 and 279.2 keV gamma rays were measured for Cu, Mo, Ag, Cd, Sn, Pr, Sm, Gd, Dy, Ho, Yb, Ta, W, Pb, Th and U employing a reflection geometry set-up with a graded shielding arrangement. A 133 cm³ HPGe detector was used to detect the scattered gamma rays. The measured cross sections were compared with the experimental values in literature and with the numerically calculated results using S-matrix theory for available cases. From the measured cross sections the form factors corresponding to 8.295 and 15.923 AA^-1 photon momentum transfer were extracted. The results are discussed in the light of the predictions of form factor formalisms.

The bound states of the H₂(j=0)-¹²C¹⁶O van der Waals molecule have been obtained, for two potential energy surfaces, by the secular equation method. A comparison is made with the eigenenergies obtained by direct numerical integration of the close-coupled equations. The latter approach has also been used to initiate a study of the effect of coupling to rotationally excited levels of H₂. For the most accurate potential, the purely rotational millimetre wave spectrum has been determined for two simple dipole surfaces. Finally, the dimer spectrum accompanying the CO fundamental has been simulated and compared to experiments.

We report R-matrix calculations for energies and width of the lowest ² Sigma resonance important for Penning and associative ionization in He-H collisions. The ab initio determination of the angular distribution of the He (2³S)-H (1²S) resonance is reported. This resonance is anisotropic with considerable d character at small internuclear separation, whereas previously it had been assumed to have an isotropic distribution.

The ratio, R, of the double ionization cross section to that of single ionization for helium by fully stripped carbon, oxygen, silicon and sulphur ions in the energy range of 2.8 to 6.3 MeV amu^-1 have been measured. The data obtained using carbon and oxygen projectiles show that in this energy range R is in better agreement with the fit given by Knudsen et al. (1984) than measurements using N⁷⁺ projectiles in the energy range 10-30 MeV amu^-1 and Kr³⁶⁺ projectiles with energies 0.5 and 1 GeV amu^-1.

Previous relative measurements of the differential cross sections for ionization of neon atoms in collisions with fast sodium atoms have been put on an absolute scale. This was done by measuring the ratio of ions and neutral atoms in a fast mixed (ions and neutrals) sodium beam and by using the well known differential cross section for elastic scattering of sodium ions by neon atoms. The measured ionization cross sections exhibit a steep rise from threshold to a flat maximum between 300 and 400 eV.

We report on the measurements of double differential cross sections, at zero degree, for the production of secondary electrons from the collision of 0.6 MeV amu^-1 Cu⁵⁺, I⁷⁺, Au¹¹⁺, U¹³⁺, Cu¹⁹⁺ I²³⁺ and Au²⁹⁺ projectiles with H₂ and He. The shapes and positions of the observed binary encounter electron peaks are compared with continuum distorted wave-eikonal initial state (CDW-EIS), impulse approximation and classical trajectory Monte Carlo calculations, as well as with predictions from an adiabatic resonant tunnelling model proposed by Fainstein and co-workers (1992). The experimentally observed binary peak positions were essentially determined by the projectile's ionic charge q and were independent of the projectile's nuclear charge Zp. Although the experimental conditions correspond to the strong perturbation regime. i.e. 1.0<or=q/V_P<or=5.9 (V_P being the projectile velocity), the CDW-EIS approach accounts well for the position and shape of the binary peak.

Two-electron capture by slow multiply charged ions colliding on rare-gas targets at keV energies populates doubly excited states of the projectile with large cross sections. Though these states are dominantly autoionizing, important radiative decay following double capture have been reported, among which the direct observation of the fluorescence from Rydberg states. These findings are important since they modify the charge and radiation balance of the multiple charge exchange reactions. A mechanism based on post-collisional effects was proposed, in which Rydberg states are fed from the quasi-symmetrical doubly excited states initially populated. A quantitative analysis of this effect is developed leading to a simple model which is applied to N⁷⁺ on He and Ar collisions. The evolution of the competition between autoionization and fluorescence yields of the Rydberg states with the internuclear distance is emphasized.

Fon and Lim (1993) have reported the 11-state and the 19-state R-matrix calculations of 1¹S-2^3,1P differential cross sections at low energies. Considerable qualitative agreement had been achieved between the R-matrix calculations and the experiments. To establish quantitative agreement, the 29-state R-matrix calculation of Sawey et al. (1990) is extended to obtain 1¹S-2^3,1P excitation cross sections and the electron-photon coincidence parameters lambda and |chi| at 500 energies ranging from the 2³P threshold to 24.2 eV for scattering angles 20, 30, 55, 60, 90, 120, 125, 140 degrees. The convergence of the R-matrix calculations in shape and magnitude of the cross sections is reexamined. The calculations are compared with the experiments and a new normalization procedure is proposed to calibrate the experimental cross sections.

The authors investigate the fivefold differential double-ionization cross section of helium by the impact of fast electrons. They consider the case where little momentum is transferred to the target so that a Born-type approximation in the projectile-target potential can be employed. Collision geometries are provided at which maxima and minima arise in the cross section and the associated ionization mechanisms are identified. Necessary kinematical conditions for the Bethe ridge in double ionization are worked out which provide a possibility for the measurement of the momentum profile of the initially bound state.

The electron impact excitation of the 5¹P state in strontium has been studied by the electron-photon coincidence technique. The Stokes parameters P₁, P₂, and P₃, of the decay transition 5¹P₁-5¹S₀ at lambda =460.7 nm have been measured for an incident electron energy of 45 eV. The measurements extend over the electron scattering angles 30 degrees to 113 degrees . The polarization correlation data have been analysed to yield the electron impact coherence parameters (EICP) for the excitation of the Sr(5¹P) state. The results of the total polarization vector provide evidence of the coherent nature of the 5¹S₀-5¹P₁-5¹S₀ excitation/de-excitation process in this relatively heavy He-like atom. The measurements provide a sensitive test for the theoretical models which involve the influence of strong electron correlation effects. Substantial agreement is found between these first measurements and the two calculations which have just become available.

Dissociative recombination of H₃⁺( nu =0) leading to H₂⁺+H^- has been measured and the results are found to agree with earlier measurements by Peart et al (1979). A discussion is given on the measurement of recombination reactions using beam techniques.

Factorization relations for rotational inelastic differential cross sections and transition probabilities for collisional excitation of rigid polyatomic targets are discussed within the sudden approximation for the special case of symmetric top molecules. The excitation of initially rotating spherical top molecules by electron impact at intermediate energies is studied in detail and the rotational rainbow structure is analysed.

Positron scattering by atomic hydrogen including positronium formation has been formulated using the R-matrix method and a general computer code written. Partial wave elastic and ground state positronium formation cross sections have been calculated for L<or=6 using a six-state approximation which includes the ground state and the 2s and 2p pseudostates of both hydrogen and positronium. The elastic scattering results obtained are in good agreement with those derived from a highly accurate calculation based upon the intermediate energy R-matrix approach. As in a previous coupled-channel static calculation, resonance effects are observed at intermediate energies in the S-wave positronium formation cross section. However, in the present results, the dominant resonance arises in the P-wave cross sections at an energy of 2.73 Ryd and with a width of 0.19 Ryd. The mechanism for the formation of this resonance and its possible experimental verification are briefly discussed. Finally, the summed elastic and positronium formation cross sections are presented and the latter compared with experiment for energies ranging from near threshold to 4.0 Ryd.

We have measured the isotope shifts of the 5d to 6p transitions in ^134-138Ba II, of which the 5d_3/2 to 6p_1/2 transition has not been measured earlier. We have also obtained values with improved accuracy of the magnetic dipole and electric quadrupole hyperfine coupling constants of the 6p ²P^o_1/2,3/2 levels in ^135,137Ba II. The measurements are performed in the red and orange parts of the optical spectrum using fast ion beam-laser spectroscopy with an improved procedure which avoids calibration of the ion velocity. A comparison with theory is made.

Dipole moment, polarizability, and their derivatives with respect to the internuclear separation are calculated for the MgC molecule by using the many-body perturbation theory method up to fourth-order. The results show that the inclusion of electron correlation is very important for this system. In particular, correlation corrections reduce the self-consistent field result for the polarizability to half, increase the dipole moment by 45% and also have dramatic effects on their derivatives. At the highest level of theory employed the calculated dipole moment is 1.86 au and the average polarizability is 56.4 au with a large anisotropy (47.5 au). The vibration-rotational dependence of the dipole moment and polarizability is analysed. Calculated values for the infrared absorption intensity, Raman scattering activity and degree of depolarization are given. The effect of electron correlation on these quantities is discussed.

A systematic study of muon capture and muon transfer has been performed in seven different H₂+SO₂ gas mixtures. From the single-exponential time structure of the muonic sulphur X-rays, one determines the lifetime of the mu p atoms under the given experimental conditions. The reduced muon transfer rates to the sulphur dioxide molecule, deduced from these lifetimes, all agree well with each other. The muonic oxygen time spectra show an additional structure as if mu p atoms of another kind were present. Comparable time structures are observed in a D₂+SO₂ mixture. For lack of a reliable theory, the data were analysed in the framework of the models of 'black and white' and 'ephemeral' muonic hydrogen atoms. The systematic study revealed an unexplained concentration dependence of the ratio of prompt muonic X-ray events between sulphur and oxygen.

The optical thickness of dense plasma, created in linear flashtubes filled with noble gases, is measured over the wavelength range 466-780 nm using a CW laser beam. The electron density and temperature radial profiles of the plasma are determined by spectroscopic methods, infrared laser interferometry and local thermodynamic equilibrium equations. Knowing those profiles, the absorption coefficients and consequently the xi ^fb factors are deduced from optical thickness measurements. These factors are compared with theoretical values of Hofsaess (1978) and with previously published experimental results. The dependence of xi ^fb on the wavelength and the plasma parameters (electron density in the range 7*10¹⁷-1.7*10¹⁸ cm^-3, temperature between 10900 and 17000 K) is discussed. The effect of the interparticle interaction on the absorption in the dense plasmas is only sensitive to the shift of the photoionization thresholds of atoms.