Table of contents

The hyperfine structure splitting of the ground state of hydrogen-like ²⁰⁹Bi is calculated for an extended nuclear charge distribution. The probability density of the odd proton in ²⁰⁹Bi is taken into account explicitly in order to compute the effect of an extended magnetization distribution on the energy splitting. The probability density of the odd proton is deduced from a mean-field model and the magnetization correction is evaluated within the Bohr-Weisskopf formalism. The authors obtained a transition wavelength of Delta lambda =242.3 nm. An estimate for the QED corrections is provided.

It is discovered and demonstrated that free ground Cr is singular in being the only 3d metal atom which develops dramatic orbital (anti-)collapse of its excited nd (3p to nd, n>or=3) orbitals. This finding is suspected to be a direct consequence of the well known fundamental irregularity in the filling of the 3d and 4s orbitals of the Cr ground-state configuration. It is also revealed that preliminary removing or excitation of the outermost 4s electron from the ground state of Cr conceals the orbital (anti-)collapse of the subsequent nd excited orbitals. All the results obtained are the first evidences for rather a delicate balance of the intra-potential of Cr for the whole nd states. The calculations are performed in the frame of the 'spin-polarized' Hartree-Fock approximation.

Experimental and theoretical values for the lifetimes of the nsnp ¹P₁ and ³P₁ levels in the Cd and Hg isoelectronic sequences are examined in the context of a data-based isoelectronic parameterization. Regularities are used to identify erroneous measurements, to examine to the degree to which theoretical predictions match large scale trends, and to make interpolative and extrapolative predictions.

The kinetic energy spectra of the fragment protons and deuterons produced in the dissociative photoionization of H₂ and D₂ by 30.5 eV photons have been measured. It is shown that the ratio of the proton to deuteron spectra can provide an experimental estimate of the autoionizing lifetime of the ¹ Sigma _u(1) superexcited state as a function of internuclear separation. The experimentally determined lifetimes are significantly smaller than theoretical predictions.

The authors report preliminary results of a study of the Hopfield series in rotationally cooled molecular nitrogen. By using angle resolved electron spectroscopy in combination with synchrotron radiation, partial cross sections and asymmetry parameters for formation of the vibrationally resolved X and A states of N₂⁺ were measured in the region of the n=6 member of the Hopfield bands. The results are compared with earlier ab initio MQDT calculations and reflect qualitative agreement for the X ² Sigma _g⁺ decay channel, but sharp disagreement for the A ² Pi _u decay channel.

Based on SCA calculations with Kepler orbits and relativistic target electron wavefunctions, the K-shell ionization caused by the low-energy proton and antiproton impacts is studied. Procedures for simulating the higher order effects are critically discussed. It is recommended to performance new experiments which would optimize the particular effects connected with antiparticle projectiles. The flatness of SCA cross sections for antiprotons in the zero energy limit might be exploited in some type of thick target experiment.

Electron emission from atom-atom collisions is analysed within the framework of both quantal and classical dynamics. The authors examine the effect of explicit electron-electron (e-e) interactions on the ejected electron spectra in hard collisions involving simultaneous excitation and ionization in the collision of two structured atoms. A double scattering sequence represented by a second-order Born approximation has been show to give a dominant contribution over the single scattering to projectile ionization. A classical simulation confirms the double scattering process is analogous to the Thomas two-step capture mechanism. The authors find good agreement between quantal and classical calculations, showing the convergence of the Born series to second order and the possibility of a classical treatment for e-e interactions in non-perturbative regimes. They also find that the shape of the ejected electron spectrum is very different from the usually assumed Lorentzian distribution.

The authors have studied electronic redistribution processes, associated with production of excited He⁺ ions, in He²⁺-He collisions at energies ranging from 1 to 300 keV amu^-1. Excited He⁺ ionic states can be formed (i) in the projectile by one-electron transfer, and (ii) in the target by removing one of the target electrons and simultaneously exciting the residual ion. At the highest energies between approximately 10 and 60 keV amu^-1 the first process is the most important one. However, for low energies below 10 keV amu^-1 they obtain the striking result that both processes are equally likely.

The close-coupling equations for the positron-hydrogen system restricted to s-wave scattering from the l=0 levels of hydrogen and positronium are written as a set of coupled momentum space Lippmann-Schwinger equations. The matrix element for positronium formation is evaluated by integrating over the multipole projected components of the momentum space wave functions. Cross sections generated in the coupled static approximation provide a verification of the method. With a larger basis, including the 1s and 2s levels of hydrogen and the 1s and 2s levels of positronium, the broad s-wave resonance at 2.6 Ryd shifts position to 3.17 Ryd. An additional resonance is observed in the cross section at an energy of 1.10 Ryd.

A method for determining the positions and widths of resonances occurring in collisions of electrons with atoms and molecules is developed using an analytic continuation of R-matrix theory into the complex energy plane. The method is particularly effective for the case of multi-channel scattering and has the important advantage of allowing the number of resonances within a given region to be determined unambiguously. Illustrations of the versatility of the technique are provided by considering multiphoton ionization of hydrogen atoms and the scattering of electrons by single-charged oxygen molecular ions. The results are compared to values obtained by fitting the eigenphase sum to a Breit-Wigner form. The advantages of the new approach are discussed. These include the ability to locate very narrow resonances and to examine the analytic structure of the scattering matrix for cases involving many scattering channels.

The behaviour of a one-dimensional system perturbed by a low frequency, periodic electric field is examined in the limit as the field frequency, Omega , tends to zero, that is the static field limit. In particular the authors obtain estimates of the widths of each member of the infinite set of resonances between any finite value of Omega and 0. In order to obtain this estimate they derive a new analytic approximation of the two-state equation of motion. Their analysis shows why recent experiments on the ionization of excited hydrogen atoms by low frequency fields failed to observe any resonances.

The present work shows that even for very high fields for which perturbation theory breaks down the even peaks in harmonic generation (HG) spectra are suppressed for a model Hamiltonian which is symmetric with respect to the reflection of coordinate space combined with translation of time by half period of the field. The proof presented utilizes the combination of Floquet theory and complex coordinate method. Numerical examples are included, which illustrate that even harmonics appear only when the above mentioned symmetry property of the Hamiltonian is broken.

The authors carried out the variational configuration interaction calculation to obtain the energies for n¹S series in He for n=7-16, and for n³S series in He for n=14-18 with B-spline basis functions. As found in their previous report, the accuracy of the results does not deteriorate with increasing principal quantum number. The accuracy of their results is better than the previous best results calculated by Accad et al. (1971). The authors give lower variational energies than Accad et al. for n¹S states (n>or=10) and n³S states (n>or=15). They also calculated the radial expectation values, the mass-polarization effect, and the expectation values of delta (r₁) and delta (r₁₂), which have not been calculated before. They can give some indication of the accuracy of the high-lying Rydberg-state wavefunctions.

A valence-universal coupled-cluster theory accounting for one- and two-electron excitation is formulated in a special form for applications to atomic systems, i.e. in terms of radial cluster amplitudes labelled by configurational indices (VU-CCSD/R method). The method is based on the general algebraic formulation for open shell systems developed by Jeziorski and Paldus (1989). The algebraic formulation is supplemented by a diagrammatic one. The completeness of the model spaces is postulated, and the wave operator is represented in Lindgren's normal ordered exponential form. Various approximations to the VU-CCSD/R method are considered. A general computer program has been developed, and extensive pilot calculations have been performed for the C²⁺ ion and, as a result of using a VU approach, for the C⁴⁺ and C³⁺. Among the states considered special attention is paid to the quasi-degenerate 2s²¹S and 2p²¹S ones. It is found that in the present formulation the number of unknowns in the equations for the cluster amplitudes is reduced by a factor of 20. The present energies are the most accurate obtained so far by a multi-reference CCSD method for a realistic (non-model) system, which make them useful for reliability studies of various MR-CC formulations. It is found that proceeding from the single-reference to the multi-reference formulation results in a change of the ground-state correlation energy by more than 1%. The impact of the choice of orbital sets on the accuracy of the results is also addressed.

Based on the generalized WKB and complex energy method, the authors study the atomic shape resonance in the IPA model, where the photoionization cross section shows maximum structure at resonance energies near the top of the barrier. Only f states of atoms around Xe and Rn have observable shape resonance structures.

The force on a multilevel atom in a 1D magneto-optical molasses with imbalanced laser intensities is theoretically investigated. The authors employ a full quantum treatment to obtain the stationary momentum distribution of the atoms, for arbitrary parameters. The force exerted the counterpropagating laser beams balances for a nonzero locking velocity. The dependence of this velocity on the intensity imbalance is found to be reduced by polarization gradient cooling. For certain values of the imbalance there are two stable velocities, and a splitting of the velocity distribution occurs. A small imbalance, or a magnetic field along the laser beams, can cause the subDoppler cooling to be lost. However, by the correct choice of parameters these effects can be made to cancel, and the sub-Doppler cooling is regained.

Autoionization processes in O₂ have been studied by measuring photoelectrons in the photon energy region between the O₂⁺ X ² Pi _g and a ² Pi _u ionic states (12.4-15 eV). The present experimental results provide comprehensive information about autoionization dynamics since the decay routes from the spin-orbit components of the neutral autoionizing states to the two spin-orbit components of the ground ionic state are resolved. The intensity ratio of the O₂⁺ X ² Pi _1/2 and ² Pi _3/2 components has been determined and strong Omega to Omega ⁺ selectivity has been observed. The observed selectivity is explained in terms of a model in which the orbital angular momentum along the internuclear axis of the positive ion core remains unchanged during the autoionization process. Using this model, Rydberg orbitals of the I, I' and I" autoionizing states are reassigned as 4s sigma _g, 3d delta _g and 3d sigma _g, respectively, and the spin-orbit constant of the J state is determined.

The authors study the classical-quantum correspondence for ionization in three-body fast ion-atom collisions. The existence of a classical limit of the quantum mechanical transition probabilities as a function of the momentum transferred to the electron during the collision, the impact parameter, the energy and angle of the emitted electron, and the initial quantum level of the target is investigated. A well behaved classical limit is shown to exist for large momentum transfers whereas ionization by small momentum transfers or at large impact parameters is shown to be classically suppressed. It is found that ionization at small impact parameters does not possess a well-defined classical limit. Modifications of classical trajectory Monte Carlo (CTMC) methods to account for non-classical ionization are suggested. Applications related to recent data for electron ejection into large angles are presented.

Gain and loss of electron kinetic energy through the mechanism of direct and inverse collisional bremsstrahlung, in the presence of a static magnetic field B, has been investigated within the framework of a non-relativistic classical theory. Interaction with ions is described by means of a Coulomb potential. Two special cases have been considered in detail, namely when the electric field of the wave is either linearly polarized along B, or circularly polarized in a plane perpendicular to B. The regimes of wave-induced electron velocities larger or comparable with the unperturbed ones have been investigated. Substantial differences from the results obtained in the weak field regime, previously considered in the literature, have been pointed out.

General expressions for the spectrum and anisotropy parameters of the 'ordinary' two-photon bremsstrahlung during electron scattering in the screened Coulomb field of the neutral atom are obtained within the frame of the non-relativistic distorted partial wave approximation.

Absolute vibrationally elastic cross sections for e-GeH₄ collisions have been determined for electrons of 1, 2, 2.5, 3, 5, 7.5, 10, 15, 20, 60 and 100 eV incidence energy over a scattering angular range of 10 degrees -130 degrees . The observed angular distributions correspond, at least qualitatively, to theoretical formulations using the continuum-multiple-scattering method, the parameter-free static-exchange-polarization approximation, and a multichannel Schwinger variational calculation. Vibrational excitation functions and the energy distribution of elastically scattered electrons reveal a shape resonance at about 2-2.5 eV. Calculations show that the resonance scattering state belongs to the t₂ representation of the T_d point group, which is also the representation of the lowest unoccupied molecular orbital (LUMO) of GeH₄. Energy loss spectra recorded in the electronic threshold region of GeH₄ reveal enhanced inelastic scattering at large angles in the loss range 7-9 eV characteristic of excitation to the lowest triplet state.