Table of contents

Accurate multiphoton detachment rates for showing the influence of resonances have been calculated using R-matrix Floquet theory. At low intensities the results are in excellent agreement with earlier single-photon detachment calculations and with experiment. At higher intensities the influence of the three-photon excitation of a resonance state on the two-photon detachment rate is shown to be important. It is also shown that the rate for absorption of three photons is substantially enhanced near the frequency which excites the residual lithium atom from the 2p to the 3s state.

The linear polarization and the collision-induced dipole moment of H(n = 2) in and collisions were measured in an energy range of 1 - 25 keV. We observe a positive electric dipole moment corresponding to an electron lagging behind the proton; however, in contrast to, for example, - He, Ne and Ar collisions the linear polarization nearly vanishes.

Initially in a collision between antihydrogen and hydrogen the electron is bound to the proton and the positron to the antiproton. Clearly, if the proton and the antiproton coincide, they cannot bind the light particles. In this letter, an upper bound value of is obtained for the critical value of the internuclear distance below which the electron and the positron cease to be bound to the nuclei as they can attain a lower energy by separating from the nuclei and forming positronium.

The diagonalization approximation is used to investigate the resonance states of converging to the n = 4 hydrogenic threshold. Computations are carried out for states of symmetries; energies and widths of several autodetaching states of the negative hydrogen ion are calculated. The partial widths associated with the various open channels both for singlet and triplet states are also given. Comparison is made with other experimental results and theoretical studies.

Transition energies and rates between the and levels of ions in the Be isoelectronic sequence (nuclear charges Z = 7-28, 30, 36, 42) are calculated in the valence and core-valence limit using the multiconfiguration Dirac-Fock method. The Breit interaction and finite nuclear mass effects are taken into account in subsequent relativistic configuration-interaction calculations. Separately optimized orbital sets are used for the initial and final states in the transitions. The transition matrix elements are evaluated with an efficient transformation technique. The calculated energy separations and transition probabilities are compared with critically evaluated experimental values and with results from other recent calculations.

Photon excitation x-ray spectator emission dynamics of the core excited carbon monoxide molecule has been studied using ab initio multiconfiguration self-consistent field calculations and a modified two-step absorption-followed-by-emission approach. The core excitations of the CO molecule and the core ionizations of the species have been considered as the precursor steps for producing the same decaying electronic states of CO. The purpose of this study is to illustrate the influence of the absorption process, using target species with different ground electronic state geometries, on the de-excitation spectra. Nuclear dynamical effects of the decaying processes of carbon and oxygen core excited states of CO to the singlet low-lying excited (final) states of and have been studied. Vibrational envelopes of the decay spectra of CO are simulated.

Total and n-partial cross sections for charge transfer in collisions are calculated for collision energies between and . A molecular expansion is employed, and semiclassical and quantal calculations are carried out including a common translation factor and a common reaction coordinate, respectively. The comparison between quantal and semiclassical cross sections and transition probabilities is discussed.

Using the effective Hamiltonian approach a complete expression is derived, which is suited for the numerical evaluation of the correction to the energy of singlet -states of the helium atom.

We report a study of bimodal behaviour of atomic wavefunctions for a transition element (Cr) and how this behaviour can be controlled and even turned into single-mode character either by (i) varying the effective nuclear charge for nonintegral values, or (ii) placing the atom in a spherical cavity of adjustable radius. Our conclusions have relevance to the emergence of valence instabilities for transition metals in the solid state. Also it is shown that the very existence of the bimodal wavefunctions provides an explanation for the frequently experienced instability of self-consistent field algorithms when a double-valley potential occurs, and suggestions are made on how to improve the numerical procedure for such calculations.

This paper presents results for the anapole magnetic field moment due to the mixing of states with opposite parity in the n = 2 hydrogen atom in an external constant electric field. The evaluation of the current densities of the four n = 2 states is carried out to first order in the field strength. These current densities and their associated magnetic fields are expanded in multipoles, including both the ordinary poloidal and the toroidal fields. The dominant contribution to the toroidal dipole (anapole) magnetic field comes from the strong mixing of the degenerate 2S, 2P states within the n = 2 shell. A brief discussion is also presented of the electronic scattering from the anapole field of a Rydberg atom. The experimental conditions under which the atomic anapole field could be detected are critically considered, in particular, in terms of the quasi-resonant dependence of the anapole field on the applied electric field.

Radiative vacancy transfer probabilities from K-shell to -subshell and M-shell were deduced for the elements Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Ta, W, Hg and Pb by measuring characteristic K x-rays from targets excited by 123.6 keV photons. Comparisons between present experimental results and theoretical predictions were made.

We study the spontaneous emission of a multilevel equidistant excited quantum oscillator (nuclei, atoms, molecules) in the case in which the symmetry of the dynamical transitions between the levels is submitted to the SU(1,1) symmetry group and compare our results with those well known for the SU(2) symmetry. It is shown that the equidistant radiator in the process of spontaneous decay in the case of SU(1,1) symmetry exhibits either only acceleration or only a decrease in spontaneous emission rates, while in the case of SU(2) symmetry the radiation pulse is similar to that for the Dicke super-radiance. The exact steady-state solution for the atomic density operator for an equidistant radiator driven by a totally incoherent broad-band (chaotic) field for a SU(1,1) symmetry group is obtained using the Fokker-Planck equation. It is found that for a strong-field limit in the case of SU(1,1) symmetry the fluctuations in the atomic population increase with increasing intensity of the external field and do not achieve their maximal value as in the case of SU(2) symmetry. Moreover, in the case of SU(1,1) transition symmetry we find that there is a superbunching effect for certain values of group parameter I, while in the case of SU(2) symmetry only bunching and antibunching occur. It is found that for vanishing values of the external field intensity the second-order intensity correlation function achieves negative values.

Using the energies and the multiconfiguration interaction wavefunctions obtained from a full core plus correlation method, the nonrelativistic dipole-length, -velocity and -acceleration absorption oscillator strengths for the transitions are calculated. In most cases, the agreement between the f-values from the length and velocity formulae is up to the fourth or fifth digit. Our acceleration f-values also agree closely with the length and velocity f-values. Combining these discrete oscillator strengths with the single-channel quantum defect theory, the discrete oscillator strengths for the transitions from the state to highly excited levels and the oscillator strength densities corresponding to the bound-free transitions are obtained.

Inner valence and Rydberg energy levels have been calculated for as a function of internuclear separation. Electron scattering from was solved for negative scattering energies in an R-matrix method which allowed the inclusion of Rydberg states in the configuration interaction calculation. Bound states were found for energies greater than 30 eV which could explain the vibrational-like peaks observed in photoelectron spectra of .

The one-electron molecule has been studied and the electronic energies of the states n = 1-5 have been determined as a function of the distance R between the two nuclei, in the Born-Oppenheimer approximation. The radial and rotational matrix elements for electron transfer in the collision have been computed and the system of 35 coupled equations describing the collision in the impact parameter approximation has been solved for an initial system composed of an incident ion and a target in the ground state. The signs of the different terms appearing in the radial matrix elements have been carefully re-examined and a clear resonance induced by the avoided crossing of the and states has been proven. The total electron transfer cross section is found to lie between those of the adjacent symmetric systems and and, in this respect, may be called `quasisymmetric transfer'.

We have used the semiclassical impact parameter method to compute cross sections for excitation and charge transfer in collisions at keV energies. Full allowance was made for the identity of the two electrons. The electronic matrix elements of the Hamiltonian were evaluated numerically but `exactly', i.e. without additional approximations, including plane-wave translation factors. Comparison is made with the results of previous calculations by Fritsch, who used a one-electron approximation. The effective cross section for emission of the He II 468.6 nm transition, following electron capture from metastable He, is presented. These results are directly relevant to the analysis of the spectra of tokamak plasmas, observed during helium beam fuelling and heating.

A comprehensive theoretical and experimental study of electron capture in collisions of with H and with C extending over the energy range to is presented. A variety of theoretical approaches were used including those based on quantal molecular-orbital close-coupling (MOCC), multielectron hidden crossings (MEHC), quantal decay and classical trajectory Monte Carlo techniques. Radiative charge transfer cross sections were computed using the optical potential/distorted wave (OPDW) and fully quantal (FQ) approaches. The MOCC, OPDW and FQ calculations incorporated ab initio potentials, nonadiabatic coupling matrix elements and transition moments computed at the configuration-interaction level. Ab initio potential surfaces in the plane of complex internuclear distance were obtained for the MEHC calculations. Merged-beam measurements were performed between and for the collision system. Diagnostics of the beam with a crossed electron beam could find no presence of a metastable component. The current results, in conjunction with previous measurements, are used to deduce a set of recommended cross sections.

Cross sections for electron capture in collisions between and laser-excited oriented atoms have been computed at intermediate energies (5-50 keV). A quantal method is used within the atomic orbital close-coupling approach as well as a purely classical model, i.e. the classical trajectory Monte Carlo method. Both methods are in good agreement over the entire energy range for total capture cross sections and partial n = 2 cross sections. At the level of state-to-state differential cross sections some differences between the two approaches become evident. The total probability for electron transfer in both methods turns out, however, always to be largest in left-hand side collisions where the projectile velocity can become parallel to (match) the electron velocity of the target.

Ab initio, multi-reference, configuration-interaction (CI) calculations of first Born cross sections for high-energy electron scattering and Waller-Hartree cross sections for x-ray scattering are presented for seven small molecules: , , and . Both elastic and inelastic cross sections are considered. We examine the sensitivity of the cross sections to various details of the treatment of electron correlation, such as the use of single- versus multi-reference CI methods, perturbative corrections to them, and the choice of basis set. There is good agreement between these and previous MRCI calculations. There is fair agreement with experiment except for and for which previously observed discrepancies remain unresolved. Higher-order Born corrections should be examined in these cases.

An empirical expression is proposed to describe the K-shell ionization cross sections by electron impact over a wide range of atomic numbers (i.e. ) and overvoltages U (i.e. ) defined as being the ratio between the incident electron energy and the ionization energy of the electrons in the K shell. The study is based on the analysis of existing experimental databases for atoms. Agreement is obtained with an accuracy of 10% over the entire atomic number and overvoltage ranges including the near-threshold region. Results are compared with those obtained with other analytical expressions.

We compute high-energy electron-impact (100-1000 eV) dissociation and photodissociation cross sections of HCl producing H and CI neutral atoms through the transition. The electron-impact dissociation cross sections are obtained in the first Born approximation, and are reported for the first time. Hartree-Fock followed by configuration interaction wavefunctions are used and vibrational degrees are considered. The dissociation cross sections as a function of the continuum energy are presented. Photodissociation cross sections are obtained and compared with previous theoretical and experimental results.

A widely used alternative to a full Breit-Pauli R-matrix calculation of electron-impact excitation is the transformation of S- or K-matrices, calculated in pure LS coupling, to intermediate coupling. Here we present a transformation method, based on multi-channel quantum defect theory (MQDT), that eliminates the problems associated with standard transformation methods and leads to accurate level-to-level electron-impact excitation cross sections. Instead of transforming the physical S- or K-matrices, we employ MQDT to generate unphysical K-matrices in pure LS coupling; we then treat all channels as open and transform these matrices to intermediate coupling. Finally, we generate the physical K-matrices from the intermediate-coupled unphysical K-matrices. To illustrate the accuracy of this method, we compare cross sections for several transitions in determined using: (i) the standard transformation method in which the LS-coupled physical S-matrices are transformed to intermediate coupling; (ii) an MQDT transformation method, used by others, in which the unphysical LS-coupled K-matrices are transformed to pure jK coupling; (iii) our intermediate coupling frame transformation (ICFT) method and (iv) a full Breit-Pauli R-matrix calculation. It is shown that our ICFT method eliminates the problems associated with the other two transformation methods and leads to cross sections that agree very well with those determined from a full Breit-Pauli R-matrix calculation. Furthermore, the ICFT method can be applied to complex atomic systems which are intractable to a full Breit-Pauli calculation.

Low-energy recombination of has been measured at the storage ring CRYRING with a high-energy precision of 15 meV. Using different electron currents the approach for correcting the variation of the ion energy due to the drag force was checked. A multi-configuration Breit-Pauli perturbation calculation using the program AUTOSTRUCTURE for the dielectronic recombination rates has been implemented. The cut-off of Rydberg states by field ionization in the analysing magnet was modified by taking radiative cascade into account. Comparison of this calculation with the measurement shows a good agreement in resonance strength but a small discrepancy in resonant energy. At very low energy, the non-resonant recombination rate is found to be a factor of 2 higher than the estimated radiative recombination rate. No indication of an electron density dependence is found.