Table of contents

The photoionization cross section of ground state Sc²⁺ is calculated using our augmented many-body-perturbation theory methodology and compared with similar results for neutral Sc. The comparison shows surprising similarities and differences which give new insight into the redistribution of oscillator strength from inner shells in response to the removal of outer-shell electrons.

Electron impact ionization of the valence 2 and inner valence 4 orbitals of nitrous oxide, N₂O, has been investigated using the (e, 2e) technique. The experiments have been performed in the asymmetric coplanar geometry, with a view to elucidating the dynamics of the ionization process. An incident energy of approximately 900 eV was used, with an ejected electron energy of 25 eV and scattering angles ranging from 2° to 10°. The kinematics for a scattering angle of 10° correspond to bound Bethe ridge conditions, and at this angle the measured cross section exhibits a deep minimum in the binary direction, similar to that observed in atomic p-orbital ionization. The valence orbital cross sections have larger recoil intensity than is the case for atomic ionization under similar kinematics, while for ionization of the inner valence 4 orbital, the recoil structure is somewhat smaller than has been observed for inner-shell ionization of atoms.

A hydrodynamical version of the time-dependent Gross-Pitaevskii equation is used to describe driven vibrations of a Bose-Einstein condensate of ⁸⁷Rb atoms in a magnetic trap. If the trap frequency is suddenly decreased, and later is suddenly returned to its initial value, the response of the condensate departs from the self-similar character that is obtained in the Thomas-Fermi approach. We show that the self-similar Thomas-Fermi modes are, in fact, unstable. Thus, the `quantum pressure' term in the hydrodynamic equations of motion can play a significant role in condensate excitation dynamics, even when its effect on ground-state properties is negligible.

General considerations and explicit computations using highly correlated Hylleraas-type wavefunctions demonstrate how doubly electronically excited autoionizing helium can be stabilized in the presence of a KrF laser at laboratory field intensities. This type of stabilization is due to the abrupt suppression of the dynamical electronic correlation as the field intensity is increased beyond a critical value. The generality of the result is stressed.

Many-body effects in photodetachment from negative ions are discussed in this article. A short survey of model and ab initio many-electron approaches applied to the study of negative ions is presented. Manifestations of the dynamic many-electron effects such as the interference between different channels, the polarization interaction, core relaxation and screening of the interelectron interaction in one-photon photodetachment processes are considered. Attention is centred on the resonances in the single-electron photodetachment cross section.

The influence of linearly and circularly polarized laser fields on the dynamics of fast electron-impact excitation in atomic helium is discussed. A detailed analysis is made in the excitation of 2 ¹S, 3 ¹S and 3 ¹D dressed states of helium target. By using a semiperturbative treatment with the Sturmian basis expansion, we take into account the target atom distortion induced by a laser field. Important differences appear between the angular distributions depending on the different states of polarization, in particular the circular polarization presents an experimental interest. We give new features (intermediate resonances) for both polarizations, concerning the n = 2 states of helium for emission and the n = 3 for the absorption, in terms of laser frequency. Qualitative differences from the case of laser-assisted elastic collisions have been found.

The results of a joint experimental and theoretical study concerning elastic electron scattering by laser-excited ¹³⁸Ba( ... 6s6p ¹P₁) atoms are described. These studies demonstrate several important aspects of elastic electron collisions with coherently excited atoms, and are the first such studies. From the measurements, collision and coherence parameters, as well as cross sections associated with an atomic ensemble prepared with an arbitrary in-plane laser geometry and linear polarization (with respect to the collision frame), or equivalently with any magnetic sublevel superposition, have been obtained at 20 eV impact energy and at 10°, 15° and 20° scattering angles. The convergent close-coupling (CCC) method was used within the non-relativistic LS-coupling framework to calculate the magnetic sublevel scattering amplitudes. From these amplitudes all the parameters and cross sections at 20 eV impact energy were extracted in the full angular range in 1° steps. The experimental and theoretical results were found to be in good agreement, indicating that the CCC method can be reliably applied to elastic scattering by ¹³⁸Ba( ... 6s6p ¹P₁) atoms, and possibly to other heavy elements when spin-orbit coupling effects are negligible. Small but significant asymmetry was observed in the cross sections for scattering to the left and to the right. It was also found that elastic electron scattering by the initially isotropic atomic ensemble resulted in the creation of significant alignment. As a byproduct of the present studies, elastic scattering cross sections for metastable ¹³⁸Ba atoms were also obtained.

A configuration-interaction (CI) calculation has confirmed an earlier prediction of positron binding to neutral copper. The single-centre CI expansion gave a binding energy of 0.003 69 Hartree and a 2 annihilation rate of 0.246 × 10⁹ s^-1. The energy and annihilation rate are different from a previous calculation using the fixed-core stochastic variational method (FCSVM) due to the slow convergence of the CI calculation with respect to the number of spherical harmonics included in the CI expansion. An extrapolation of the CI calculation to the infinite spherical harmonic limit improves the quality of agreement with the earlier FCSVM calculation. The present calculation highlights the importance of an explicit treatment of the strong electron-positron correlations which are present when positrons interact with atoms.

A systematic method of treating multiplets in the density functional theory is presented. An optimized potential method is introduced for the lowest-lying multiplets. A method proposed recently by the author is used to generalize the Krieger, Li and Iafrate (KLI) approximation for multiplets. The generalized exchange-only KLI exchange potential leads to results as good as the Hartree-Fock results. Inclusion of the simple local Wigner correlation results in an improvement only in the total energies, whereas the multiplet splitting becomes even worse. The multiplet separation can be approximated by a very simple expression containing the average density and the difference of the Kohn-Sham potentials.

The spectrum of five times ionized xenon (Xe VI) has been observed in the 270-7000 Å wavelength range. We proposed 18 new energy level values of the 5s5p5d configuration, all the energy level values for the 5p³ configuration and three energy level values of the 5s5p6s configuration. The observed configurations were theoretically interpreted by means of Hartree-Fock relativistic (HFR) calculations and least-squares fit of the energy parameters to the observed levels. The parameter values are compared with results from HFR calculations.

New approximations to the partition function of diatomic molecules are given. The semiclassical Wigner-Kirkwood expansion truncated at the second or fourth order diverges as T . Various methods of scaling the Wigner-Kirkwood expansion are given which avoid this disadvantage and give sufficient accuracy in the low-temperature region. In addition, two modifications of the Pitzer-Gwinn method are discussed. Most molecular potentials can be considered almost harmonic near the minimum. For the scaling of the Wigner-Kirkwood expansion, therefore, a functional form is chosen, which would be exact for every temperature in the case of a purely harmonic potential. In the high-temperature limit the scaled and the conventional Wigner-Kirkwood expansion become identical.

The cross sections for quenching by H₂ of the 4p´[1/2]₁ and 4p´[1/2]₀ states of Ar I, corresponding to emission lines at 696.5 and 750.4 nm, are obtained in a gas discharge using emission spectroscopy in conjunction with a laser-induced fluorescence measurement of the atomic hydrogen density, and found to be (1.3±0.3) × 10^-19 and (1.5±0.3) × 10^-19 m², respectively. The cross sections for the quenching of H(n = 4) and H(n = 5) by H₂ are obtained using emission spectroscopy and the known quenching cross section for H(n = 3) and are found to be 3.1(±0.1)(-0.9) × 10^-19 and 8.3(±0.7)(-2.5) × 10^-19 m², respectively, where the first uncertainty is the random error and the second uncertainty is a possible systematic error.

Calculation of the three-body recombination rate is reformulated in a manner in which the velocity contribution is more explicit and the recombination rate is spectrum analysed for electron velocity distributions under ultra-fast laser irradiation. Electron distribution functions are calculated by using a two-dimensional Fokker-Planck simulation. The shape of the recombination rate spectrum with respect to the velocity is found to be well described by vf(v) where f(v) is the pitch-angle-averaged velocity distribution. It is shown visually that the increase in the slow electron population effectively enhances the recombination rate. The effect of a model energy loss on the recombination rate is also discussed.

We study spin and polarization correlations in atomic photoionization from np_J subshells, including the first retardation corrections to the dipole approximation. This extends previous work on distributions of unpolarized photoelectrons ejected by polarized photons (Bechler A and Pratt R H 1989 Phys. Rev. A 39 1774, 1990 Phys. Rev. A 42 6400). A nonrelativistic Pauli-Schrödinger approach in a self-consistent central potential is used, neglecting spin-orbit coupling. There are nontrivial correlations, even in this nonrelativistic approximation, and even without taking into account spin-orbit coupling, provided the electron is ejected from a subshell with a definite value J of the total angular momentum. (However, if summation over J is performed, one would have to include the spin-orbit coupling in order to obtain any remaining spin and polarization correlations in the nonrelativistic approach). Explicit formulae are given in terms of dipole and quadrupole radial matrix elements and phase shift differences. Results are compared with exact numerical calculations. Corrections to the dipole approximation are generally small at low energies and low Z, but increase with energy and atomic number. Including the first retardation corrections significantly improves agreement.

The spectrum of tellurium was photographed on a 3 m normal incidence spectrograph in the 300-2100 Å wavelength region using a triggered spark source. Three configurations, namely 5p³, 5s5p5d and 5s5p6s, of trebly ionized tellurium (Te IV), all involving two-electron excitations, were investigated. Seven out of ten levels reported previously were confirmed, some with a change of configuration assignment, the J-value of one level was changed from (3/2) to (5/2) and two levels were rejected. The new observations have resulted in establishing 35 out of 36 energy levels belonging to these three configurations. Least-squares fitted and Hartree-Fock calculations involving multi-configuration interactions were carried out to interpret the observed spectrum. 155 new lines have been classified in the Te IV spectrum.

The triple differential cross section (TDCS) for double photoionization of calcium atoms has been studied, in the region of the Ca 3p3d giant resonance, for equal and unequal energy sharing by the two electrons. The measurements were performed at two different photon energies corresponding to different resonance symmetries, and the results show that the TDCS is strongly resonance dependent. For a fixed total energy the TDCS, at a relative angle of 180° for the two electrons, has been found to increase approximately linearly with increasing energy of one of the electrons.

We have studied the characteristics of an atom laser created by removing atoms from a Bose-Einstein condensate (BEC) via a two-photon Raman process. In contrast with outcoupling by RF transitions, Raman outcoupling offers spatial selectivity within the condensate and delivers significant momentum transfer to the outcoupled atoms. We develop a version of zero-temperature mean-field theory for BECs which includes the internal state of the atom and the effect of laser fields focused into the cloud. For a model condensate we find that the outcoupled matter wave develops sharp peaks at spatial positions where the two-photon Raman process is resonant with the hyperfine splitting of the atomic ground state. As time proceeds the width of the outcoupled wave expands and a steady stream of matter waves travels outside the condensate volume.

We have studied the classical hydrogen atom in two dimensions under an intense electric field with radial symmetry (1.5 degrees of freedom). We have obtained a classical ionization suppression at high angular momentum with the radial model similar to the quantum estimation for the one-dimensional (1D) hydrogen atom in a linearly polarized field and we compare these results with the smoothed 1D Coulomb potential model. We have obtained two generalized Kepler maps for several values of the eccentricity (e) of the trajectories.

The symmetrized contribution of E-type spin-rotation interaction to conversion between spin modifications of E- and A₁-types in molecules with C_3v-symmetry is considered. Using the high-J descending of collisional broadening for accidental rotational resonances between these spin modifications, it was possible to co-ordinate the theoretical description of the conversion with (updated) experimental data for two carbon-substituted isotopes of fluoromethane. As a result, both E-type spin-rotation constants are obtained. They are roughly one and a half times more than the corresponding constants for (deutero)methane.

We study phase-matching conditions for high-order harmonic generation as well as high-order sum- and difference-frequency mixing processes in strong laser fields, using a graphical approach described in Balcou et al (1997 Phys. Rev. A 55 3204-10). This method is based on the analysis of the different quantum paths that contribute, with different phase properties, to the single-atom response. We propose a simple numerical method to disentangle the quantum paths contributing to the generation process. We present graphical maps of the phase matching around the laser focus, which allow one to predict the geometries that optimize the conversion efficiency of the process considered. The method is applied to the study of sum- and difference-frequency mixing processes. The qualitative predictions of the graphical phase-matching approach are confirmed by numerical propagation calculations.

A time-dependent approach for treating electron-hydrogen scattering is reported that utilizes a fully correlated two-electron wavefunction represented on a three-dimensional lattice using the basis-spline collocation method. The lattice, time-dependent approach obviates the need for consideration of the three-body Coulomb boundary conditions, avoids the use of severe approximations such as those of perturbation theory for slow collisions, and provides a relatively dense representation of the one- and two-electron continua. Probabilities for excitation and ionization are computed by projection onto lattice eigenstates of the H atom. Partial cross sections for excitation and ionization are obtained and compared with results of other theoretical methods for the ¹S and ³S channels.

A complete set of spin-averaged parameters, including the differential cross sections and three Stokes parameters, were obtained from a series of superelastic scattering experiments on the 4 ²P state in potassium. The four scattering parameters have been measured at an effective electron energy of 54.4 eV on the ground state and over an angular range from 0° to 145°. The experimental results are compared with a convergent close-coupling (CCC) calculation and several distorted-wave calculations together with previously reported measured differential cross sections. The excellent agreement between the CCC theory and the present measurements resolves a long-standing anomaly between theory and experiment at this energy in potassium.

Stimulated by the need to solve numerically the time-dependent Schrödinger equation in the most efficient manner we study analytically the distribution over angular momentum eigenstates of an initially Gaussian wavepacket in a linearly polarized laser field, evolving in time, both in momentum and length gauge. Two extreme cases are examined in detail, namely when the laser field is either very strong or else very weak in comparison to the confining potential experienced by the wavepacket. These cases help identify the most advantageous choice of gauge for calculations on laser-driven atoms in a variety of circumstances.

The static molecular third-order polarizabilities () are calculated for the five stable isolated-pentagon-rule isomers of C₇₈. The method utilized is based on the finite-field approach coupled with semiempirical polarization calculations on all-valence electrons. Our results indicate that the increase of the curvature of C₇₈ reduces its third-order polarizability.

In this work, we report on a theoretical study of electron-CS₂ collision in the low- and intermediate-energy range. More specifically, the elastic differential and integral cross sections as well as the grand total (elastic+inelastic) cross sections in the 0.05-100 eV range are reported. A complex optical potential consisting of static, exchange, correlation-polarization plus absorption contributions, derived from a fully molecular wavefunction, is used for the electron-molecule interaction. The Schwinger variational iterative method combined with the distorted-wave approximation are applied to calculate the scattering amplitudes. The comparison between the calculated results and the existing experimental and theoretical results is encouraging.

We propose a new analytical method to solve for the nonexactly solvable Schrödinger equation. Successfully, it is applied to a class of spiked harmonic oscillators and truncated Coulomb potentials. The utility of this method could be extended to study other systems of atomic, molecular and nuclear physics interest.

Two distinct mechanisms are investigated for transferring a pure ⁸⁷Rb Bose-Einstein condensate in the |F = 2, m_F = 2 state into a mixture of condensates in all the m_F states within the F = 2 manifold. Some of these condensates remain trapped whilst others are output coupled in the form of an elementary pulsed atom laser. Here we present details of the condensate preparation and results of the two condensate output coupling schemes. The first scheme is a radio-frequency technique which allows controllable transfer into available m_F states, and the second makes use of Majorana spin-flips to populate all the manifold sub-states equally.

A bipolar spherical harmonics expansion of the modified Faddeev equation is used to calculate the K-matrices of the e⁺+H scattering for up to five open channels and for total angular momentum L4. Partial cross sections and differential cross sections including hydrogen production channels are reported. Comparison with previous calculations are made whenever feasible. Good agreement is observed.