Table of contents

Adiabatic wavefunctions and dipole moments of Rydberg states perturbed by a neutral atom have been investigated. Wavefunctions are equal to sums of products of Coulomb wavefunctions and differ very substantially from any single Coulomb wavefunction. Using the Kirchhoff-integral method (Fabrikant I I 1993 J. Phys. B: At. Mol. Opt. Phys.26 2533) we calculate the potential curves of two adiabatic Rydberg states for the Rb*(n≃30) + Rb(5s) system: one dominated by the ³S symmetry near the ground-state Rb atom, and the other dominated by the ³P symmetry.

When an excited atomic electron interacts with a neutral perturbing atom or molecule that possesses a shape resonance, it generates a characteristic class of Born-Oppenheimer potential curves that rise with internuclear distance. We document this effect, and predict the existence of a diverse class of stable, strongly bound atom-atom and atom-molecule states that result from this phenomenon. For the specific case in which Rb is the perturbing atom, we show that such states should be observable in the spectroscopy of an ultracold gas or condensate.

Recently, a universal shape function for cross sections of ionization by charged particle impact has been established. Here, we show how an analogous shape function for multiple ionization by photons can be constructed along the same lines. It is argued that this shape function provides an excellent parametrization of photoionization cross sections in cases where all electrons are ionized. In more complicated situations, the parametrization can still be useful, although its applicability may be limited.

New coplanar (e, 3e) experiments for double ionization (DI) of helium (1s²) and argon (3p²) at about 600 eV incident energy, and under moderate momentum transfer to the target (~0.8 au), are presented. It is shown that, when the data are sorted according to the Bethe-ridge condition (i.e. the magnitude of the momentum transfer is equal to that of the sum momentum of the emitted electrons), the (e, 3e) angular distributions clearly bear the signature of the target initial two-electron momentum density, |ψ(p₁,p₂)|². We show, for the first time in DI, the different signature due to the s or p character of the ionized electrons. For He, the results are qualitatively compared with a state-of-the-art first-order theory.

Most of the information about the environment of the early Universe comes to us from radiation emitted from atoms and molecules. An understanding of the relevant atomic and molecular processes is needed to correctly interpret this radiation. Atomic and molecular process also control the evolution of the early Universe. In this paper, we review the atomic and molecular processes that are important in the early Universe.

The ionization efficiency curve up to 16 eV, the total electron scattering cross section up to 15 eV, the electron excitation cross sections for metastable 5 ³P_0,1,2 and resonance 5 ¹P₁ levels up to 11 eV have been measured for cadmium atoms. The energy dependence of the total cross section has revealed two distinctly well resolved resonance features at energies of 0.33 and 3.74 eV, respectively. The total excitation cross sections measured for the 5 ³P_0,1,2 and 5 ¹P₁ levels refine the earlier measured optical excitation functions for these levels. These results show the possibility of measuring, in the same experiment, the total cross section of electron scattering by cadmium atoms, the ionization efficiency and the low-lying level excitation function, as well as providing their absolute calibration.

We consider the 1s-1s charge-exchange process in collisions between a positronium and proton (anti-proton) in the presence of a relatively weak low-frequency electromagnetic field. The consideration shows that, because of the field-induced modification of the (final) continuum states of the colliding system, the capture process can be accompanied by substantial energy exchange with the field. This exchange first of all results in the change in the energy spectrum of the outgoing positron (electron). Some arguments are also given that the presence of the field might influence the total capture cross section.

In this paper, we study the entanglement properties of a pair of electrons ejected simultaneously from an atom following the absorption of a single photon. We show that, in the absence of spin-orbit interaction (SOI), the non-separability of the state of two photoelectrons in a pair depends upon the spins of the atom and of its residual dication. The two electrons produced in a double photoionization experiment may always be entangled, on the other hand, if the SOI is taken into account. In the latter case, entanglement will be affected by, among other things, the photoionization dynamics and the state of polarization of the absorbed radiation.

The scattering length a associated with two-body interactions is the relevant parameter for near threshold processes in cold atom-atom collisions. For this reason zero-range potentials are traditionally used to model collective behaviour of dilute collections of bosons. The model is also used to compute three-body recombination rates, where it gives an a⁴ law. In this paper we examine the applicability of the zero-range model to real physical systems. Hyperspherical adiabatic potentials obtained from the zero-range model are compared with published potentials based on realistic two-body interactions. From these comparisons it is possible to determine the regions where the model applies.

A significant effect of the acceleration of electrons by the ion field (AEIF) on the widths of spectral lines in high-density plasmas was first studied in one of our previous papers. That theory was successfully applied for explaining H_α widths measured in an underwater laser-produced plasma. In the present paper an advanced theory of the effect of the AEIF on shifts of spectral lines is developed. Its results are applied to two sets of experiments where H_α shifts were measured in high-density plasmas. For the experiment in the underwater laser-produced plasma, where the temperature was relatively low (T = 0.77 eV), we eliminated a factor of two discrepancy between the standard theory (ST) of the shifts and the experimental shifts and achieved excellent agreement with the experimental shifts. We also demonstrated that, for the condition of this experiment, another new source of the shift - the dipole ionic-electronic shift (DIES) introduced previously - plays an even more important role than it had been thought in the earlier paper on the DIES. We provide a more convincing proof of the existence of the DIES. For the experiments in a gas-liner pinch, where the temperature was significantly higher (up to 10 eV) than in the above experiment, we eliminated a discrepancy - up to a factor of two - between the ST of the shifts and the experimental H_α shifts. For the conditions of this experiment, practically the entire difference between the results of the present theory and the results of the ST of the shifts is due to the AEIF. Thus, we showed that the development of the advanced AEIF theory is important for explaining the experimental shifts in high-density plasmas. We also clarified some issues concerning a comparison of our theory with an experiment in a laser-driven pressure cell and with other theoretical works.

We have investigated electron emission for transfer ionization (TI) and total electron emission, for He and H₂ targets. Double differential cross sections in angle and energy of the ejected electrons have been obtained covering the angular range from 0° to 175° and energies from 2 to 140 eV. Pure ionization differential cross sections, derived by subtraction, show an increasing contribution to the electron emission at low energies. The main observed structure, a cusp in the forward direction for electrons emitted with velocities close to the incident projectile velocity, presents a symmetric shape for TI. This fact, together with the similar shape of the cusp for both He and H₂ targets, suggest the presence of a two-electron intermediate state centred at the projectile leading to cusp formation in TI. The cusp for pure ionization, in contrast, shows a strong asymmetry, and a different shape for each target. The differential electron emission measurements allow us to estimate the cross sections for capture to doubly excited states resulting in autoionization of the projectile, which contributes to the TI channel. We obtained a much larger cross section value for H₂ than for He targets, in contrast with known cross sections for true double capture having the same order of magnitude for both targets.

The traditional implementation of R-matrix theory to electron scattering and photoionization requires the computationally demanding diagonalization of dense symmetric matrices of Hamiltonian elements. To make this task more efficient and to improve the convergence of the R-matrix basis, we develop an alternative R-matrix theory based on partitioning the eigenvectors and eigenvalues into a group of accurately known low-lying ones and derive an approximation to account for the remainder. We give the appropriate expressions for the R-matrix, its energy derivative, and the dipole matrix in this basis. Scattering from a diffuse hydrogen-like oxygen target with n⩽8 states is used to illustrate the convergence of the method.

The fine-structure splitting of resonance atomic levels causes essential effects on the amplitude of a two-colour frequency mixing in a magnetic field. The fine-structure effects are analysed in the scattering cross section, and particularly in the circular dichroism phenomena, such as the elliptical polarization of the generated wave, when both incident waves are linearly polarized, and the dependence of the cross section on the helicity of one of the incident waves when the other one is linearly polarized. Together with general analytic expressions for the amplitude, the numerical data are presented for three different double-resonant routes involving two dipole and one quadrupole interactions of atoms with a field. The results may be useful for the polarization control of the process in an atomic vapour and for the frequency-mixing spectroscopic studies of atomic structure.

Self-consistent calculations of plasma effects on atomic structure, based on the ion-sphere model, are presented. The electron impact ionization cross section is calculated, using a distorted-wave approximation, for partially ionized gold plasmas at different temperatures and densities. The results show large plasma effects on collisional ionization at high density and low temperature. The plasma medium leads to enhancement of the ionization cross section by a factor of up to two.

The performance of the subspace density functional theory in its local density approximation is investigated by applications to the excitation energies of small molecules. The different schemes, concerning the choice of finite basis sets, for treating in a balanced way the ground and excited state calculations, are discussed. Unlike the conventional atom centred basis sets, we used off centred basis sets whose parameters were determined by invoking the minimum principle for the subspace energy. For the molecules under consideration (H₂, HeH and LiH) such basis sets consisting only of s-type Gaussians, optimized for each subspace energy, were found to provide a reasonable agreement of the excitation energies computed with those obtained by the configuration interaction methods with extended basis sets.

A systematic study of the triple differential cross section for the electron impact ionization of magnesium is presented. Complete sets of theoretical results using both the first Born and the distorted wave Born approximation are given for a range of asymmetric kinematical regimes. How the physical significance of the different approximations enter the character of the cross sections will be explicitly demonstrated. Comparison is made with experiments of the Maryland group and suggestions are made for new experiments.

We study the limitations for entanglement due to collisional decoherence in a Bose-Einstein condensate. Specifically we consider relative number squeezing between photons and atoms coupled out from a homogeneous condensate. We study the decay of excited quasiparticle modes due to collisions in condensates of atoms with one or two internal degrees of freedom. The time evolution of these modes is determined in the linear response approximation to the deviation from equilibrium. We use Heisenberg-Langevin equations to derive equations of motion for the densities and higher correlation functions which determine the squeezing. In this way we can show that decoherence due to quasiparticle interactions imposes an important limit on the degree of number squeezing which may be achieved. Our results are also relevant for the determination of decoherence times in other experiments based on entanglement, e.g. the slowing and stopping of light in condensed atomic gases using dark states.

In this paper we demonstrate, through numerical calculations, the possibility of determining the field dependence of the ac Stark effect through the interaction of an atom (or molecule) with a bichromatic ultrafast laser field. The time-dependent Schrödinger equation is solved for a single active electron confined to move in a square-well potential distorted by an intense Stark field in combination with a perturbative probe field which connects a pair of levels through photon absorption. The aim is to determine the field parameters that allow the dependence of the dynamic shift of the optically connected levels on the Stark field to be mapped out in real time. The calculations show that this can be achieved when the duration of the probe field is at least as short as the half-cycle period of the Stark field. An experimental realization of the proposed measurement scheme and its limitations are discussed. It is also possible, in principle, to adopt the ac Stark effect to determine the absolute phase of the carrier wave of an ultrafast laser pulse.

Photodetachment of the Ps^- ion by low-energy photons (ℏω<<m_ec²) is considered. Our present analysis is based on the dipole approximation which simplifies the original problem. The angular correlation between the two vectors which represent the initial photon polarization (or photon propagation ) and momentum of the emitted electron is studied in detail. It is shown that the angular distribution of the emitted photoelectron can be represented by a simple one-parameter formula. The photodetachment of two-particle neutral system Ps (e⁺e^-) and complete (i.e. three-particle) photodissociation of the Ps^- ion are also briefly discussed. In fact, the approach developed here can be used to determine photodetachment cross-sections for an arbitrary Coulomb three-body system.

The dynamics of a Bose-Einstein condensate (BEC) in a time-dependent harmonic trapping potential is determined for arbitrary variations of the position of the centre of the trap and its frequencies. The dynamics of the BEC wavepacket is soliton-like. The motion of the centre of the wavepacket, and the spatially and temporally dependent phase (which affects the coherence properties of the BEC) multiplying the soliton-like part of the wavepacket, are analytically determined.

We report an ab initio study of the dynamics of the reaction F + D₂→DF + D, where D₂ is initially in a rotationally excited state (j = 2). The possibility of obtaining ultra-cold molecules and of investigating reaction dynamics at ultra-low temperature relies on the production of molecules in a well defined quantum state and it is important to know the relative efficiency of the rotational quenching and of the chemical reaction. We examine here a reaction with an activation barrier, the reaction of F with D₂, and we find that quenching dominates the reaction when the initial rotational level lies energetically below the barrier, so severe trap loss may occur before the reaction can take place.

High-resolution Doppler-free saturated absorption spectroscopy has been employed to determine accurate (about ±1 MHz) isotope shifts for twenty-seven 4d²5s²-4d²5s5p transitions of Zr I in the green, blue and yellow-red spectral regions. These transitions, which involve an electron jump 5s→5p, represent a class in which the field shift (FS) component is larger than the specific mass shift (SMS). The SMSs and FSs were separated using a King plot procedure, in which the SMS of the 468.78 nm reference transition was estimated from scaled ab initio calculations. The SMSs for these transitions are found to lie between zero and about three times the normal mass shifts. Using refined multiconfigurational fine-structure calculations and scaled pseudo-relativistic Hartree-Fock estimates, FSs and SMSs have been computed and found to be in broad agreement with the experimentally determined values. A detailed comparison of the calculated FSs and SMSs with the accurate experimental values for this large sample of zirconium transitions is found to provide stringent tests of the Vinti integral and electron density values calculated from the radial wavefunctions and also of the admixture coefficients of the eigenvector components determined from the multiconfigurational fine-structure calculations. Finally, the results of this work have been used to predict the SMS and FS values for 20 relevant even- and odd-parity Zr I configurations.