Table of contents

In recent years there have been many studies of multiple ionization of closed shell rare gas atoms by intense laser fields. Until now no similar work has been done in the study of more diverse targets such as negative ions where low binding energies and strong electron correlations could yield distinctive behaviour. We present the first results of ionization of more than one electron from a range of atomic negative ions by intense laser pulses. Although these pulses are long by modern standards, and tend to produce sequential ionization in atoms, the positive ion yields from the negative ions do not depend predictably on the ionization potentials. This suggests that there may, intriguingly, be an alternative mechanism enhancing double ionization at low intensities.

A procedure to extract the two complex amplitudes that govern the He photodouble ionization process from the experimental data is proposed. The results are compared with the predictions of the convergent close coupling and hyperspherical R-matrix with semiclassical outgoing wave theories.

Because of its fundamental role in cold hydrogen plasmas such as those in certain astrophysical or fusion reactor environments, and as a prototype for the study of similar processes in other cold plasmas, three-body, diatomic association (often referred to by the general term three-body recombination) is considered. In three-body collisions involving two hydrogen atoms and a proton this process results in the formation of H₂ or H₂⁺ while the third, scattering particle carries away the excess energy. To reach an understanding of its dynamics and effectiveness, we present the first study of this process by using an extended coupled-channel formalism, treating the rotations of the associating particles through the sudden approximation. We describe the three-particle configuration space by a large set of relevant bound and continuum states, the latter being obtained by discretization of the vibrational continuum in a large quantization space. The association rate coefficients, resolved in final vibrational states, are calculated and show that the production of H₂⁺ is significantly faster than that of H₂ due to the strong charge transfer between the corresponding continua and a favourable distribution of highly excited vibrational states in the case of H₂⁺.

Recent progress in calculations of QED radiative corrections to parity nonconservation in atoms is reviewed. The QED vacuum polarization, the self-energy corrections and the vertex corrections are shown to be described very reliably by different methods used by different groups. All new calculations have recently converged to very close final results. Each separate radiative correction is very large, above 1% for heavy atoms, but, having different signs, they partly compensate each other. Our results for the radiative corrections for all atoms are presented. The corrections are −0.5% for ¹³³Cs and −0.70% for ²⁰⁵Tl, ²⁰⁸Pb and ²⁰⁹Bi. The result for ¹³³Cs reconciles the most accurate atomic experimental data for the 6s–7s parity nonconservation amplitude in ¹³³Cs of Wood et al (1997 Science275 1759) with the standard model.

We have investigated L-shell photoionization (PI) of ¹S ground-state and ³P^o metastable states of the Be-like ion, B⁺, in the photon energy range 22.4–31.3 eV, at an experimental energy resolution of 25 meV. Absolute PI cross sections have been measured using a photon–ion merged-beams arrangement at the Advanced Light Source. Detailed calculations using the semi-relativistic Breit–Pauli R-matrix approach suggest a fraction of 29% of metastable ions in the primary beam of the experiment. Excellent agreement is observed between the theoretical predictions and the experimental high-resolution PI cross sections. The present results are compared with earlier experimental and theoretical work. The cross section for PI of B⁺ ground-state ions can be very well described by an analytic formula derived from quantum defect theory.

Experimental magnetic sublevel scattering-angle-integrated cross sections following ionization–excitation of He(1s²)¹S to He⁺(2p)²P^o in e⁻ + He and H_m⁺ + He (m = 1–3) collision systems have been determined using a combination of total cross sections and polarization fraction measurements in the extreme ultraviolet range. The derived magnetic sublevel cross sections, σ₀ and σ₁, for M_L = 0 +/- 1 have been studied over a wide range of velocities (2–8.5 au) for electron impact on helium. These results are compared with previous experimental proton cross sections (2–6 au) as well as new measured data for hydrogen molecular impact (1.4–4.0 au) for equi-velocity. In addition, our electron and proton measurements are compared with earlier theoretical predictions, our recent second-Born calculations fully including off-shell energy terms, and present predictions from a hybrid first-order and second-order distorted-wave plus R-matrix (close-coupling) model (DWB1 + RMPS and DWB2 + RMPS). Finally, we have extended our polarization measurements for H⁺ + He collisions and found excellent agreement between theory and experiment at nearly all impact energies. However, the present second-Born and DWB1 + RMPS results deviate slightly from the experimental electron data while the DWB2 + RMPS calculations tend toward the experimental data of Forand et al (1985 J. Phys. B: At. Mol. Phys.18 1409).

Absolute differential cross sections were measured at 135° for the elastic and the vibrationally inelastic electron scattering from threshold to 12 eV, with emphasis on the threshold region. In addition, relative dissociative electron attachment spectra were measured from 0.1 to 3.5 eV. Structures of vibrational origin were observed at energies below 1 eV, well below the ²Π shape resonance, in the cross sections for the excitation of vibrational overtones and for dissociative attachment. They are generally narrower and deeper than similar structures in CO₂. The structures are absent when the N≡N stretch is co-excited. The structures are interpreted in terms of vibrational Feshbach resonances supported by a state of the anion where an electron is temporarily loosely bound in a spatially diffuse cloud by a combination of dipolar and polarization forces around a molecule with an excited bending and/or N–O stretch vibration.

Excitation of the 3¹P state of magnesium by 40 eV electrons has been studied experimentally using the polarized photon-scattered electron correlation method over a wide range of scattering angles (10°–120°). Measured Stokes parameters are consistent with the small angle (≤20°) data of Brunger et al (1989 J. Phys. B: At. Mol. Opt. Phys.22 1431–32). The measured linear Stokes parameters, and the circular polarization calculated from these parameters, assuming coherent excitation, are generally well produced by recent theoretical models, convergent close coupling, R-matrix with pseudo states, relativistic distorted wave and first-order many-body theory. Despite extensive testing, directly measured circular polarization data yielded low values at most scattering angles beyond 20°. Differential cross sections for elastic scattering and excitation of the 3¹P and 3³P states were also measured over the angular range 10°–140° and compared with theoretical predictions.

We apply a new implementation of the convergent close-coupling (CCC) method to electron–helium scattering. The target states are obtained from one-electron He⁺ box-based eigenstates rather than the usual Laguerre-based orbitals. The utility of the new method is demonstrated for 50 eV electron-impact ionization of helium with three different energy sharings between the two outgoing electrons. Excellent agreement is found between previous and new CCC predictions, and also with experimental data.

Detailed molecular structural information of the living state is of enormous significance to the medical and biological communities. Since hydrated biologically active structures are small delicate complex three-dimensional (3D) entities, it is essential to have molecular scale spatial resolution, high contrast, distortionless, direct 3D modalities of visualization of naturally functioning specimens in order to faithfully reveal their full molecular architectures. An x-ray holographic microscope equipped with an x-ray laser as the illuminator would be uniquely capable of providing these images. A quantitative interlocking concordance of physical evidence, that includes (a) the observation of strong enhancement of selected spectral components of several Xe^q+ hollow-atom transition arrays (q = 31, 32, 34, 35, 36, 37) radiated axially from confined plasma channels, (b) the measurement of line narrowing that is spectrally correlated with the amplified transitions, (c) evidence for spectral hole-burning in the spontaneous emission, a manifestation of saturated amplification, that corresponds spectrally with the amplified lines, and (d) the detection of an intense narrow (δθ_x ∼ 0.2 mrad) directed beam of radiation, (1) experimentally demonstrates in the λ ∼ = 2.71–2.93 Å range (ℏω_x ∼ = 4230–4570 eV) the operation of a new concept capable of producing the ideal conditions for amplification of multikilovolt x-rays and (2) proves the feasibility of a compact x-ray illuminator that can cost-effectively achieve the mission of biological x-ray microholography. The measurements also (α) establish the property of tunability in the quantum energy over a substantial fraction of the spectral region exhibiting amplification (Δℏω_x ∼ 345 eV) and (β) demonstrate the coherence of the x-ray output through the observation of a canonical spatial mode pattern. An analysis of the physical scaling revealed by these results indicates that the capability of the x-ray source potentially includes single-molecule microimaging, the key for the in situ structural analysis of membrane proteins, a cardinal class of drug targets. An estimate of the peak brightness achieved in these initial experiments gives a value of ∼10³¹–10³² photons s⁻¹ mm⁻² mrad⁻²/(0.1% bandwidth), a magnitude that is ∼10⁷–10⁸-fold higher than presently available synchrotron technology.

Accurate atomic data, such as fine structure energy levels and oscillator strengths of different ionization stages of iron ions, are important for astrophysical and laboratory plasmas. However, some important existing oscillator strengths for ions with an open 3d shell found in the literature might not be accurate enough for practical applications. As an example, the present paper checks the convergence behaviour of the energy levels and oscillator strengths of Fe VIII by systematically increasing the 3pⁿ–3dⁿ (n = 1, 2, 3 and 6) core–valence electron correlations using the multiconfiguration Hartree–Fock method. The results show that one should at least include up to 3p³–3d³ core–valence electron correlations to obtain converged results. Large differences are found between the present oscillator strengths and other theoretical results in the literature for some strong transitions.

We investigate the dynamics of vortices in repulsive Bose–Einstein condensates in the presence of an optical lattice (OL) and a parabolic magnetic trap. The dynamics is sensitive to the phase of the OL potential relative to the magnetic trap, and depends less on the OL strength. For the cosinusoidal OL potential, a local minimum is generated at the trap's centre, creating a stable equilibrium for the vortex, while in the case of the sinusoidal potential, the vortex is expelled from the centre, demonstrating spiral motion. Cases where the vortex is created far from the trap's centre are also studied, revealing slow outward-spiralling drift. Numerical results are explained in an analytical form by means of a variational approximation. Finally, motivated by a discrete model (which is tantamount to the case of the strong OL lattice), we present a novel type of vortex consisting of two pairs of antiphase solitons.

We have developed a recollision model approach, dedicated to the investigation of C⁺ subject to high-intensity laser fields. Double photoionization is investigated for 780 nm light at intensities between 10¹⁴ and 10¹⁶ W cm⁻². Using the recollision model we obtain a ratio between double and single ionization of 0.13% when only a single recollision takes place. Through a careful analysis of the approach, we investigate what approximations are needed in order to apply the recollision model to strong-field double ionization of a more complex atom.

Collisionless fragmentation of non-rotating Ni_n (n = 4–14) clusters is studied using micro-canonical molecular dynamics (MD) computer simulations. The clusters are modelled by an embedded-atom potential energy surface. The distributions of the channel-specific fragmentation probabilities, and the global and channel-specific fragmentation rate constants are computed and analysed as functions of the internal energy and size of the clusters. The trends derived from the dynamical calculations are compared to the fragmentation energy patterns, those of the Rice–Ramsperger–Kassel (RRK) and statistical approaches. The rate constants are an order of magnitude smaller for the RRK model than with both the MD and transition-state theory approaches. The results are also compared with the other multi-channel fragmentation works.

Total and partial single-electron-capture cross sections for H⁺ + Ca collisions have been calculated by the semiclassical impact-parameter method. A two-centre atomic basis expansion was used in the impact ion energy range 1–100 keV. The present total cross sections are in good agreement with previous experimental data. The most important contributions to the total cross sections come from the capture in the n = 2 shell in almost the entire energy range studied in this work, but the contribution from the n = 1 level is increasingly important at energies higher than 60 keV.

Energy spectra of electrons detached in collisions of Cl⁻ and Br⁻ with atomic hydrogen and deuterium have been measured for laboratory frame ion energies between 0.2 and 8.0 eV. Their shapes agree very well with the predictions of nonlocal resonance theory. Both types of structure predicted by the theory are observed. They are the 'v steps', at ro-vibrational thresholds, and the 'S steps', which are a consequence of interchannel coupling, which raises the cross section when a higher vibrational channel closes. They exhibit the behaviour predicted by theory both when the collision energy is varied and upon isotope substitution. The 'v steps' move to higher electron energies with higher collision energy and when hydrogen is substituted by deuterium, reflecting the higher maximum energy available to the electron. The positions of the S steps do not depend on collision energy, and are essentially equal to differences of vibrational energies of the product molecules HCl, DCl, HBr and DBr. The relative cross sections for formation of low vibrational levels (i.e., emission of fast electrons) are smaller in the deuterated compounds, reflecting the slower motion of D compared to H and consequently preferred detachment at high internuclear separations.

We study the efficiency of evaporative cooling of a trapped gas of caesium atoms in the hydrodynamic regime by the numerical solution of classical kinetic theory equations. The results of the numerical simulation are compared to our experimental observations of evaporative cooling of magnetically trapped ¹³³Cs atoms in the F = 3, m_F = −3 state. The simulation accurately reproduces our experimental performance and indicates that the reduction in cooling efficiency as the gas enters the hydrodynamic regime is the main obstacle to the realization of Bose–Einstein condensation (BEC) in this state. The simulation is used to explore alternative routes to BEC.

The cross sections for the emission of dispersed fluorescences in the ultraviolet and visible ranges by excited hydrogen atoms and NH radicals in the photoexcitation of ammonia have been obtained as a function of incident photon energy in the range of 13–40 eV, in which five superexcited states have been found. Three of these states around 22, 30 and 33 eV are doubly excited states, which have been investigated in detail for the first time and the others are single-hole one-electron superexcited states around 15 and 26 eV. It is remarkable that the doubly excited states around 30 eV give as large oscillator strengths for Balmer fluorescences as the nearby single-hole one-electron superexcited states around 26 eV that are built on the (2a₁)⁻¹ ion core, which seems not to be amenable to the independent electron model.