Table of contents

The convergent close coupling method has been applied to positron–helium scattering for energies below the positronium formation threshold (17.8 eV). Convergence is studied as a function of the Laguerre basis size. Rapid convergence to the accurate phase shifts given by Van Reeth and Humberston (1999 J. Phys. B: At. Mol. Opt. Phys. 32 3651–67) is demonstrated. The resultant cross sections are in excellent agreement with the experimental data of Stein et al (1978 Phys. Rev. A 17 1600–8) and Mizogawa et al (1985 Phys. Rev. A 31 2171–9).

Results of molecular dynamics simulations of fission reactions and are presented. The dependence of the fission barriers on the isomer structure of the parent cluster is analysed. It is demonstrated that the energy necessary for removing homothetic groups of atoms from the parent cluster is largely independent of the isomer form of the parent cluster. The importance of rearrangement of the cluster structure during the fission process is elucidated. This rearrangement may include transition to another isomer state of the parent cluster before actual separation of the daughter fragments begins and/or forming a 'neck' between the separating fragments.

We discuss the possibility of using a three-level ladder atomic system, with a final autoionizing state, for the efficient generation of short radiation pulses in the extreme ultraviolet through a process of sum-frequency mixing in the presence of maximal coherence. The results of numerical calculations are presented, investigating the effects of photoionization, dynamic Stark shift and electron impact dephasing on the conversion efficiency in a single atom model. The results of this investigation can be useful in identifying a real candidate for a possible experiment.

Experimental M-shell nickel spectra in the 14.4–16.5 nm region from the JET tokamak (from both divertor and limiter configurations) and from the reversed field pinch RFX have been simulated. These spectra include lines from five ionization states, namely from K-like Ni⁹⁺ to P-like Ni¹³⁺ ions. For the JET limiter configuration the spectrum upper wavelength limit was higher (18.0 nm) and lines from Si-like Ni¹⁴⁺ ions were also observed. Collisional–radiative (CR) models have been built for these six Ni ions, considering electron collisional excitation and radiative decay as the main populating processes for the excited states. These models give photon emission coefficients (PECs) for the emitted lines at electron density (n_e) and temperature (T_e) values corresponding to the experimental situations. Impurity modelling is performed using a 1D impurity transport code, calculating the steady state radial distribution of the Ni ions. The Ni line brightnesses are evaluated in a post-processing subroutine and simulated spectra are obtained. The spectrum from a single ion, in the absence of blendings, depends only on the T_e and n_e values in the emitting shell of the ionization state considered. On the other hand, the superposition of these spectra depends on the experimental conditions, as a consequence of the fact that the ion charge distribution depends not only on the radial profiles of T_e and n_e, but also on the chosen ionization and recombination rate coefficients and on the radial profiles of the impurity transport coefficients in the region of the emitting shells. Since the aim of the paper is the investigation of the atomic physics of the M-shell ions, the section discussing the plasma physics phenomena is purposely limited. For each experimental spectrum a few simulations are presented, since a unique choice has not been found by selecting the input parameters of the transport code. The effect of the T_e and n_e values on the emitting shells as well as the influence of line blendings on the single-ionization-degree spectra are stressed. These, in turn, are then compared with the predictions. For the n_e range considered the PECs are practically independent of n_e. The T_e dependence is much reduced due to the fact that the spectral fits performed are actually comparisons of line ratios. The agreement found between experimental and simulated single-ionization-degree spectra gives confidence in the atomic data used in the CR models.

In a combined experimental and theoretical effort, we have investigated dissociative attachment and vibrational excitation in low-energy electron collisions with chlorine molecules. Using the laser photoelectron attachment method, we have measured the energy dependence of the cross section σ_DA(E) for dissociative electron attachment (Cl⁻ formation) over the range 0–195 meV with an energy width of 1–3 meV and for Rydberg electron transfer at high principal quantum numbers (n>67). Near zero energy, the cross section shows a behaviour compatible with the threshold law for p-wave attachment via the ²Σ_u⁺ resonance, reaches a maximum around 50 meV and declines towards higher energies. These findings are in good agreement with the results of semi-empirical R-matrix calculations. Measured rate coefficients k_nl for Cl⁻ formation due to electron transfer from K^**(nl) Rydberg atoms were found to be nearly constant for high principal quantum numbers (n>67) in contrast to the behaviour expected within the quasi-free electron model for p-wave attachment. The R-matrix calculations are extended to describe electron attachment through the ²Π_g and ²Π_u resonances, and recommended absolute cross sections for dissociative attachment to chlorine molecules at room temperature are provided over the energy range 0–9 eV. Furthermore, we predict cross sections for vibrationally inelastic electron scattering through the ²Σ_u⁺, ²Π_g and ²Π_u resonances.

A new quantum mechanical expression for calculating electron-impact broadening is obtained for intermediate coupling. It would be especially useful for calculations of widths and shifts of fine structure spectral lines of multicharged ions where LS coupling breaks down.

We present a two particle model to explain the mechanism that stabilizes a bunch of positively charged ions in an 'ion trap resonator' (Pedersen et al 2001 Phys. Rev. Lett. 87 055001). The model decomposes the motion of the two ions into two mappings for the free motion in different parts of the trap and one for a compressing momentum kick. The ions' interaction is modelled by a time delay, which then changes the balance between adjacent momentum kicks. Through these mappings we identify the microscopic process that is responsible for synchronization and give the conditions for that regime.

We investigate systems of identical bosons with the focus on two-body correlations. We use the hyperspherical adiabatic method and a decomposition of the wavefunction in two-body amplitudes. An analytic parametrization is used for the adiabatic effective radial potential. We discuss the structure of a condensate for arbitrary scattering length. Stability and timescales for various decay processes are estimated. The previously predicted Efimov-like states are found to be very narrow. We discuss the validity conditions and formal connections between the zero- and finite-range mean-field approximations, Faddeev–Yakubovskii formulation, Jastrow ansatz and the present method. We compare numerical results from the present work with mean-field calculations and discuss qualitatively the connection with measurements.

Multiple-charged Kr ions have been studied using monochromatized synchrotron radiation combined with a coincidence technique. The multiple-charged ions formed through ionization of the inner sub-shells were detected by a time-of-flight mass analyser and the photoelectrons emitted from those orbitals were observed by a cylindrical mirror electron energy analyser. The coincidence measurement between the energy-selected photoelectron and the photoion specified the individual multiple ionization channels of Kr. The Kr 3d ionization produces doubly and triply charged ions. The Kr 3p ionization mainly yields triply charged Kr ions via M₂₃M₄₅N₂₃ Coster–Kronig decays, and the M₂₃M₄₅N₁ decays generate considerably quadruply charged ions. The 3s ionization significantly produces quadruply charged ions via super-Coster–Kronig decays with successive Auger transitions.

We present a fully relativistic theoretical treatment of the process of dielectronic recombination (DR) with emission of two photons. In the case of KLL DR into hydrogen-like ions, the system is not stable until both electrons are de-excited with the emission of two photons. On the basis of a projection operator formalism, perturbative expressions for the cross sections are derived. The results for the total cross section for hydrogen-like uranium are presented and compared to those which have been obtained in a theory considering only one photon transition. In addition, the angular distribution of the two photons and their correlation are examined.

We present a comprehensive calculation of 3D dynamic stabilization (DS) of ground-state hydrogen in superintense circularly polarized laser pulses. Three laser-pulse envelopes have been considered: Gaussian, sech, and Lorentzian. The ionization probability at the end of the pulse P_ion was calculated for a range of high frequencies ω ranging from 0.65 to 8 au, for peak fields up to about 60 au (depending on ω), and for full width at half maximum pulse lengths τ_p extending from 0.25 to 100 cycles (depending on ω). This is a very accurate calculation, very much more time consuming than its linear polarization counterpart. For Gaussian and sech pulses we find prominent DS and substantial atomic survival under conditions where our nonrelativistic, dipole approximation calculation is expected to be valid. For Lorentzian pulses there is no DS in the range studied, and we explain the reasons. We find that the evolution of the atom is adiabatic and amenable to single-state Floquet theory, up to very large peak fields (several au), and down to very short pulses (few cycle, subfemtosecond). The general case of nonadiabatic pulses is interpreted in terms of the multistate Floquet theory. We compare the results for P_ion in the cases of circular and linear polarization and find a surprising resemblance, when represented as a function of the peak intensity. Our results indicate the possibility of observing DS experimentally with the VUV–FEL light sources that are now in test operation, or with the attosecond pulses obtained from high harmonic generation, in a state-of-the-art experiment, however.

We report the results of coupled channels calculations of cross-sections for torsionally elastic and inelastic transitions in E-type methanol (CH₃OH), with helium as the colliding partner. The dependence of the CH₃OH–He interaction potential on the internal rotation (torsional) angle was determined using second-order many-body perturbation theory. The methanol basis comprised levels belonging to the ground torsional state (ν = 0) and the first excited torsional state (ν = 1). The collisional 'propensity rules' observed in the case of torsionally elastic collisions were found not to apply to torsionally inelastic transitions between states of ν = 0 and 1. We assessed the effect of the torsional coupling on the torsionally elastic cross-sections and found changes of no more than about 30% at the highest collision energy considered (500 cm⁻¹). The cross-sections for torsionally inelastic transitions were found to be typically two orders of magnitude smaller than for torsionally elastic transitions.

A stepwise laser excitation method has been used to probe individual ro-vibrational levels in the molecular hydrogen metastable c(2p) ³Π_u manifold of states. Metastable states produced by electron-impact excitation are subsequently excited by the absorption of a single UV laser photon, producing a complex triplet nd Rydberg spectrum observed via field ionization and autoionization. The spectrum has been analysed to determine quantum numbers of the states associated with the transitions. Excitation functions for individual ro-vibrational states in the c(2p) ³Π_u manifold are then determined as a function of electron-impact energy.

The influence of the target density on the electron-capture (EC) processes in collisions of fast ions with atoms and molecules is considered. The partial EC cross sections σ_n on the principal quantum number n of the scattered projectile, as well as the total σ_tot = Σ_nσ_n values, are calculated for highly charged ions interacting with gaseous and solid targets in the energy range of E = 100 keV u⁻¹ to 10 MeV u⁻¹. It is shown that with the target density increasing, the populations of the excited states of the scattered projectiles, formed via the EC channel, are drastically suppressed due to projectile ionization by the target particles and, as a result, the total EC cross sections decrease by orders of magnitude at low energies, while the reduction is less prominent at high energies.

We present a quantum dynamical treatment of the vibrationally elastic scattering off tetrafluorocarbon (CF₄) molecules by very low energy positrons. The present calculation is based on the fixed-nuclei approximation and the interaction potential between the positron and the molecular target includes the ab initio electrostatic and the target correlation-polarization effects. The latter contribution is considered by means of a parameter-free model potential based on the density functional theory. The cross sections for this system are obtained by solving multichannel close-coupling equations for the wavefunction of the scattered positron, while no positronium formation channel is taken into account by the present calculation. The elastic cross section, the scattering length (A₀) and the location of a virtual state for the total system are obtained and discussed in detail.

The effect of differing the datasets used in the modelling of the Ni-like Gd x-ray laser (XRL) is examined through the 1.5D hydro-atomic code, EHYBRID. Two atomic datasets, including energy levels and radiative and collisional excitation rates, are used as input data for the code. It is found that the behaviour of the XRL is somewhat different than might be expected from superficial examination of the atomic data. The similarities in the gain profiles at low densities are found to have encouraging implications in our attempts to model XRLs.

We investigate mean-field effects in two-component trapped Fermi gases in the superfluid phase, in the vicinity of s-wave Feshbach resonances. Within the resonance superfluidity approach (Holland et al 2001 Phys. Rev. Lett.87 120406) we calculate the ground state energy and the heat capacity as functions of temperature. Heat capacity is analysed for different trap aspect ratios. We find that trap anisotropy is an important factor in determining both the value of heat capacity near the transition temperature and the transition temperature itself.

New electron-impact differential cross-section (DCS) and DCS ratio measurements for the excitation of the four levels making up the 3p⁵4s configuration of argon are reported at incident electron energies of 14, 15, 17.5, 20, 30, 50 and 100 eV. These cross-sections were obtained using a conventional high resolution electron spectrometer. Elastic electron scattering from argon was used as a calibration standard. Electron–helium DCSs were used to determine the instrumental transmission of the spectrometer. Further checks of the relative shape of these DCS measurements were made using the method of gas mixtures (Ne mixed with Ar). We also present results from new calculations of these DCSs using the R-matrix method, the unitarized first-order many-body theory, the semi-relativistic distorted-wave Born approximation, and the relativistic distorted-wave method. Comparison with available experimental DCSs and DCS ratios is also presented.

The formation of fluorescent and metastable fragments from four diatomic molecules, i.e. O₂, N₂, NO and CO, has been investigated in the vacuum ultraviolet region. The neutral particles are detected by using a microchannel plate stack, where retarding electric potentials prevent charged particles from reaching the detector. Every diatomic molecule investigated here shows the formation of fluorescent and metastable fragments in particular photon energy regions. Three Rydberg states of O₂ converging to O₂⁺(a ⁴Π_u) undergo both neutral dissociation processes forming fluorescent [O⁺(⁴S)]3s ³S₁ and metastable [O⁺(⁴S)]3s ⁵S₂; the yield curve for each fragment is determined. Direct and cascade formation of the fluorescent [N⁺(³P)]3s ⁴P fragment from N₂ are separated, and it is found that the dissociation of the 4s σ Rydberg state converging to N₂⁺(C ²Σ_u⁺) preferentially produces [N⁺(³P)]3p fragments, but not or weakly the [N⁺(³P)]3s ⁴P fragment. High -n Rydberg states converging to NO⁺(c ³Π) and CO⁺(D ²Π) undergo neutral dissociation described by the core ion model, resulting in large peaks for neutral particle formation.

An investigation of the formation channels and properties of ion fragments following electron-impact dissociative ionization of the CCl₂F₂ molecule using electron kinetic energies in the 0–100 eV range is reported. Measurements of ion appearance potentials (APs) and nascent translational energy distributions were made on a supersonic expansion of CCl₂F₂ in a time-of-flight mass spectrometer. A discussion of the correlation between the channel APs, the precursor bond characters as calculated from the population analysis, and the low-resolution photoelectron spectrum of the CCl₂F₂ molecule is presented.