Table of contents

An accurate pseudostate summation technique is used to evaluate the multiphoton absorption coefficient for an atom as a function of the frequency of incident photons. The effect of polarization of the incident radiation on the absorption coefficient is shown by comparing the results obtained for the linearly polarized light with those obtained for the circularly polarized light.

We measured state-selective absolute electron capture cross sections for collisions of He-like C⁴⁺, N⁵⁺ and O⁶⁺ ions with molecular hydrogen by means of photon emission spectroscopy at impact energies ranging from 4 keV amu^-1 down to energies as low as 5 eV amu^-1. In this low-energy region the total cross sections are found to be weakly dependent on the projectile energy, whereas the state-selective ones change drastically with varying impact energy. Up to now no experiments were available to benchmark theoretical predictions. In our method we combine for the first time decelerated projectile beams with the possibility of measuring state-selective electron capture.

Compared with existing measurements for state-selective cross sections our deduced cross sections are in perfect agreement in the overlapping energy region (E>>100 eV amu^-1). Good agreement is also found on comparing our results with existing total cross sections. In comparison with theoretical calculations some remarkable and unexpectedly large discrepancies of orders of magnitude are found towards lower impact energies.

An effective Hamiltonian approach is used to analyse QED contributions to the fine structure of 2 ³P_J levels in helium to order mα⁷. Formulae for leading contributions are derived and their numerical values presented.

The stochastic variational method is used to investigate the stability of the (m⁺, e^-, e⁺) system as a function of the m⁺/m_e mass ratio. The system was found to be stable for 0.697 78⩽m⁺/m_e⩽1.6343. These mass limits correspond to stability for energy values of the (m⁺,e^-) subsystem satisfying 0.205 498⩽E(m⁺,e^-)⩽0.310 196 (energies in Hartree). These energy limits correspond roughly to the ionization potentials of neutral atoms that are known to bind a positron. The (m⁺, e^-, e⁺) system can be regarded as an analogue of a typical positronic atom since the structure of the (m⁺, e^-, e⁺) system as a function of E(m⁺,e^-) is seen to be reminiscent of the structure of positronic atoms as a function of the parent atom ionization potential.

We consider a ground-state hydrogen atom interacting with a three-colour electromagnetic field, taken as the superposition of a fundamental frequency ω and two consecutive odd harmonics of order 2p-1 and 2p + 1 (p⩾2 and integer), with constant relative phase differences. We study in the lowest-order perturbation theory the process of ionization due to the net absorption of energy 2pℏω. The ionization rate is controlled by the relative phase difference δ between the harmonics, which modifies the interference terms. We present an analysis of both the angular electron distribution and the total rate, which shows that the interference effects are influenced by the order of the harmonics and their intensity ratio.

We present the elastic and charge exchange cross sections for slow (meV) H⁺ + D(1s) collisions based upon a recently formulated adiabatic representation. This representation avoids many of the usual `translational factor' issues, and those that remain have no significant effect for scattering in the n = 1 manifold. The cross sections are converged at the 1% level. The various Feshbach and shape resonances that form in the isotopic gap and in the charge transfer region are identified and compared with previous work. An exact analytical expression for the ratio of H⁺ + D(1s) and D⁺ + H(1s) charge exchange cross sections suitable for astrophysical simulations is presented.

We employ a three-body classical trajectory Monte Carlo (CTMC) method to calculate the recoil-ion momentum distribution at its kinematic threshold in ion-atom ionization collisions. We analyse how this threshold is intertwined by dynamical constraints to the electron capture to the continuum cusp in the electron double differential cross section. We compare these calculations with those from a full quantum-mechanical description and explore how these structures depend on the interactions among the three particles in the final state.

The multiphoton ionization of hydrogen in strong, nearly parallel microwave and static fields of comparable strength is a fundamental problem of great complexity. The survival probability against ionization shows rapid oscillations which have remained unexplained to date. We show that the experimental oscillations can be explained qualitatively and quantitatively based on just one assumption of a relatively small misalignment of the nearly parallel microwave and static fields. Moreover, these rapid oscillations can be used as a very sensitive diagnostic of the small misalignment of the two fields in future experiments.

Superelastic electron scattering involving the collisional de-excitation of laser-excited ¹³⁸Ba (...6s6p ¹P₁) atoms to the (...6s²¹S₀) ground state has been used to measure electron impact coherence parameters for the related (...6s²¹S₀)-(...6s6p ¹P₁) inelastic process. Measurements of the natural parameters: γ and P_lin⁺ were made for excitation at impact energies of 5, 7.5, 9, 10 and 15 eV. Experimental data are compared with available theoretical results.

Absolute photon emission cross sections following electron capture reactions have been measured for C²⁺, N³⁺, N⁴⁺ and O³⁺ ions colliding with Li(2s) atoms at keV energies. The results are compared with calculations using the extended classical over-the-barrier model by Niehaus. We explore the limits of our experimental method and present a detailed discussion of experimental errors.

The generalized oscillator strengths (GOSs) for the transitions from ground states to the excited states np⁵(n + 1)s of Ne (n = 2), Kr (n = 4) and Xe (n = 5) have been calculated as a function of the momentum transfer squared using the random-phase approximation with exchange and using the Hartree-Fock approximation. The positions of the characteristic minima and maxima in the GOSs have received particular attention in the evaluation. They are found to agree excellently with the experimental results. Exchange and correlation effects between the electrons in different subshells are discussed and are found to influence the positions of the minima insignificantly.

The time evolution of driven two-level systems in the far-off-resonance regime is studied analytically. We obtain a general first-order perturbative expression for the time-dependent density operator which is applicable regardless of the coupling strength value. In the strong-field regime, our perturbative expansion remains valid even when the far-off-resonance condition is not fulfilled. We find that, in the absence of dissipation, driven two-level systems exhibit coherent population trapping in a certain region of parameter space, a property which, in the particular case of a symmetric double-well potential, implies the well known localization of the system in one of the two wells. Finally, we show how the high-order harmonic generation that this kind of system displays can be obtained as a straightforward application of our formulation.

Electronic excitation and capture processes are studied in collisions involving systems with only one active electron such as the alkaline (Li)-proton in the medium-energy region (0.1-15 keV). Using the semiclassical impact parameter method, the probabilities and the orientation parameter are calculated for transitions between initial and/or final oriented states. The results show a strong asymmetry in the probabilities depending on the orientation of the initial and/or final states. An intuitive view of the processes, by means of the concepts of propensity and velocity matching rules, is provided.

The spectroscopic parameters (R_e, D_e, ω_e) for the ground state of the weakly bound Hg-rare-gas (He, Ne, Ar, Kr, Xe) van der Waals molecules have been calculated at the valence coupled-cluster with single and double excitations and perturbative contribution of connected triple excitations level, using quasirelativistic energy-consistent small-core pseudopotentials and large atom-centred basis sets. The theoretical potentials have been obtained without fitting to any experimental data. Very good agreement of the theoretical results with available experimental data has been found.

X-ray spectra of 2l-3l' transitions in the elements krypton (Z = 36), zirconium (Z = 40), niobium (Z = 41) and molybdenum (Z = 42) from charge states around neon-like have been observed from Alcator C and Alcator C-Mod plasmas. Accurate wavelengths (±0.5 mÅ) have been determined with reference to neighbouring aluminium, silicon, sulfur, chlorine and argon lines with relatively well known wavelengths. Line identifications have been made by comparison to ab initio atomic structure calculations, using a fully relativistic, parametric potential code. Calculated wavelengths and oscillator strengths are presented for 2l-3l' transitions in neon-like ions and neighbouring Na-, Mg-, F- and O-like satellites. Electric quadrupole transitions in neon-like ions of the form 2s-3d and 2p-3p are found to be relatively bright, and their measured intensities agree well with the results of collisional-radiative modelling. The 2p-3s magnetic quadrupole transition is also relatively intense in the absence of collisional de-excitation at tokamak densities (10¹³-10¹⁵ cm^-3). For krypton, transitions with upper levels up to n = 9 have been observed, and the measured wavelengths and intensities have been compared with calculations. For some neon-like transitions with nearly degenerate upper levels, there can be substantial configuration interaction which alters the line intensities.

Poelstra et al (1994 J. Phys. B: At. Mol. Opt. Phys. 27 781-94) have presented a calculation of the Wannier exponent for the four-particle threshold break-up in double ionization of H^- by positron impact. These authors find the threshold power law to be governed by a planar break-up configuration with only one unstable mode. This result has recently been questioned by Kuchiev and Ostrovsky (1998 Phys. Rev. A 58 321-35), who find two unstable modes for the same configuration. The reason for this disagreement remained unclear. In this comment we show both by an intuitive argument and by a simple calculation that it is an unfortunate reduction of coordinates performed in the calculation by Poelstra et al that causes this disagreement.