Table of contents

We present results of accurate ab initio Floquet calculations of lower-order harmonic generation by argon at the KrF laser fundamental frequency which exhibit large enhancement of the third, fifth and seventh harmonics at intensities corresponding to field-atom resonances. The resonant effects occur at relatively low laser intensities. These are the first reported results for a complex atom taking full account of electron correlation and give a quantitative description of the resonant enhancement. We infer that similar enhancements will occur for other multi-electron atomic systems. The single-atom response calculations demonstrate the importance of resonant enhancement of harmonic generation in multi-electron atoms and suggest a possible low-cost method of sustainable harmonic generation.

New coplanar (e, 3-1e) experiments on double ionization (DI) of helium and molecular hydrogen at about 600 eV incident energy, and with asymmetric energy sharing among the two ejected electrons are presented. The results are compared with state-of-the-art first-order theories, and are discussed in terms of DI mechanisms. Evidence is given of the great importance of second- or higher-order effects in the projectile-target interaction, even at the high incident energy considered.

The quantum jump statistics for the three stable isotopes of Mg⁺ have been studied experimentally. Single ions of magnesium were held and laser cooled in a Penning trap. Quantum jumps were observed by collecting the resonance fluorescence near 280 nm from the cooled ion.

The jump rate was analysed to find ratios of the average time the ion spends fluorescing, `on' time, to the average time it spends in the dark state, `off' time: for both ²⁴Mg⁺ and ²⁶Mg⁺ this value was found to be 15.9±1.2, in agreement with a theoretical value of 16.0. The presence of hyperfine structure in ²⁵Mg⁺ means that while the jump rate for ²⁴Mg⁺ and ²⁶Mg⁺ remains unaltered for both available cooling transitions, for the odd isotope the on to off ratio is predicted to be 11.8 and 22.8 for the lower and upper transitions respectively. Experimental values for these ratios were found to be 10.2 ± 0.7 and 22.0 ± 0.6.

The occurrence of so-called nuclear jumps in ²⁵Mg⁺ is investigated qualitatively. These rare jumps are produced as a result of the hyperfine splitting, which is only present in the odd isotope. Although the data indicate some evidence for this type of jump, it is not possible to draw any firm conclusions due to the small number of events observed.

The strong-field approximation, amended so as to allow for rescattering, is used to calculate high-order above-threshold ionization (ATI) spectra. The single-active-electron binding potential is modelled by a zero-range potential. The emphasis is on enhancements of groups of ATI peaks that occur for sharply defined laser intensities. The enhancements are traced to multiphoton resonance with the ponderomotively upshifted continuum threshold. Good agreement both with experimental data and with numerical simulations using the three-dimensional time-dependent Schrödinger equation for an optimized one-electron binding potential is observed. The physical reason for the close agreement of the results of the two, apparently so different, models is discussed. For quantitative agreement with the experimentally observed positions of the resonances, an `effective' continuum threshold has to be introduced. The effects of focal averaging are evaluated and discussed. Resonant enhancement of high-order harmonic generation is also considered.

The charge-transfer processes of Co³⁺ with Ar and N₂ were studied in a quadrupole RF ion trap with the mean collision energy of about 5.6 eV. The total rate coefficients were measured to be 7.85(0.72)×10^-11 cm³ s^-1 for Co³⁺ with Ar at equivalent temperature of 1.8×10⁴ K and 4.38(0.50)×10^-11 cm³ s^-1 for Co³⁺ with N₂ at equivalent temperature of 1.4×10⁴ K, respectively.

We have investigated cross sections for the excitation of the 2p³3s ⁵S^o, ³S^o, 2p³3p ⁵P, ³P, 2p³3d ³D^o, 2p³3s' ³D^o, 2p³3s'' ³P^o and 2s2p^5 3P^o states by electron impact using a modified R-matrix method based on the B-spline representation of the scattering orbitals in the energy region from threshold to 100 eV. The 26 spectroscopic bound and autoionizing states of atomic oxygen have been included in the close-coupling expansion. We have also carried out calculations in 8 and 16 state close-coupling approximations and in the Born approximation. We have obtained an accurate representation of the target wavefunctions on the basis of non-orthogonal orbitals, and we compare the present results with other calculations and experiments. Significant discrepancies in the shape and magnitude of cross sections for some of the transitions are noted.

The energy eigenfunctions of a confined hydrogen atom have a simple coalescence property near the centre, and an inflexion property at the boundary and nodal points. Using these properties, simple wavefunctions are developed for the lowest energy state and first excited state with a given l, and also for the ground state perturbed by a multipolar potential. The predictions for the energies and multipolar polarizabilities are in close agreement with the numerically obtained accurate values.

Several peaks have been observed, in the electron energy range up to 20 eV, due to Coster-Kronig electrons from the autoionizing Rydberg states emitted from 2 MeV u^-1 Si⁵⁺ ions excited through a carbon foil. To obtain a better understanding of these low-energy electron spectra and their production mechanisms, we synthesize the expected electron spectrum which is compared with the observed electron spectrum. Two theoretical methods, namely the perturbation theory of Z-expansion (MZ code) and the multiconfiguration Hartree-Fock method (Cowan code), are used to calculate the necessary Auger electron energies and emission rates. It has been found that the 1s²2pnl-1s²2s ²S and 1s²2s2pnl-1s²2s^2 1S decays give the most significant contributions to the observed electron spectrum from threshold up to the 20 eV region. The synthetic electron spectra have also been found to reproduce the observed electron spectra quite nicely when the charge distributions of the projectile Si ions after thin carbon foils are taken into account.

Electron impact excitation collision strengths of doubly ionized atomic neon (Ne III) are calculated in the close-coupling approximation using the state-of-the-art R-matrix method. Twenty-eight 2s²2p⁴, 2s2p⁵, 2s²2p³3ℓ (ℓ = s,p,d), and 2s²2p³4s LS target terms, which comprise 56 fine-structure levels, are retained in the present calculations. The effective collision strengths for astrophysically important lines in the Ne III spectra are obtained by averaging the electron collision strengths, for a wide range of incident electron energies, over a Maxwellian distribution of velocities. Results are presented for electron temperatures (T_e in kelvins) in the range 3.0⩽log T_e⩽6.0, applicable to many laboratory and astrophysical plasmas. The present results provide information on several important astrophysically observed lines in the Ne III spectrum necessary in order to quantitatively analyse existing extreme-ultraviolet solar spectra.

First order distorted wave Born approximation (DWBA) triple differential cross sections are reported for low-energy electron-impact ionization of the inner 3s and outer 3p shells of argon. Previous DWBA works have demonstrated that experiment and theory are not in accord for low energy ionization of inert gases and here we investigate the importance of exchange scattering. Different approximations for treating exchange scattering are investigated. It is shown that exchange scattering is particularly important for 3s ionization. Even with a proper treatment of exchange, the first order calculations are still not in satisfactory agreement with experiment. Consequently higher order effects will have to be included to achieve a satisfactory description of the low-energy ionization process. We also investigated both the Hartree-Fock and optimized potential methods for calculating atomic wavefunctions and static potentials and found that both methods produced almost the same cross sections.

4f collapse in rare earth ions produces an enormous level density due to the occurrence of near-degeneracies of 4f/5p and 4f/5s binding energies. As a result, there is a complete breakdown of the single-particle approximation and the level and line spectra satisfy many of the distributions associated with random matrix theory. Similar behaviour has been previously reported in spectra associated with wavefunction contraction effects or atoms containing open 4f subshells; here the effects of combining both phenomena are explored. In particular it is shown that the adjacent level separations satisfy a Wigner distribution, while transitions between them obey the Porter-Thomas distribution. Detailed Hartree-Fock calculations show that there is a strong correlation between line strength and transition energy. This correlation is pursued and parametrized for the particular case of 4f→5d, 5p→5d and 5s→5p transitions in Sm IX within the unresolved transition array model.

With the aim of elucidating general trends for the autoionizing decay of the Rydberg series Rg (n_cp_1/2⁵nℓ'[K' = ℓ'±1/2]_J) (Rg = Ne, Ar, Kr, and Xe; n_c = 2-5), converging to the fine-structure-excited ionization limit Rg⁺ (n_cp_1/2^5 2P_1/2), we have carried out calculations of the resonance widths for all ℓ'[K']_J series with ℓ' = 0-5 within the single-electron Pauli-Fock approach. The ℓ'-dependence of the reduced widths Γ_r = n^*3Γ_n (n* = n-µ_ℓ), which is independent of n at large n, is obtained, and the dominant decay paths are characterized. Based on these findings, expressions for a `generalized' reduced width Γ_gr are presented for the ℓ'[ℓ'±1/2]_J resonance series in which the main dependence on ℓ' is removed and which provide estimates of the autoionization widths towards higher ℓ'. The calculated reduced resonance widths are compared with experimental results.

We use many-body theory to find the asymptotic behaviour of second-order correlation corrections to the energies and positron annihilation rates in many-electron systems with respect to the angular momenta l of the single-particle orbitals included. The energy corrections decrease as 1/(l + ½)⁴, in agreement with the result of Schwartz, whereas the positron annihilation rate has a slower 1/(l + ½)² convergence rate. We illustrate these results by numerical calculations of the energies of Ne and Kr and by examining results from extensive configuration-interaction calculations of PsH binding and annihilation.

A theory is developed that allows a qualitative explanation and quantitative calculation of the widths and shifts of the two-photon Doppler-free transitions from the ground states of alkali atoms to Rydberg n ²S and n ²D states. The widths and shifts result from collisions between Rydberg atoms and ground-state alkali atoms, either of the same or of other species. The collisions are represented by means of a pseudopotential that is a function of theoretically calculated free-electron elastic scattering resonant and nonresonant phase shifts, which depend upon the Rydberg electron momentum. The momentum, in turn, is correlated with the electronic position within the Rydberg atom. This theory assumes classical straight-line trajectories and takes into consideration the multicollisional nature of the Rydberg-electron-perturbing-atom interaction within the Rydberg atom. Impact theory calculations of the linewidths and shifts for K**(n S)-K, K**(n S)-Rb, Rb**(n S)-Rb, Rb**(n S)-K, K**(n D)-K, K**(n D)-Rb, Rb**(n D)-Rb and Rb**(n D)-K show substantial agreement with experiment for 10⩽n⩽40.

In this paper we present the results and discuss the applicability of an impact theory of lineshapes, developed by Henry and Herman for the pressure-induced broadening and shifting of transitions from the ground state to various Rydberg states of alkalis, to the situation in which the perturbing atoms are ground-state rare gas atoms. This theory, which has successfully described the collisional broadening of Rydberg states by alkali atoms, can without major alterations predict the widths and shifts of Doppler-free two-photon transitions to nS and nD states of Rb and Na, 6⩽n⩽30, perturbed by He, Ne, Ar, Kr and Xe. The results in all cases show reasonable agreement with experiment, with the transitions to Na**(nD) states being somewhat less satisfactory than the others.

In this study we perform Dirac-Fock CCSD(T) (coupled-cluster approach with single and double substitution (non-iterative triplets)) electric field gradient (EFG) calculations with self-consistent inclusion of the Gaunt operator as a first correction to the non-covariant Coulomb repulsion operator. Al in AlF and AlCl and Tl in TlH served as test systems for mainly non-relativistic and strongly relativistic systems. The inclusion of the Gaunt term is expected to have only little direct influence on the EFG since it mainly affects the innermost nearly spherically symmetric shells, but due to non-spherical molecular density shifts upon the self-consistent orbital relaxation a pronounced SCF/CCSD(T)/Gaunt EFG shift of 0.1 au was observed in TlH. In the light systems studied, the effect is very small and still negligible. Thallium itself possesses no ground-state nuclear quadrupole moment (NQM), but the results show that for very accurate determinations of heavy-element NQMs the Gaunt contribution to the EFG should be taken into account.

Relative probabilities of monopole shake processes are calculated in the sudden perturbation approximation for atoms with Z between 4 and 71. Most of the calculated data are presented compactly using a simple two-parameter interpolation formula for Z-dependences of shake probabilities per electron.

Probabilities for adiabatic or near-adiabatic state transformation within a highly excited shell of Li(n = 25) were studied experimentally and theoretically for a time dependent electric field, (t), and a constant magnetic field, . The fields were sufficiently weak and the time dependence slow enough such that only states belonging to the chosen shell were involved. The studies show that the dynamics are governed by the approximate hydrogenic character of the system in most cases, but for some specific time dependences it is influenced strongly by core interactions as expressed through the quantum defects, δ_l. The s-state is effectively decoupled from the rest of the n = 25 manifold due to a very large quantum defect. However the quantum defects of the p, d and f states are shown to play a decisive role in the dynamics. The core interactions lead to avoided crossings, non-adiabatic state transformations, and possibly even phase-interference effects. When a resonance condition pertaining to the hydrogenic character of the system is fulfilled, a linear Stark state is transformed completely into a circular Stark state oriented along _f.

Quadrupole oscillator strengths (QOS) for He and the isoelectronic ions from Li⁺ to Ne⁸⁺ are reported for all possible n ¹S-m ¹D, n ³S-m ³D, n ¹P-m ¹P and n ³P-m ³P transitions involving states with m <7 and n <7. The calculations are based upon explicitly correlated wavefunctions that lead to variational energies only nano-Hartrees above the best literature values. The results extend significantly both the accuracy and range of the QOSs available for two-electron atomic species.

The R-matrix method is used to calculate the elastic and the excitation cross sections from the ground state X ²Π to the four electronically excited states A ²Σ⁺, 1 ⁴Σ^-, 1 ²Σ^- and 1 ²Δ of the SH radical. Configuration interaction (CI) wavefunctions are used to represent the target states. In our target CI model, we keep 1σ, 2σ, 3σ and 1π orbitals fully occupied and the remaining seven valence electrons are free to occupy 4σ, 5σ, 6σ, 2π and 3π molecular orbitals. This model gives an equilibrium bond length, R_e, of the X ²Π state equal to 2.568 a₀ which is in good agreement with the experimental value 2.535 a₀ and a SH equilibrium dipole moment of 0.795 D which is close to the experimental value of 0.758 D. Scattering calculations are performed in the static-exchange, the static-exchange plus polarization and the 5-state models. The vertical excitation energies lie in the range 4.01-7.42 eV. We find a bound state of SH^- of ¹Σ⁺ symmetry with an electron affinity of 1.37 eV at R_e for the 5-state model. We also find two shape resonances in ¹Π symmetry and ³Π symmetry with a common configuration 2π³6σ, and two core-excited shape resonances of ¹Σ⁺ and ³Σ⁺ symmetries with A ²Σ⁺ as the parent state. Born correction is applied for dipole allowed transitions to account for higher partial waves excluded in the R-matrix calculation. Cross sections are given for scattering energies up to 10 eV.