Table of contents

The strong-field dynamics of diatomic molecules with antibonding valence orbitals is investigated within classical and quantum mechanical model calculations. Due to the orbital symmetry, ejected electrons never return to the core if a linearly polarized laser field acts perpendicularly to the molecular axis. However, elliptical polarization can be used to achieve recollision so that high-order above-threshold ionization and high-order harmonic generation become maximal at nonzero ellipticity. Smaller ellipticities favour higher harmonic orders. For given ellipticity the probability for electron recollisions depends strongly on the orientation of the molecule. Thus, ellipticity can be used as a control parameter to select a range of orientations for recollision-induced processes.

Absolute photoionization cross sections for the population of the Kr II 4p⁴np states when the exciting-photon energy corresponds to the first four 3d⁹np resonances were calculated beyond the two-step model for the first time. Good agreement between computed and measured photoionization cross sections proves the importance of taking into account the interference between different resonance channels in understanding the dynamics of the Auger decay of the Kr I 3d⁹np resonances.

The phenomenon of strong laser field atomic stabilization is discussed. Earlier suggested models and mechanisms of stabilization are described: Λ- and V-type interference stabilization of Rydberg atoms, adiabatic (Kramers–Henneberger) and high-frequency stabilization of neutral atoms and negative ions, and so on. Both numerical and analytical approaches to the description of these phenomena are discussed. In this context, ab initio numerical solutions of the nonstationary Schrödinger equation, obtained by several groups of authors, are overviewed. Based on the most modern and recent solutions of this type, mechanisms of stabilization of a hydrogen atom are shown to vary with varying intensity and frequency of a laser field. Such an evolution and applicability condition of various stabilization mechanisms is described. Limitations arising due to relativistic effects are discussed. Existing experiments on strong-field stabilization are overviewed and their interpretation is considered.

The relativistic coupled cluster theory is employed to calculate the hyperfine structure of the ²S _1/2, ²P _1/2, ²P _3/2, ²D _3/2 and ²D _5/2 states of singly ionized calcium. The importance of correlation effects is highlighted. Our results are compared with other theoretical calculations and experiments.

We consider the influence of a weak low-frequency electromagnetic field on Compton scattering of a high-energy photon by an electron which is initially bound in the ground state of a light atomic target. It is shown that this influence can be very substantial for the Compton scattering with a large (on the target scale) momentum transfer to the target when the electron, as a result of the scattering, makes a transition into a high-energy continuum state. It follows from our consideration that a weak low-frequency field can pronouncedly modify both spectra of emitted electrons and those of outgoing high-frequency photons. The modification of the latter spectra means that in the bound–free Compton scattering on a light target a high-energy photon can be indirectly, but rather effectively, coupled to a weak low-frequency field, using the target electron as a mediator.

A detailed investigation is made of low energy positronium–hydrogen s-wave elastic scattering, using the Kohn variational method, with very flexible scattering wavefunctions including all inter-particle correlations. The singlet and triplet scattering lengths are calculated and found to be a⁺ = 4.311a ₀ and a⁻ = 2.126a ₀ respectively. The phase shifts are believed to be within less than 0.5% of the exact values, and they are in good agreement with the results of previous calculations.

A 3D lattice based on a high-power CO₂ laser is considered in the context of laser cooling and trapping of atomic calcium. We expect to be able to realize a system with >10 000 lattice sites each with more than 100 atoms, and the facility to laser cool all the atoms into the vibrational ground state using the intercombination line. The configuration allows the production of an array of small Bose–Einstein condensates, enabling an investigation of the build-up of phase coherence as a function of atom number.

The interaction of a laser with potassium atoms is investigated by monitoring the radiation of the 5P–4S transition under the two-photon 4S_1/2 –6S_1/2 excitation. For low number density and laser intensity the observed radiation is parametric axial or conical, depending on the laser detuning from the two-photon resonance, the number density and the laser intensity. With increasing laser intensities and number densities, an additional delayed component is observed corresponding to a two-step four-wave mixing with the participation of internally generated photons. A two-photon quantum interference effect is observed for the total (axial and conical) parametric four-wave mixing emission. A deviation from the proposed model under certain experimental parameters is attributed to a two-step four-wave mixing and the phase mismatch of the parametric processes.

In a paper with a similar title, Yamanaka has calculated the mass polarization effect (to first order in μ/M) for several low-lying states of the two-electron atoms and ions with atomic number Z from 2 to 10. Here we improve the previous results by using Hylleraas variational wavefunctions with up to 560 terms and extend the calculation to include some additional states and the Z = 1 ground state. In addition, we compute the second-order effect using the method of pseudostate summation. A nonperturbative method of computation is also discussed and used as a check.

Two-photon ionization of atomic hydrogen with an excess photon is revisited. The non-relativistic dipole approximation and Coulomb Green function (CGF) formalism are applied. Using the CGF Sturmian expansion straightforwardly, one gets the radial transition amplitude in the form of an infinite sum over Gauss hypergeometric functions which are polynomials. It is convergent if all intermediate states are in the discrete spectrum. In the case of two-photon ionization with an excess photon, when photoionization is also possible, intermediate states are in the continuum. We performed the explicit summation over intermediate states and got a simple general expression for the radial transition amplitude in the form of a finite sum over Appell hypergeometric functions, which are not polynomials. An Appell function may be expressed as an infinite sum over Gauss functions. In the case of ionization by an excess photon, Gauss functions are transformed to give a convergent radial transition amplitude for the whole region. The generalized cross sections for two-photon above-threshold ionization of atomic hydrogen in the ground state and excited states calculated by us agree very well with results of previous calculations. Generalized cross sections for two-photon ionization of positronium in the ground state are obtained by scaling those for atomic hydrogen.

The capture of a target electron to the projectile continuum with simultaneous photon emission, known as radiative ionization (RI), is investigated theoretically for fast, highly charged projectiles colliding with light target atoms. Based on the impulse approximation in its post form, the features of radiative ionization are studied and contrasted with the related processes of radiative electron capture and nonradiative electron capture to continuum (ECC). A large RI cusp asymmetry is found, increasing strongly with projectile charge but decreasing with projectile velocity. However, in contrast to ECC, the cusp is skewed to the high-energy side. Results are shown for 20 MeV amu⁻¹ Kr³⁶⁺ + H and for 100 and 400 MeV amu⁻¹ U⁹²⁺ on H and N.

Mass-to-charge spectra of ions formed in ionization of freon-12 molecules, CF₂Cl₂, by protons with energies of E_p = 10 keV and 1 MeV are studied for the first time. Relative probabilities of different dissociation channels of the ionized freon-12 molecule, are obtained and compared with similar data for freon-12 ionization by electrons, positrons and photons. Absolute total and partial experimental cross sections for the formation of different fragment ions are evaluated for the case E_p = 10 keV. Neutral chlorine atom detachment is found to be the main fragmentation channel at both energies studied. Calculations of geometry and binding energies of fragment ions as well as energies required for activating various dissociation channels are carried out. A qualitative explanation is given of the observed fragmentation spectra.

Argon energies of J = 1 odd and J = 2 even states, oscillator strengths of the two lowest J = 1 odd states, and line strengths for transitions between J = 1 odd and J = 2 even excited states are calculated with the configuration-interaction + many-body-perturbation-theory method. The results are in good agreement with experiment.

We report static dipole (hyper)polarizability values for silicon. Relying on finite-field Møller–Plesset perturbation theory and coupled-cluster calculations with large Gaussian-type basis sets of near-Hartree–Fock quality, we find electron correlation effects to be small for both properties. We estimate the mean dipole polarizability at = 37.4 ± 0.1e²a ₀²E_h⁻¹ and = (4.3 ± 0.1) × 10⁴e⁴a ₀⁴E_h⁻³.

Cross sections for the single ionization of Kr XI–XIX ions by electron impact are calculated in the Coulomb–Born approximation by the code 'ATOM' and compared with experimental data and other theoretical results for Kr XI, Kr XII and Kr XIX. Single electron impact ionization is the sum of the contributions from direct ionization and from excitation of the autoionizing levels. The first part includes only ionization from the 3s, 3p and 3d shells since ionization from the 1s, 2s, 2p shells results in double ionization. The autoionizing levels (1s²2s2p⁶3s²3p⁶3d^mnl and 1s²2s²2p⁵3s²3p⁶3d^mnl) decay via autoionization and finally contribute to single ionization. The results are found to be in satisfactory agreement with the experimental results obtained in a crossed electron–ion beam experiment in the energy range from threshold to about 6 keV. The case of ions formed in metastable states is studied as they were observed experimentally.

Using a perturbative approach to the independent particle approximation (IPA) breaking effects we find analytical equations which enable us to obtain the high energy behaviour beyond IPA. The IPA breaking contributions to the cross sections are expressed as the IPA matrix elements of relatively simple operators. The equations for the cross section ratios are shown to be valid over a broad energy interval. We also obtain the quantitative criteria, which enable us to predict which of the couplings are significant for the fixed values of the photon energy. We compare our perturbative results to our numerical calculations. Accounting for IPA breaking effects is shown either to eliminate or to diminish strongly the discrepancy between the experimental data and the results of IPA calculations.

The resonance and near-threshold structures of low-energy electron interaction with Sr and Ba atoms have been investigated by carrying out respectively 11-state and 21-state close-coupling calculations within the R-matrix scheme. Part of the core–valence correlation is included by exciting one core electron to the valence orbitals. Results for the low-energy electron scattering and the photodetachment of the negative ions are presented. For Sr, the results show that there is a ²D shape resonance around 0.8 eV above the elastic scattering threshold and a remarkable structure just above the first ³P^o excitation threshold. For Ba, two ²D shape resonances are predicted, just above the elastic scattering threshold and the ³D excitation threshold, respectively.

We have measured the alignment A₂₀ of Xe⁺ ions after L₃ photoionization via the anisotropy of Lℓ x-radiation using linearly polarized synchrotron radiation with energies of 4.814 and 5.055 keV, i.e. 27 and 268 eV above the L₃ ionization energy, respectively. The alignment values are −0.061(43) and −0.044(70) for the respective photon energies. The experimental values are smaller in magnitude by roughly a factor of 2–3 than the theoretical ones which have been calculated for an extended energy range of photons within the Hartree–Fock approximation.

For a Bose-condensed gas confined in a magnetic trap and in a two-dimensional (2D) optical lattice, the non-uniform distribution of atoms in different lattice sites is considered based on the Gross–Pitaevskii equation. A propagator method is used to investigate the time evolution of 2D interference patterns after (i) only the optical lattice is switched off, and (ii) both the optical lattice and the magnetic trap are switched off. An analytical description on the motion of side peaks in the interference patterns is presented by using the density distribution in a momentum space.

The lifetimes for the 1s2sns ⁴S states of lithium with n from 3 to 8 are calculated using variational wavefunctions in Hylleraas coordinates. Comparisons are made with other theoretical and experimental values.

We have observed broadening and shift of the 2S–4S Doppler-free two-photon transition of atomic Li by collisions with neon and argon buffer gases in a heat-pipe oven. Measured broadening and shift rates are presented and compared with theoretical predictions calculated in the impact approximation using three different interaction potentials. A superposition of polarization and modified Fermi potentials gives good agreement with the experimental data.

We discuss the concept and design of effective atom–atom potentials that accurately describe any physical processes involving only states around the threshold. The existence of such potentials gives hope to a quantitative, and systematic, understanding of quantum few-atom and quantum many-atom systems at relatively low temperatures.

Plasma screening and nonideal collective effects on the electron-impact excitation of hydrogen-like ions are investigated in nonideal plasmas. The interaction potential in nonideal plasmas is given by the pseudopotential model, taking into account the screening effect as well as the collective effect. The screened radial atomic wavefunctions and energy eigenvalues for the 1s and 2p states of the hydrogen-like target ions in nonideal plasmas are obtained by the variational principle and perturbational method. The analytic form of the screened Coulomb focusing factor is obtained, including the plasma screening and nonideal collective effects in order to investigate the correct behaviour of the excitation cross section near the excitation threshold. The excitation cross section including the screening, collective, and Coulomb focusing effects are obtained as a function of the nonideality plasma parameter, Debye length and kinetic energy of the projectile electron. It is found that the scaled 1s → 2p excitation cross sections including the plasma screening and collective effects are found to decrease with increasing the nonideality plasma parameter.

Absolute electron-impact total cross sections (TCSs) for C₄H₆ isomers (1, 3-butadiene and 2-butyne) and for hexafluoro-2-butyne (C₄F₆) have been measured in a linear electron-beam transmission experiment as a function of impact energy, between 0.5 and 370 eV. For the C₄H₆ molecule distinct differences in the shape and magnitude of TCS energy dependences of both isomers are apparent at low energies. In the 1, 3-butadiene TCS curve, the enhancement below 1.2 eV, the narrow maximum centred around 3.2 eV and the dominant broad peak at 9–9.5 eV are easily distinguishable; a very weak shoulder is located near 8 eV. The TCS energy function of 2-butyne demonstrates a remarkable resonant-like maximum located near 3.6 eV and much broader enhancement centred at 8 eV. Above 50 eV both C₄H₆ curves merge together. The hexafluoro-2-butyne TCS has the maximum near 8 eV and very broad enhancement spanning between 20 and 80 eV; weak shoulders near 3 and 6 eV are also noticeable. The effect of perfluorination is indicated.

The systematic trend of Stark widths along spectral series is discussed. Attention is drawn to earlier studies of regularities and systematic trends of Stark broadening parameters.

The systematic trends of Stark widths across spectral series of Pb II are discussed.