Table of contents

Motivated by the stability problem for doubly charged negative atomic ions, we study the minimal charge required for an atomic nucleus to bind N electrons. Employing large and diffuse basis sets and multireference configuration interaction computations, is determined for systems up to N = 19 corresponding to elements from the lowest three periods of the periodic table. For those elements X considered here, our results rule out the stability of , and give strong evidence that no other stable atomic dianions exist.

The convergent close-coupling (CCC) formalism is employed to calculate the fully resolved triply differential cross section (TDCS) for helium double photoionization. This is the first ab initio calculation of the kind where all three gauges of the electromagnetic interaction produce TDCS within a few per cent of each other. Comparison is made with the experimental data in the range of photon energies 10 - 53 eV above the threshold, both for equal and unequal energy sharing. The present calculations agree very well, both in shape and where available in magnitude, with the experimental data obtained at the BESSY storage ring (Berlin, Germany) (Schmidt et al 1996 J. Electron Spectrosc. Relat. Phenom. 79 279 and references therein). In addition, excellent agreement is found with the calculations of Pont and Shakeshaft (1995 Phys. Rev. A 51 R2676).

The ionization of potassium Rydberg atoms with by pulsed unidirectional electric fields, termed half-cycle pulses (HCPs), with various well characterized shapes (rectangular, triangular and sawtooth) and durations comparable to the classical electron orbital period is investigated. The experimental results are compared with classical trajectory Monte Carlo (CTMC) simulations and the classically scaled results of quantum calculations undertaken at n = 5. The data show that in the intermediate regime, , the threshold field for ionization becomes sensitive to the shape and rise time of the HCP, increasing with increasing rise time. For each pulse shape, the agreement between the experimental measurements and both the CTMC and quantum calculations is very good.

Interaction of a moderately intense , infrared , short pulse (1 ps) free-electron laser beam with a gold surface is found to cause an intense electron emission (on the order of ) and a corresponding `hot' electron spectra (up to a few eV). We show that heating of the electrons through classical acceleration by the laser electric field in the presence of collisions is a possible explanation for such observations.

The R-matrix with pseudo-states (RMPS) method has been used to study electron-impact excitation of helium from the ground state and the metastable state for total energies up to 40 eV. The overall agreement with recent convergent close-coupling (CCC) calculations of Fursa and Bray is very satisfactory. The remaining disagreements are apparently associated with differences in the description of the target structure, which is superior in the present work compared to the frozen-core approach used in the CCC calculation. Severe discrepancies remain between the RMPS/CCC predictions and some experimental data, particularly for excitation from the state.

The analytic continuation of Crother's uniform semiclassical wavefunction to below threshold has been extended to include resonant states for L = 1 and L = 2, where L is the total angular momentum azimuthal quantum number of the two excited electrons. Using the Bohr-Sommerfeld complex quantization rule with two complex transition points the complex eigenvalues for doubly excited states of are calculated. The resonance energy positions, which are parametrized in terms of an irrational azimuthal quantum number for and an integer for , are in good agreement with the experimental results of Buckman et al.

Three-dimensional potential energy functions (PEFs) have been generated for the X, a, and b states of using the internally contracted multireference configuration interaction approach. Analytic forms of the PEFs were employed in calculations of the vibrational energy levels, vibrational wavefunctions and Franck-Condon factors for the hypothetical direct ionization process . For the state the Renner-Teller problem has been solved and the pattern of the bending levels analysed. The collinear charge separation path yielding has been calculated for 14 electronic states. The electronic ground state of was found to have a barrier height of 1.4 eV, in good agreement with the experimentally detected onset of this charge separation process. The shapes of the close-lying potential energy functions indicate that for energies higher than about 4 eV above the electronic ground state, dissociation processes from these states will be accompanied by complicated coupling effects.

The resonance energies and widths for doubly excited states of beryllium-like ions are determined by calculating the density of resonance states using the stabilization method. A model potential is used to represent the interaction between the outer electrons and the core electrons. Products of Slater orbitals are used to represent the two-electron wavefunctions. We present results for the two lowest doubly excited states for Z = 5-10, and Z = 12. Comparisons are made with other calculations in the literature.

We develop a semirelativistic distorted-wave Born approximation applicable to the study of (e,2e) processes at intermediate energy. Using this method, we investigate the influence of relativistic interactions for the continuum and bound electrons on the parameters describing spin-polarized (e,2e) experiments on xenon at intermediate energy and for asymmetric kinematics.

High-resolution crossed beam laser spectroscopy has been performed on an atomic beam of natural vanadium. The hyperfine structure was measured in highly resolved spectra for 25 transitions from the first metastable states, , to two high-lying multiplets, and . This yielded the hyperfine A(J) and B(J) coefficients of 16 fine structure states, including the six previously unstudied levels in the multiplet. Excellent agreement was found between these values and those obtained from atomic beam magnetic resonance experiments for the metastable states. In addition, the hyperfine structure of was measured in eight transitions to the upper state multiplet . This resulted in the evaluation of hyperfine coefficients for all 10 levels of the upper and lower states, previously unknown, as well as the first optical isotope shift measurements for vanadium.

Accurate static dipole polarizabilities and hyperpolarizabilities are calculated for the ground states of the boron, carbon, nitrogen, oxygen and fluorine atoms. Anisotropies are reported for the P-state atoms. The computations are based on a finite-field procedure using energies computed with basis sets of contracted Gaussian-type functions and the CCSD(T) method, that is the coupled cluster method in the space of single and double substitutions corrected non-iteratively for the effects of triple substitutions. These are the first electron-correlated hyperpolarizabilities reported to date for all but one of these atoms.

Measurements of the threshold photoelectron spectrum of helium have been made at the Gas Phase beamline at Elettra Synchrotron, Trieste. The energy region studied spans the thresholds of the satellite states, beginning with N = 2, up to the threshold. The energy resolution in the new spectrum is five times better than that of previous measurements and has enabled us to resolve satellite states with N = 13-19 for the first time. The part of the spectrum in the region of the double-ionization threshold exhibits a cusp-like feature in accordance with predictions of the Wannier model for threshold photo-double ionization. This region has been used to examine the role of doubly excited neutral states in single-ion formation and also to compare with previous experiments the ratio of the discrete and continuum amplitudes of the cusp. The use of the threshold photoelectron technique as an accurate and convenient method to calibrate the energy scales of optical monochromators for synchrotron radiation is noted.

The previously reported observation of stable negative Yb ions has been reinvestigated. No evidence is found for the existence of such ions or for long-lived metastable ions. This conclusion is limited to Yb ions with binding energies larger than 3 meV.

We present simulations of the two-colour harmonic response of a one-dimensional model atom exposed to a superposition of two laser fields of varying relative intensities and of non-commensurate frequencies. We show that this interaction can be used to efficiently generate a quasi-continuum of very closely spaced harmonics and explain how such a scheme can be implemented experimentally. We show how the generation process can be described in terms of the beat frequencies between the two fields. Finally, we discuss the limitations of the - interaction (which has been studied extensively elsewhere).

The two- and three-photon resonant ionization of Ca has been studied. Calcium atoms in the thermal atomic beam were ionized from the ground state by three-photon absorption. The ion signals exhibited resonances attributed to the two-photon transition and three-photon transitions due to , , and , autoionizing states. A simple theoretical model was used to reproduce the main features of the observed spectra due to the two-photon resonant ionization.

The derivation of the transformative complex absorbing potential (TCAP) method and its performance are discussed. This approach was developed in a previous paper (1995 J. Phys. B: At. Mol. Opt. Phys. 28 1475) and illustrates the relation between complex absorbing potentials (CAPs) and the smooth exterior scaling (SES) method. Starting from an energy-dependent CAP one arrives at an SES-like Hamiltonian via an elementary similarity transformation. Developing this idea further leads to the so-called TCAP equation. It differs from the SES Schrödinger equation in two respects. Firstly, the potential is not transformed and, secondly, an additional correction term appears. Neglecting this rather small term leads to a Hamilton operator that is easy to apply to time-dependent as well as time-independent problems. This Hamiltonian can be extended order by order ending up in the full SES Hamiltonian. By numerical application it is demonstrated that the TCAP approach is very efficient and can easily be generalized to the multi-dimensional case for which formulae are provided.

The differential cross sections for electron transfer in -Na(3s) collisions in the 0.4-14 keV energy range are studied using the impact parameter semiclassical method and a 28-state molecular orbital basis model including a common translational factor. The calculated differential cross section (DCS) for electron transfer to Li(2s) and Li(2p) and the three Stokes parameters of the Li(2p) final state are in excellent agreement with the measured DCS at 1 keV and 3 keV. The energy dependence of the final-state polarization for electron transfer to Li(2p) is explained using animations of the density and current of the electronic wave function complemented by time-dependent molecular state populations. We find that different transfer mechanisms dominate at low and high collision velocities.

The presence of an axially symmetric angular distribution of energetic protons in astrophysical plasmas can be detected by measuring the linear polarization of the lines emitted from collisionally excited levels of atoms. Another signature of proton beams can be found in the study of line profiles. For a quantitative interpretation of the observations of the line one needs to know all the relevant integrated and differential cross sections and the polarization fraction.

Coupled-state calculations based on a two-centre expansion in atomic orbitals and pseudostates were carried out for protons colliding with H(1s) in the impact energy range 1-100 keV. Differential cross sections for excitation and capture from the level n = 1 to the level n = 3 as well as the total and differential polarization fraction of the line are presented. It is shown that the differential cross sections reach a maximum for an angle varying from 1 to degree when the collision energy varies from 1 to 40 keV. For 1 keV collisions, this leads to large recoil velocities and, as a consequence, to a large Doppler broadening. At the same energy, the resulting polarization fraction is found to have only small variations with the deflection angle.

We present the results of the many-body treatment of the bremsstrahlung (BrS) process in collisions of fast electrons with metal clusters. We concentrate our attention on the range of photon frequencies characteristic for plasmon excitations in clusters. The dynamic polarizability of a cluster in this frequency range is very large and it determines the value and behaviour of the cross section of the process as a function of various parameters. When calculating the cross section of the process, we treat the dynamic response of the cluster in the random-phase approximation with exchange, using wavefunctions of the Hartree-Fock jellium model and also in the resonance plasmon approximation. The latter approach is well applicable for the treatment of collective excitations of electrons in clusters. We analyse the spectral and angular distributions of the emitted photons, as well as the collision velocity dependence of the cross section for small sodium clusters and demonstrate that collective electron excitations determine the main features of the BrS cross section.

An approach to calculate elastic differential cross sections of atoms and ions at small angles is shown. The key point of this approach is selecting a suitable slope before integrating the Dirac radial equations. The slope is extracted from the normalization coefficient of continuum state solutions in an effective Coulomb field. The approach is utilized to investigate the influence of high partial-wave phase shifts on the cross sections at small angles. As is known, these phase shifts are mainly produced by long-range forces such as the polarization interaction or the Coulomb interaction or both. Calculations using mercury as an example show the effect of high partial waves and different cut-offs on the small-angle differential cross sections in the first Born approximation at 25 eV. These calculations, at 1500 eV, are compared with the recent measurement with the angle-resolved electron energy-loss spectrometer in Xu's group. Furthermore, calculation results are given at the intermediate energies of 50, 100, 300 and 500 eV where the experimental data are available. Comparisons between the theory and experiment over 9-1500 eV show that further studies at the incident energy of about 100 eV are needed.

The energy spectrum of scattered electrons in K-shell ionization of medium to heavy atoms by relativistic electrons has been calculated for certain kinematic conditions. In this calculation the final state is described by a non-relativistic multiple-scattering wavefunction of Das and Seal multiplied by suitable spinors. Very satisfactory agreement with experiment is obtained.

The distorted wave Born approximation is used to calculate the He (e, 2e) triple differential cross section at 64.6 eV in the different kinematical geometries, namely: (i) non-coplanar symmetric scattering angles with symmetric as well as asymmetric energy sharing; (ii) coplanar asymmetric with one scattering angle fixed and the other one variable; (iii) coplanar asymmetric with angular separation between the two outgoing electrons constant. The spin average static exchange (with the exchange part taken in equivalent local approximation) interaction potential is used. Two important characteristic features of the observed differential cross sections in non-coplanar geometry, namely: (i) a common point in the cross section curve for the two outgoing electrons at with the incident direction projected on the detection plane for all the energy sharing states and (ii) a deep minimum for the angular separation of between the detection plane and the electron beam have been fully supported. In coplanar asymmetric geometry, results are in agreement with the scaled up (by a constant factor of 1.8) convergent close-coupling cross sections.

For the first time absolute cross sections for electron impact ionization of the hydrogen-like ions B⁴⁺, C⁵⁺, N⁶⁺ and O⁷⁺ at electron energies from below threshold up to about 6 keV have been measured using the crossed-beams technique. All measured cross sections are in very good agreement with available distorted-wave exchange calculations and the semiempirical Lotz formula. The behaviour of the classically scaled cross sections along the hydrogen isoelectronic sequence is discussed. A recently developed scaling technique allows us to predict all non-relativistic cross sections along the isoelectronic sequence on an absolute scale.

A crossed beam technique is used to measure ionization cross sections of metallic atoms. Relative values of cross sections of single, double and triple ionization of magnesium have been successfully measured over the 0-700 eV range with an estimated accuracy of 11%, 19% and 25%, respectively. Absolute values of cross sections have been obtained by normalization to a theoretical value at high electron energy. The theoretical value is calculated by using the generalized oscillator strength approximation. Results are compared to previously published values and, for single ionization in particular, a comparison with theoretical cross sections is performed.

We have measured the differential and integral cross sections for electron impact excitation of the , , and states of CO in the 10-15 eV impact energy region. These measurements fill the gap which existed between the low-energy (threshold to about 12 eV) data of Zobel et al and the intermediate energy (20-50 eV) results of Middleton et al. Recent electron time-of-flight measurements are utilized to place the relative cross sections (deduced from energy-loss spectra) on the absolute scale.

The cross sections are in excellent agreement with the results obtained by Zobel et al at the overlapping impact energies and always smoothly extrapolate to their low-energy results. Extrapolation to the intermediate energy domain and comparison with the results of Middleton et al is not so clear.

Comparison with theoretical differential and integral cross sections of Sun et al and the integral cross sections of Morgan and Tennyson reveals angular and energy dependences which resemble those found for the experimental results but there are serious discrepances concerning the absolute magnitudes.

Recombination of ions with free electrons has been investigated in a merged-beams experiment at the UNILAC of GSI in Darmstadt. Special emphasis was put on the recombination at very low energies in the electron-ion centre-of-mass frame. At we found an enormous recombination rate exceeding the theoretical expectations for radiative recombination by a factor of 365. For further investigation of this enhancement, the electron density and the magnetic field guiding the electron beam were varied. While an increase of the electron density by a factor of 10 had little influence, the measured rate coefficient increased significantly with the magnetic field strength.

We report calculations of Rydberg states for spherical metallic clusters. These calculations demonstrate that centrifugal barrier effects operate, but that they act differently from the situation in atoms. Instead of orbital collapse, one sees a characteristic flattening of the effective potential: for and it occurs for the Rydberg electron, and for and , the Rydberg electron, which become very sensitive to core polarization effects. The effect of the centrifugal barrier on the quantum defects has been explored for various values of angular momentum.

A high resolution 36 mÅ (FWHM), optically thin emission study of the and (3, 2) Rydberg bands excited by electron impact at 100 eV has been completed in the extreme ultraviolet (EUV). A model of the perturbed rotational line intensity distribution of the bands shows the effects of electronic state mixing between the Rydberg state and the valence state. By normalizing the model to the published predissociation yield for the laboratory spectrum can be used to determine the predissociation yields for each rotational level of and 4. Based on a 25% accuracy of the model fit to the measured signal intensities it is found that the predissociation yields of the rotational levels increase as the percentage of character increases. On the other hand, the predissociation yields of the rotational levels reach a maximum for . The mean predissociation yields for and 4 levels are a function of temperature and are found to be 0.41 and 0.58, respectively at 300 K. The -dependent predissociation yields indicate that the emission cross section is a function of temperature. The remainder of the bands forming the progressions -5) from and -6) from were studied at 64 mÅ (FWHM) resolution. Using this composite spectrum of the two progressions the electron impact emission cross sections of the and 4 levels at 300 K were determined and compared with previous results.

We experimentally investigate decoherence by spontaneous emission in the quantum delta kicked rotor. Caesium atoms in a magneto-optic trap are initially cooled, and then subjected to a pulsed standing wave of near resonant light. As the detuning from the atomic transition is varied, we observe a change in the momentum diffusion after the quantum break time.