Table of contents

We have extended the laser probing technique at the CRYRING storage ring to measurement of the extremely long lifetime (28 s) of the metastable 3d²(³P)4s b ⁴P_5/2 level in Ti II. The result obtained demonstrates the power of this method for investigation of such long-lived levels. This is the first experimental lifetime investigation of metastable states in Ti II.

The stabilization dynamics of model He beyond the dipole approximation and with two active electrons is investigated in the presence of a high-intensity and high-frequency laser pulse. We show that the magnetic-field component of the laser pulse and the electron–electron interaction jointly suppress the dichotomy of the wavefunctions as well as the atomic stabilization.

We propose a model for the triple photoionization of Be in which a core 1s electron absorbs the photon γ + 1s²2s²Be → ₁p + 1s2s²Be⁺ and the valence 2s² electrons are shaken off into continuum due to the sudden change of the core potential. We decompose the double shake-off amplitude into a single shake-off 2s² → nss and a subsequent electron impact ionization of the doubly charged Be²⁺ ion s + 1sns Be²⁺ → ₂l + ₃l + 1s Be³⁺. The latter process is described by the T-matrix of inelastic electron scattering on the 'semi-hollow' 1sns Be²⁺ ion in the monopole singlet channel. The convergent close-coupling method is used to evaluate the T-matrix.

We present ab initio calculations for argon in a KrF laser field for intensities up to around 10¹⁴ W cm⁻². We study the dynamics of Floquet field-atom states in laser pulses with time-varying amplitudes. We show how the pulse length and shape can be used together with knowledge of the Floquet states and quasienergies to control directly the atomic populations both during and at the end of the pulse. The calculations describe experimentally viable coherent control methods and the analysis is applicable to other field-atom systems. These are the first such ab initio calculations that we are aware of for a many-electron system.

Early theoretical work emphasized the gross trend of the total correlation energy of neutral atoms with atomic number Z. This was later improved quantitatively, by focusing on the number of pairings between antiparallel-spin electrons in the same main shell (i.e., K, L, M...shells). Here, this viewpoint is pressed, and correlation energies associated with s–s, s–p and p–p pairings are extracted. The s–s energy turns out to be rather insensitive to a change from K to L shells, but a small increase is found to occur in the M shell. Also, the s–p and p–p pairing energies show a small increase from the L to the M shells. The s–s and s–p energies are ≈1 eV, the s–s contribution being the larger. The p–p energy is ≈1/3 eV. The effect of removing or adding one electron to the neutral atom has also been analysed. Wavefunction overlap and wavefunction localization appear to control the values of the pairing energies.

The hyperspherical partial wave approach has been applied here in the study of double photoionization of the helium atom for equal-energy-sharing geometries at 20 eV excess energy. Calculations have been done both in length and velocity gauges and are found to agree with each other, with the CCC results and with experiments and to exhibit some advantages for the corresponding three-particle wavefunction over other wavefunctions in use.

We account for the different symmetries of the 2 ^1,3S helium excited states in a quasiclassical description of the knockout mechanism augmented by a quantum shake-off contribution. We are thus able to formulate the separate contributions of the knockout and shake-off mechanisms for double photoionization for any excess energy from the 2 ^1,3S states. Photoionization ratios and single-differential cross sections calculated for the 2 ^1,3S excited states of helium are found to be in very good agreement with recent theoretical results.

We predict a dynamical classical superfluid–insulator transition in a Bose–Einstein condensate (BEC) trapped in combined optical and axially symmetrical harmonic potentials initiated by the periodic modulation of the radial trapping potential. The transition is marked by a loss of phase coherence in the BEC and a subsequent destruction of the interference pattern upon free expansion. For a weak modulation of the radial potential the phase coherence is maintained. For a stronger modulation and a longer holding time in the modulated trap, the phase coherence is destroyed thus signalling a classical superfluid–insulator transition. The results are illustrated by a complete numerical solution of the axially symmetrical mean-field Gross–Pitaevskii equation for a repulsive BEC. Suggestions for future experimentation are made.

The differential and total cross sections for single charge exchange in p + He collisions have been calculated by means of the Born distorted wave (BDW-4B) approximation. The BDW-4B model is a four-body quantum-mechanical distorted-wave theory. The computations of total cross sections are carried out at impact energies ranging from 40 keV to 15 MeV. The differential cross sections are calculated at impact energies covering 50, 100, 150, 200, 293, 300 and 400 keV. The theoretical results obtained for the differential and total cross sections are compared with the available experimental data and good agreement is found.

The recently proposed semiempirical mass formula for spherical Coulomb crystals is generalized to include spheroidal deformations and Maclaurin-type rotations. The external focusing frequencies are adjusted in such a way that in the interior the forces due to focusing and Coulomb repulsion are cancelled out exactly. With the help of molecular dynamics calculations isometric configurations of non-spherical Coulomb crystals formed due to a non-spherical trapping potential or due to rotation are found to have less energy than the spherical ground state with the same number of particles. Possible collective multipole excitations of the neutral background are discussed.

It is shown that a macroscopic superposition state of radiation, strongly interacting with an ensemble of two-level atoms, is removed, generating a coherent state describing a classical radiation field, when the thermodynamic limit is taken on the unitary evolution obtained by the Schrödinger equation. Decoherence appears as a dynamical effect, in agreement with a recent proposal (Frasca 2001 Phys. Lett. A 283 271). To prove that this effect is quite general, we show that this same behaviour appears when a superposition of two Fock number states is also considered. Higher order corrections are computed, showing that this result tends to become exact in the thermodynamic limit. It appears as a genuine example of intrinsic collapse of the wavefunction.

The absorption cross section and the ionization quantum yield of H₂O have been measured using a synchrotron radiation source between 9 and 22 eV. Comparison between the two curves highlights competition between relaxation processes for Rydberg states converging to the first à ²A ₁ and to the second ²B ₂ excited states of H₂O⁺. Comparison with D₂O absorption and ionization yields, derived from Katayama et al (1973 J. Chem. Phys.59 4309), reveals specific energy-dependent deuteration effects on competitive predissociation and autoionization relaxation channels. Direct ionization was found to be only slightly affected by deuteration.

A generalized Floquet formalism combined with Bloch's wave operator methodology is applied to the study of adiabatic versus sudden transport mechanisms in molecular responses to ultrashort and intense laser pulses. The model considers the laser pulse as a whole to be periodic, instead of the optical cycle as in standard Floquet treatments. This leads to a better understanding of the dynamics and a considerable simplification in terms of the number of generalized Floquet eigenstates needed to accurately describe the dynamics. The method is illustrated for the phenomenon of above threshold dissociation and inelastic transitions in H₂⁺ in the presence of ultrashort (a few femtoseconds) and intense (of the order of TW cm⁻²) laser fields. It is found that, while the dissociation probabilities and fragment kinetic energy spectra can be accurately described by a single generalized Floquet eigenstate within the adiabatic limit, an accurate description of inelastic transitions requires more than one generalized Floquet state, due to the role of non-adiabatic dynamics.

We present a computational approach to the outcoupling in a simple one-dimensional atom-laser model, the objective being to circumvent mathematical difficulties arising from the breakdown of the Born and Markov approximations. The approach relies on the discretization of the continuum representing the reservoir of output modes, which allows the treatment of arbitrary forms of outcoupling as well as the incorporation of nonlinear terms in the Hamiltonian, associated with interatomic collisions. By considering a single-mode trapped condensate, we study the influence of elastic collisions between trapped and free atoms on the quasi steady-state population of the trap, as well as the energy distribution and the coherence of the outcoupled atoms.

We present a theoretical model for describing the dynamics of Bose–Einstein condensates in anharmonic trapping potentials. To first approximation the centre-of-mass motion is separated from the internal condensate dynamics and the problem is reduced to the well known scaling solutions for the Thomas–Fermi radii. We discuss the validity of this approach and analyse the model for an anharmonic waveguide geometry which was recently realized in an experiment (Ott et al 2002 Preprint cond-mat/0212220).

The active region for emission of radiation by an electron driven by a strong laser field in the proximity of a stationary scattering centre is localized in space and time. It is argued that the extension of this region can be controlled by changing the velocity of the electron, and that information on this extension is contained in the duration and in the spectrum of the emitted radiation pulse.

Vibrational excitation (VE) of HF by low-energy electrons has been investigated experimentally and theoretically. A new nonlocal resonance model has been constructed based on ab initio calculations of the coupling between a discrete state and continuum states. VE and resonant elastic cross sections have been calculated for a set of initial vibrational states of the molecular target. New high-resolution measurements of VE cross sections for the transitions v = 0 → 1,..., 4 have been carried out. The calculated cross sections are in good agreement with the experimental data, indicating that the mechanisms responsible for the rich threshold structures found in the collision cross sections of HF are well understood.

The evolution of atomic solitary waves in Bose–Einstein condensate under adiabatic changes of the atomic scattering length is investigated. Variations of amplitude, width, and velocity of the soliton are found for both spatial and time adiabatic variations. The possibility of using these variations to compress solitons up to very high local matter densities is shown both in the absence and in the presence of a parabolic confining potential.

We examine coherent effects in a four-level Vee scheme. We show that the system can be made to display a number of different phenomena, including the inhibition of two-photon absorption and the production of multiple dark states. We show that this latter effect is due to the independent control of the Autler–Townes splitting of the upper and lower probe field by independent driving fields. We suggest possible experimental systems in rubidium for the observation of such effects.

The electric dipole moment functions of iodine E0_g⁺–A1 _u and E0_g⁺–B''1 _u transitions in the range of 3.2–4.2 Å have been determined for the first time by simulating the luminescence spectra from the E0_g⁺,v_E = 9, J_E ≈ 55 rovibrational level and the measured ratios of the total intensities of the E0_g⁺–A1 _u, E0_g⁺–B''1 _u and E0_g⁺–B0 _u⁺ transitions.

A relationship between the auxiliary density, (r), defined within the framework of the weighted density approximation and the kinetic energy modulating factor, A_N([ρ(r)]; r), which appears in the local-scaling transformation version of density functional theory is presented. This relationship imposes the condition of positiveness on the kinetic energy modulating factor and this, in turn, leads to an important mathematical condition on any approximate kinetic energy density functional. It is shown that two well-known approximate kinetic energy density functionals do not satisfy the above relationship at distances very close to the nucleus. By forcing a given approximate kinetic energy density functional to obey the above condition, both the kinetic and exchange energies can be obtained within a framework similar to that of the weighted density approximation. Results on some closed-shell atomic systems provide support for those ideas.

A dark soliton becomes unstable when it is incident on a background density gradient, and the induced instability results in the emission of sound. Detailed quantitative studies of sound emission are performed for various potentials, such as steps, linear ramps and Gaussian traps. The amount of sound emission is found to be a significant fraction of the soliton energy for typical potentials. Continuous emission of sound is found to lead to an apparent deformation of the soliton profile. The power emitted by the soliton is shown to be parametrized by the square of the displacement of the centre of mass of the soliton from its density minimum, thus highlighting the significance of the inhomogeneity-induced soliton deformation.

The rotationally excited 2 ¹P (L = 1) and 2 ³P (L = 1) states in the ^∞He, ³He and ³He helium atoms are considered by using our multi-box variational procedure (Frolov 2001 Phys. Rev. E 64 036704). In particular, the total non-relativistic energies of these states for the ^∞He atom have been determined to very high accuracy. These energies are −2.123 843 086 498 101 357 17 au (2 ¹P state) and −2.133 164 190 779 283 204 14 au (2 ³P state). A number of bound state properties for these two states in the ^∞He atom are also presented. The total energies of the 2 ¹P (L = 1) and 2 ³P (L = 1) states in ³He and ⁴He have been determined. Some astrophysical applications of the rotationally excited 2 ¹P (L = 1) and 2 ³P (L = 1) states in the helium atoms are briefly discussed.

In a crossed-beam experiment, differential cross sections (DCS) for elastic and vibrationally inelastic scattering of electrons from propane have been measured in the energy range E = 0.5–10 eV. Absolute values are obtained by using e⁻–He scattering as a reference system. For elastic scattering, the measured DCS are analysed by phase-shift analysis and modified effective-range theory and then extrapolated into the experimentally inaccessible regions. In this way, the complete set of elastic DCS for E = 0–10 eV and θ = 0°–180° is obtained. Integral and momentum transfer cross sections are determined, which are compared with the results of electron transmission and swarm experiments. For vibrationally inelastic scattering, the observed vibrational excitation (VE) processes are divided into three groups: (i) CH_x stretching modes, with a mean energy loss of ΔE = 366 meV; (ii) CH_x deformation modes, with ΔE = 175 meV; (iii) CC carbon chain vibrations, with ΔE = 122 meV. Absolute differential and integral cross sections are determined for each of the three mode groups. Two principal VE mechanisms are responsible for the observed processes: direct VE for energies below 3 eV and resonant VE for energies around 7–8 eV. The Born dipole approximation is used to analyse the data in the region of direct VE at low energies.

The stability of an attractive Bose–Einstein condensate on a joint one-dimensional optical lattice and an axially symmetrical harmonic trap is studied using the numerical solution of the time-dependent mean-field Gross–Pitaevskii equation and the critical number of atoms for a stable condensate is calculated. We also calculate this critical number of atoms in a double-well potential which is always greater than that in an axially symmetrical harmonic trap. The critical number of atoms in an optical trap can be made smaller or larger than the corresponding number in the absence of the optical trap by moving a node of the optical lattice potential in the axial direction of the harmonic trap. This variation of the critical number of atoms can be observed experimentally and compared with the present calculations.

A heuristic approach is used to find the bound electron energy levels for systems of three or more bodies in atoms. A Hulthèn potential is used to describe the screening of electrostatic effects. The relevant screening parameter is found to depend on the number of binary interactions and on the usual energy and kinetic momentum quantum numbers. Our approach is inspired by the dimensionality theory developed to describe multiple-electron interactions and their dynamical correlations. As a consequence of our choice of both hyperspherical coordinates, well adapted to radial correlations, and the Hulthèn potential with radial symmetry, a good precision is achieved for the computation of the energy levels of the helium atom (ground state and simply excited states), and also of those of He-like atoms, particularly for K shells. We can also satisfactorily reproduce the K energy levels for complete atoms with various Z-values, and we recover the Moseley series for the first ionization potentials of Li-, Be-, B-, O-, and Na-like systems.