Table of contents

Double ionization of Ne by 25 fs, 1.0 PW cm⁻² laser pulses has been explored in a kinematically complete experiment using a 'reaction microscope'. Electrons are found to be emitted into a narrow cone along the laser polarization (ε), much more confined than for single ionization, with a broad maximum in their energy distribution along ε. Correlated momentum spectra show both electrons being ejected into the same hemisphere, in sharp contrast to predictions based on field-free (e, 2e) recollision dynamics, but in overall agreement with recent semiclassical calculations for He.

For elastic scattering of ¹³³Cs atoms by ⁸⁵Rb and ⁸⁷Rb atoms, interacting via the X ¹ Σ⁺ and a ³Σ⁺ molecular states of RbCs, calculations are presented of the scattering length and the effective range and also of two volume parameters, one for low energy p-wave scattering and one for the energy of an ensemble of fermions. The calculations are performed by various methods, with several representations of the interatomic potentials, including new ab initio potentials presented here. It is shown how the calculated data may be corrected to include the influence of the long range interactions, thus removing the need for numerical solutions of the radial Schrödinger equation at very large interatomic separations. The sensitivity of the results to the choice of potentials, to the van der Waals dispersion coefficient C₆ and to the reduced mass of the colliding atoms is explored, and is found to be great.

The dynamics of a bright matter wave soliton in a quasi one-dimensional Bose–Einstein condensate (BEC) with a periodically rapidly varying time trap is considered. The governing equation is based on averaging the fast modulations of the Gross–Pitaevskii (GP) equation. This equation has the form of a GP equation with an effective potential of a more complicated structure than an unperturbed trap. In the case of an inverted (expulsive) quadratic trap corresponding to an unstable GP equation, the effective potential can be stable. For the bounded space trap potential it is showed that bifurcation exists, i.e. the single-well potential bifurcates to the triple-well effective potential. The stabilization of a BEC cloud on-site state in the temporary modulated optical lattice is found. This phenomenon is analogous to the Kapitza stabilization of an inverted pendulum. The analytical predictions of the averaged GP equation are confirmed by numerical simulations of the full GP equation with rapid perturbations.

Using the explicit numerical solution of the axially symmetric Gross–Pitaevskii equation, we study the oscillation of the Bose–Einstein condensate (BEC) induced by a periodic variation in the atomic scattering length a. When the frequency of oscillation of a is an even multiple of the radial or axial trap frequency, respectively, the radial or axial oscillation of the condensate exhibits resonance with a novel feature. In this nonlinear problem without damping, at resonance in the steady state the amplitude of oscillation passes through a maximum and minimum. Such a growth and decay cycle of the amplitude may keep on repeating. Similar behaviour is also observed in a rotating BEC.

We studied the ionization probability of Na atoms in an intense Ti–sapphire laser at 800 nm versus the laser pulse duration. The ionization probabilities for the Na(3s) and Na(3p) states were found to vary non-monotonically with the pulse duration at a constant peak laser intensity. The abnormal (non-monotonic) pulse duration dependence was traced to the competition between resonant and non-resonant multiphoton ionization processes, which occurs when the frequency broadening due to the finite pulse duration is compatible with the energy difference between the excitation energy and multiphoton energy.

Lifetimes (the inverse of the total decay rates) of several 3p and 3d levels in Ca-, Ar-, Cl- and Si-like ions of Fe that decay only by electric-dipole-forbidden transitions have been measured optically using a heavy-ion storage ring, observing either near-UV or EUV light. In several cases, more than one decay contributes to a given decay curve, which complicates the analysis. The lifetime results, with a precision ranging from 0.8 to 10%, compare well with some theoretical predictions.

In a crossed-beam experiment, vibrational excitation (VE) of ethane by electron impact has been measured for collision energies in the range E = 1– 10 eV. The energy resolution is 30 meV (FWHM) in the energy-loss mode. The observed VE processes are divided into three groups: (i) CH stretching modes with a mean energy loss of ΔE = 365 meV; (ii) CH₃ deformation modes with ΔE = 180 meV; (iii) CC carbon chain vibrations with ΔE = 125 meV. Absolute differential and integral cross sections are determined for the three groups of vibrations. The results are interpreted by two principal VE mechanisms: direct excitation for energies below 3 eV and resonant excitation for energies around 7–8 eV. At low energies (E < 3 eV) the present results are compared with the results from swarm experiments, while for E ≥ 3 eV the present measurements resolve some discrepancies between the results of two previous beam experiments. The present work complements a previous study by the same authors on elastic e–C₂H₆ scattering in this energy range.

We measure and simulate numerically the Hanle effect and non-zero field level crossing signals in ⁸⁵Rb and ⁸⁷Rb atoms in a magnetic field at room temperature. Diode laser radiation from 4 mW cm⁻² to 3.3 W cm⁻² tuned to the D₂ absorption line of each isotope excites atoms into all the excited-state hyperfine levels simultaneously inside the unresolved Doppler profile. Polarization fluorescence detection is used to observe dark and bright resonances, as well as non-zero field level crossing resonances, for several excitation lines. A broad spectral line excitation model is applied to analyse the measured signals. The non-linear Zeeman effect is included in the model for both ground and excited states. Although the applied magnetic field does not exceed 80 G, several hyperfine levels of the excited state show a substantial deviation from the linear Zeeman effect.

The sensitivity of molecular vibrational population dynamics, governing the CO laser operated in fundamental and overtone transitions, to vibration-to-vibration rate constants is investigated. With this aim, three rate constant sets have been used, differing in their completeness (i.e. accounting for single-quantum exchange only, or for multi-quantum exchange with a limited number of rate constants obtained by semiclassical calculations, and, finally, with an exhaustive set of rate constants including asymmetric exchange processes, as well) and in the employed interaction potential. The most complete set among these three is introduced in this paper. An existing earlier kinetic model was updated to include the latter new data. Comparison of data produced by kinetic modelling with the above mentioned sets of rate constants shows that the vibrational distribution function, and, in particular, the CO overtone laser characteristics, are very sensitive to the choice of the model. The most complete model predicts slower evolution of the vibrational distribution, in qualitative agreement with experiments.

Theoretical study of the ionization of sodium clusters by an intense laser pulse with a photon energy close to the surface plasmon resonance is presented. The proposed model describes the resonance channel of cluster ionization via excitation of a multi-plasmon collective electron state that can subsequently decay through a single electron emission. The method of separation of the centre of mass coordinate from other electronic degrees of freedom has been employed to take into account the highly excited electronic states. The autoionization decay rate of the multi-plasmon excited state together with the energy and angular distributions of outgoing electrons has been obtained.

Motivated by the observation of the predissociation of H₂ triplet states in low vibrational levels, we evaluate the contribution of triplet dissociative states to the low-energy dissociative recombination of H₂⁺ in vibrational levels where v = 0, 1. The triplet states contribute mainly via radial couplings between monoexcited Rydberg configurations without curve crossings. The total contribution of the six triplet states leading to dissociation in H(n = 1) + H(n = 2) is found to be smaller by at least one order of magnitude than the contribution of the lowest doubly excited singlet state, (2pσ_u)²¹ Σ _g⁺.

The dark soliton created in a Bose–Einstein condensate becomes grey in the course of time evolution because its notch fills up with depleted atoms. This is the result of quantum mechanical calculations which describe the output of many experimental repetitions of creation of the stationary soliton, and its time evolution terminated by a destructive density measurement. However, such a description is not suitable to predict the outcome of a single realization of the experiment where two extreme scenarios and many combinations thereof are possible: one will see either (1) a displaced dark soliton without any atoms in the notch, but with a randomly displaced position, or (2) a grey soliton with a fixed position, but a random number of atoms filling its notch. In either case the average over many realizations will reproduce the mentioned quantum mechanical result. In this paper we use N-particle wavefunctions, which follow from the number-conserving Bogoliubov theory, to settle this issue.

Closed-orbit theory provides a general approach to the semiclassical description of photo-absorption spectra of arbitrary atoms in external fields, the simplest of which is the hydrogen atom in an electric field. Yet, despite its apparent simplicity, a semiclassical quantization of this system by means of closed-orbit theory has not been achieved so far. It is the aim of this paper to close that gap. We first present a detailed analytic study of the closed classical orbits and their bifurcations. We then derive a simple form of the uniform semiclassical approximation for the bifurcations that is suitable for an inclusion into a closed-orbit summation. By means of a generalized version of the semiclassical quantization by harmonic inversion, we succeed in calculating high-quality semiclassical spectra for the hydrogen atom in an electric field.

We have measured all detectable cationic and anionic fragments in single-channel mode from N₂O as a function of photon energy in the vicinity of the nitrogen 1s core-level threshold. Due to the high degree of localization of the core electrons, the two excitations N_t1s → 3π^* and N_c1s → 3π^* show high levels of site-selective behaviour. The observed partial ion yield for the sole anionic fragment, O⁻, in conjunction with the partial cation yields, confirms our previous demonstration of anion-yield spectroscopy as a unique tool to identify core-level shape resonances.

We report integral, differential and momentum transfer cross sections for elastic scattering of low-energy electrons by CF₃X (X = Cl, Br, I) molecules. We use the Schwinger multichannel method with pseudopotentials (Bettega et al1993 Phys. Rev. A 47 1111) at the static exchange approximation. Our calculations cover the energy range between 5 and 30 eV. We compare our results with available theoretical and experimental results for CF₃Cl and CF₃I, and in general find good agreement. In particular, our results show the shape resonances belonging to the A₁ and E representations of the C_3v group that have been reported by previous work.

We derive a semiclassical theory for the detection of matter-waves. This theory draws on the theories of semiclassical optical detection and fluid mechanics. We observe that the intrinsically dispersive nature of matter-waves is important in deriving such a theory.

The recent version of the shift contribution to the hydrogen lines proposed by Oks (2002 J. Phys. B: At. Mol. Opt. Phys.35 2251) is shown to overestimate any dipole ionic–electronic shift effect. Moreover, it is not needed for the interpretation of experiments with the best diagnostics, and is inconsistent with recent full numerical simulations.