Table of contents

We investigate the relative particle number squeezing produced in the excited states of a weakly interacting condensate at zero temperature by stimulated light scattering using a pair of lasers. We shall show that a modest number of relative number-squeezed particles can be achieved when atoms with momentum k, produced in pairs through collisions in the condensate, are scattered out by their interaction with the lasers. This squeezing is optimal when the momentum k is larger than the inverse healing length, k>k₀. This modest number of relative number-squeezed particles has the potential to be amplified in four-wave-mixing experiments.

We discuss the interpretation of delayed electron emission from excited clusters as a statistical process analogous to thermionic emission from a hot filament. We argue that transition state theory is not a good theoretical framework for electron emission. Instead the calculation of emission rates may be based on detailed balance and theoretical or experimental cross sections for electron capture, but there can be large uncertainties in theoretical estimates of cross sections. We emphasize the conceptual simplicity obtained with the introduction of the microcanonical temperature. In experiments, the energy distribution is often so broad that it is essential to account for its modification by depletion, which for a very broad distribution leads to a decay rate inversely proportional to time. Another complication is photon emission, and we present estimates of the radiation intensity based on a simple model of a cluster as a sphere containing a gas of free electrons.

In the analysis of experiments, we first discuss the information about cluster dynamics obtained from studies of photoelectron spectra. However, we focus mainly on a detailed analysis of measurement of the rate of delayed electron emission and its dependence on the cluster excitation. Often the parameters of a statistical description, derived from fits to measurements, have appeared to be inconsistent with estimates from theory or from independent experiments. We analyse a measurement of laser-induced electron emission from small Nb clusters and find that inclusion of anharmonic effects in the heat capacity and, even more important, of the competition by radiative decay leads to more reasonable parameters in the statistical description than obtained from the original analysis. The most detailed studies have been performed for fullerene anions. For most of the measurements, radiative cooling is not significant, but it is important to take into account the finite width of the energy distribution, deriving from the initial heating in an oven. Measured cross sections for electron attachment can be applied in lifetime calculations, and an improved analysis leads to the conclusion that the experiments are consistent with the interpretation of electron emission as thermionic emission.

A formulation for calculating the polarization correlations of the triple-differential cross section for incident electrons and emitted photons in the electron bremsstrahlung from atoms using the relativistic partial-wave method is given. With the partial-wave method we calculate the unpolarized triple-differential cross section and the polarization correlation C₂₀₀ of the bremsstrahlung from atoms with atomic number Z = 1, 47, and 79 for incident electrons of kinetic energies T₁ = 180 and 300 keV. Our partial-wave results are compared with the partial-wave results of Shaffer et al (Shaffer C D, Tong X-M and Pratt R H 1996 Phys. Rev. A 53 4158) the partial-wave results of Keller and Dreizler (Keller S and Dreizler R M 1997 J. Phys. B: At. Mol. Opt. Phys.30 3257), and other approximate and experimental results.

In this paper we report semiclassical approximate values of the Stark parameters for several lines of Pb II. They were calculated using a set of wavefunctions obtained from Hartree-Fock relativistic calculations. Stark widths for 43 lines of Pb II arising from ns ²S_1/2, np ²P_1/2,3/2, nd ²D_3/2,5/2 and 5f  ²F_5/2,7/2 levels of Pb II have been calculated in this way. Stark widths are presented as a function of temperature for an electron density of 10¹⁷ cm^-3. Our results are compared with available experimental data. Stark shifts of rare lines with experimental values given in the references have been studied. Common regularities for the Stark width of the resonance lines of singly ionized lead have been found and are discussed.

Emission spectra of the 2-v'' progression (1⩽v''⩽6) in the B ²Σ⁺→X ²Σ⁺ system of ¹³C¹⁶O⁺ have been photographically recorded at a resolution that was high enough to achieve a clear separation of the spin components for most of the observed rovibronic transitions. Least-squares methods were used to reduce the measured line wavenumbers to the molecular constants appearing in the effective Hamiltonian taken from the work of Amiot et al (Amiot C, Maillard J P and Chauville J 1981 J. Mol. Spectrosc.87 196-218). Merging of the data for the v' = 2 progression with previous measurements in the v' = 0 and 1 progressions resulted in considerably improved equilibrium molecular constants, RKR potential curves, Franck-Condon factors and r centroids for the B→X system of ¹³C¹⁶O⁺.

Triple differential cross sections for the ionization of hydrogen atoms by electron impact have been calculated for an incident electron energy of 27.2 eV following hyperspherical partial wave theory for asymmetric geometries. Calculated results are, generally, in good agreement with experiment and with results of some other well known theories. This study clearly indicates that the hyperspherical partial wave theory is a very effective approach for low-energy ionization studies.

Three-body dynamics are investigated for single ionization of hydrogen by 50 and 100 eV positron impact. The momentum distributions of the electron, the projectile and the recoiling target nucleus are investigated. The present results show that, despite the large mass of the target nucleus, it actively participates in the dynamics of the ionizing collisions. In particular, a strong correlation between the positron and the recoil-ion momenta is found, which is especially important at the lower collision energy of this study.

We have studied two- and three-photon ionization of the 3s and 3p (m_L = 0) states in Ar⁷⁺ at an intensity of 10¹⁵ W cm^-2. The 4f state appears prominently in the 3s two-photon ionization spectrum due to light-induced continuum structure. The three-photon ionization rate for the 3p state is strongly influenced by the 4s resonance after absorption of a single photon and the np and nf Rydberg series after absorption of two photons. The calculations demonstrate the efficiency of the R-matrix Floquet approach for highly charged ions in x-ray laser fields.

We discuss two different approaches for the approximate treatment of the bremsstrahlung (BrS) process of non-relativistic electrons on many-electron atomic/ionic targets, described by a total amplitude which is a sum of ordinary and polarizational BrS amplitudes. The approaches are based on the so-called `stripping' effect; they are useful for calculation of the BrS spectra for photon energies greater than the outer shell thresholds.

Extensive configuration interaction calculations are presented for the 2s²2p ²P^o_J-2s2p^2 4P_J' intercombination lines in C II, N III and O IV. Both valence correlation and core polarization effects are included, the latter being significant in achieving agreement between length and velocity forms of the oscillator strengths of related allowed transitions.

Refinements to the ab initio results are made by incorporating small corrections to the Hamiltonian matrix elements so as to reproduce the experimental energy spectra.

The present calculations agree quite closely with the independent MCHF calculations of  Tachiev and Froese Fischer (2000 J. Phys. B: At. Mol. Opt. Phys.33 2419), and support the experimental results of Träbert et al (1999 J. Phys. B: At. Mol. Opt. Phys.32 L491) rather than those of Fang et al (1993a Phys. Rev. A 48 1114, 1993b Astrophys. J.413 L141).

We propose a scheme for generating two-photon states for a cavity field. In the scheme the cavity initially contains a small coherent or thermal field. Then a resonant two-level atom is sent through the cavity. After an appropriate interaction time the atom is state-selectively measured, which may collapse the cavity field onto a two-photon state. A three-photon state of rather high fidelity can also be generated with a single atom.

We present calculations of the vibrational excitations of three of the normal modes of the cyclopropane molecule in the gas phase: the ν₁ C-H stretching mode, the ν₂ H-C-H scissoring mode and the ν₃ C-C stretching mode. The excitation processes occur by electron-impact inelastic scattering within the energy range where resonant scattering occurs, according to the existing experiments.

The quantum calculations employ the single-centre-expansion-close-coupling method to treat the scattering process and the vibrational contributions are obtained within the adiabatic nuclear vibration approximation scheme.

The results are shown to be in rather good accord with the experiments and the most likely microscopic mechanisms which preside over the excitation of these modes by electron impact at resonances are discussed in some detail.

We investigate the behaviour of a dilute quasi-two-dimensional, harmonically confined, weakly interacting Bose gas within the finite-temperature Thomas-Fermi approximation. We find that the thermodynamic properties of the system are markedly different for repulsive and attractive interactions. Specifically, in contrast to the repulsive case, there appears to be a phase transition when the atoms interact with an attractive pseudo-potential, in the sense that there is no self-consistent solution for the normal ground state below a certain temperature T^⋆. These numerical findings are supported by analytical investigations of the thermodynamics of the system in the complex fugacity plane, and within the random-phase approximation.

The absolute total cross section for electron-hexafluoropropene (C₃F₆) scattering has been measured over the energy range from 30 to 370 eV in a linear electron-beam transmission experiment under single-collision conditions. Between 30 and 70 eV the total cross section appears to be relatively high and nearly constant, and above 70 eV it decreases monotonically with increasing energy. Such behaviour of the total cross section energy function seems to be generally true for perfluorinated targets. Some other features in electron scattering cross sections for perfluorinated targets are noted and discussed.

The integral elastic e^--C₃F₆ scattering cross section has been calculated using an independent atom method with a static + polarization model potential. Results for impact energies above 70 eV are in good agreement with values evaluated from experimental (total and ionization) cross-section data.

Absolute photoionization cross section profiles, branching ratios and asymmetry parameters of xenon have been calculated with the time-dependent density functional theory (TDDFT) approach in a wide photon energy range, from threshold up to 740 eV. The relativistic TDDFT (RTDDFT) equations have been implemented employing a B-spline finite basis set with a non-iterative algorithm for the calculation of the response-induced potential, thus eliminating the well known convergence difficulties associated with their iterative solution. The use of the gradient-dependent LB94 exchange-correlation potential, which allows the existence of bound Rydberg states, has permitted the description, for the first time at RTDDFT level, of the autoionization resonances. Generally, an excellent reproduction of experimental results is obtained, always at least as accurate as that obtained with the computationally more expensive relativistic random phase approximation approach, making the B-spline RTDDFT formulation a very promising approach for the calculation of photoionization processes in heavy-atom systems.

The phase dependence of the photon momentum vector k and the direction of an applied electric field E, which is a signature for an atomic T-odd interaction, is investigated for the first time using a fibre interferometer. Atomic thallium vapour sits in one arm of the interferometer and is subjected to laser radiation tuned near to the 6p_1/2-6p_3/2 transition at 1281 nm. The results show no evidence of a k · E effect down to the level of 0.06±0.1 µrad for a field of 10 000 V m^-1. This sets a limit on the T-odd admixture coefficient δ_T = (±0.9±2)×10^-3. Some important sources of systematic effects are discussed as well as the factors which limit the present sensitivity.

We report the vacuum-ultraviolet absorption spectrum of doubly ionized rubidium observed in a dual laser plasma experiment. The spectrum exhibits discrete structure due to 3d-nl excitations superimposed on the underlying 4s and 4p continua. The total photoionization cross section, computed within the framework of Fano theory, shows very good agreement with experiment.

Ab initioR-matrix techniques are combined with multi-channel quantum defect theory to determine the properties of arbitrarily highly excited electronic states of nitric oxide. Results are obtained for l = 0-4 channels associated with the four lowest NO⁺ target channels, namely ¹Σ⁺, ³Σ⁺, ³Π and ³Δ. Energy and bond length variations of the quantum defect functions for all target states are reported. Certain small surface amplitude R-matrix poles also serve to determine the energies of valence states. Diabatic potential curves for Rydberg states converging to the ¹Σ⁺ and ³Σ⁺ target states and for the valence states are presented. Systematic jumps in the pσ quantum defects for all target states as the bond length increases beyond R = 2.5 au are attributed to Rydbergization of the antibonding 2pσ* molecular orbital.

Angular distributions of recoiling H₂⁺ and H⁺ ions following collisions of slow O⁵⁺ and H₂ were measured. The recoil ions originating from single or double electron capture were detected at angles in the range 20°-130°. At projectile velocities lower than 0.1 au, two groups of peaks are clearly visible. A first structure, centred at ~9.5 eV, corresponds to a double capture at relatively large impact parameters (b≈5 au). At higher fragment energies, another structure is clearly observed. The latter group of peaks shifts towards lower energies as the detection angle increases. Calculations using classical kinematics equations show that these peaks are due to single or double capture at very small impact parameters (b<1 au). Our calculations reproduce the position of the peaks, as well as the relative cross sections as a function of the detection angle.

Single-electron capture in collisions of 9 keV × q Ne⁸⁺ and Ne⁷⁺ ions with He has been studied using cold-target recoil-ion momentum spectroscopy. With an improved apparatus a longitudinal momentum resolution of 0.07 au has been achieved. This momentum component is directly proportional to the difference in the binding energy of the active electron between the final and the initial state. For the first time state-resolved differential cross sections have been determined with respect to the main quantum number, subshell level and spin state of the captured electron. A comparison with recent theoretical results for energy levels in Be-like Ne is given.

The Sturmian expansion of the first-order Dirac-Coulomb Green function (Szmytkowski R 1997 J. Phys. B: At. Mol. Opt. Phys.30 825) is employed to derive an analytical formula for the magnetizability of the relativistic hydrogen-like atom in the ground state. The Gordon decomposition of the magnetizability is carried out, allowing one to identify its diamagnetic and paramagnetic parts.

Two-dimensional photoelectron spectroscopy, in which photoelectron yield is measured as a function of both photon and electron energy, has been used to investigate rotational transitions associated with vibrational autoionization in molecular hydrogen, following excitation by synchrotron radiation. The energy resolution achieved in this study was sufficient to separate individual rotational transitions and hence determine the change in rotational quantum number between the initial, neutral and final, ion states. Rather than concentrate on the rotational decay routes of particular autoionizing Rydberg states, advantage has been taken of the comprehensive nature of the two-dimensional photoelectron spectra (2DPES) to perform a more general type of analysis. Constant transition energy spectra (CTES) were extracted from the 2DPES corresponding to specific rotational transitions J''-J = 1-1, 2-2, 3-3 and ΔJ = + 2, between the initial, neutral and final, ionic states. The ΔJ = 0 spectra proved to be broadly similar, once an allowance was made for the different amounts of rotational energy involved, and the comparison highlighted areas in which differences occurred. In many cases low-n high-υ interlopers, Rydberg states converging on higher vibrational ionic thresholds than members of the main series, were found in these spectral regions, suggesting that these Rydberg states have a significant effect on the rotational spectrum. Analysis of the ΔJ = + 2 CTES revealed a tendency for these low-n high-υ interlopers to feature strongly in these spectra and this has been tentatively associated with the relatively long lifetime of these states against autoionization.

Based on sum rule examinations of second moments, we point out that the correlated single-electron momentum properties reported by Sarsa and co-workers are inconsistent with the electron-pair intracule and extracule properties given by the same authors based on the same wavefunctions.