Table of contents

It is proposed that the binding energies of positrons to a number of atoms be determined by the charge transfer reaction A^-+e⁺ Ae⁺+e^-. This reaction can be realized in the laboratory by directing a negative ion beam through a positron trap and measuring positively charged ions downstream. Unique identification of Ae⁺ is possible when the electron affinity, positron affinity and neutral atom ionization potentials satisfy a few simple identities. This technique could be used to investigate the existence of positron binding for a number of elements such as Si, Fe, Cu, Ge, Ru, Ag, Sn, Sb, Ta, W, Au, Pb and Bi.

A generalization of the Keldysh ionization amplitude derived previously which incorporates rescattering is used to calculate above-threshold ionization spectra for medium-high intensity (~10¹⁴ W cm^-2). Quantum interferences are identified and shown to be capable of causing above-threshold ionization peaks to rise and fall by an order of magnitude when the laser intensity changes by only a few per cent. The calculations are carried out for a zero-range binding potential. Comparison with available three-dimensional accurate calculations for a Coulomb potential shows sufficient agreement suggesting that predictions based on the zero-range potential are qualitatively reliable.

We report on studies of Ps subjected to strong subpicosecond laser pulses. We find substantial deposition of population in excited states extending over a broad range of intensities. The possible usefulness as a source of excited Ps to spectroscopy and the formation of antihydrogen is discussed.

Theoretical results are presented for electron-impact ionization of He(1s²), leaving the ion in the n = 2 excited states. The projectile-target interaction is described in the first-order plane-wave Born approximation, whereas the interaction between the ejected electron and the residual ion is treated non-perturbatively within the convergent close-coupling formalism. Comparison of the predictions from two entirely independent approaches, formulated either in coordinate or in momentum space, provides a basis for further systematic improvement of the models.

We report experimental results related to the effect of laser polarization on the angular distributions of ions produced by the interaction of high-intensity short laser pulse radiation with CS₂, OCS, CO₂ and CH₃I. Using time-of-flight mass spectrometry and laser pulses of 200 fs at 800 and 400 nm and 500 fs at 248 and 497 nm we have observed strong laser-induced alignment of the singly and multiply charged fragments along and/or perpendicular to the direction of the electric field of the laser light. The dependence of the observed effects on laser wavelength and intensity has been studied. The results are shown to contribute to the elucidation of the alignment mechanism.

We present experimental results for doubly differential electron emission in transfer ionization produced by H⁺ projectiles on a He target at 70 keV impact energy. Energy spectra were obtained at emission angles of = 0°, 20° and 50°, and compared with spectra of total electron emission, these being dominated by single ionization. A cusp-shaped emission of electrons at the velocity of the emerging H⁰ projectiles, and an enhanced electron emission from binary-encounter collisions are observed in the transfer ionization spectra.

It is shown that recently measured cross sections for double ionization of negative ions (H^-, O^- and C^-) possess a universal shape when plotted in suitable dimensionless units. The shape can be represented with a simple analytical function, following the same principles as those which have been utilized during the establishment of a universal shape function for single ionization (Rost and Pattard 1997 Phys. Rev. A 55 R5). Thereby, it is demonstrated that direct double ionization dominates the cross section for the targets considered.

We derive exact thermodynamic identities relating the average number of condensed atoms and the root-mean-square fluctuations determined in different statistical ensembles for the weakly interacting Bose gas confined in a box. This is achieved by introducing the concept of auxiliary partition functions for model Hamiltonians that do conserve the total number of particles. Exploiting such thermodynamic identities, we provide the first, completely analytical prediction of the microcanonical particle number fluctuations in the weakly interacting Bose gas. Such fluctuations, as a function of the volume V of the box are found to behave normally, in contrast with the anomalous scaling behaviour V^4/3 of the fluctuations in the ideal Bose gas.

Atomic interferometry was born recently, towards the end of the 1980s. Its development has been extremely fast, new techniques being pioneered independently and almost simultaneously in different laboratories all over the world. Nowadays, these techniques have reached a high level of sophistication, opening a wide area of fundamental and practical applications. In this paper the general architecture of interferometers in which matter waves are coherently manipulated is described. Various realizations of atom and molecule interferometers are reported, together with the major results obtained with each type of interferometer. Finally, new trends and perspectives are given. Whilst the techniques seem to be almost completely achieved, new developments are coming up, such as the use of new and non-ordinary sources. Forthcoming applications are numerous. They deal with the most fundamental aspects of quantum mechanics, with the metrology of fundamental constants, with the use of interferometers as very sensitive probes of external interactions and inertial effects, with atomic nanolithography, etc.

Employing the animated crossed-beams technique, absolute cross sections for electron impact single ionization of the iron isonuclear sequence are measured for charge states q = 1-6 for electron energies from threshold up to 1 keV, as well as for the intermediate charge states q = 9,10 up to an electron energy of 5 keV. The cross sections observed for Fe^q+ ions in charge states q = 1 - 4, 9 and 10 show a significant ionization signal below the respective ground state threshold resulting from ions in excited, long-lived metastable states in the parent ion beam. In the case of Fe⁵⁺ and Fe⁶⁺ no metastable components are found. The experimental results are in good agreement with the theoretical predictions if excitation-autoionization processes are taken into account. Our measured cross sections are also in good agreement with experimental results of other groups.

Using the crossed-beams technique, cross sections for electron impact double ionization of Fe^q+ ions in charge states q = 1 - 6 (except 2) are measured in the energy range from the respective ionization thresholds up to 1 keV, partly up to 5 keV. The data for Fe⁴⁺, Fe⁵⁺ and Fe⁶⁺ show significant contributions from inner-shell processes. Using a high-resolution energy-scan technique, we study narrow resonances in the cross sections for Fe⁴⁺ and Fe⁵⁺. Besides contributions from excitation-autoionization processes, a remarkable number of structures arising from resonant-excitation triple-autoionization processes are observed. The results are compared with the scaling laws of Fisher et al and the semiempirical formula published by Shevelko and Tawara, respectively.

Ab initio variational calculations are made of the elastic scattering and positronium formation cross sections for positron-helium scattering below the first excitation threshold of the target. Three variants of the Kohn variational method are used with very flexible trial wavefunctions and a very accurate helium wavefunction. The elastic scattering and positronium formation cross sections are believed to have converged to within 5% and 10%, respectively. The calculated positronium formation cross section displays a similar energy dependence to that of recent experimental results but with a 25% difference in magnitude. However, the theoretical total cross section, both above and below the positronium formation threshold, agrees with the experimental measurements to within 10%. Threshold effects in the s-wave cross sections manifest themselves as a significant feature in the total cross section. Similarities with the corresponding results for positron-hydrogen scattering are discussed.

It was proposed by two of us (Wiseman H M and Collett M J 1995 Phys. Lett. A 202 246) that a device producing a coherent atomic wave (that is, an atom laser) could be built using dark-state cooling of atoms in a trap. Here we present a more complete analysis of this system, including the effects of atomic interactions through dipole-dipole coupling and hard-core collisions. We show that for reasonable physical parameters the dipole interactions have little contribution to the atomic dynamics. In contrast, the hard-core collisions are likely to dominate the laser linewidth. In fact, it would appear necessary to select (or produce) a species with a hard-core scattering radius of much less than one nm in order for the output of the device to be phase coherent.

The angular dependence of emission of L x-rays following photoionization at 22.6 and 59.5 keV in ₈₂Pb has been investigated by measuring the intensity ratios I(L1)/I(L), I(L)/I(L) and I(L)/I(L) at different angles varying from 50° to 140°. The measurements were taken using ¹⁰⁹Cd and ²⁴¹Am radioisotopes as photon sources and a Si(Li) detector. The measured intensity ratios for various L x-rays are found to be angle independent within experimental error. This is contrary to the large anisotropic emission of L1 and L x-rays following photoionization as reported by Kahlon et al (1990a J. Phys. B: At. Mol. Opt. Phys.23 2733; 1990b Pramana-J. Phys.35 105) and by Ertugrul (1996 Nucl. Instrum. Methods B 119 345).

Average energy shifts of K and K x-ray satellite lines and K/K intensity ratios have been experimentally evaluated for a ⁸⁰Se target bombarded by various projectiles with atomic numbers in the region 3Z_p16 at a given projectile energy of 3.3 MeV/u. The data have been analysed using an approach, based on the results of extensive single-configuration Dirac-Fock (DF) calculations, performed within the multiconfiguration DF method for various distributions of holes in subshells of a ⁸⁰Se ion. This provides information about the average number of holes in various subshells of ⁸⁰Se ions at the time of emission of the K x-ray lines. The primary average number of L- and M-shell holes in a ⁸⁰Se target produced at the moment of collisions with projectiles and the average ionization probabilities per electron for L and M shells have been deduced by means of a simple statistical scaling procedure which accounts for all processes that modify the number of L- and M-shell holes prior to the K x-ray emission. It has been found that the average ionization probabilities per electron for L and M shells increase considerably with Z_p². For larger Z_p the average ionization probability per electron is significantly higher for M shells than for L shells. On the other hand, for small Z_p the ionization probability per electron seems to be slightly higher for L shells than for M shells. Moreover, the dominant role of the simultaneous multiple ionization of L and M shells accompanying the ionization of a K shell has been observed for projectiles with Z_p>3.

The alignment of the 2p⁵3s²²P_3/2 autoionizing state in sodium after electron-impact excitation was determined via the angular anisotropy of the intensity I() of the autoionization electrons. For emission angles 90°, the intensities I() were measured by rotating the incident electron gun, whereas magnetic-field deflection was applied for larger angles. Near-threshold negative-ion resonance structures in the alignment of an autoionizing state were observed for the first time and are discussed in detail. R-matrix calculations for incident electron energies from threshold to 80 eV successfully reproduced the broad-scale energy dependence of the alignment. The cross section ratio for the fine-structure components 2p⁵3s²²P_3/2 and ²P_1/2 were also measured and reproduced satisfactorily by theory.

The three-dimensional angular distribution of electrons in two-photon ionization of atoms by an elliptically polarized laser beam, from an initial state with arbitrary angular momentum J_i, is expressed in terms of six frequency-dependent invariant atomic parameters. The specific effect of the field polarization on the angular distribution (the elliptic dichroism, ED) is discussed for the general case of elliptic polarization. The necessary conditions for a non-zero ED are established and its physical origin is explained. The general theory is illustrated by a simple analytical calculation for the -potential model. Numerical calculations of the angular distribution parameters for H and Cs atoms are also performed and the dependence of ED magnitude on the atomic quantum numbers and the photon frequency is discussed for different frequencies.

Angle-resolved resonant electron spectra following vibrationally resolved C 1snl ( = 0,1) excitations in CO have been measured. The observed groups of resonant Auger lines exhibit great similarity in shape to the C-KVV group of non-resonant Auger lines and the observed angular distribution behaviour reflects the symmetry-dependent anisotropies of the excitation process. The molecular progressions in the C 1s ^* resonant Auger spectrum could be clearly identified due to their different angular distribution behaviour. Furthermore, it could be shown that the screening energies for spectator electrons over binding energy have a linear dependence between 3p, 4p and threshold as was expected for the higher p-symmetry Rydberg orbitals.

We present the theoretical calculation of the differential scattering cross section for elastic electron-helium scattering in the presence of a bichromatic CO₂ laser field. The two components of the laser field have the frequencies and 2, which are out of phase by an arbitrary scale . The calculation is done in the first Born approximation taking into account both the direct and the exchange scattering amplitudes and using the closure approximation. The dressing of the target states is found to be considerable in the region of small scattering angles. There are important modifications of the scattering cross section as a function of the phase angle . The calculated spectra of the scattered electrons (for l>0 and l<0) are in mirror symmetry for = 0 versus = and symmetric for = /2 with respect to l = 0. The second harmonic does not contribute to the scattering in the small angle region, but modifies it strongly for higher scattering angles. Finally, we make a comparison between our semiclassical approach and the quantal approach of Rabadan et al (1996 J. Phys. B: At. Mol. Opt. Phys.29 163-175).

The cross sections for the dissociative electron attachment (DEA) to ozone have been measured. Care has been taken to collect all the high kinetic energy fragments such that the present absolute DEA cross section measurements are believed to be the most accurate measurements made across the range from 0 to 10 eV for ozone. Our results have been interpreted with the help of both existing data in the literature and quantum chemical calculations performed on the neutral and negative-ion states of ozone.

The concept of effective, position-dependent reduced mass is introduced for three-body molecules. It varies continuously between the asymptotic limits and generates a scaled Born-Oppenheimer-type Hamiltonian that contains a substantial amount of the adiabatic correction. A close analytical expression for this correction is obtained, without the explicit calculation of the nuclear kinetic energy matrix element. The approach accounts for the isotope effect and allows the first successful Born-Oppenheimer-type calculation on e^-e⁺e^-.

The high-resolution (full width at half maximum = 0.11 Å) emission spectrum of the deuterium molecule produced by electron-impact excitation at 100 eV has been measured in the wavelength range 1200 - 1660 Å. In conjunction with the experimental measurement, transition probabilities of and D band systems are calculated. Synthetic spectra based on the calculated transition probabilities are in very good agreement with the optically thin experimental spectra. While centrifugal effects on the overall Lyman band emission intensity in D₂ and H₂ are similar, the effect of rotation-vibrational perturbations between the B ¹_u⁺ and C ¹_u⁺ states on spectral intensities is found to be less significant in D₂. Excitations via isotropic (J = 0) and anisotropic (J = ±2, and 0) interactions for the dipole forbidden E, F ¹_g⁺-X ¹_g⁺ transition are inferred from the measured spectra. Excitation cross sections of the B ¹_u⁺-X ¹_g⁺ and C ¹_u-X ¹_g⁺ bands, along with the estimated E, F ¹_g⁺-B ¹_u⁺ cascade cross section, for D₂ are obtained.

The fixed-core stochastic variational method has been used to demonstrate the existence of an electronically stable ground state for potassium positride, KPs, with a binding energy of 0.003 275 Hartree. The 2 annihilation rate of 2.001 × 10⁹ s^-1 was almost the same as the annihilation rates computed previously for LiPs and NaPs. Analysis of the radial expectation values, the annihilation rates and the e^--e⁺ correlation function reveal that KPs can be regarded as a Ps cluster moving in the field of the neutral atom.

A general expression for the cross sections of inelastic collisions of fast (including relativistic) multicharged ions with atoms which is based on the generalization of the eikonal approximation is derived. This expression is applicable for a wide range of collision energy and has the standard nonrelativistic limit; in the ultrarelativistic limit it coincides with Baltz's exact solution (Baltz 1997 Phys. Rev. Lett.78 1231) of the Dirac equation. As an application of the obtained result the following processes are calculated: the excitation and ionization cross sections of a hydrogen-like atom; the single and double excitation and ionization of a helium-like atom; the multiple ionization of neon and argon atoms; and the probability and cross section of K-vacancy production in the relativistic U⁹²⁺-U⁹¹⁺ collision. Simple analytic formulae for the cross sections of inelastic collisions and the recurrence relations between the ionization cross sections of different multiplicities are also obtained. Comparison of our results with the experimental data and the results of other calculations are given.

The resonance energies and widths of Mg 3pn¹D^e and ¹F^o doubly excited states are determined by calculating the density of resonance states using the stabilization method with the B-spline-based configuration interaction (BSCI) approach. The effects due to the intrashell core excitation and intershell core-valence interactions are taken into account in the BSCI calculation by using a parametrized long-range core-polarization potential. Comparisons are made with the existing theoretical and experimental results.

Electron momentum spectroscopy has involved the verification of an ab initio model for the target-ion overlap, or Dyson orbital, in a calculation of a high-energy (e,2e) reaction by comparing the resulting cross section with experimental data. We describe an inverse method of extracting the overlap directly from experimental data by using a quantum-mechanically constrained statistical fitting procedure in conjunction with previously verified reaction models.

The optical emission from a laser produced plasma (LPP) generated by a 10 640 Å laser irradiation, with a flux of 2 × 10¹⁰ W cm^-2, on Pb targets was recorded and analysed between 2000 and 7000 Å; spectral lines emitted by the first two ionization species of Pb were studied in a wide range of plasma parameters. First experimental values of the atomic transition probabilities for ten spectral lines of Pb III have been determined by measurement of emission line intensities from an optically thin LPP of Pb in an atmosphere of Ar. The studied plasma has a temperature of about 25 300 K and an electron density of 10¹⁷ cm^-3. Local thermodynamic equilibrium conditions and plasma homogeneity has been checked.

The positions and the widths of 21 ²P^o resonances in Be II between 1 ¹S₀ and 2 ¹P thresholds are calculated using a saddle-point complex-rotation method. Relativistic and mass polarization corrections to the autoionization energy are included. Below threshold (1s2s)³S our results agree well with other calculations and experiments. We make an identification of states above the (1s2s)³S threshold. For the region above 2 ³S, our present results for Be II are contrasted with those for Li I (Wu L J and Chung K T 1997 J. Phys. B: At. Mol. Opt. Phys.30 1173). We discuss the dependence of the widths on the atomic number.

The 4d inner-shell absorption spectra of Te, Te⁺, Te²⁺ and Te³⁺ have been recorded with the dual laser produced plasma technique. Discrete structure due to 4d5p transitions has been observed in the 37 - 45 eV region and identified using Hartree-Fock calculations within the superposition of configurations method. For comparisons of theoretical and experimental spectra, the method approximates a calculation of the resonant part of the photoionization cross section. It was also necessary, especially in the higher ion stages, to include absorption from excited states of the ground configurations. From these, synthetic spectra were produced which generally agree well with the experimental data.

Hydrogen ionization in soft collisions with relativistic charged projectiles is considered for the ranges of parameters: (a) vZ<<v², v₀<<v<c; (b) Z<<v, v₀<<v<c (here Z is the projectile charge, v the collision velocity, v₀ the Bohr velocity in the hydrogen ground state, the Lorentz factor and c = 137 is the speed of light). Analytical expressions have been obtained for differential cross sections of hydrogen ionization accompanied by the emission of soft electrons. It is found that the electric-dipole interaction contributes most to the soft emission. An asymmetry in the angular distribution of soft electrons is found which can be interpreted as a result of two effects: (i) an absorption of virtual photon momenta by the atomic system, (ii) so-called `post-collision interaction'. These results are generalized to describe analytically soft-electron emission from helium colliding with relativistic projectiles. This generalization gives a simple fit to experimental data on both differential and total cross sections for helium single ionization in a wide range of projectile charges and velocities.

For the ionization of atoms the possibility of using radial orbitals of the initial and final states calculated for different Hamiltonians is demonstrated. For a limited number of final states, the additional orthogonality constraints should be imposed on the continuum radial wavefunction taking into account the orthogonality between the final state of the ion + electron and initial excited states of the atom. Within the framework of the relaxed-orbital Hartree-Fock approximation agreement between the length and velocity forms can be achieved if the orthogonality constraints of the wavefunctions of excited states with those of all lower-lying states of the same symmetry are correctly fulfilled.

The formula = _oo-_oc(_cc+)^-1_co is familiar in quantum defect theory: is the reactance matrix for electron-ion collisions; is a matrix with partitioning for open-open, open-closed, closed-open and closed-closed elements; and = tan( ) where is a diagonal matrix of effective quantum numbers in the closed channels. In the simplest case it may be assumed that is a slowly varying function of the energy.

This formula has usually been obtained by assuming only a pure Coulomb potential at radial distances rr₀, with r₀ finite. This paper discusses the derivation of a similar formula for the case in which long-range non-Coulomb potentials are included, behaving like r^--1 for rr₀, with 1.

The fully differential cross sections for the electron-impact ionization of atomic targets, prepared in a given circular state using laser-pumping, reveal a dependence on the inversion of the helicity of the exciting photon. This `dichroism' effect was shown to be strongly dependent on the geometrical arrangements of the experiment both theoretically and experimentally. In addition, as shown theoretically in this paper, the dichroism may also vanish at certain `non-geometrical' points that can be deduced analytically within the first Born approximation. More elaborate calculations using the distorted-wave Born approximation confirm this analysis. On the basis of this study, we further suggest a possible explanation for a structure observed recently in the state-resolved fully differential cross sections for a sodium target.

A systematic study of the formation of CD_n²⁺ and CH_n²⁺ dications in fast charge-stripping collisions with Ar atoms was conducted. The experimental method was based on the detection of the D (or H) fragments of the molecular ion of interest, and thus reducing the effect of the fraction of molecular ions containing the ¹³C isotope and other beam impurities. We observed long-lived CD₂²⁺, CD₄²⁺, and CD₅²⁺ dications. In the same process neither long-lived CD²⁺ nor CD₃²⁺ were observed. The mean lifetime of CD₂²⁺ was determined to be 4.0±_1.1^1.3 µs, and those of CD₄²⁺ and CD₅²⁺ were longer than 2.1 and 3.3 µs, respectively. The production cross sections of CD_n²⁺ from different CD_m⁺ beams were measured. Long-lived CD₂²⁺ was formed from all CD_m⁺ beams (m2) and also directly from the rf ion source. In contrast, CD₄²⁺ and CD₅²⁺ were formed only from CD₄⁺ and CD₅⁺, respectively.

The relative cross section for the excitation of the 2p_u state of H₂⁺ by 400 eV electrons as a function of energy loss has been measured and calculated. Electrons scattered at 18° with energy losses ranging from 30 to 72 eV were detected in coincidence with H⁺ ions of 7.7 eV. The H⁺ ions were detected at 72° relative to the beam direction. Because of the rapid dissociation of the excited state, it follows that the detected ions were produced by target H₂ molecules with internuclear axes oriented at 72°. The relative cross sections were calculated using prolate spheroidal coordinates and fitted to the data with one parameter. The general shape of the energy-loss curve is in agreement with the measurements.

We present a study of the effects of temperature upon the excitation frequencies of a Bose-Einstein condensate formed within a dilute gas with a weak attractive, effective interaction between the atoms. We use the self-consistent Hartree-Fock Bogoliubov treatment within the Popov approximation, and we compare our results to previous zero-temperature and Hartree-Fock calculations. The metastability of the condensate is monitored by means of the l = 0 excitation frequency. As the number of atoms in the condensate is increased, with T held constant, this frequency goes to zero, signalling a phase transition to a dense collapsed state. The critical number for collapse is found to decrease as a function of temperature, the rate of decrease being greater than that obtained in previous Hartree-Fock calculations.