Table of contents

The low-energy (below 10^-3 au) capture processes in hydrogen-anti-hydrogen scattering have been investigated using the plane wave approximation. The capture cross sections for the different excited states (up to the principal quantum number N = 24) of the protonium (Pn) atom, keeping the positronium (Ps) atom in its ground state, have been explicitly calculated. The present cross sections for the excited states of the Pn atom (N = 24, L = 0) are found to be in fair agreement with those of Jonsell et al (Jonsell S, Saenz A, Froelich P, Zygelman B and Dalgarno A 2001 Phys. Rev. A 64 052712). The s-state capture cross sections for the N = 17-23 excited states are found to be quite significant. The p-state cross sections are also not negligible.

We introduce nonlinear canonical transformations that yield effective, solvable Hamiltonians of multiphoton down-conversion processes, and we define the associated non-Gaussian multiphoton squeezed states as the coherent states of the multiphoton Hamiltonians. We study in detail the four-photon processes and the associated non-Gaussian four-photon squeezed states. The realization of squeezing, the behaviour of the field statistics, and the structure of the phase space distributions show that these states realize a natural four-photon generalization of the two-photon squeezed states.

More accurate term values, a few line strengths and transition probabilities for the E2 transitions of the iron group highly stripped ions Fe XVI, Co XVII and Ni XVIII are reported. We have used the fully relativistic Dirac-Hartree-Fock orbitals to form the Slater determinants to represent different atomic states and included the effect of Coulomb correlation using a fully ab initio all-order many-body coupled-cluster method. In this method, atomic wavefunctions are decomposed in terms of the amplitudes of exciting clusters of a finite number of particles. We have included all the single, double and partially triple excitations from the core in our calculation with a large basis of 98 bound and continuum orbitals within a maximum energy of 500 a.u. chosen through a highly improved methodology. It is found that the percentage errors in term values are highly correlated with the nuclear charges (Z).

Electron-pair uncertainty relationships of generalized Heisenberg type in quantum mechanics are formulated. They express restrictions imposed by quantum theory on the electron-pair intracule (relative motion) and extracule (centre-of-mass motion) densities in the two complementary spaces by means of the corresponding statistical moments. These relations are shown to reduce to an elementary or Heisenberg form when the intracule-extracule isomorphism holds. The validity of this isomorphism is also discussed for some special systems.

Recent research has shown that there are a number of atoms and atomic ions that can bind a positron. The number of atoms known to be capable of binding a positron has expanded enormously in recent years, with Li, He(³S^e), Be, Na, Mg, Ca, Cu, Zn, Sr, Ag and Cd all capable of binding a positron. The structure of these systems is largely determined by the competition between the positron and the nucleus to bind the loosely bound valence electrons. Some systems, such as e⁺Li and e⁺Na, can be best described as a Ps cluster orbiting a charged Li⁺ or Na⁺ core, while others such as e⁺Be consist of a positron orbiting a polarized Be atom. In addition, a number of atoms (Li, C, O, F, Na, Cl, K, Cl, Cu, Br) can bind positronium and a few systems capable of binding two positrons have also been identified. These positron-binding systems decay by electron-positron annihilation with the annihilation rate for e⁺A systems largely determined by the parent atom ionization potential.

The photon-ion merged beam technique has been used to measure the ionization cross sections of I^-, Cs⁺, Ba⁺ and Ba²⁺ ions in the energy region from 40 to 185 eV, which is dominated by photoexcitation from the 4d shell (the experimental data are available at http://www.ifa.au.dk/amo/atomphys/atomphys.htm). Within the experimental accuracy, the total oscillator strengths for these ions and for the I⁺, I²⁺, Xe⁺ and Xe²⁺ ions, that were recently studied by some of the present authors, are identical, with the contribution from the 4d → np or nf resonances becoming more important with increasing nuclear charge and ionization stage, and with the maximum value of the continuum cross section increasing as the nuclear number is enlarged. The present data support the assumption that the 4f wavefunctions contract gradually with increasing ionization, both along isonuclear and isoelectronic sequences, for ionic charges ranging from -1 to +2.

Differential cross sections for the scattering of electrons by a short-range potential in the presence of an intense, linearly polarized, low-frequency field are obtained in three different ways: within the Kroll-Watson approximation, within the impulse approximation and through an exact Floquet calculation. Examples are given where the cross section for small-angle scattering with emission or absorption of photons is larger by several orders of magnitude than predicted by the Kroll-Watson approximation. The enhancement can be ascribed to off-shell contributions and is correctly reproduced by the impulse approximation. However, no such enhancement is found in model calculations for helium. The accuracy of the impulse approximation in the vicinity of a resonance is also studied.

We present a quantum dynamical treatment of the vibrational excitation of gaseous methane molecules from single collisions with low-energy electrons. The exact vibrational coupled equations are solved using the four different normal modes of the molecular target and the numerical bound states of each mode. The interaction forces are treated exactly for the static effects while exchange, correlation and polarization contributions are added via parameter-free model potentials. The equations are solved in a body-fixed reference frame using a single-centre expansion formulation and the ensuing inelastic, partial cross sections are compared with experiments and with earlier calculations.

Measurements of line profiles of the hydrogen Hα transition from a well diagnosed gas-liner pinch are reported. The discharge is diagnosed using Thomson scattering and simultaneous spectroscopic measurements are performed giving the plasma parameters 0.5×10¹⁸ cm^-3⩽n_e⩽2.5×10¹⁸ cm^-3 and 6 eV⩽k_BT_e⩽10.5 eV. The Stark width and shift are determined and compared to a selected set of experimental and theoretical data. The difference between the present results and former data, which were measured with the same device, is critically discussed.

Theoretical results from multi-state nonrelativistic R-matrix (close-coupling) calculations are presented for electron-impact excitation of molybdenum from the (4d⁵5s)⁷S ground state. The dependence of the results on the details of the theoretical model, particularly on the structure description and the number of coupled states, is analysed. These results represent the first step in an ongoing programme to obtain atomic data for spectroscopic modelling of a molybdenum-oxide lamp displaying strong transitions of Mo I.

We study classical bremsstrahlung from neutral atoms and investigate the origins of the structures in the energy dependence of the differential cross section ω dσ/dω and the asymmetry parameter a₂, using soft-photon limit results in terms of elastic scattering. We show that trapping features in elastic scattering do not contribute to the structures. Rather, these structures result from the suppression of contributions to the radiation from certain angular momenta at certain energies. This corresponds to the association of structures in quantum bremsstrahlung with zeros in certain angular momentum matrix elements at certain energies.

The Ne⁺ 2p⁴nℓ satellite states lying between the Ne⁺ 2s main line and the Ne²⁺ 2p⁴(³P,¹D,¹S) doubly charged ion states have been studied. The study combined the high-energy resolution of the threshold photoelectron technique and of the gas phase photoemission beamline at the Elettra synchrotron radiation source. The threshold photoelectron (TPE) measurements show many more states than observed in previous photoemission or optical measurements. The TPE spectrum in the region of the Ne²⁺ 2p⁴(¹D) ionization potential shows a clear cusp-like shape, the typical signature of threshold photodouble ionization. This feature is found to be well represented by an asymmetric power law as in the He²⁺(¹S) case.

We calculated the electric-field splitting of the 1snd ¹D and ³D levels of the ³He I spectrum for n = 3,4 and 5 and evaluated the anticrossings of singlet and triplet Stark substates. The anticrossing spectra observable after selective excitation of singlet states with asymmetric charge and current distributions are discussed. The anticrossing spectra of ³He I differ significantly from those of ⁴He I due to the hyperfine interaction and, therefore, provide a valuable means for investigations on collisional excitation of He atoms.

The aim of this paper is to examine the role of excited states and multi-electron interactions in molecular ionization by strong laser fields. We present new data on the ionization and dissociation of iodine molecules that reveal important aspects of the strong field-molecule interaction in the short pulse regime. Our data, along with previous studies, is inconsistent with the simplest and commonly accepted model of molecular ionization in a strong laser field and this has led us to examine closely the individual ionization steps. We have found that a molecule can ionize into several distinct configurations predominantly through multi-electron interactions and the abundance of such configurations is dependent on internuclear separation. The ionization appears to be dominated by pairs of states with gerade and ungerade symmetry, as they have a large dipole coupling and the transition is near resonant with the strong laser field. In a one-electron molecule, this pair consists of the ground and first excited state whereas in a two-electron molecule this corresponds to the lowest lying pair of ionic states. In this paper, we propose a framework for organizing the numerous ionization pathways based on the electronic configuration of the initial charge state.

Neutral photodissociation of CO has been investigated using synchrotron light in the range 19-26 eV by observing dispersed fluorescence from excited neutral C atoms. Follow-up ab initio calculations support the observed neutral carbon excitation functions, which to a large extent are associated with the CO Rydberg series converging to the CO⁺ C and D states.

We have investigated a new scheme for excitation of the 5s, J = 2 metastable level of Kr (5s[3/2]_{J = 2}) which can be readily extended to other rare gases. In the scheme, an ultraviolet (UV) lamp is used to create a population of Kr atoms in the 5s[3/2]_{J = 1} level in a gas cell. The excited atoms are then pumped to the 5p[3/2]_{J = 2} level, using 819 nm light from a Ti:sapphire laser, from which they decay to the metastable state with a branching ratio of 77%. We made two striking observations: (1) the laser power required to saturate the second step decreases markedly as a function of gas cell pressure, and (2) the UV photon flux is converted with very high efficiency (≈10%) to metastable atom flux. A Monte Carlo study of the scattering of UV photons in the cell reproduces the trends observed. The understanding achieved points to the design of a higher flux source of metastable atoms.

We report the results of the experimental determination of the appearance energy values AE(X^{n +} /X) for the formation of multiply charged Ne, Ar and Xe ions up to n = 4 (Ne), n = 6 (Ar) and n = 8 (Xe) following electron impact on Ne, Ar and Xe atoms using a dedicated high-resolution electron impact ionization mass spectrometer. The data analysis uses the Marquart-Levenberg algorithm, which is an iterative, nonlinear least-squares-fitting routine, in conjunction with either a two-function or a three-function fit based on a power threshold law. This allows us to extract the relevant AEs and corresponding exponents for a Wannier-type power law from the measured near-threshold data. The values of the AEs determined in this work are compared with other available experimental and spectroscopic values of the AEs and the extracted exponents are compared with other available experimental data and with the predictions of the various Wannier-type power law models.

In two dimensions the Gross-Pitaevskii equation for a cold, dilute gas of bosons has an energy-dependent coupling parameter describing particle interactions. We present numerical solutions of this equation for bosons in harmonic traps and show that the results can be quite sensitive to the precise form of the coupling parameter that is used.

The ground state of atoms in an optical lattice is found using a Gutzwiller approximation. In particular we examine systems with large numbers of atoms per site. The number squeezing in the ground state is calculated in terms of lattice parameters. The approximation successfully describes the state of an arbitrary number of identical lattice sites at zero temperature, for both the superfluid and Mott insulator phases.