Table of contents

A joint theoretical–experimental study of the transfer ionization process p + He → H⁰ + He²⁺ + e⁻ is presented. For the first time all particles in the final state have been detected in triple coincidence. This fully differential measurement is in good agreement with a theoretical model where the target is described by a wavefunction containing both radial and angular correlation terms.

Ejected-electron excitation functions for the lowest-lying (3p⁵4s²)²P_3/2 and (3p⁵4s²)²P_1/2 autoionizing states in potassium have been measured for incident-electron energies from threshold to 500 eV and 200 eV, respectively, with improved energy resolution. Strong resonances have been observed and well resolved in the excitation functions of both states near their excitation thresholds. R-matrix (close-coupling) calculations were performed to analyse and tentatively assign these features. A remarkable similarity between the spectrum of the core-excited negative-ion potassium states and the excited states of atomic argon is noted.

The experimental cross sections for the various decay channels of the doubly excited states of H₂ after photon absorption have been modelled as a superposition of scaled absorption profiles. Limits of the adiabatic hypothesis for the dissociation of these states were put into evidence, and some non-adiabatic mixings could be quantified. The various dissociation channels have been identified, state by state, and the various dissociation yields determined when the dipole moment was known.

Energy levels of laser-excited Au I Rydberg states n = 25–38 in n²S_1/2, n²P_1/2,3/2 and n²D_3/2 series have been measured by means of millimetre-wave spectroscopy in the frequency range 80–300 GHz. In this n region, Rydberg progressions were smooth and regular within the ±1 MHz measurement accuracy, and quantum-defect Rydberg–Ritz coefficients for these series have been obtained in a single-channel analysis. The doublet fine-structure splittings of P terms for n = 14–36, combining old and new data, have been fitted by an expansion formula. An improved value of the first ionization potential for Au I is V_IP = 74 409.11 ± 0.03 cm⁻¹.

We have studied theoretically the atomic structure and ground-state photoionization cross-section in a model of endohedral Mg (Mg@C₆₀) with particular emphasis on doubly excited bound and autoionizing (AI) states. We modelled Mg@C₆₀ as a system of two active electrons outside a frozen, doubly charged ionic core, confined to a spherically symmetric potential well of finite depth. We employed a suitable B-spline basis set within a CI approach to describe the two-electron states of the confined system. In addition, we have calculated the ground-state photoionization cross-section in a limited energy range above the first ionization threshold. We found that doubly excited perturbers appear in each regular singly excited Rydberg series of bound states. They are composed of two-electron configurations built with those one-electron orbitals mostly affected by the confining potential. Moreover, the low-lying part of the AI spectrum for each symmetry analysed is perturbed by states novel to the confined system, whose configuration expansion is again dominated by the one-electron orbitals mostly affected by the confining potential and whose AI widths vary over a wide range of values. Finally, the ground-state photoionization cross-section in a limited energy range (a few eV) above the first ionization threshold is structured by the AI resonances in a way markedly different from the corresponding Mg atom cross-section.

The non-dipole contribution to spin polarization of photoelectrons from Xe, Cs and Ba 3d_5/2 and 3d_3/2 levels is calculated. The calculation is carried out within the framework of a modified version of the spin-polarized random phase approximation with exchange. The effects of relaxation of excited electrons due to the 3d-vacancy creation are also accounted for. It is demonstrated that the parameters that characterize the photoelectron angular distribution as functions of the incoming photon energy, although being predictably small, acquire additional peculiarities when the interaction between electrons that belong to the 3d_5/2 and 3d_3/2 components of the spin–orbit doublet is taken into account.

Ab initio potential energy curves of the Rydberg NeH molecule in the presence of cylindrical spatial confinement were computed by the method of multi-reference configuration interaction with extended basis sets. The influence of the applied potential to the structures and spectra of the ground and excited states of NeH was analysed in terms of perturbation theory. In addition, the phenomenon of field-induced ionization was discussed.

Systematically enlarged multiconfiguration Dirac–Fock wavefunctions have been used to calculate the transition energies, oscillator strengths, emission rates and branching ratios for the fine-structure forbidden M1 and E2 transitions between the levels of the low-lying 3s²3p⁵, 3s3p⁶, 3p⁴3d and 3p⁴4s configurations in Ar II. Large wavefunction expansions had to be applied since these transitions happened to be very sensitive with respect to configuration interactions and relativistic effects. By applying a sufficiently large expansion for the wavefunctions, however, our theoretical data agree well with those obtained in precision measurements and, therefore, might be used in the identification of further (observed) lines. We also report the energies and rates for the dipole-allowed E1 transitions, for which a number of previous experiments and computations are available from the literature.

In this paper, we examine the possibility of generating bright and dark solitons, as well as multi-pulse solitary waves in Bose–Einstein condensates. Starting from the linear limit of the problem, where such solutions are known explicitly, we use 'dynamic continuation' through the temporal variation of the scattering length, which can be realized in experiments upon using external time-dependent magnetic fields. We examine the stability of the resulting solutions both through direct numerical simulations and through numerical continuation of the relevant steady states and linear stability analysis. Analytical results, illustrating the nature of the bifurcation of the nonlinear solutions, are found to be in good agreement with the numerical findings.

The total cross section for ionization and simultaneous ionization–excitation of helium by electron impact has been revisited within the framework of first-order approximations for the projectile–target interaction. A systematic study of the sensitivity of theoretical results on the details of the computational model, specifically the treatments of the projectile–target interaction, the initial bound state and the ejected-electron–residual-ion interaction reveals the difficulties of making highly reliable predictions.

The energy spectra of the ground and low-excited states of a positronium negative ion Ps⁻ in a quantum dot (QD) with a parabolic confinement are investigated as a function of the confinement strength ω₀ by means of the exact diagonalization of the Hamiltonian matrix. The results for the two-dimensional (2D) disc-like QD and the three-dimensional (3D) spherical QD are compared with each other: in general, the energies of the states and the interval between states decrease with the reduction of the space dimensions.

The paper presents a new alternative exact quantum expression of the x-component of the dipole matrix element between the Stark states of a hydrogen atom in terms of the Jacobi polynomials by transforming the hypergeometric functions appearing in the standard quantum formula. The new quantum formula readily leads to analytic study and numerical computation for such large values of the parabolic quantum numbers for which difficulties had earlier been encountered. The paper goes on to derive an approximate but simple quantum formula of the dipole matrix element in terms of the ordinary Bessel functions and demonstrates its remarkable accuracy for transitions ranging from that between the Stark levels of the lowest lying states to that between the Stark levels of nearby Rydberg states. The formula enables accurate numerical computation to be performed over an extended range of large parabolic quantum numbers that had earlier defied evaluation. The expressions given in this paper in essence solve the problem of determination of analytic behaviour and numerical computation of the dipole matrix element for transitions between the Stark levels of nearby Rydberg states. The paper also presents, for the first time, a derivation of the formula of the correspondence principle method from the quantum expression without appealing to any classical or semiclassical argument, and clarifies the conditions of its applicability.

We investigate the focusing dynamics of a trapped Bose–Einstein condensate. A comparison between the interacting and noninteracting case is made where self-similar solutions can be assumed. In addition, we discuss the stability of the cloud when local supersonic velocities are induced.

In order to understand the line broadening of the 2p → 2s transition of Li atoms in He gas as a function of pressure and temperature we have calculated the emission spectra of LiHe and LiHe₂ based on quantum chemically computed potential energy surfaces and transition moment functions. A full quantum treatment of the red wing is compared with the results of simplified methods (e.g., the quasistatic theory) and with recent experiments.

A pulsed low-pressure capillary discharge operated in neon, helium and hydrogen gas mixture was used as the source of a high-density plasma with predominantly protons as ions for the measurement of the spectral line profiles of 13 isolated Ne I lines. An electron density of 4.8 × 10²² m⁻³ with an estimated uncertainty of 10% was determined from the profiles of three He I lines, while an electron temperature of 33 000 K, with an uncertainty of 12%, was measured using the relative intensities of O II impurity lines. The experimental widths of symmetric Ne I profiles are compared with data of two semiclassical calculations using quasi-static and ion impact approximations. The average ratio of measured to theoretical line widths is approximately the same for both theoretical approaches, showing good agreement in either case. This agreement with both theories must be considered, however, to be accidental, for the following reason. In one theory, the electron impact widths are relatively larger with small quasi-static ion contribution, while the other approach produces relatively smaller electron and larger ion impact widths. The asymmetric He I line profiles prove that the semiclassical theoretical approach with quasi-static ion approximation describes better the experimental Stark profiles of He I and Ne I lines. Contrary to the recently reported experimental Ne I line shapes, all studied profiles are symmetric within experimental uncertainty. Possible causes for the line asymmetry and discrepancy of Stark widths with data of this experiment are traced to the plasma source inhomogeneity and plasma diagnostics. Good agreement with several other experiments is detected.

Applying the animated beam method the total cross sections (TCS) for double ionization (DI) of helium by electrons are measured in the collision energy region from threshold to 3 keV. The TCS for the same process are calculated at intermediate and high incident energies in the first Born approximation (FBA). The radial and angular correlations between the bound electrons and repulsion between the ejected electrons are accounted. In order to go beyond the FBA the scattered electron is considered as a particle being in the Coulomb field of the nucleus, the charge of which is screened by the ejected electrons. On the basis of the obtained results some conclusions concerning the behaviour of the TCS in the intermediate and high-energy regions are drawn.

The absorption spectrum accompanying the Zn (4¹P₁–4¹S₀) 213.8 nm resonance line in the presence of 1.613 × 10¹⁹ atoms/cm³ of Kr is carefully measured and the absolute absorption coefficient per atom pair is derived. The rainbow satellites associated with ZnKr (¹0⁺ ← X) free–free transitions on both line wings are observed and utilized for probing the excited state potential curve by means of quantum simulation of the spectrum. Best available theoretical potential curves were employed as the input data. It is found that the simulated red wing profile including the satellite agrees with the experimental one quite well. However, in the case of the blue satellite a considerable correction of the ¹0⁺ state potential at short interatomic distances proved to be necessary in order to reproduce the spectral position and the shape of the experimental band. Van der Waals constants describing the long-range behaviour of the excited states ¹0⁺ and ¹1 potential curves are derived from the near red wing profile analysis.

We study the ionization of a hydrogenic target by the impact of moderate energy electrons and in the presence of a laser field. This process, generally referred to as the laser-assisted (e, 2e), is treated non-relativistically as follows: we evaluate the S-matrix elements using Volkov and Coulomb–Volkov wavefunctions for describing respectively the continuum states of the scattered and the ejected electrons. The dressed state of the target is described by a time-dependent wavefunction derived from a perturbation treatment (of the laser field). With the aid of numerical results we explore the dependences of the fully differential cross sections on the laser field properties such as the strength, the frequency and the polarization direction, as well as on the continuum electron energies and their emission angles. Furthermore, we assess the interplay between the laser influence and the role of scattering from the residual ion by comparing the cross sections from (one-electron) ionic targets and varying the charge state of the residual ion.

Shallow donor muonium states with small hyperfine frequencies, recently observed in II–VI semiconductor compounds, have a number of unique features that present both opportunities and challenges in understanding muon spin dynamics in the presence of Heisenberg spin exchange. First, the shallow muonium state in CdSe with hyperfine frequency ω₀/2π ≈ 0.1 MHz is already in the high field regime even in the earth's magnetic field, where only two precession frequencies are observable by the muon spin rotation (μSR) technique. Second, unlike in the case of more conventional muonium species with a larger hyperfine frequency, the μSR signal of shallow muonium states can be observed even in the transition region, between the slow spin–flip regime and the fast spin–flip regime, where the spin–flip rate and the hyperfine frequency are comparable. The muon spin dynamics in the transition region has not been theoretically explored previously, mainly because normal muonium in vacuum gives no observable signal in this region. Third, in the case of shallow muonium states, the incoherent process defined to be those spin–flip collisions that cause changes in muon spin precession frequencies, becomes crucially important in the transition region, where the incoherent process is entirely negligible in more conventional muonium species. By taking incoherent multiple collisions into account, an analytical expression for the time evolution of the muon spin polarization in Mu is derived, where Mu undergoes repeated spin–flip collisions. Comparisons with Monte Carlo calculations show that the analytical expression obtained in this work can reliably be used to analyse experimental data for shallow donor states not only in the slow spin–flip regime, but also in the transition region up to the onset of the fast regime. The present work confirms a recent experimental finding that, in the transition region, the initial phases of the two precession components of shallow donor states exhibit a strong and unexpected dependence on the spin–flip rate. Such a rate-dependent initial phase is expected to be widely observed in Mu species with hyperfine frequencies less than several tens of megahertz. In order to understand spin dynamics in the transition region in a wider and more general context, Monte Carlo calculations are also carried out for more conventional muonium species with large hyperfine frequencies in which no observable signal can be seen in a transverse field in the transition region and in a large part of the fast regime.

Differential cross sections (DCSs) for electron-impact excitation of the 4f¹⁴6s6p ¹P₁ resonance state of ytterbium were measured at 10, 20, 40, 60 and 80 eV incident electron energies (E₀) and scattering angles (θ) between 2° and 150°. The absolute DCS scale for the ¹P₁ state was determined through normalization of its relative DCSs to the optical oscillator strength using the forward scattering function method. DCSs for elastic electron scattering by ytterbium were measured at the same energies for θ between 10° and 150°. Elastic-to-inelastic intensity ratios were obtained from energy-loss spectra recorded with overall energy resolution of 65 meV (FWHM) and angular resolution of 1.5°. Both inelastic and elastic DCSs were extrapolated to 0° and 180° and numerically integrated to yield integral, momentum transfer and viscosity cross sections. Our results are compared with scarce experimental and theoretical data.

We have developed a multi-coincidence technique to study MCI–molecule collisions. Using this technique, complicated reaction processes and collision dynamics in Kr⁸⁺ + N₂ collisions have been successfully revealed in the energy region below 200 eV/u. The reaction processes in the single-, double-, triple- and quadruple-charge changing collisions are resolved and three low-energy collision phenomena, 'peak-shifting' of molecular ions, 'peak-splitting' and 'anisotropic fragmentation' of fragment ion pairs, are found in the time-of-flight spectra of target ions. By careful analysis of kinematics of all the products, it is concluded that the charge transfer in the Kr⁸⁺ + N₂ collisions is dominated by multi-electron capture processes followed by electron emission and the collision dynamics is characterized by the transverse momentum transfer from the ion to the molecule and the appearance of the 'anisotropic fragmentation'.

Dipole selection rules and electron–electron correlation effects in double ionization of He by 6 MeV proton impact were studied by analysing the correlation function differential in the polar emission angles of both electrons. In the data features are observed with a level of detail which is comparable to fully differential cross sections. Therefore, this method opens unexpected possibilities of studying effects due to dipole selection rules and electron–electron correlations in unprecedented detail.

The triple differential cross sections (TDCSs) of electron-impact ionization for ions (Li⁺ (1s²) and K⁺ (3p⁶)) and atoms (He (1s²) and Ar (3p⁶)) have been calculated at coplanar asymmetric geometry using the distorted wave Born approximation method. The features of binary peaks in the TDCSs of ions are compared with those of corresponding atoms. Differences of positions of binary peaks between ions and atoms indicate that the usual single-binary encounter mechanism, which is a good description of the electron collision with atoms, cannot describe the electron collision with ions well. A double-binary encounter mechanism is proposed to describe the electron collision with ions.

We construct an optical dipole trap using a Fabry–Perot interferometer as a power buildup cavity. Large power enhancement allows us to produce 9 mK deep potential wells with 48 µm spot size and 100 nm detuning from the rubidium D1 resonance. The optical trap is loaded from a dark-spot magneto-optical trap which, in turn, is loaded from a low-velocity intense source of ⁸⁵Rb atoms. Under typical experimental conditions, there are 1.4 × 10⁶ atoms in 2000 antinodes of the optical trap. Average atom density is 1.1 × 10¹² cm^-3. The number of trapped atoms is limited by the atom density, or trapping volume. The standing-wave configuration with tight longitudinal confinement has much smaller trapping volume compared with the equivalent travelling wave. A method for converting the trap to a travelling wave-like configuration using phase modulation is proposed.