Table of contents

We studied fully differential cross sections for single ionization of helium by 1 GeV/u U⁹²⁺ impact with emission of low-energy electrons (∼2–10 eV) in collisions with 'intermediate' momentum transfers (∼0.5–1 au). These collisions belong to a new collision regime (for which the basic dynamics of ion–atom collisions have never been explored before) where the impact velocity approaches the speed of light and the minimum momentum transfer is very small, but the ratio of the projectile charge to the collision velocity is of the order of unity. It is shown that higher order terms in the interaction between the projectile and the active target electron do not affect the ionization dynamics. In contrast, the interaction between the projectile and the target core has a profound impact on the shape of the fully differential cross section, changing the binary-to-recoil-peak ratio by two–three times.

We report on calculations of electron-impact broadening of Sr⁺ spectral lines in ultracold neutral plasmas. Optical absorption imaging of an ultracold Sr plasma has recently been used to infer detailed in situ information on the ion dynamics. We modify our recent treatment of collisional broadening in He metastable lines to the collision of electrons from Sr⁺ ions. The required scattering matrix elements are obtained from an ab initio R-matrix calculation.

We investigate the possibility of inducing a bosonic current which is rotational in a pseudo 1D quasi-condensate confined in an optical toroidal trap. The stability of such a current is also analysed using a hydrodynamics approach. We find that such a current is uniform when the circular symmetry is preserved and energetically stable when the modes of elementary excitations are restricted to one dimension. This scheme allows us to distinguish between a quasi and a true condensate by measuring the rotational spectra of the sample.

The competition between autoionization and neutral dissociation (ND) of the vibronic Rydberg states 2σ⁻¹_u(c⁴Σ⁻_u)(ns/nd)σ_g³Σ⁻_u(v = 0, 1) has been investigated using an experiment of photon-induced fluorescence spectroscopy (PIFS) and the simultaneous measurement of the photo ion yield. The experiments were performed with monochromatized synchrotron radiation from BESSY II with a very narrow bandwidth of ΔE = 1.5 meV at 23 eV where it was possible to observe single rotational branches. The experiments are compared to ab initio calculations for the partial autoionization and dissociation widths and to the corresponding simulations of the spectroscopic shape of the rovibronic branches. For more precise calculations of the dissociation widths, diabatic potential curves of the 2σ⁻¹_u(c⁴Σ⁻_u) states of the O⁺₂ are extracted from the adiabatic ones and improved calculations for molecular orbitals of the Rydberg and Auger electrons are used.

Even though spin-dependent effects are naturally associated with relativistic effects, it has been known for some time that significant spin asymmetries for electron-impact ionization are possible in a completely non-relativistic model if the J-state of the residual ion can be experimentally resolved. In the lowest order implementation of the same model, the spin asymmetry would vanish if the J-states of the ion are not experimentally resolved (i.e., summed over). Consequently, it is perhaps possible to search for relativistic effects by looking at asymmetries for which the final ion J-state is not resolved. There is also some experimental evidence that relativistic effects might be important for large scattering angles which are inaccessible to current experimental set-ups. If this is the case, relativistic effects might be seen in a double differential cross section measurement which integrates over all angles. Very recently, some significant experimental spin asymmetries for electron–xenon scattering have been reported for a doubly differential spin-asymmetry measurement in which the final J-state was not resolved. The purpose of this paper is to investigate whether or not these experimental results indicate relativistic effects.

We have theoretically investigated the focusing of a launched cloud of cold atoms. Time-dependent spatially-varying magnetic fields are used to impart impulses leading to a three-dimensional focus of the launched cloud. We discuss possible coil arrangements for a new focusing regime: isotropic 3D focusing of atoms with a single-impulse magnetic lens. We investigate focusing aberrations and find that, for typical experimental parameters, the widely used assumption of a purely harmonic lens is often inaccurate. The baseball lens offers the best possibility for isotropically focusing a cloud of weak-field-seeking atoms in 3D.

We present a general procedure, based on the Holstein–Herring method, for calculating exactly the leading term in the exponentially small exchange energy splitting between two asymptotically degenerate states of a diatomic molecule or molecular ion. The general formulae we have derived are shown to reduce correctly to the previously known exact results for the specific cases of the lowest Σ and Π states of H⁺₂. We then apply our general formulae to calculate the exchange energy splittings between the lowest states of the diatomic alkali cations K⁺₂, Rb⁺₂ and Cs⁺₂, which are isovalent to H⁺₂. Our results are found to be in very good agreement with the best available experimental data and ab initio calculations.

Interaction of a lossless two-level atom with a monochromatic (classical) field of radiation is considered, as the atom initially possesses a translational state with a number of equidistant and discrete momenta. It is shown that the Rabi oscillations in such an atom evolve as a sequence of collapses and revivals, if the coupling wave deeply saturates the optical transition. Between revivals, the populations undergo subrevivals. Approximate analytical formulae are obtained taking the initial momentum distribution in the form of two shifted Gaussians or a Besselian. A possible experimental realization of such revivals is discussed.

The adiabatic correction for the hydrogen–antihydrogen interaction in the ground state has been calculated using the Born–Handy method. The variational wavefunction was a linear combination of explicitly correlated Gaussian functions. It has been found that the correction grows rapidly for the nuclear separation approaching the critical distance. It is hypothesized that it becomes infinite at the point where the bound-state solution of the leptonic Schrödinger equation with fixed nuclei changes to a continuum wavefunction. This effect is nonphysical and means that there is no reasonable way to extend the adiabatic ground-state potential energy curve below the critical distance in a strict manner.

This paper presents the results of semi-classical calculations of rate coefficients of (n − n')-mixing processes in collisions of Rydberg atoms H*(n) with H(1s) atoms. These processes have been modelled by the mechanism of the resonant energy exchange within the electron component of the H*(n) + H collisional system. The calculations of the rate coefficients, based on this model, were performed for the series of principal quantum numbers, n and n', and atomic, T_a, and electronic, T_e, temperatures. It was shown that these processes can be of significant influence on the populations of Rydberg atoms in weakly ionized plasmas (ionization degree ⪅10⁻⁴), and therefore have to be included in appropriate models of such plasmas.

We have studied the effect of post-collision interaction (PCI) on the Auger line shape after photoionization as a function of the emission angle relative to the primary photon beam and its polarization vector. For Ne K-LL Auger lines the photon energy was in the 900–1200 eV region, for Ar L-MM lines it was in the 320–440 eV region. Our calculations, which were made on the basis of the eikonal approach, were found to be in good agreement with our measurements at these energy ranges. The angular dependence of peak asymmetry is greater when the energy difference of the photoelectron and the Auger electron is smaller: for Ar at 440 eV photon energy it even changes the sign, i.e. the PCI line distortion disappears completely at a certain angle.

The energies and bound-state properties of the bi-muonic helium-3 and helium-4 atoms in their ground 1¹(S = 0)-states are determined to very high accuracy. It is shown that the lowest order QED (and relativistic) effects play a significantly larger role in the case of bi-muonic ³Heμ₂ and ⁴Heμ₂ atoms than in the two-electron He-atoms. In particular, the effect of vacuum polarization and corresponding energy shifts for the ground 1¹(S = 0)-states in the bi-muonic helium-3 and helium-4 atoms have been evaluated.

Au L_α and L_β and Ag L-shell x-ray production cross-sections by electron impact have been measured in the incident energy region from near threshold to about 25 keV. Thin films with thick aluminium substrates were used as targets in the experiments. The effect of directional and energy spreading of the electron beam within the active films and x-ray enhancement due to backscattering electrons and bremsstrahlung photons from the substrates are corrected by means of Monte Carlo simulations. The corrected experimental data provided by this method are compared with calculated cross-sections from a PWBA theory with Coulomb, relativistic and exchange corrections and with other experimental data available in the literature.

Electron capture and loss cross sections for U²⁸⁺ colliding with H₂, N₂ and Ar were measured at 3.5 and 6.5 MeV/u. These data were used to benchmark n-body calculations using the classical trajectory Monte Carlo method. The n-body calculations include electrons on both nuclear centres and all electron–electron and electron–nuclear interactions between each centre. For the U²⁸⁺ ion, 36 electrons were incorporated in the calculations (4s²4p⁶4d¹⁰4f¹⁴5s²5p²), while for the H, N and Ar targets all electrons were used except those for the K-shell of Ar, leading to 39-, 45- and 54-body calculations, respectively. Projectile electron loss was predicted for U²⁸⁺ at energies from 2 to 150 MeV/u. Only for the H-target did the projectile electron loss cross section decrease approximately as E⁻¹. The heavier targets exhibited slower energy dependences, contrary to the E⁻¹ prediction of one-electron theories. Moreover, the collisional interactions are quite strong with an average of 1.64 and 2.88 electrons removed from the U²⁸⁺ ion at 10 MeV/u in each collision with N and Ar, respectively. These data and calculations were used to assess the vacuum requirements for the SIS-100 synchrotron ring under construction at GSI-Darmstadt. For the residual gases expected to be in the ring, the U²⁸⁺ lifetime was found to be essentially constant as a function of projectile energy, leading to very stringent vacuum requirements.

Five-fold differential cross sections for electron-impact double ionization of the 3s electrons of magnesium have been calculated in the second Born approximation in the impulsive regime. Comparing these results with calculations carried out in the first Born approximation demonstrates the dominant contribution of the second Born term. The second Born calculation shows that the contribution of the two-step 2 (TS2) process becomes large under the condition where sequential binary collisions on the Bethe ridge can occur. The effect of electron correlation in the initial target state is also examined by using a configuration interaction wavefunction.

Collisions between tightly confined atoms can lead to ionization and hence to loss of atoms from the trap. We develop second-order perturbation theory for a tensorial perturbation of a spherically symmetric system and the theory is then applied to processes mediated by the spin–dipole interaction. Redistribution and loss mechanisms are studied for the case of spin-polarized metastable helium atoms and results obtained for the five lowest s states in the trap and trapping frequencies ranging from 1 kHz to 10 MHz.

We investigate theoretically the application of tailored incoherent far-infrared fields in combination with laser excitation of a single rovibrational transition for rotational cooling of translationally cold polar diatomic molecules. The cooling schemes are effective on a timescale shorter than typical unperturbed trapping times in ion traps and comparable to obtainable confinement times of neutral molecules.