Table of contents

Photoionization cross sections for both atomic helium and molecular hydrogen are calculated using a time-dependent close-coupling method. The total electronic wavefunction for the two-electron system is expanded in six dimensions, where four dimensions are represented on a radial and angular lattice and a coupled channels expansion is used to represent the other two dimensions. Double photoionization cross sections are obtained for both He and H₂ for a range of photon energies above the complete fragmentation threshold. Comparisons are made with absolute experimental measurements.

We study the dynamics of Bose–Einstein condensates in time-dependent microtraps for the purpose of understanding the influence of the mean field interaction on the performance of interferometers. We identify conditions where the nonlinearity due to atom interactions increases the sensitivity of interferometers to a phase shift. This feature is connected with the adiabatic generation of a dark soliton. We analyse the robustness of this phenomenon with respect to thermal fluctuations, due to excited near fields in an electromagnetic surface trap.

The vibrationally resolved partial ionization cross sections of the X, A and B states of N₂⁺ in the photon energy range ∼20–35 eV have been characterized by photoelectron spectroscopy. Vibrational intensity ratios of the X, A and B states have been determined. The vibrational intensity ratios of the X and B states show a wealth of both sharp and broad features. In accordance with previous experimental and theoretical studies, these structures have been assigned either to one-electron processes, like the interchannel coupling between the 3σ_g and 2σ_u ionization channels and the autoionization of excited Rydberg states, or to two-electron processes, like the recently observed non-Rydberg doubly excited resonances (NRDER). The present study provides direct information on the autoionization of the NRDER states and the experimental evidence of their predicted, but not yet verified, selective decay to the X, A and B states of N₂⁺.

We have carried out a 129 close-coupling level Dirac–Coulomb R-matrix calculation for the electron-impact excitation of Ni-like Xe. We have utilized this data to generate the spectral signature of Xe²⁶⁺ in terms of feature photon-emissivity coefficients . We have compared these with those generated using semi-relativistic plane-wave Born excitation data, which forms the heavy species baseline for the Atomic Data and Analysis Structure (ADAS). We find that the Born-based give a reasonable qualitative description of the spectral signature but that, quantitatively, the R-matrix-based differ by up to a factor of 2. The spectral signature of heavy species is key to diagnosing hot plasmas such as will be found in the International Thermonuclear Experimental Reactor.

In this paper, we study the implementation of the Berry approach as expressed in his seminal publication (1984 Proc. R. Soc. Lond. A 392 45) within Born–Oppenheimer molecular systems (1927 Ann. Phys., Lpz.84 457). This was done with the purpose of revealing the relation between the various time-dependent magnitudes in the adiabatic limit and their time-independent counterparts. The two main results are: (1) in the adiabatic limit, a single Born–Oppenheimer state becomes an eigenstate of the system during the excursion of a system along a given contour, and (2) at the end of a closed contour, the topological phases associated with the states that moved adiabatically along a closed contour are well presented by the diagonal D-matrix as obtained from the time-independent treatment. The theoretical study is supported by a detailed numerical study carried out for two ab initio molecular systems, namely the (Na, H₂) and the (H, H₂) systems.

The accuracy of accounting for the Breit effects in calculations of atoms and molecules in the framework of the two-component relativistic effective core potential (RECP) formalism is analysed theoretically and illustrated in calculations. Contributions of the Breit interaction between different core and valence shells are studied in the framework of the four-component SCF approximation for uranium and plutonium. It is shown that two-electron Breit effects between valence electrons can be neglected for the calculation of valence energies in systems containing actinides with 'chemical accuracy' (1 kcal mol⁻¹ or 350 cm⁻¹), whereas large core–core and core–valence Breit contributions can be efficiently described by one-electron RECP operators. Different versions of generalized RECPs with the Breit interaction taken into account are constructed for uranium and plutonium and compared for accuracy with the corresponding all-electron four-component calculations.

Recent measurements of the elastic differential cross section in the backward direction in electron krypton scattering just above the inelastic threshold of 4p⁵5s J = 2 state at 9.915 eV illustrated the importance of including absorption effects in any theoretical calculation. Subsequently, there have been theoretical predictions of a substantial reduction in the backward direction of the elastic differential cross section in electron krypton scattering in the intermediate energy range up to 100 eV when absorption is included. In this paper we present the first measurement of the elastic differential cross section for krypton in the backward direction in this intermediate energy range; these measurements are generally in good agreement with the recent theoretical predictions.

Disorientation and disalignment of neon excited atoms in the fine-structure 2p₂ level (in Paschen notation) of the 2p⁵3p configuration have been investigated in a helium–neon glow discharge at temperatures between 17 and 300 K. The determined rate coefficients for disorientation, R_do, and disalignment, R_da, due to helium atom collisions show similar temperature dependences of T^1.6 below 77 K with the ratio about R_do:R_da = 2:1. This fact indicates that both the excitation transfer cross sections between the m_J = 1 and m_J = −1 states, σ_1–1, and between the m_J = ±1 and m_J = 0 states, σ₁₀, due to helium atom collisions have similar energy dependences of E^1.1 with the ratio about σ_1–1:σ₁₀ = 2.5:1 below 10 meV.

Wannier-type threshold theory predicts power dependence, σ ∼ E^μ, for the cross section σ of the break up of a quantum particle into several charged fragments (E is the excess energy above the fragmentation threshold). The threshold indices μ are calculated for the processes (electron impact ionization and photoionization) that lead to n = 6, 7 or 8 electrons receding from a charged core.

The photoabsorption spectrum of Kr-like Y³⁺ ions in the very soft x-ray/extreme ultraviolet spectral range where single (4s–np) and double (4p4p–nℓn'ℓ') electron excitations dominate has been measured using a dual laser plasma photoabsorption technique. The corresponding photoionization cross section has been computed within the framework of the configuration interaction Pauli–Fock approach. The calculations confirm that the 4s–5p resonance drops below the 4p ionization threshold so that the first member of the 4s–np photoionization resonance series becomes 4s–6p. Also, whereas the first such series member in Rb⁺ and Sr²⁺ appears as a window resonance, in Y³⁺ it switches to a normal, slightly asymmetric absorption peak. In addition, although the 4s–5p_1/2 resonances in the 4p⁵_1/2 and 4p⁵_3/2 continua are mirror images in Rb⁺ and Sr²⁺ and hence cancel each other, they exhibit quite different shapes in Y³⁺ and hence we observe the residual signature of the 4s–6p_1/2 resonance as a weak partner to the main 4s–6p_3/2 resonance.

Absolute differential cross sections have been measured for elastic electron scattering by molecular oxygen in the backward direction over the scattering-angle range 100°–180°, at selected incident energies in the range 7–20 eV. These measurements employed the magnetic angle-changing technique with a newly designed conical-solenoid source for the localized magnetic field. The measured differential cross sections are presented together with previous experimental work and the results of theoretical calculations. Integrated elastic and momentum transfer cross sections have been deduced using the measured differential cross sections for backward scattering together with existing differential cross sections for forward scattering.

We have applied the Breit–Pauli B-spline R-matrix method described earlier (J. Phys. B: At. Mol. Opt. Phys.37 (2004) 2173) to the treatment of e–Ar collisions. An individually optimized, term-dependent set of non-orthogonal valence orbitals is used to account for the strong term dependence in the one-electron orbitals. Concentrating on the near-threshold regime for excitation of the 3p⁵4s manifold, we obtain excellent agreement with benchmark experimental data for the production of metastable atoms and for excitation of the VUV-emitting J = 1 levels. A detailed partial-wave resolved resonance analysis shows that many of the structures listed by Buckman and Clark (Rev. Mod. Phys.66 (1994) 539) are composed of overlapping resonances. These, together with their principal components and decay channels, are identified and, in some cases, alternative classifications are suggested.

Total and partial photoionization cross sections of the Be-like C²⁺ ion leading to the C³⁺ 2l and 3l(l = 0, 1) states from the ¹S ground state have been calculated. The lower members of all autoionizing Rydberg series converging to the C³⁺ 2p, 3s, 3p, and 3d thresholds are identified and their energies and widths are determined. Except for the resonances converging to the C³⁺ 2p threshold, all of the resonance series are perturbed by overlapping series converging to other thresholds. In addition, the 4s4p ¹P resonance is found to lie just below the C³⁺ 3d threshold. The calculations show excellent agreement between length and velocity gauges. Comparison with experiment for the photoionization of a mixture of the ¹S ground state and ³P metastable states of C²⁺ in the photon energy range 40.8–56.9 eV shows good agreement with the present theoretical results; for the comparison, a calculation of photoionization of the metastable state was also performed. The present calculations are also compared with other previous theoretical and experimental results.