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The radiative charge exchange processes in H⁺ + H(1s) and He⁺(1s) + H(1s²) collisions at intermediate ion–atom impact velocities were treated in this paper from a spectroscopic aspect as new sources of UV and VUV emission. These processes were characterized by the cross-section spectral densities. In the case of hydrogen the corresponding spectral density was calculated for the wavelength λ and impact velocity v in the ranges 1.823 nm ⩽ λ ⩽ 217.537 nm and 0.141v₀ ⩽ v ⩽ 1.414v₀ where v₀ is the atomic unit velocity. Based on these calculations the photon fluxes, generated due to the interaction of weakly ionized low pressure hydrogen plasma with H⁺ ion beams, were estimated. It was shown that these fluxes in the UV and VUV domain were strong enough for the spectroscopic measurement. In the case of helium the photon fluxes were estimated in the range λ > 30 nm. It was found that they are smaller than those in the case of hydrogen but still at a substantial level.

Differential cross section (DCS) data for excitation of the 4 ¹P^o state (2.93 eV) of Ca atoms by electrons have been obtained using a crossed electron–atom beam technique. The measurements were divided into two groups. The first one consists of measurements at 10, 20, 40, 60 and 100 eV impact electron energies from 1° to 10° scattering angle interval. The second group of measurements are those at larger scattering angles (10°–150°) at the same energies except 100 eV. Absolute values have been obtained by normalizing generalized oscillator strengths (GOS) to optical oscillator strength (OOS) in the minimum momentum transfer squared (K²) limit. Integral, momentum transfer and viscosity cross sections have been determined from DCSs. A comparison is made with existing relativistic distorted wave calculation.

The production of anionic and cationic fragments of NO has been measured after photoexcitation in the vicinity of the nitrogen 1s threshold. The partial ion yield for the anionic fragment O⁻, in conjunction with the partial cation yields, can clearly distinguish the shape resonances from doubly excited states. On the N 1s → π* resonances, which are composed of the three intermediate states ²Δ, ²Σ⁻ and ²Σ⁺ in the order of increasing photon energy, the intensity ratios for the NO⁺⁺, O⁺ and O⁺⁺ fragments are very different from the total ion yield. An explanation for this behaviour in terms of symmetry is proposed.

Resonance structures in the photoionization of atomic phosphorus for the ejection of 3p or 3s electron from the ground 3s²3p³⁴S^o and excited metastable ²D^o and ²P^o states have been studied in the R-matrix approach. Photoionization cross sections and photoelectron angular distribution asymmetry parameters are dominated by Rydberg series of autoionization resonances converging to several ionic thresholds. There are window resonances due to the 3s3p³(⁵S^o)np, 3s3p³(³D^o)np and 3s3p³(³P^o)np Rydberg series in the inner-shell photoionization from the ground state. Autoionizing states are identified and analysed to obtain resonance positions E_r, effective quantum numbers n* and widths Γ_r using a procedure of eigenphase gradients. Our results for the 3s3p³(⁵S^o)np window resonances are in excellent agreement with the experimental values. Total and partial cross sections together with asymmetry parameter for leaving the ion in the ground 3s²3p²³P state have been calculated in both length and velocity formulations that exhibit very good agreement with each other.

Partial wave collision strengths are presented for low-energy electron-impact transitions in Fe II between the 3d⁶4s a⁶D^e ground state and the 3d⁷ a⁴F^e, 3d⁶4s a⁴D^e and 3d⁷ a⁴P^e low-lying excited states. The collision strengths are calculated in LS coupling using a new Fortran 95 R-matrix program including all terms of the 3d⁶4s, 3d⁷, 3d⁶4p, 3d⁵4s² and 3d⁵4s4p configurations in the close coupling expansion of the collision wavefunction. Special emphasis is given to the inclusion of configuration interaction (CI) effects both in the target and in the collision wavefunctions. In both cases series of calculations are carried out where additional CI terms are included systematically. It is found that in order to obtain close to converged low-energy partial wave collision strengths two-electron excitations from the 3p shell to the 3d shell as well as pseudo s and d orbitals must be included in the CI expansions. Also resonance effects in low-energy partial wave collision strengths are found to depend sensitively on the representation of the d orbitals in the CI expansion of the collision wavefunction. Finally, CI models for both the target and collision wavefunctions are defined which can be used in proposed calculations to obtain accurate total collision strengths and effective collision strengths for transitions between LS-coupled terms and between fine-structure levels of Fe II.

Highly energetic negative ions have been observed by irradiating water and heavy water micro droplets with ultrashort (35 fs) and highly intense ((0.8–1.2) × 10¹⁹ W cm⁻²) laser pulses. The creation of negative hydrogen (H⁻), deuterium (D⁻) as well as oxygen (O⁻) ions is suggested due to a process in which first their formation takes place in the hot plasma followed by an acceleration together with the positive ions. The negative ions are pulled by the energetic positive ions and propagate with them contributing to the quasineutrality of the beam. The observed phenomenon is discussed on the basis of possible formation and acceleration mechanisms of negative ions.

The behaviour of reactive scattering at ultracold temperatures is explored by calculating the real and imaginary parts of the scattering length for the reaction of F with a molecule composed of a pair of pseudo-hydrogen atoms of arbitrary mass. The origin of a low energy feature in the cross section for the reaction of F with H₂ and its absence for the reaction with D₂ is investigated. Close-coupling calculations of the scattering matrix show that the F–H₂ feature arises from the presence of a virtual state associated with the van der Waals well in the entrance channel and that the virtual state is responsible for the enhanced zero temperature rate coefficient of the F–H₂ reaction. For a mass of about 1.12 hydrogen masses the virtual state turns into a zero energy resonance and the corresponding zero temperature rate coefficient is 1 × 10⁻⁹ cm³ s⁻¹ despite an energy barrier of 300 K. Evidence in support of the virtual state is also provided by the detection of a deep Ramsauer–Townsend minimum in the elastic component of the total cross section for F–H₂ which the present calculations predict to occur at low energies.

We present a phase-space method for the Bose–Hubbard model based on the Q-function representation. In particular, we consider two model Hamiltonians in the mean-field approximation; the first is the standard 'one-site' model where quantum tunnelling is approximated entirely using mean-field terms; the second 'two-site' model explicitly includes tunnelling between two adjacent sites while treating tunnelling with other neighbouring sites using the mean-field approximation. The ground state is determined by minimizing the classical energy functional subject to quantum mechanical constraints, which take the form of uncertainty relations. For each model Hamiltonian, we compare the ground state results from the Q-function method with the exact numerical solution. The results from the Q-function method, which are easy to compute, give a good qualitative description of the main features of the Bose–Hubbard model including the superfluid to Mott insulator. We find the quantum mechanical constraints dominate the problem and show there are some limitations of the method particularly in the weak lattice regime.

The stability of a low-temperature Bose–Einstein condensate with attractive interactions in one- and three-dimensional double-well potentials is discussed. In particular, the tunnelling dynamics of a condensate under the influence of a time-dependent potential gradient is investigated. The condensate is shown to collapse at a critical potential gradient which corresponds to a critical number of atoms in one of the two wells. The sensitivity of this tunnelling-induced collapse could provide a useful tool in the study of condensates with attractive interactions.

Using a crossed-beams set-up, we have studied the electron energy spectra due to Penning and associative ionization in thermal energy collisions of state-selected metastable Ar*(4s ³P₂, ³P₀) and Kr*(5s ³P₀) atoms with ground-state Hg atoms at an electron energy resolution of 30 meV. In all three cases the energy spectra exhibit a prominent peak located close to the energy difference between the metastable excitation energies and the Hg ionization energy; they show a sharp drop to lower and an extended tail towards higher electron energy with shapes which differ for the three systems. The energy-integrated ionization cross section for Ar*(³P₀) exceeds that for Ar*(³P₂) by about 30%. Excitation transfer to autoionizing Hg** states, resulting in a sharp additional structure, is only observed—as a weak channel—in Ar*(³P₀) + Hg collisions. For the three investigated systems calculations of the electron energy spectra have been carried out within the local optical potential approximation. As input we use ab initio potential curves for the ionic exit channels as well as for the complex entrance channel potentials. Using these potentials (involving a scaled width for Kr* + Hg), impressive agreement with the experimental electron spectra is observed. In contrast, earlier empirical model potentials for the entrance channel are found to be inadequate. The calculated ionization rate coefficients at T = 300 K amount to (3.72, 5.26) × 10⁻¹⁰ cm³ s⁻¹ for Ar*(³P₂, ³P₀) + Hg, respectively.

The Laguerre-based convergent close coupling method is used to calculate the electron-impact excitation cross sections of the auto-ionizing states 2s2s¹S, 2s3s¹S and 2s3s³S, various ionization, as well as other cross sections for scattering from the 1¹S, 2¹S and 2³S initial states of helium. The calculations are done within the S-wave model. Unlike the case of bound-state excitation, convergence studies need to take into account occasional mixing between the core manifolds which can severely affect the structure of the auto-ionizing states and those corresponding to ionization with excitation. A method to take this into account is given and it is shown that convergent results may be obtained. Comparison with the 'Z = 2' time-dependent calculations of Pindzola et al (1999 Phys. Rev. A 59 4390–8) is satisfactory.

Cross sections for the multi-ionization of He and Li are presented for impact energies in the range of 50 to 1000 keV/amu. These are calculated using the eikonal initial state approximation to represent the input and exit channels of the active electrons. The ionization process is simulated in a variety of ways, most notably an attempt to account for the effects of electron correlation via the inclusion of a continuum density of states (CDS) term. Inadequacies, of the CDW formulation at small impact parameters, and of the models themselves, are discussed and conclusions are drawn on what repercussions this has for the cross sections calculated.

Photochemical formation of caesium hydride in a room temperature cell containing Cs vapour and H₂ under illumination of the mixture by laser radiation tuned to the D₂ atomic resonance line of Cs is reported. The chemical reaction is accompanied by dramatic changes in magnitude of the resonant absorption and fluorescence of Cs atoms in H₂ environment as compared with a pure Cs vapour case. A theoretical model, which takes into account all the optical and chemical processes involved at these densities, is developed. The effective rate of a two-step chemical reaction (1 ± 0.3) × 10⁻¹⁰ cm³ s⁻¹ is determined from the fitting of experimental results to this model, exploiting the laser intensity dependence of the fluorescence signal. Continuous (several hours) illumination of the cell by radiation tuned to the Cs D₂ line results in decrease of the hydrogen pressure in the cell, which is an additional proof for the formation of CsH molecules. The proposed investigation technique is applicable for a large variety of resonant photochemical reactions. Possible application for isotope separation is discussed.

Multiple photoionization of Xe near the 3d_5/2-shell threshold photoionization region is studied by threshold electron–ion coincidence spectroscopy. The coincidence spectra of Xe³⁺ to Xe⁷⁺ ions exhibit characteristic profiles associated with multi-step post-collision interactions in Auger cascades following 3d_5/2-shell threshold photoionization. The Auger cascade decay channels leading to the formation of multiply charged ions are deduced from the energies of the profile peaks, which increase gradually with increasing charge state. The formation of Xe³⁺ to Xe⁵⁺ ions is found to arise from cascades of normal Auger decays, whereas the formation of Xe⁶⁺ and Xe⁷⁺ ions involves double Auger decays. The branching ratio of double to normal Auger decays is estimated to be 0.25 (±0.1) for the decays following the creation of 3d_5/2-hole states in Xe.

The computation of first-order relativistic corrections to electrical response properties has been implemented into the Dalton program at the level of closed-shell coupled-cluster theory, within the framework of the direct perturbation theory (DPT) of relativistic effects. Calculations of the first-order relativistic DPT corrections to the static and frequency-dependent dipole polarizability (α) and second dipole hyperpolarizability (γ) of the Ne atom illustrate possible applications of the new code.

The closed form of the first order non-linear differential equation that is satisfied by the effective range within the variable phase formulation of scattering theory is discussed. It is shown that the conventional method of determining the effective range, by fitting a numerical solution of the Schrödinger equation to known asymptotic boundary conditions, can be modified to include the first order contribution of a long range interaction.

Semiempirical methods are used to characterize and parametrize the effects of intermediate coupling and configuration interaction on the energy levels of the 3s3p, 3p² and 3s3d configurations for ions in the Mg isoelectronic sequence. These parametrizations are then used to predict the branching fractions for the 3s3p–3p² and 3s3p–3s3d transition arrays. The predictions are compared with the MCHF calculations for S V and Fe XV, and good agreement is obtained. The application of this method to deduce transition probabilities and oscillator strengths from lifetime measurements is discussed.

In a recent paper, Brida et al (2002 J. Phys. B: At. Mol. Opt. Phys.35 4751) reported the first experimental realization for testing of de Broglie–Bohm (dBB) theory against standard quantum mechanics (SQM), based on a suggested scheme presented in Golshani and Akhavan (2001 J. Phys. A: Math. Gen.34 5259). However, some details of their experiment are not compatible with our suggestion. Therefore, their conclusion, in which the experimental data are in complete agreement with SQM but show some deviations from dBB predictions, cannot be considered as a final verdict.

We reply to Akhavan and Golshani's comment on our double-slit experiment addressed to test standard quantum mechanics versus de Broglie–Bohm theory. We show that their results, albeit representing a relevant contribution to the discussion on the realizability of a test of de Broglie–Bohm theory, do not allow us to draw any conclusion about our experiment since their wavefunction does not describe well the photon states produced by our set-up. On the other hand, we give arguments of why our experiment can be considered a good realization of the theoretical proposal of Ghose (1999 Foundations of Quantum Theory and Quantum Optics ed S M Roy), Ghose et al (2001 Phys. Lett. A 290 205) and Golshani and Akhavan (2001 J. Phys. A: Math. Gen.34 5259).