Table of contents

The photon-energy dependence of the oxygen-anion (O⁻) yield has been measured following photoexcitation of CO just above both the C and O K-shell ionization thresholds. The observed exponentially decaying yields above both edges are attributed to a post-collision interaction (PCI) between the emitted photoelectron and the CO molecule. Using a modified semi-classical atomic model, the near-threshold spectra can be reproduced if an energy shift is included. The shift in photon energy above the C K edge is greater than above the O K edge. The difference is attributed to a site-dependent PCI effect related to the polarity of the valence orbitals of the CO molecule.

Structures of vibrational origin were discovered in vibrationally inelastic electron–CO₂ cross sections in the energy range 0.4–0.9 eV, well below the ² Π _u shape resonance. They appear in the excitation of higher vibrational levels, in particular the highest members of the Fermi polyads of the type (n, 2m, 0) with n + m = 2–4. The lowest two structures, at 0.445 and 0.525 eV, are narrow; higher-lying structures are broader and boomerang-like. The structures are absent when the antisymmetric stretch is co-excited. The structures are interpreted in terms of a wavepacket of the nuclei reflected from a potential surface of the CO₂⁻ anion in a bent and stretched geometry. A state emerging from the virtual state upon bending and stretching and the state resulting from bending the ² Π _u shape resonance are discussed as possibly being responsible for the structures.

The theory of the interaction of the H₂⁺ molecular ion with an intense short laser pulse is modelled by solving the time-dependent Schrödinger equation for the electronic degree of freedom while the nuclear motion is described classically. This method allows us to discuss the influence of the pulse duration on the respective weights of ionization and dissociation.

We discuss selection rules due to discrete dynamical symmetries of the Schrödinger equation with an explicit time-dependent external interaction of finite duration. A symmetry property of the time development operator is derived. From this property we derive selection rules for final state populations applicable to non-perturbative laser–atom interactions within or beyond the dipole approximation. The symmetry relation also leads to selection rules for high harmonic generation.

We study the truncated Wigner method applied to a weakly interacting spinless Bose-condensed gas which is perturbed away from thermal equilibrium by a time-dependent external potential. The principle of the method is to generate an ensemble of classical fields ψ(r) which samples the Wigner quasi-distribution function of the initial thermal equilibrium density operator of the gas, and then to evolve each classical field with the Gross–Pitaevskii equation. In the first part of the paper we improve the sampling technique over our previous work (Sinatra et al2000 J. Mod. Opt.47 2629–44) and we test its accuracy against the exactly solvable model of the ideal Bose gas. In the second part of the paper we investigate the conditions of validity of the truncated Wigner method. For short evolution times it is known that the time-dependent Bogoliubov approximation is valid for almost pure condensates. The requirement that the truncated Wigner method reproduces the Bogoliubov prediction leads to the constraint that the number of field modes in the Wigner simulation must be smaller than the number of particles in the gas. For longer evolution times the nonlinear dynamics of the noncondensed modes of the field plays an important role. To demonstrate this we analyse the case of a three-dimensional spatially homogeneous Bose-condensed gas and we test the ability of the truncated Wigner method to correctly reproduce the Beliaev–Landau damping of an excitation of the condensate. We have identified the mechanism which limits the validity of the truncated Wigner method: the initial ensemble of classical fields, driven by the time-dependent Gross–Pitaevskii equation, thermalizes to a classical field distribution at a temperature T_class which is larger than the initial temperature T of the quantum gas. When T_class significantly exceeds T a spurious damping is observed in the Wigner simulation. This leads to the second validity condition for the truncated Wigner method, T_class − T ≪ T, which requires that the maximum energy _max of the Bogoliubov modes in the simulation does not exceed a few k_BT.

A fast method is presented for calculating the net emitted spectrum of a spherical sulphur plasma, from which the bulk of the radiation originates in the S₂ B³ Σ _u⁻ → X³ Σ _g⁻ molecular transition. The radiative transfer calculation needs as input the local spectral emission and absorption coefficients of the transitions. Calculations are presented using both a quantum-mechanical (QM) and a semiclassical (SC) method for generating these coefficients. It is found that the fine structure of QM coefficients, which is absent in the smooth SC data, may have a profound influence on the transport calculation. The circumstances where this occurs are identified and discussed. Measured spectra of the sulphur lamp are presented and compared with calculations, and a number of differences are found which may be due to an incorrectly modelled temperature profile or non-local thermal equilibrium (LTE) effects.

The absolute single-and double-photoionization cross sections of singly charged Fe ions have been measured from 15.8 to 180 eV using the merged-beam technique. The data yield information about the photoionization continua and the resonance structures resulting from excitation of the outer 3d and 4s electrons as well as the inner 3p and 3s electrons. The vast majority of the Fe⁺ target ions were present in the ground-state configuration, 3d⁶4s, and term,⁶D. The experimental data have been compared with several calculations, for example R-matrix calculations from the Opacity Project and data obtained using the central-field approximations. The experimental data are available at http://www.ifa.au.dk/amo/atomphys/atomphys.htm.

The electron impact single detachment process on Li⁻ targets was studied using the storage ring CRYRING located at the Manne Siegbahn Laboratory in Stockholm, Sweden. The Li⁻ ions, first stored in the storage ring, were merged with a cold, 1.4 cm in diameter, electron beam. The neutral Li atoms, originating from the process under scrutiny, were recorded by an energy-sensitive surface barrier detector in order to measure the relative electron single detachment cross section. The findings are the following. The cross section increases smoothly above the 1.4 eV detachment threshold and reaches a maximum at about 12 eV. At even larger energies a slow decrease, which follows the ln (E)/E energy dependence predicted by the Bethe–Born approximation, is observed.

Isotopic selectivity calculations are carried out for minor calcium isotopes against the major isotope ⁴⁰Ca for the single-resonance two-step and double-resonance three-step photoionization schemes with narrow-band lasers by using spectral simulation (SS) and modified spectrum (MS) approaches. The results of these calculations are compared with the density matrix (DM) results reported in the literature. It is noted that the values of isotopic selectivity from the SS approach do not agree with those from the DM approach whereas the MS approach, considering hole burning in the Doppler-broadened atomic spectrum, predicts selectivity values which are in good agreement with the DM results. It is argued that one can adequately use the simple MS approach rather than the complex DM approach for the calculation of isotopic selectivity of multi-step photoionization with single-frequency lasers.

The isomerization barrier between the two most stable C₃H isomers has been considered in this work as a test case for density functional theory (DFT) calculations. A wide range of exchange and correlation functionals are checked and their results are compared to very accurate state-of-the-art coupled-cluster benchmark calculations. Special care is devoted to basis set effects as well as to the role played separately by exchange and correlation energies. A few exchange–correlation functionals analysed here are able to approximate the exact result within the so-called 'chemical accuracy' (± 1 kcal mol⁻¹) and some of them even reach the most stringent 'calibration accuracy' (± 1 kJ mol⁻¹). However, a warning message is also given, since the wide spread of the results is large enough to prevent a general agreement between DFT and exact values.

We study the interaction of a three-level atom with a one-mode cavity field in the presence of a Kerr-like medium. The atom is initially prepared in a momentum eigenstate and the field in the squeezed state. We obtain the constants of motion and the wavefunction for the atomic system of Λ-configuration. Also, we calculate the momentum increment, the momentum diffusion and the field entropy. The analytic results are employed to perform some investigations of the temporal evolution of the momentum increment and the field entropy. The effect of the detuning and the Kerr-like medium is analysed. The results show that the effect of the detuning and/or the Kerr-like medium changes the quasi-period of both the momentum increment and the field entropy evolution and entanglement between the atom and the field. Conclusions and discussion are given and illustrated by numerical results.

The radiative recombination of a free electron into excited states of bare, high-Z ions and the subsequent photon decay are studied in the framework of the density matrix theory. Emphasis is placed, in particular, on the angular correlation between the recombination and the decay photons. The general expression for the photon–photon angular correlation function is derived, based on Dirac's equation as appropriate for high-Z ions. Computations for the dependence of the photon–photon correlation function on the nuclear charge and the projectile energies are carried out for the capture into the 2p_3/2 level and the subsequent Lyman-α₁ (2p_3/2 → 1s_1/2) radiation.

Electron-impact excitation collision strengths for transitions among doubly excited levels up to the n = 3 shell (excluding the 1s 3l3l' configurations) of lithium-like argon and iron have been calculated using a radiation- and Auger-damped, intermediate-coupling frame transformation, R-matrix approach. Collision strengths have also been calculated for transitions between all singly excited levels up to the n = 5 shell for the same systems.

The Maxwell-averaged effective collision strengths are estimated to be accurate to within 20% at temperatures 5 × 10⁴–5 × 10⁸ K for Ar¹⁵⁺ and 10⁵–10⁹ K for Fe²³⁺. These results are of substantially improved precision compared to previous studies.

The data relate to the analysis of soft x-ray helium-like spectra in both astrophysical and fusion thermal plasmas. We summarize the sensitivity to the new data of the spectral simulations which are matched to experiment in current spectral analysis procedures. Also, we present some brief results of modelling using the presented data.

This work reports numerical experiments intended to clarify the internal equilibration process in large molecules, following vibrational excitation. A model of an amorphous and oxygenated hydrocarbon macromolecule (∼ 500 atoms)–simulating interstellar dust—is built up by means of a chemical simulation code. Its structure is optimized, and its normal modes determined. About 4.5 eV of potential energy is then deposited locally by perturbing one of the C–H peripheral bonds, thus simulating the capture of a free H atom by a dangling C bond. The ensuing relaxation of the system is followed for up to 300 ps, using a molecular mechanics code. When steady state is reached, spectra and time correlation functions of kinetic energy and bond length fluctuations indicate that most normal modes have been activated, but the motion remains quasi-periodic or regular. By contrast, when the molecule is violently excited or embedded in a thermal bath (modelled by Langevin dynamics), the same markers clearly depict chaotic motions. Thus it appears that even such a large system of oscillators is unable to provide the equivalent of a thermal bath to any one of these, barring strong resonances between some of them. This conclusion is of consequence for the interpretation of astronomical infrared spectra.

Collateral numerical experiments show that (a) relaxation times increase as perturbation energy decreases by spreading through the system; (b) energy deposited in the highest-frequency modes does not relax preferentially into the lowest-frequency modes but follows specific paths determined by near resonances and coupling strength; (c) energy deposited in the lowest-frequency modes is able to flow up the whole frequency ladder, albeit less easily than in the opposite direction.

We studied the ionization and fragmentation of CO₂ in collisions with 5.9 MeV u⁻¹ Xe¹⁸⁺ and Xe⁴³⁺ ions utilizing a position-and time-sensitive multi-particle detector. By coincident measurement of the momenta of correlated fragment ions the applied method yields a kinematically complete description of the fragmentation process. Of special interest are the 'Coulomb explosion' (CE) processes CO₂ − → C^p+ + O^q+ + O^r+ for which the released kinetic energy as well as angular correlations are determined. The measured angular spectra are—provided that the molecular vibrations of CO₂ are taken into account—in reasonable agreement with a simple CE model. This model is, however, insufficient to explain the observed energy distributions. Apparently the detailed electronic structure of the intermediate highly charged CO₂ ion plays an important role.

The mutual angular distributions of the two ejected electrons following direct photodouble ionization have been measured for D₂ at an excess energy (E) of 25 eV using linearly polarized light. These (γ, 2e) 'triple' differential cross sections (TDCSs) were obtained for asymmetric electron energy conditions with energy sharing ratios (R = E₂ /E₁) of R = 24, 11.5, 4 and 2.57. In all cases the 'reference' electron (energy = E₁) was oriented along the direction of the electric field vector () and detected in coincidence with a second electron (energy = E₂) coplanar with and the photon beam direction (k_γ). For comparison, helium TDCSs were obtained for the same E and R values under nearly identical spectrometer conditions. These show very good agreement with the results of hyperspherical-ℜ-matrix with semi-classical outgoing waves calculations, thus providing even more confidence in the D₂ TDCSs where there is as yet no accurate ab initio theory. The similarities and differences between the experimental results associated with the two targets are qualitatively discussed in terms of Feagin's model (Feagin J M 1998 J. Phys. B: At. Mol. Opt. Phys.31 L729).

Absolute total cross sections (TCSs) for electron scattering from two C₃H₄ isomers (allene and propyne) and from propane (C₃H₈) have been measured in a linear electron-beam transmission experiment for impact energy ranging from 0.5 to 370 eV. Low-energy TCS functions for C₃H₄ are dominated by prominent structures: a resonant-like enhancement of the cross section for allene peaks at around 2.3 and 3.4 eV for propyne, while very broad enhancement is centred at 9.5–10 and 8.0–8.5 eV for allene and propyne respectively. Some supplementary weak features are also discernible. The general shape of cross sections for both C₃H₄ isomers is similar except that the cross section enhancements are at different locations. In addition, the TCS for propyne has been compared with cross sections for other open-chain hydrocarbons with three carbon atoms, i.e. propene (C₃H₆) and propane (C₃H₈), and the effect of the multiplicity of the C–C bond on the low-energy scattering is demonstrated.

An error in the above paper has been brought to our attention. On page 5103 (line 3 from the bottom of the page), it states that: 'For example, the first excitation channel (n = 2) in positron-Li²⁺ scattering will not be open until the positron energy reaches an energy of about 85 eV and the rearrangement channels (positronium-formation) will only be open at an even higher energy.' The energy 'about 85 eV' is imprecise and should read '6.75 Ryd (~91.8 eV)'.