Table of contents

The formulation of the time-dependent close-coupling method is extended so that energy and angle differential cross sections for the double photoionization of helium may be obtained. The fully quantal method now yields absolute total integral, energy differential, and angle differential cross sections. A detailed comparison is made with the absolute synchrotron measurements of Bräuning et al (1998 J. Phys. B: At. Mol. Opt. Phys.31 5149-60) for triple differential cross sections at 20 eV excess photon energy. The agreement between theory and experiment is excellent.

In a search for quasi-bound states, or resonances, high-resolution measurements (ΔE~25 meV) of the total positron scattering cross sections have been carried out in the energy region of the first electronically excited states of H₂, N₂, CO and Ar. In the case of H₂, a recent calculation by Varella et al predicts such a resonance in the total elastic cross section near the excitation threshold for the B ¹Σ_u⁺ state. We find no experimental evidence for the existence of this resonance and also find similar negative results for the other atomic and molecular targets.

A hyperspherical L²-integrable basis set is presented for use in expanding the semiclassical time-dependent close-coupling equations. We discuss advantages of the method in the light of other conventional L²-integrable basis expansion methods and the previously implemented hyperspherical energy-discretization method. By means of the present basis set, electron emission cross sections of autoionizing states of helium due to energetic proton and anti-proton impact are evaluated for demonstration. We find contrasting behaviours of the electron emission cross sections in the He^**(N = 3) manifold due to proton impact and to anti-proton impact caused by the interference between the two final continuum states of He(1s) + e and He(2s) + e. An argument based on the sudden approximation accounts for the disappearance of the difference in the total electron emission cross sections.

The H₃⁺ is a key ion in the interstellar chemistry. The rate of recombination of H₃⁺ with electrons, α, plays a pivotal role in the level of ionization of interstellar plasmas. Over the past 50 years a number of studies of this recombination have been carried out, yet they did not lead to an unambiguous value of α. We report observation of the rate of de-ionization of H₃⁺ as low as α_eff ~ 3×10^-9 cm³ s^-1 at hydrogen density decreasing down to 5×10¹⁰ cm^-3. This implies that the dissociative recombination of H₃⁺ under interstellar conditions is very slow with α_DR<<10^-8 cm³ s^-1. This value is in agreement with theoretical predictions and astronomical observations, and provides an explanation of the `enigma of H₃⁺'.

It is shown that quadrupole matrix elements corresponding to the np→f transitions in Xe have strong maxima near ionization thresholds very similar to the well known maxima in the dipole nd→f transitions. The latter manifest themselves as giant resonances in the total photoionization cross section and appear due to the double-well shape of the effective potential for the f partial waves. The quadrupole resonances have the same origin because, as we show, the short range phase shift of the f partial waves as a function of photoelectron energy is increasing by π in both dipole and quadrupole resonances. The quadrupole resonances reveal themselves in strong deviations of angular distributions and spin polarization of photoelectrons near ionization thresholds from the predictions given by the electric dipole approximation.

At low energies, the major pathway for the electron impact dissociation of H₂ is through excitation to b ³Σ_u⁺. Ab initio calculations using the adiabatic nuclei, energy balance model of Stibbe and Tennyson (1998 New J. Phys. 1 2.1) of total cross sections, angular differential cross sections, energy differential cross sections and double differential cross sections for the electronic ground state initial vibrational v = 0 level, dissociating into continuum states are presented. The formal expressions needed for such calculations, which involve three fragments in the exit channel, are derived.

We observed quite different spectra for electromagnetically induced absorption (EIA) in two cases where the polarizations of the lasers was orthogonally linear and counter-rotating circular, respectively. By using the density matrix equation we were able to try to confirm that the difference between them may result from the spontaneous transfer process occurring between subsystems within the system. The experimental results agree qualitatively well with the simulated spectra in the Doppler-broadened system.

Absolute cross sections for electron impact ionization, dissociative excitation (DE) and dissociative ionization of N₂⁺ ions are measured in the energy range from threshold to 2500 eV. The animated crossed electron-ion beam method has been employed. The individual contributions of ionization products (N₂²⁺) and dissociation fragments (N⁺), which have both identical mass-to-charge ratio and average velocity, are deduced from the analysis of product velocity distributions. Particular attention was paid to determining the transmission efficiency for dissociation fragments, since their collection was incomplete during the measurements. Threshold energies and kinetic energy released to dissociation fragments are measured. The role of states contributing to different reactions is discussed. For DE, the present results are found to be much smaller than the results of Peterson et al (1998). For ionization (single and dissociative), a satisfactory agreement with their result is obtained as well as with the prediction of Kim et al (2000) obtained in the binary-encounter Bethe approximation.

We present a comparative picture of the (e, 3e) process on H^-, He and Li⁺. These three targets have isoelectronic structure with increasing nuclear Coulomb interaction. The study has been done at high incident energy, E₀≈5.6 keV, very small scattering angle and low ejected electron energies. The actual scattering angle is adjusted so that the momentum transfer and the energies of the ejected electrons are the same in all three cases. The result is that the final states are identical except for the charge on the bare nucleus. The value of the momentum transfer relative to the reciprocal of the atomic size is found to profoundly affect the angular distribution of the five-fold differential cross section.

With exact wavefunctions for a non-relativistic electron interacting with a two-mode intense photon field propagating in two perpendicular directions, obtained by solving a Schrödinger-like equation, we predict a previously unknown `spin-other-orbit' effect of photon modes. A multiphoton ionization experiment to test this effect is suggested. Transition rates which manifest this effect to be measured in the experiment are calculated and presented graphically. A detailed comparison between this effect and the well known spin-other-orbit effect of electrons is included.

It is demonstrated that ultrafast ionization of a surface layer of an originally transparent solid material, initialized by a moderate intense optical field, can be used to realize an ultrafast gate. The transition time generally is more than one order of magnitude faster than the driver pulse. It is shown that even driver pulses of the order of tens of femtoseconds are capable of switching pulses from the infrared to the vacuum ultraviolet on a sub-10 fs time scale. Thus, this is one of the fastest optical gates. It can be used for the measurement of the temporal dynamics of femtosecond pulses.

Multiple electron emission from argon by 1 MeV proton impact is studied using the CDW-EIS model. Knowledge of the contributions to the total emission from all the orbitals of the target atom and the inclusion of secondary processes like Auger transitions allow us to give a quantitative explanation of recent experiments where the electron emission spectra was taken in coincidence with the recoil-ion charge state.

The critical temperature and condensate fraction of a trapped interacting Bose gas are investigated when both atom-atom interaction and finite-size effects are taken into account. A canonical ensemble is used to obtain the equations on the condensate fraction for the trapped interacting Bose gas near and below the critical temperature. In our approaches corrections due to atom-atom interaction and finite-size effects are obtained simultaneously for the critical temperature and condensate fraction of the system. An analytical high-order correction to the condensate fraction is given.

Absolute total electron-impact ionization cross sections from threshold to 220 eV are reported for the formation of positive ions from a range of chlorocarbons (one to five carbon atoms), including all chlorine-substituted methanes and ethanes. Correlations between the measured ionization cross section, ionization potential and molecular polarizability volume are explored and compared with data for the perfluorocarbons and mixed halocarbons. A C-Cl bond additivity cross section determined previously for mixed halomethanes has been refined to fit the experimental data for the higher chlorocarbons. Maximum cross sections predicted using bond additivity contributions are shown to be in agreement with experiment for a wide range of molecules to better than ±10% accuracy, and in most cases to better than ±5%. The experimental data are compared with the predictions of the Deutsch-Märk and binary-encounter Bethe models.

We report absolute differential electron scattering cross sections (DCSs) of molecular oxygen (O₂) at 20, 30, 50 and 100 eV impact energies (E₀). DCSs were measured for inelastic features in the 6.5-12.5 energy-loss range including the Schumann-Runge (SR) continuum, the longest band (LB) and the second band (2B) along with the summed DCS for the third band (3B) plus the 10.76 and 11.03 eV energy-loss features. Measurements were performed at scattering angles of 0°, 5°, 10°, 15°, 20° and 25° using a conventional electrostatic electron energy-loss spectrometer. Inelastic electron energy-loss spectra of O₂ were measured giving relative inelastic DCSs, which were then normalized via the relative flow technique using inelastic He and Ne transitions as standards. Past works (Trajmar et al 1972, Shyn et al 1994) have shown good relative agreement while displaying significant discrepancies in absolute terms. The current normalization technique represents an improvement over those employed in previous inelastic O₂ DCS work. As such, the present results serve to firmly establish the absolute normalization of these DCSs. Furthermore, the current results are able to provide an additional test of the relative angular distributions of previous works throughout the important low-angle region where the DCSs undergo the most rapid change. This paper also represents the first measurements of inelastic O₂ DCSs to extend right down to the forward-scattering limit.

Beam-foil spectra of fluorine-like potassium, K XI, have been recorded in the wavelength region between 30 and 700 Å. Eighteen spectral lines have been identified as transitions between levels in the 2p⁴3s, 3p, 3d and 4f configurations. Our identifications are supported by isoelectronic comparison and by theoretical interpretation of observed level values.

A three-body classical trajectory Monte Carlo method is used to study charge transfer between highly charged bare ions (Ne¹⁰⁺, Ar¹⁸⁺, Fe²⁶⁺, Kr³⁶⁺ and Xe⁵⁴⁺) and neutral hydrogen at collision energies between 1 eV amu^-1 and 100 keV amu^-1. The x-ray emission resulting from these reactions can be used as a diagnostic tool to study the charge transfer processes occurring in plasmas. For low-energy collisions (<100 eV amu^-1), the electrons are captured into states characterized by large principal quantum numbers and low angular momentum. A result of this non-statistical behaviour is the increase in Lyman series x-rays being emitted by captured electrons moving directly from high-n states to the K-shell. Calculated results for the enhancement of K-shell x-ray emission at low energies compare favourably with measurements made at the Lawrence Livermore National Laboratory using the electron beam ion traps EBIT-II and SuperEBIT.

The relative, coplanar angular distributions of electrons, produced in an electron-impact double ionization of helium (e,3e reaction), have been measured at 1.1 keV impact energy. The momentum transfer was 0.45 au and the two `ejected' electrons were detected with the same energy of 10 eV. The general features of the angular distributions are discussed. The data are analysed in different angular modes which allows a detailed comparison with state-of-the-art calculations. For high incident energy and small momentum transfer, as in the present case, the (e,3e) cross section can be related to the single-photon double ionization (PDI). We exploit this fact and compare the present findings with the PDI and identify the contribution of non-dipole effects.

The existence of the permanent electric dipole moment (EDM) of an atom implies the simultaneous violation of parity and time-reversal symmetries. Two experiments based on laser cooling have been proposed to search for the EDM of atomic ytterbium. We propose three different relativistic many-body approaches to calculate the EDM of atoms with strongly interacting configurations. Two of them are based on the configuration-interaction method and the third is an algebraic many-body perturbation theory that explicitly uses configuration state functions. We use these three approaches to calculate the EDM of atomic ytterbium arising from an electron-nucleus tensor-pseudotensor interaction. The results of these calculations are in excellent agreement with each other.

We carried out an exact density matrix analysis for the nonlinear Hanle effect in an open V-type level system. Steady-state analytical solutions of the Liouville equation have been obtained and the dependence of the populations of the upper and lower states on the applied magnetic field is discussed. These curves, corresponding to experimental ones when fluorescence or absorption is detected, are dependent on the excitation and decay rates and can be used as a tool for analysing population inversion in laser transitions.

NO dispersed VUV-fluorescence excitation (λ_fl = 134-152 nm) was measured as a function of the exciting-photon energy between 16.9 and 19.6 eV using monochromatized synchrotron radiation with a bandwidth of 6.5 meV. A two-dimensional fluorescence excitation spectrum was obtained, where the intensity is plotted as a function of both fluorescence wavelength and exciting-photon energy. Fluorescence from NO⁺ A ¹Π (v = 0,1,2) vibronic levels and fluorescence from excited dissociation fragments N I (3s ²P_J) was observed simultaneously. Fragment state selective predissociation and autoionization of the NO: (c ³Π)3pπ (v = 0) and (c ³Π)3pσ (v = 0) Rydberg states were observed simultaneously.

We systematically study the effect of the inclusion of the frequency-independent Breit interaction - treated perturbatively as well as variationally - on Dirac-Fock total energies and ⟨r^k⟩ expectation values for He- and Be-like ions using an algorithm recently described by Reiher and Hinze. Fully numerical, highly accurate solution methods are employed throughout for solving the Dirac-Fock-Coulomb-Breit (DFCB) equations. These methods allow us to investigate the change of the wavefunction upon inclusion of the Breit interaction in the self-consistent field procedure of an atomic structure program.

The dependence of expectation values on different finite-nucleus model (FNM) potentials for the electron-nucleus attraction is also studied. It is shown that, in general, the choice of the FNM for the electron-nucleus potential hardly affects the difference between Dirac-Fock-Coulomb (DFC) and DFCB expectation values. In the case of pointlike nuclei one should treat the Breit interaction variationally instead of perturbatively.

For energy eigenvalues, we find that the difference between the Breit interaction treated self-consistently or perturbatively is negligibly small - even for highly charged ions - if an extended nucleus model is used. The data given may serve as a reference for more approximate treatments involving, for example, basis-set approaches with limited basis-set size. In this extensive study of the SCF effect on the Breit interaction, highly accurate, variationally obtained DFCB total energies and ⟨r^k⟩ expectation values are given for different models of the electron-nucleus interaction.

The R-matrix method is used to calculate the elastic and the excitation cross sections from the ground state X ²Π to the four low-lying electronically excited states a ⁴Σ^-, A ²Δ, B ²Σ^- and C ²Σ⁺ of methylidyne (CH) radical. Configuration interaction (CI) wavefunctions are used to represent the target states. In our CI model we keep the 1σ orbital doubly occupied and the remaining electrons are free to occupy the 2σ,3σ,4σ,1π,2π,3π and 1δ orbitals. This model gives an equilibrium bond length, R_e, of X ²Π state equal to 2.113 a₀ which is in excellent agreement with the experimental value of 2.116 a₀ and a CH equilibrium dipole moment of 1.53 D which is close to the experimental value of 1.46±0.06 D. Scattering calculations are performed in the static-exchange, static-exchange plus polarization, 5-state and 6-state models. Our best 6-state model also includes the D ²Π state. The vertical excitation energies lie in the range 0.32-7.29 eV and agree within three per cent of the experimental values. We find a bound state of CH^- of ³Σ^- symmetry with an electron affinity of 0.61 eV at R_e. Below 1 eV there are shape resonances in ¹Σ⁺ and ¹Δ symmetries. Both of these resonances have the configuration 3σ²1π². Born correction is applied for dipole-allowed transitions to account for higher partial waves excluded in the R-matrix calculation. Cross sections are given for scattering energies up to 10 eV.

The photoabsorption spectrum of a tin laser-produced plasma has been recorded in the 23-33 eV region using the dual laser plasma technique. 4d→5p transitions from the 4d¹⁰5s²5p ground state of Sn II and from the 4d¹⁰5s² ground and 4d¹⁰5s5p excited states of Sn III were observed and new features identified with the aid of multiconfiguration Hartree-Fock calculations. The relative intensities of transitions in Sn III indicate a departure from local thermodynamic equilibrium conditions in the absorbing plasma.

Electron-impact excitation collision strengths for transitions between all singly excited levels up to the n = 4 shell of helium-like argon and the n = 4 and 5 shells of helium-like iron have been calculated using a radiation-damped R-matrix approach. The theoretical collision strengths have been examined and associated with their infinite-energy limit values to allow the preparation of Maxwell-averaged effective collision strengths. These are conservatively considered to be accurate to within 20% at all temperatures, 3×10⁵-3×10⁸ K for Ar¹⁶⁺ and 10⁶-10⁹ K for Fe²⁴⁺. They have been compared with the results of previous studies, where possible, and we find a broad accord.

The corresponding rate coefficients are required for use in the calculation of derived, collisional-radiative, effective emission coefficients for helium-like lines for diagnostic application to fusion and astrophysical plasmas. The uncertainties in the fundamental collision data have been used to provide a critical assessment of the expected resultant uncertainties in such derived data, including redistributive and cascade collisional-radiative effects. The consequential uncertainties in the parts of the effective emission coefficients driven by excitation from the ground levels for the key w, x, y and z lines vary between 5% and 10%. Our results remove an uncertainty in the reaction rates of a key class of atomic processes governing the spectral emission of helium-like ions in plasmas.

Photo-double ionization (PDI) of helium at 40 eV above the threshold has been studied for unequal energy-sharing in the complementary kinematics obtained in two measurements by the interchange of the kinetic energies E₁↔E₂ of the two photoelectrons. The triple differential cross sections (TDCS) were measured in the plane perpendicular to the photon direction using the multicoincidence end-station of the gas-phase photoemission beam-line of the Elettra storage ring. The measured TDCSs were compared with previous experimental results using a practical parametrization proposed by Cvejanovic and Reddish (2000 J. Phys. B: At. Mol. Opt. Phys.33 4691) and with predictions of the 3C and convergent close-coupling calculations. Satisfactory agreement with the previous experimental data was found. The comparison with the two theoretical models shows that the TDCSs is the complementary kinematics still present a challenge for PDI theory.