Table of contents

The authors analyse the classical dynamics of near-collinear electron configurations in helium. The dynamics turns out to be fully chaotic. An application of periodic orbit quantization techniques yields the energy of doubly excited states with high accuracy. The analysis shows that near-collinear intra-shell resonances are associated with an asymmetric stretch like motion of the electron pair rather than the symmetric stretch motion along the Wannier ridge.

Cross sections have been calculated for single ionization in collisions between protons and helium atoms, in the range of projectile energies 25-100 keV. The semiclassical impact parameter method was employed, with a basis comprising 22 states on the (He) target and 29 states on the projectile. Good agreement is obtained with the measurements of Shah et al. (1989). The inclusion of pseudo-continuum states on the helium centre is found to significantly modify the total cross section for capture into the bound states of hydrogen, leading to improved agreement with the measurements.

The authors have calculated the cross sections for the production of (2lnl'), with n=3, 4 and 5, and (3l3l') states by double electron capture in C⁶⁺-He collisions at low impact energies. Their values for these cross sections are very close to the experimental data. The present cross sections for the series (2lnl') follow a n^-3 scaling law at intermediate impact velocities (v approximately=0.4 au) as observed in electron spectroscopy experiments.

Differential, integral and momentum transfer cross sections have been obtained, using an independent-atom model along with partial waves, for the elastic scattering of electrons by the carbon tetrafluoride molecule over an incident energy range 100-700 eV. The present method yields more structure in DCS at large angles and energies than obtained experimentally.

The authors present a method for using basis functions which do not satisfy the boundary conditions for confined systems in which the boundary surface is of arbitrary shape and complexity. They show that the existence of a variational solution which does satisfy the boundary conditions is possible only if the basis set is linearly dependent on the boundary surface. This criterion also suggests two methods of approximating solutions when this condition is not satisfied. The variational solutions are shown to be the orthogonal set of eigenfunctions of a projected Hamiltonian. The projected subspace is the set of null eigenvectors of the boundary surface overlap matrix. The method is applied to the problem of a hydrogen atom in an infinitely strong spherical box displaced from the centre of the well. Results are compared with those of previous authors.

A method is presented for adapting scattering calculations performed with the molecular R-matrix method to find bound states based on the atomic method of Seaton. Quantum defect theory is used to determine initial energy grids and to determine whether all the bound states have been located. This method is particularly suited to the Rydberg states of electron plus molecular ion systems. The authors calculate and assign the lowest 33 electronic states of the HeH molecule. Previously on 14 of the lowest (n<5) bound states have been fully characterized, with several states omitted. They suggest that the omitted states give rise to some of the observed but previously unexplained weak transitions. Vibrational motion is included in their calculations within the adiabatic approximations. Effects arising from short-range correlations and nuclear motion are shown to be very significant for the lowest electronic states. Transition energies amongst the excited states agree with accurate spectroscopic determinations to better than 50 cm.

The spectrum of iodine was photographed in the 400-2000 AA wavelength region on 3 m, 6.5 m and 10.7 m normal incidence spectrographs using triggered spark and sliding spark sources. On the basis of these observations all levels of the 5s²5p², 5s5p³ and 5s²5p6s configurations and all but one level (J=4) of the 5s²5p5d configuration in I IV have been established. An earlier analysis has been found completely incorrect. Multiconfiguration interaction calculations were used to interpret and confirm the analysis; 74 lines have been classified in this spectrum.

The non-resonance contributions to the self-broadening of the helium singlet lines 2¹S-n¹P, n=3, 4, 5; 2¹P-n¹S, n=3, 4, 5, 6 and 2¹P-n¹D, n=3, 4, 5 are calculated within a semiclassical impact model of pressure broadening. The S-matrix elements for the ¹S, ¹P and ¹D levels are determined from the first-order coupled equations of time-dependent perturbation theory in which the collision between the excited helium emitter and ground state helium perturber is described in terms of adiabatic molecular potentials based upon model potential expressions for the interacting atoms. If the resonance and non-resonance contributions are assumed to be additive, the agreement between theory and experiment is generally better than 30%. However for transitions involving the highly excited states 5¹S, 6¹S and 5¹D the theoretical results are higher than experiment by up to 50%.

Alignment of M-subshell vacancy states after photoionization in Pb by 5.959 keV photons has been reported. The present results contradict the predictions of Cooper and Zare (1969) that, after photoionization of inner shells, the vacancy state has equal population of magnetic substates and the subsequent X-ray emission is isotropic, but confirm the predictions of the calculations of Flugge et al. (1972) that the atomic inner-shell vacancies produced after photoionization are aligned and the X-ray emission from the filling of vacancies in states with J>1/2 is anisotropic.

This paper presents the results of studies into the spatial behaviour of the gradient force acting on a three-level Lambda -atom in a two-frequency standing light wave. The effect of spatially inhomogeneous optical pumping and light-induced level shifts results in the wavelength-averaged force becoming other than zero. The magnitude of the rectified force is comparable with that of the gradient force in a single-frequency standing light wave.

A general and computationally efficient (analytical) approach evaluating the transition probabilities for vibration-translation (VT) exchange, involving higher vibrational states, in diatomic gases is presented. The role of the slope of the repulsive part of the intermolecular potential on the transition probability is also investigated. Examples of numerical results are given for a number of transitions in nitrogen and oxygen molecules in the ground electronic states. The approach can be used for VT exchange involving diatomic molecular ions.

The authors present evidence that doubly differential distributions of electrons produced by ion-atom ionizing collisions of intermediate energies do not show a concentration in the sense of a dominant emission centred at the velocity of the saddle point of the potential extended between the two Coulomb centres, as given by the projectile ion and the residual target ion. This evidence is sustained by measurements performed with protons of 52 and 103 keV interacting with He. Their electron distributions are represented in terms of the doubly differential cross section given by the number of electrons per unit volume in velocity space, that is not affected by transformation into different reference frames. The authors do, however, not deny the important role that the two-centre saddle potential plays in the ionization mechanism. This leads to an enhanced forward emission which, however, is distributed in the velocity interval between zero and the projectile velocity.

A new interpretation is given to experiments on double-atom excitation in He-He collisions that were focused on the relative population and coherence of atomic substate combinations. It is shown that a proposed two-state propensity rule may describe basic characteristics of the measurements. More generally by choosing the quantization axis perpendicular to the beam axis it can be understood why the two-state propensity model is successful. It also turns out that the diagonal elements of the new density matrix are independent of the kinetic energy. From experimental excitation parameters the angular part of the atomic charge density for double-atom excitation is viewed by fixing the direction of one of the excited electrons and showing the angular charge distribution of the other. The shape of the atoms changes dramatically with the collision energy, especially if the direction of the first electron is 'frozen' in the direction of the initial relative velocity.

Ejected electron spectra from the doubly-excited helium-like ions produced by the double electron capture collisions of bare ions B⁵⁺ and C⁶⁺ with He have been measured using zero-degree Auger electron spectroscopy with a high energy resolution. Electrons from the configuration of (2lnl') have been observed for n>or=2 in B⁵⁺-He collisions and n>or=3 in C⁶⁺-He collisions. Experimental energy values of ejected electrons from the (2l2l') and (2l3l') states of B³⁺ ions and the (2l3l') states of C⁴⁺ ions are determined by comparing the observed spectra with the theoretical calculations. Spectral features have also been observed at different collision energies. The population mechanisms to those states are briefly discussed.

The presence of the magnetic quadrupole term in addition to the electric dipole one in the multipole expansion of the radiation field is reflected in the angular distribution of the X-ray transition. To extend the previous works the authors studied the angular distribution of different L₃ X-ray lines of Ba, Sm and Er experimentally for proton impact at 0.23, 0.28 and 0.35 MeV bombarding energies respectively, using a Si(Li) detector. The anisotropy parameter ratios of the Ll and L alpha transitions were found to be 5.2+or-2.8, 5.5+or-0.7 and 6.9+or-1.5. Angular distributions of Tb L-shell X-rays were also measured by a high resolution crystal spectrometer for 0.556 MeV amu^-1 nitrogen ions, a value of 3.5+or-2.5 was obtained for the anisotropy parameter ratio. These ratios deviate significantly from the theoretical ones.

An investigation of the 'Hornbeck-Molnar ionization' process between laser-excited Cs and ground state Na atoms has been performed in a crossed beam apparatus. Caesium atoms in the 8d, 10s, 9d, 11s and 10d levels respectively have been obtained by two-step resonant CW laser excitation. The relative collisional cross sections and the dissociation energy of the NaCs⁺ molecular ion have been obtained from quantitative analysis of the produced atomic and molecular ions. The experimental results are compared with the theoretical values calculated using a model developed by Janev and Mihajlov (1980).

A simple ionization model has been used to compute metastable beam populations for atomic ions formed in low density, high electron temperature ECR plasma type ion sources. Metastable fractions for each charge state of atomic carbon, nitrogen and oxygen have been evaluated. Computed metastable fractions are found to be in reasonable agreement with experimental data.

The role of the electron translational factors (ETF) in the semiclassical theory of charge transfer is addressed with respect to orientation effects and possible propensity rules in s-p and p-p capture processes. It is shown that orientation effects can be understood in the light of the intuitive classical 'rolling ball' and 'velocity matching' arguments and analysed using the Larsen-Taulbjerg first-order approximation to the ETF. Emphasis is placed on investigating resonant p-p transfer which is a crucial test case for the effects of ETF on orientation dependences. The predictions are exemplified by four-state close-coupling calculations for Na(3p_m)+Na⁺ to Na⁺+Na(3p'_m) resonant transfer taking full account of ETF. Some problems involved in comparing experimental data at small scattering angles with theoretical predictions are discussed.

An ab initio, parameter-free model interaction is computed for the H₂O molecule as a target in low-energy electron scattering processes. The static, exchange and polarization correlation forces are evaluated according to computational procedures developed earlier by the author and collaborators specifically for polyatomic targets. The single-centre description of the target wavefunction was obtained via a fairly extended expansion on a basis set of Slater orbitals and the coupled equations of the body-fixed collision dynamics were solved at several energies, from approximately=1 eV up to 20 eV. Angular distributions for total cross sections, rotationally elastic and inelastic cross sections are compared with the available data and found to be in good agreement with them. The behaviour of the various eigenphase sums is also examined and the possible existence of broad shape resonances is discussed.

The author investigates the eikonal theory of charged-particle potential scattering in the presence of a laser when the collision time is much smaller than the laser period while the translational velocity of the particle is much larger than its classical oscillatory velocity in the field. The author's detailed numerical calculations show that for small scattering angles, previous eikonal results of the Kroll-Watson type, based on a stationary phase approximation (SPA), are inaccurate for differential cross sections and become worse as one increases the range and/or the strength of the potential. The author replaces the SPA by a different approximation, ideally suited to small-angle scattering and obtain excellent agreement with the 'exact' numerical results.

Over the years there have been many theoretical studies of the electron-hydrogen-atom scattering problem including some recent highly sophisticated works. While agreement between experiment and theory is satisfactory for low energies, experiment and theory are not in accord in the intermediate energy range. For the case of perturbation series calculations, the feasibility of distorted-wave calculations exact to second order has been demonstrated. Previous calculations have not included second-order exchange and in this paper it is shown that this effect is the primary cause for the disagreement between experiment and theory. When second-order exchange is included, reliable results for elastic scattering are obtained for incident electron energies of 30 eV and higher, and for 2s and 2p excitation reliable results for quantities depending on magnet sublevel differential cross sections are obtained for 50 eV and higher. The existing experimental data indicate that there is still a problem with the phases of the 2p amplitudes for scattering angles greater than 70' at 54 eV.

The R-matrix method is used to calculate collision strengths for electron impact excitation of Cl IX from its ground state (²P⁰). Configuration interaction wavefunctions are used to represent twelve target states which are included in the calculations. Finally the effective collision strengths have been calculated for electron temperatures in the range (5*10³-10⁵) by assuming a Maxwellian electron velocity distribution for the incident electrons. This is the first detailed calculation on this ion in which the effects of exchange, channel couplings and short-range correlation effects are taken into account.

The relativistic polarized orbital theory of the elastic electron and positron scattering from closed-shell atoms is formulated in a similar way to that used in the non-relativistic case. The scattering equations involving the relativistic polarization potential for both types of projectile and also the exchange-polarization terms for the electron projectile are derived. The relativistic formula for the positron-atomic-electron annihilation coefficient Z_eff is presented.