Table of contents

Analysis of the partial Fermi hole in the 2³S state of helium and the ground states of Li and Be shows that the radius of the Fermi hole increases with the distance of the reference electron from the nucleus, and that there exists a non-zero lower limit for the radius. The limit increases in a regular manner for the two-, and three- and four-electron isoelectronic sequences. The radius of the Fermi hole in the ground states of the Li and Be isoelectronic sequences varies with the atomic number but is independent of the number of electrons. The latter observations stands in contrast to the differences in the sizes of the electron clouds of each isonuclear pair.

The relation between the off-shell Coulomb wavefunction in the momentum representation and the off-shell Coulomb T matrix, derived from standard formulae in quantum scattering theory, is investigated. The on-shell singularities of these quantities are examined and it is shown how they may be regularised in a consistent way by modifying the standard formulae, to take account of the long-range nature of the Coulomb potential in terms of distorted plane and spherical waves in the momentum representation.

The positron-hydrogen-molecule scattering processes have been investigated using the lab-frame rovibrational close-coupling approximation, retaining two vibrational ( nu =0, 1) and three rotational (j=0, 2, 4) states in the coupling scheme. The effect of higher vibrational states is found to be marginal. The results for the total integrated, pure vibrational and rotational excitation cross section have been reported in the energy range 0.6 to 6.0 eV. The effect of including the vibrational states enhances the total cross sections. The present results for the total integrated cross sections favour the recent measured values of Charlton et al. (1983).

Energy distributions of electrons ejected in collisions of He metastable atoms with He metastable atoms have been measured. The ratio of the ionisation cross sections of He atoms for He(2³S)-He(2³S), He(2¹S)-He(2³S) and He(2¹S)-He(2¹S) systems has been estimated to be 2:1:8.

In configurations containing open s subshells, far-configuration mixing ('crossed second-order effects') leads to different electron change densities at the nucleus for the various Russell-Saunders terms and thus to different field isotope shifts. For nd^N(n+1)s (n+1)p configurations (n=3, 4, 5; N=1-10) the electron charge densities D of pairs of terms belonging to parents with the same multiplicity calculated using the Hartree-Fock computer code of Froese Fischer (1978). Usually, the term with the higher multiplicity has the larger density. The density difference Delta D increases linearly with the number of d electrons up to the half-filled shell and then decreases. The terms belonging to the parent with the higher multiplicity show larger values of Delta D than those belonging to the parent with the lower multiplicity. The effect increases with the principal quantum number n; it depends on the spins of the parent and grandparent terms but not on their angular momenta. The Hartree-Fock results of Delta D were used to calculate field shift differences between the corresponding terms. The comparisons of calculated and experimental field shift differences shows that the non-relativistic Hartree-Fock method correctly predicts their sign and order of magnitude.

The hole formalism has been used to elucidate exchange correlation effects in the ²S state of the lithium isoelectronic series. This approach provides a very simple physical picture of the effects of electron correlation from the point of view of the reference electron. In particular the calculations demonstrate that the probability of finding any two electrons with the same spin is reduced at both small and large interelectronic distances with a concomitant increase of the probability of their being at intermediate distances. Also a general tendency for the radius of the Fermi hole to increase as a function of r₁ has been shown together with the existence of a non-zero limit. Finally the authors have found that the polarisation of the Fermi hole is spherically symmetric with respect to the nucleus and has very little dependence on the distance of the reference electron from the nucleus.

Calculations are reported of lifetimes for ¹S Rydberg states in Ca I based on previously published MCHF wavefunctions for the ¹S states. To obtain these lifetimes, MCHF calculations for ¹P Rydberg states have been carried out and are reported. The question of the identification of the 3d4p ¹P perturber in the 4snp ¹P series is discussed and f values are reported for seven 4s²¹S to ¹P transitions. The calculations suggest a new view of the problem concerning the whereabouts of the perturber. Comparisons are made with the results of MQDT analysis.

Configuration interaction wavefunctions are used to calculate the wavelength and oscillator strength (length and velocity forms) for the 4s²¹S to 4s4p ¹P^o resonance transition in calcium. Good agreement between the velocity form and experiment is obtained. However, the length form overestimates the oscillator strength by approximately 10%. The dominant interactions which influence the transition under study are s² to d² and s² to ss' in the initial state and sp to dp and sp to s'p in the final state. The importance of including the non-fine-structure relativistic terms as well as the usual non-relativistic terms in the Hamiltonian in order to determine the configuration mixing is investigated. The importance of including core-valence correlation effects in the initial- and final-state wavefunctions is also investigated.

Spectral lineshape narrowing of atomic vapour emission in a bufferless, wall-coated absorption cell has been analysed using the same formalism developed to describe sub-Doppler linewidths, Dicke narrowing, in a buffered cell. Within this formalism lineshapes are generated using Monte Carlo techniques. Dramatic linewidth reductions in the bufferless cell, compared with the buffered cell, occur because of the high probability that Doppler shifts before and after a particular wall collision will cancel each other. The formalism described allows more physical insight than the classic one-dimensional box model previously invoked to explain spectra narrowing in bufferless, wall-coated cells. Additionally, this formalism reveals a correlation between post-collision velocities and intercollision times in the buffered cell resulting in modifications of the standard Dicke analysis of these cells.

Relative partial photoionisation cross sections and the photoelectron angular distribution parameter beta have been measured for the 4d electrons of atomic silver in the energy range 14<or=h nu <or=140 eV. Data are found to be in good agreement with the results calculated in the relativistic random-phase approximation (RRPA) for the neighbouring atom palladium. Comparison with solid-state data reveals a dominant atomic character of the 4d electrons in metals. This is especially evident for the beta parameter. However, differences occurring around the Cooper minimum at 130 eV show the influence of the metallic state on the properties of the 4d electrons.

Following excitation to the B(1) ³ Pi _u// state of Na₂ by the 4880 and 4965 AA lines of an argon-ion laser, the collision-induced system (2) ¹ Sigma _g⁺-A(1) ¹ Sigma _u⁺ was recorded at high resolution by Fourier-transform spectrometry. The A(1) ¹ Sigma _u⁺ approximately b(1) ³ Pi _u interactions were studied in detail. The electronic part of the interaction matrix element (³ Pi _0,u mod H_so mod ¹ Sigma _u⁺) is independent of nu (b ³ Pi ) for the Li Li et al. (1984) numbering and is found to be 5.91+or-0.14 cm^-1.

The authors calculate the probability of nonadiabatic transitions in a multiple-crossing Landau-Zener model, in which a diabatic state mod 0) crosses a set of diabatic states mod k) (k=1, 2, . . ., N) with a constant velocity. It is shown that the probability P that the system stays in mod 0) after the multiple crossings under the condition it starts from mod 0) is exactly given by P=exp(-2 pi Sigma _k=1^N J_k²/h(cross) mod u mod ) irrespective of the configuration of mod k), where J_k is the off-diagonal coupling constant between mod 0) and mod k( and u is the velocity of the change of the energy differences.

A two-electron model of direct excitation in Li-Na collisions is developed, based upon one-electron eigenstates of the separated atoms. A valence-bond-type two-electron atomic basis, antisymmetrised and modified by the translational phase factors, is used in the expansion of the electronic scattering state. The time-dependent Schrodinger equation is solved by the impact parameter method assuming a rectilinear trajectory and constant relative velocity of the alkali cores. Solutions of the resulting close-coupled equations have been obtained in the high-energy limit which defines a model with complete neglect of exchange contributions. Various sets of basis functions, Li(2s/2p)Na(3s/3p), involving single and/or double (simultaneous) excitation of the alkali atoms have been used. The aim was to obtain first estimates of Li(2p) and Na(3p) excitation cross sections and polarisations as functions of collision energy and to probe the details of the direct-excitation mechanism. The single-excitation predictions match recent experimental cross sections at 50-60 keV centre-of-mass energy. The double-excitation contributions are significant but the present calculations tend to overemphasize their importance. Excitation out of the collision plane, an allowed two-electron excitation process, is important at centre-of-mass energies above 100 keV.

The absolute populations of C⁵⁺ ions emerging in excited states from carbon foil targets of thicknesses ranging from 10 to 125 atoms.AA² have been determined for incident beams of 24, 35 and 43 MeV C⁴⁺. For a given beam energy the population of 5⁺ ions emerging in the 2p state was found to follow the total 5+ charge fraction as the foil thickness was increased, suggesting that the cross section for excitation of ground-state electrons was larger than the cross section for electron capture by the bare nucleus. The authors found that in the cases studied, the 2p population formed (17+or-5)% of the total 5+ charge fraction. Populations for other excited states are also reported. The results are consistent with the size of the density effect previously observed in projectile charge-state distributions for the same collision system.

The angular variation of the asymmetry for the 1s-2s transition in the scattering of spin-polarised electrons (of 35, 54.4 and 100 eV energies) with spin-polarised hydrogen atoms is studied using a two-potential localised exchange approach. The results are compared with the other available calculations of Edmunds et al. (1983) and McDowell et al. (1984).

An R-matrix calculation for electron-helium scattering is described in which the eleven lowest target states of helium are included, using configuration interaction wavefunctions. Excitation cross sections are calculated up to 30 eV from the 1¹S ground state and the metastable 2³S and 2¹S states of helium to the higher n=2 states, and, for the first time in an ab initio calculation, to the n=3 states. The authors confirm their preliminary finding that the explicit coupling of the n=3 channels significantly reduces the cross sections calculated in an earlier five-state approximation, by up to 50% for n=1 to n=2 transitions at energies above the n=3 thresholds. Using the new cross section data, rate coefficients are calculated for temperatures up to 50000K for the transitions considered.

A series of successively more accurate wavefunctions (of the natural orbital form) for the helium-atom ground state (the best yielding 98.6% of the correlation energy) is used to analyse the 1200 eV non-coplanar symmetric (e, 2e) data for helium with the plane-wave impulse approximation. Particular attention is focused upon the determination of the ratio for populating the n=2 (2s and 2p) and 1s ions states. It is seen that the cross section ratio (at low recoil momentum) converges satisfactorily as additional target natural orbitals are utilised for the calculation of the overlap function. The convergence of the cross section ratio at high azimuthal angles is seen to be much slower.

Electron attachment cross sections are reported in the electron energy range 0-150 meV, and at energy resolutions of 5-8 meV (FWHM), for the molecules 2-C₄F₈ (perfluoro-2-butene), c-C₅F₈ (perfluorocyclopentene), c-C₆F₆ (perfluorobenzene) and C₇F₈ (perfluorotoluene). Use is made of the Kr photoionisation technique, and measured attachment lineshapes are converted to cross sections by normalisation through absolute attachment rate constants. As in previous results on 2-C₄F₆, CFCl₃, CCl₄ and SF₆, the present four molecules exhibit resolution-limited onsets at threshold consistent with the s-wave nature of the attachment, and a neutral-negative-ion curve crossing at zero energy.

Distorted-wave calculations are reported for positron impact excitation of the helium atom. 1¹S-2¹S(2¹P) transitions are studied. Closed-form expressions for scattering amplitudes are obtained. Present results are compared with the earlier theoretical estimates.

The authors present classical trajectory (CT) calculations for the diffusion and viscosity coefficients in the temperature range 77.3-1100K using the HFD1 surface of Fuchs et al. (1984). They have also performed energy-sudden (ES) calculations for this surface and classical infinite-order sudden (MM) calculations for both this surface and their HFD2 surface. The authors CT results agree with the measurements of Kestin et al. (1979) for the viscosity coefficient over the entire range of temperatures reported (298-973K), and for the diffusion coefficient in the temperature range 298-763K. Their error limits are +or-1% for viscosity and +or-2% for diffusion. However the authors' values consistently exceed by about 2% the measurements of Trengove and Dunlop (1982) for diffusion in the temperature range 277-323K, where their error estimate is +or-0.2%. Comparison of the ES results with the CT and the MM results indicates that the ES and MM (IOS) approximations are reasonable for viscosity but less reliable for diffusion. The MM results for both diffusion and viscosity are generally consistent with the quantal infinite-order sudden results for both surfaces, which suggests that quantal effects for these coefficients are relatively small. From the MM results it may be inferred that the HFD1 surface is better than the HFD2 surface for these properties.