Table of contents

The equations of the random-phase approximation (RPA), completely describing excitation energies and transition matrix elements for N-electron atomic systems, are reformulated in the language of Hartree-Fock theory, for single-particle orbitals. RPA is shown to be fully equivalent to the solution of a coupled Hartree-Fock-like problem, in the field of a (N-1)-electron system. By neglecting coupling effects, a convenient computational model of the literature is recovered.

The levels of an atomic multiplet can be exactly fitted by a model Hamiltonian comprising a set of scalar products of tensor operators. When this set is incomplete, a least-squares fit leads to errors Delta _i that do not exhibit a normal distribution. This analysis suggests that normal distributions cannot be expected when the levels of more complex atomic systems are being fitted.

A new absolute measurement of the cross section for the process Ne⁺+e to Ne²⁺+2e has been made at electron energies from threshold to 2 keV using the crossed electron and ion beams technique. The total error at a 90% confidence level is estimated to be +or-5%, increasing to +or-7% at energies greater than about 1 keV. The present cross section is about 6% greater than that measured by Dolder et al. (1963) near the peak, increasing to 11% at higher energies. The cross section of Muller et al. (1980) has the same shape as the present data from threshold to 400 eV, but is about 20% lower in magnitude. Above this energy it falls more rapidly with increasing energy and is more than 30% lower at 800 eV. The recent measurement of Achenbach et al. (1984) is in excellent agreement with the present data.

The bound-free transition form factor ( kappa mod exp(iq.r) mod nlm) is calculated directly in spherical coordinates for both hydrogenic and Slater-type orbitals. Using the exact Coulomb wave (r mod kappa ) associated with the field -Z/r, extremely concise analytical expressions are derived for the integral irrespective of either the triad nlm or of the quantisation axis. Results are given in terms of Appell hypergeometric polynomials F₁ of two variables.

The Laporte-Platt degeneracies for the spectroscopic terms of the configurations l^N are equivalent, when l=2, to the repeated eigenvalues found by diagonalising a delta-function interaction delta (r_i-r_j) for electrons i and j. For l>2 a limited generalisation is possible, since terms of maximum multiplicity remain degenerate while much of the degeneracy of the other terms is lost. This is put in the context of group theory, the techniques of which are used to account for the invariably null matrix elements of delta (r_h-r_i) delta (r_i-r_j). Configurations involving inequivalent electrons lead to generalisations of the Laporte-Platt degeneracies, and the use of the interaction Sigma delta (r_i-r_j) provides checks on the matrix elements of the Coulomb interaction.

High-lying odd-parity levels of neutral ytterbium are investigated by the method of selective three-step laser spectroscopy. Excitation from the ground state 4f¹⁴6s² is performed by means of three dye lasers and the Rydberg levels are detected by using the field ionisation method. Three different levels are used as starting levels of the transitions to Rydberg levels. Thus Rydberg levels pertaining to 6snp J=0-2 and 6snf J=2, 3 series as well as some 4f¹³5d²6s levels are observed. The new experimental data are analysed by means of the multichannel quantum defect theory. The theoretical analysis consists of a J by J parametrisation of the observed energy spectra. The perturbations of the Rydberg series 6snp J=1, 6snp ³P₀ and 6snf J=3 by levels of the 4f¹³5d²6s configuration are clearly interpreted. The limitations of the MQDT models derived solely from energy data are discussed. A revised value of the first ionisation limit is proposed.

The authors study cooperative effects on the fluorescence spectrum of several atoms in an intense radiation field. In the secular approximation the spectrum contains three lines, as in the single-atom case. The authors evaluate the strengths of these lines for an arbitrary number of atoms without collisions. In the case of two atoms, collisional interactions with a perturber bath are included. The interatomic coherence in the absence of collisions leads to an enhancement of the line strengths, which is most pronounced for the central Rayleigh line. When collisions are present, the strengths of the two sidebands can actually be diminished by the cooperative effects.

The ratio of the intensities of the Lyman alpha (n=2 to 1), Balmer alpha (n=3 to 2) and Balmer beta (n=4 to 2) fine structure components of hydrogen-like ions emitted from plasmas of various atomic number Z is examined. The fine structure intensities of Lyman alpha can only be deduced assuming statistical equilibrium between the fine structure sub-levels of quantum state n=2 for electron densities N_e>or approximately=10¹³Z⁹ cm^-3, while the fine structure intensities of Balmer alpha and beta can only be deduced assuming statistical equilibrium between the sub-levels of n=3 and 4 respectively for N_e>or approximately=10¹²Z⁹ cm^-3. At low electron densities (N_e<or approximately=10³Z⁹ cm^-3 for Lyman alpha, N_e<or approximately=10⁷Z⁹ cm^-3 for Balmer alpha and beta), the fine structure intensities are given by a coronal-like model, where collisional processes into the sub-levels are balanced by radiative de-excitation. At intermediate electron densities, the Lyman alpha, Balmer alpha and Balmer beta fine structure ratios can vary rapidly with N_e and should be useful as electron density diagnostics.

The velocity selective optical pumping method has been extended to the selection of three components of the atomic velocities. Three non-coplanar pumping beams and a detection beam are used, all of them originating from the same laser. The signal proceeds from crossed non-linear effects depending on each of the pumping-beam intensities. This method has been demonstrated experimentally with metastable neon atoms and signals have been obtained due to almost motionless atoms in space.

The He II alpha photoelectron spectra for ionisation from the 5d subshells of metal atoms in the Hg II, Tl I and Pb II halides have been recorded at two angles to unpolarised photon beams. From these measurements asymmetry parameters beta for the 5d^-1 ionisation have been determined. The variation of beta with electron kinetic energy is similar to that observed previously for 5d^-1 ionisation of atomic mercury. Differences are attributed to the variation in Z rather than to the molecular environment. The 5d^-1 ionisation energy of TlF is shown to be consistent with a simple classical model of chemical shifts.

A trajectory method has been developed for the solution of the one-electron Schrodinger equation in atomic scattering problems. The method is based on the fluid dynamical interpretation of the quantum mechanical probability density and current. The time evolution is described by trajectories of particles, which represent the probability distribution. The trajectories are calculated by solving classical equations of motion with forces depending on the probability density. Results are obtained for the differential and total ionisation and charge exchange cross sections for the scattering of H⁺ on H.

Differential and integral cross sections for five transitions (1s-ns (n=2, 3, 4) and 2s-ns (n=3, 4)) of the hydrogen atom by positron and proton impact are calculated, in the framework of the distorted-wave approximation. A comparative study is made by taking distorted waves (Coulomb waves) either in the initial channel or in the final channel or in both the channels. The energy range studied for positrons is from 100 to 500 eV and for protons from 100 to 500 keV. The computed results are compared with the plane-wave results and other available theoretical results.

Within the frame of relativistic Hartree-Fock theory, a two-channel model is proposed for the elastic scattering of electrons (in the energy range 10-200 eV) from noble-gas atoms. The coupling between the two channels is described by a dipole potential obtained from a charge polarisation potential of the Temkin type. Numerical results have been obtained for the differential and total scattering cross sections from He and Ne. The authors find excellent agreement with recent experimental data. Further, they compare their results with those from a one-pseudo-state calculation, from calculations using various local potential models and from one-channel relativistic Hartree-Fock calculations.

The relativistic R-matrix method is used to calculate elastic and inelastic cross sections for electrons incident on caesium atoms with energies from 0-3 eV. In addition to the total cross sections, results are presented on the differential cross sections, sigma , and the spin polarisation, P_y, of the scattered electrons as a function of energy at the scattering angles 10 degrees , 50 degrees , 90 degrees and 150 degrees . The calculation reveals a wealth of resonances around the ²P_1/2 and ²P_3/2 thresholds. The resonances are analysed in detail and their role in the scattering process is discussed.

Results for the low-energy (0-5 eV) scattering of electrons by atomic Tl are presented. The resonance structure is analysed and a comparison is made between a non-relativistic and a relativistic R-matrix calculation which takes into account parts of the Breit-Pauli Hamiltonian. Total cross sections for the transitions (6p)²P_1/2 to (6p)²P_1/2,3/2, (7s)²S_1/2, (7p)²P_1/2,3/2 and (6d)²D_3/2,5/2 are given.

Numerical results for the spin polarisation S_P and the scattering asymmetry S_A are presented for the elastic and inelastic scattering of electrons from Hg and Tl atoms. The interpretation in terms of the so-called 'fine-structure effect' indicates that spin flips by exchange may represent an important polarisation mechanism, in addition to the conventional Mott scattering process.

The technique of collision-induced non-radiative transitions has been used to study the excitation transfer in neon brought about by electron-atom collision processes. The experimental results are in agreement with the selection rules suggested by a theoretical analysis for excitation transfer by this process. These selection rules are: (i) the core configuration is conserved and (ii) N+N'>or approximately=K>or approximately= mod N-N' mod , where N=l, j, J. Here, l and j, respectively, refer to the orbital and total angular momentum quantum numbers of the running electron and J is the total angular momentum quantum number for the atom. N' refers to the similar quantum numbers for the final state involved in the excitation transfer and K=1 for dipolar interactions and 2 for quadrupole interactions. The electron-atom collision process seems to be a more efficient mechanism of excitation transfer than atom-atom collision processes.

Quantitative results have been obtained for vibrational excitation of both H₂ and D₂ by electron impact at 10 eV. A strong isotope effect is observed for the lowest levels where the X ² Sigma _u⁺ resonance produces the main contribution. The B ² Sigma _g⁺ resonance is only observed weakly in the high vibrational level cross sections. Vibrational excitation was also observed at 5 eV where only the X ² Sigma _u⁺ resonance is present. The near-threshold cross section of the b ³ Sigma _u⁺ state was determined with the aim of observing the B ² Sigma _g⁺ resonance in this channel. No such contribution was detected, the cross section appeared to be dominated by direct scattering.

The rate coefficients, beta , for the attachment reactions of electrons with c-C₇F₁₄, CH₃Br, CF₃Br, CH₂Br₂ and CH₃I have been determined under truly thermal conditions at temperatures in the range 200-600K using the flowing afterglow/Langmuir probe technique. The beta values (in units of CM³ s^-1) at 300K are 6.8*10^-8, 6*10^-12, 1.6*10^-8, 9.3*10^-8 and 1.2*10^-7 respectively. The beta for all of the reactions increase with temperature and approach the theoretical maximum value of beta at high temperatures. From Arrhenius plots, activation energies for the reactions have been determined to be 40, 300, 80, 50 and 25 meV respectively. All of the reactions, except for that with c-C₇F₁₄ (for which electron attachment is apparently non-dissociative), proceed by exoergic dissociative attachment producing halogen atomic negative ions. The data are compared with previous data where these are available.