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A new band system of CuBr involving two new electronic states has been observed in the spectral region 5900-6300 AA. This is the first observation of a transition involving two excited states in any diatomic copper halide. The transition appears to be Delta Lambda =0 type. The vibrational assignments of the bands have been confirmed by measuring the isotopes shifts. The vibrational constants for the two states for ⁶³Cu⁷⁹Br are omega _e'=300.35 cm^-1, omega _e^"=241.52 cm^-1, omega _e'x_e'=0.42 cm^-1, omega _e"x_e"=1.46 cm^-1 and nu _e=16267.55 cm^-1. A partial rotational analysis of the (0,0) band has also been carried out.

Independent-atom model calculations have been carried out and total cross sections for e⁺+H₂ scattering have been obtained via the optical theorem in the intermediate- and high-energy region. The authors results are in very good agreement with the recent experimental data of Charlton et al. (1980) beyond 100 eV.

Supplementing the well known variational expansions in terms of harmonic oscillator states and Glauber's (1963) coherent states, the authors show the existence of another type of expansion in terms of coherent states. Employing this expansion form, they derive, using the generator coordinate formalism, an orbit method similar to that recently developed by Davis and Helles (1979). Then they apply it to calculations of nuclear bound and resonance states in several anharmonic one-dimensional model potential systems (i.e., Morse, double-minimum and cubic anharmonic potential systems).

Wavelengths and atomic parameters for both dielectronic and inner-shell satellite lines of the type 1s²nl-1s2l'nl have been calculated for O VI. The atomic data for these satellite lines with n=2 were calculated in a multiconfiguration intermediate-coupling scheme and are compared with previous results. The fine-structure splitting is also compared with previous detailed calculations. Calculations for the dielectronic satellite lines with n=3, 4 and 5 have been performed in a multiconfiguration LS coupling scheme. The intensities of the n=6 satellites are derived from these data and thus an estimate of the contribution of the dielectronic satellites to the resonance line is obtained. New calculations for the excitation rates for both the inner-shell satellites, excited from 1s²2s and 1s²2p, and the resonance line 1s²¹S-1s2p ¹P of O VII are also presented. The intercombination and forbidden lines 1s²-1s2p ³P, ³S are not considered.

Oscillator strengths for electric dipole transitions within the ground complex of the aluminium isoelectronic sequence have been calculated up to Np⁸⁰⁺ using the relativistic parametric potential method. The electrostatic interaction between all states of the complex is explicitly introduced. The Z dependence of the f values throughout the sequence is analysed for the particular example of transitions between the odd levels J=¹/₂ and the even levels J=³/₂. Intracomplex correlation effects and relativistic effects simultaneously become very important at a sufficiently high stage of ionisation, where it is no longer possible to represent the wavefunctions by a pure state in the jj coupling scheme. Numerous anticrossing effects appear in the complex; they give rise to several important irregularities in the trend of the f values. Therefore, simple theoretical arguments cannot be used ab initio to predict the evolution of the f values along the sequence; furthermore the interpolation of the results within the sequence is a somewhat difficult problem which calls for a previous analysis of the systematic trends of the energy levels.

The effect of electron correlation on the energies of the LS states determined by the 4f² electronic shell is studied for the Pr³⁺ ion by means of a non-relativistic configuration interaction method. The excited orbital set included orbitals of s, p, d, f, g and h symmetry. To speed up the convergence and to reduce the impact of the virtual excitations to the continuum, the radial functions in these orbitals were partially optimised in a series of MCHF calculations. It is shown that the 4f5f configurations are dominant in the description of intrashell correlation effects for the 4f² shell. The inclusion of these correlation effects also resulted in a considerable improvement of the theoretical multiplet splittings over the HF results. However, intershell correlation effects have to be taken into account to get satisfactory agreement.

The authors have used an optogalvanic detection scheme, in conjunction with a two-step pulsed laser excitation, to investigate high-lying (Rydberg) krypton levels. The excitation scheme starts from the metastable 1s₅ level, populated in the He-Kr discharge tube and uses either the 2p₂ or the 2p₃ level as an intermediate level. The transient variation of the voltage across the discharge (duration about 50 mu s) is detected as a function of the second-step laser frequency. The energy of the levels of the odd series nd(3/2)₂ and nd(7/2)₃ have been measured for n=15 to 25 with an accuracy of 0.03 cm^-1. Members of the even series nf(7/2)_J, nf(9/2)_J and np(5/2)₃ have also been identified and the corresponding energies measured. In the latter case, the excitation scheme involves a collisional excitation transfer from the laser populated intermediate levels to the closely lying 3d₄' level.

Using laser optical pumping and magnetic resonance, Lande factors for one level of the 2p⁵3p configuration of neon, six levels of the 3p⁵4p configuration of argon and five levels of the 4p⁵5p configuration of krypton have been measured with an accuracy of about 3*10^-4. These values are compared with other experimental and theoretical values.

It is shown that the dispersion dependence of the rotation of the plane of polarisation of weak radiation passing through a three-level atomic medium may be free of Doppler broadening, on the account of a velocity-dependent Stark-shift which is created by saturating the wave on an adjacent transition. In the case of two-photon resonance the use of opposing light beams with different frequencies gives the possibility for an essential increase of the signal on account of the resonance gain on an intermediate transition, without additional Doppler broadening.

Three-photon ionisation probability rates have been calculated for circularly and linearly polarised incident radiation applying time-dependent perturbation theory to the lowest non-vanishing order in the framework of the quantum defect method generalised to the spin-orbit coupling scheme. The infinite summations over the unperturbed atomic states have been performed on the basis of the Green's function formalism. Electron spin polarisation has been evaluated and the possibility of obtaining ejected polarised electrons in the direction of the incident radiation wavevector through three-photon ionisation by circularly polarised radiation is shown.

The X-ray linear attenuation coefficient of single-crystal silicon has been measured to within 0.5% using an energy dispersive method. Atomic photoelectric cross sections are derived from the experimental results and compared with theoretical calculations. Results are given for the Cu and Mo K lines.

Incoherent scattering functions have been experimentally determined for 84.4 and 123.6 keV photons in the angular range 30-110 degrees for the elements Sc, Y, La, Dy and Pb. The experimental values of the scattering functions have been compared with those estimated on the basis of non-relativistic SCF Hartree-Fock wavefunctions. Good agreement is observed between theory and experiment within experimental errors.

Synchrotron radiation in the 1060-2000 AA region has been used to measure the average absorption and fluorescence cross sections of NO and to determine approximate photodissociation quantum yields. Several vibrational levels of the D ² Sigma ⁺, E ² Sigma ⁺ and B' ² Delta states have high fluorescence quantum yields. The C ² Pi and B ² Pi states do not fluoresce (to the detection limits) when the excitation energies are above the first dissociation limit. In general, the fluorescence yields decrease with increasing photon energy. The present quantitative fluorescence measurements are compared with spectroscopic observations and are found to be reasonably consistent.

The photoabsorption cross section of CO has been measured in the 1060-1550 AA region using synchrotron radiation. The oscillator strengths for the transitions from CO(X ¹ Sigma ⁺) to various excited states are obtained from these data. Fluorescence from excited CO was observed in the 1150-3000 and 3000-8000 AA regions. The quantum yields for the production of fluorescence from the A ¹ Pi and b ¹ Sigma ⁺ states are unity because their excitation energies are below the dissociation limit. The C ¹ Sigma ⁺, nu =0 level has significant fluorescence quantum yields both in the UV and visible regions, but the yields for the E ¹ Pi , nu =0 and C ¹ Sigma ⁺, nu =1 levels are very small. The C ¹ Sigma ⁺, nu =1 level is presumably predissociated. The cross sections for the production of fluorescence from the a' ³ Sigma ⁺, d ³ Delta _i, e ³ Sigma ^- to a ³ Pi and B ¹ Sigma ⁺, C ¹ Sigma ⁺ to A ¹ Pi transitions upon excitation from the X ¹ Sigma ⁺ have also been measured.

The time-dependent sudden approximation is applied to dipole-dipole scattering. It is shown to be reducible to an analytic form of extreme simplicity, allowing rotor-rotor inelastic cross sections to be calculated with minimal effort. The possibility of correcting the method to allow for adiabaticity is explored, and an optimal procedure is given. Comparison with close-coupling calculations for HF-HF is made, and two classes of transition are identified, those of 'resonant' and 'non-resonant' character, respectively. For the latter, the adiabatic correction is successful, while for the former it fails badly. A physical explanation for this failure is offered.

The doubly excited quartet spectrum of Be II, produced by foil excitation of fast Be⁺ ion beams, has been studied in the wavelength region 600-5000 AA. On the basis of new wavelength determinations for the quartet transitions it has been possible to identify the lower lying, doubly excited levels 1s2snd ⁴D(n=3-5), 1s2pnd ⁴D degrees (n=3-5), 1s2p3p ⁴S, ⁴P and ⁴D, and 1s2s4p ⁴P degrees besides obtaining further evidence for the identification of the 1s2s4s ⁴S and 1s2s3p ⁴P degrees levels recently proposed by Mannervik et al. (1981). Recent theoretical calculations support the present identifications.

A modified binary-encounter approach (BEA) is applied to the calculation of cross sections of single-impact ionisation for protons and He⁺ ions incident on H, N, O and the noble-gas atoms. The consideration of the charge transfer cross section is shown to be important in deriving the ionisation cross section from the corresponding electron removal cross section. The influence of the effective charge of the He⁺ ion is also investigated.

Cross sections for the formation of ⁷Li²⁺ by collisions between ⁷Li⁺ ions have been measured for interaction energies between 19.3 and 87.6 keV. In this energy range the cross section rises monotonically from 1.48*10^-18 to 9.35*10^-18 cm². In efforts to compare these results with analogous measurements for helium, four scaling laws were investigated. None was entirely successful although simple classical scaling gave the best fit between measurements for He and Li⁺.

The author considers the second-order Born approximation for a charge transfer process to study electron capture to continuum states of a fast bare charge after a collision with a hydrogenic atom. By relaxing usual approximations, he is able to take full account of the corrections to the nu ^-12 asymptotic behaviour of the cross section with impact velocity nu , and consequently to obtain the angular dependence of the electron distribution around the projectile. There is a dominating nu ^-14 ln nu anisotropic term, which is due to a contribution to the transition amplitude coming from the neighbourhood of the pole in the free-particles Green's propagator. The large value of the corrections to the nu ^-12 dependence of the cross section casts doubts on the validity of such asymptotic expansions for non-relativistic velocities.

Cross section measurements are reported for electron capture by multiply charged lithium, nitrogen and neon ions from atomic hydrogen. The ion energies depend on the ion charge state i and typically range from 3i keV to 12i keV. Experimental technique and resulting uncertainties are discussed in detail. Good agreement is obtained with published experimental results where comparison is possible. Discrepancies are observed, however, in comparison with available theoretical calculations.

Charge transfer between H₂⁺ ions in specific vibrational states and H₂ molecules has been investigated within the energy range 8-1000 eV using photoelectron-photoion coincidence spectroscopy. Results up to the sixth vibrational level show that cross sections vary significantly with vibrational energy in a complicated manner which is not in accord with multistate impact parameter theory.

Two recent pseudostate calculations on elastic electron-hydrogen and electron-helium scattering are examined. It is shown that the pseudostates used fail, most severely, to represent short-range interactions. Despite this failure the pseudostate cross sections agree very well with experiment at the lower energies. This agreement and its subsequent deterioration with increasing energy are explained in terms of a Pauli exclusion mechanism which can deter a slowly moving incident electron from sampling the region of short-range interactions. With increasing impact energy the exclusion mechanism becomes less and less effective in preventing penetration into the domain of short-range interactions with the result that the failure of the pseudostate approximation is unmasked. The proposed mechanism supports the view that exchange coupling of the ground state to excited states is very important at lower energies. It is recommended that the reliability of pseudostate sets be judged by how well they can reproduce second Born amplitudes. Indeed, it was by this means that the short-range deficiencies of the pseudostates studied in the present work were clearly revealed.

A semiclassical explanation of the behaviour of the triple differential cross sections in the symmetric case is given. It is shown that the long-range interaction of the outgoing particles results in a change in the angles of their asymptotic paths when the particles are assumed to behave classically. Good agreement with experiment is obtained.

An electron impact experiment is presented which allows a better understanding of C^-/CO formation. Structures observed in the sigma =f(E) curve for C^-/CO indicate that the ² Sigma ⁺ resonant state ( nu =0 at 10.04 eV) is predissociated by another resonant state leading to C^-(⁴S)+O(³P). Using a deconvolution technique, several contributions to the dissociative attachment cross section are clearly separated: predissociation of the nu =1 and nu =2 levels of the CO^-2 Sigma ⁺ resonant state and direct dissociative resonant processes leading to C^-/CO are observed.

Three-body electron-ion recombination in a molecular ambient gas is investigated theoretically. The limited success attained by a classical treatment is lost when necessary account is taken of the discreteness of the negative energy levels of the recombining electron. Thus the three-body recombination coefficients calculated in carbon dioxide and in water vapour differ from those measured by Warman et al. (1979) and Sennhauser et al. (1980) being much lower and falling off far more steeply with increase in temperature. It is noted that the ions involved in the laboratory study, (CO₂⁺)_n and H₃O⁺(H₂O)_n, are open to dissociative recombination and it is pointed out that this process and three-body recombination supplement one another with striking consequences. The mechanism is in two stages: firstly, collisions between the electrons and the ambient gas molecules populate the Rydberg levels; secondly, the electrons in the Rydberg levels participate in radiationless transitions leading to dissociation in the same way as free electrons. A suitably descriptive name for the mechanism is collisional dissociative recombination. Detailed calculations give gas-density-dependent recombination coefficients in quite good agreement with those measured.