Table of contents

It is demonstrated that improved results can be obtained from second-order Rayleigh-Schrodinger calculations of electron correlation energies using very large basis sets by introducing a scaling parameter in the zero-order Hamiltonian. This scaling parameter may be determined from calculations taken to third-order using basis sets of moderate size.

The convergence of the expansion in powers of 1/n, where n is the principal quantum number, of radial hydrogenic matrix elements is examined. Conditions for convergence are obtained.

The spectral profile of the strongly resonance-broadened transition lambda 585.2 nm of Ne I has been studied in absorption. A narrow-band tunable dye laser probed a weak discharge in natural neon cooled with liquid nitrogen. The Lorentzian width extrapolated to zero density was found to be 108+or-20 MHz, consistent with the value 114+or-4 MHz calculated from the natural widths of the levels. There is no indication of the excess width found in earlier emission experiments in helium and neon. A preliminary result of (1.7+or-0.2)10^-21 MHz atom^-1 m³ was obtained for the broadening constant of lambda 585.2 nm in absorption at 77K. This is consistent with the value found in emission.

Some confusion has developed with regard to the correct procedure necessary to compare charge exchange to the continuum (CEC) theories and experiment for ion-beam-foil collisions where the emitted electrons have velocities v_e approximately=v_i. In order to make such a comparison, the authors emphasise that (a) the instrumental resolutions in angle and absolute value of velocity must be folded into the theoretical double differential cross section, and (b) that if the emission of CEC electrons occurs only because of interaction with the outermost atomic layers, those electrons originating from further inside the foil must be subtracted as background. Unfortunately, these considerations were not properly accounted for by Steckelmacher and colleagues (1978), as they compared their recent results with theory and published experiments.

Cross sections for one-electron capture by C⁺, N⁺ and O⁺ ions in H and H₂ have been investigated within the energy range 0.1-13 keV from measurements using a tungsten tube furnace to provide a target of highly dissociated hydrogen. Measured cross sections are believed to pertain to ground-state ions except in the case of O⁺-H collisions where accurate measurements were precluded by the presence of metastable species in the O⁺ ion beam.

Exit channel zero-angle and small-angle spectra for the single-electron capture collisions N²⁺-He, Ne, Ar are reported and assigned.

The effect of ignoring terms of higher order than 1/k_i² in the e^--H elastic-scattering amplitude is examined in relation to the modified Glauber approach. The good agreement with the experimental data obtained within the eikonal-Born series (EBS) at 50 eV is due to the higher-order terms hidden in the second Born term (f_B2) and that the 'residual' EBS obtained by ignoring these terms leads to poor results. As expected, these higher-order terms are not important at higher energies (>or approximately=200 eV).

Five-state R-matrix calculations are used to calculate the differential cross sections lambda , chi , A₀^col, A₁₊^col, A₂₊^col, mod O_1-^col mod and theta _min for electron excitation of the 1¹S to 2¹P state of helium at 81 eV. The results are compared with recent experimental results of Hollywood, Crowe and Williams (1979).

The relative differential cross section for e^--He scattering with the excitation of the 2³S state has been measured as a function of energy at various scattering angles. It is confirmed that the most prominent structures around 22.5 eV are due to an S, a P and a D resonance, the widths of which rise slowly from S to D.

An electron impact process leading to the dissociation of molecular nitrogen through the intermediate formation of a temporary negative ion is described. The process is observed by detection of a narrow band of slow electrons ejected from N^-(³P). The cross section for dissociation has a vertical onset just above the dissociation limit of N₂ and reaches a peak value of 2.5*10^-18 cm²+or-50%.

The crossed-beam technique has been used to measure differential cross sections of vibrationally elastic ( Delta nu =0)e-SF₆ scattering. The maximum energy was about 10 eV. Distinct regions of dominating interaction mechanism are observed in the energy dependence. Below 1 eV, the cross section appears to be determined essentially by direct scattering while in the 2-3 eV range and around 7 eV the scattering is dominated by resonances. The cross sections are obtained in absolute units. In the region of 1 eV, vibrationally elastic scattering and vibrational excitation, associated with different experiments, are of equal magnitude ( approximately=10^-15 cm²).

Upper and lower bounds on the scattering length of electrons on mu ^-p atoms, which differ by 7.9*10^-15 cm, are obtained. The scattering length turns out to be rather small, being -0.7*10^-13 cm.

The authors have calculated oscillation strengths and transition probabilities for transitions between the 1s²2s², 1s²2s2p and 1s²2p² states, namely: ¹S₀^e to ¹P₁⁰; ¹P₁⁰ to ¹D₂^e; ¹P₁⁰ to ¹S₀^e and ³P_J⁰ to ³P_J'^e. For allowed transitions, use of elaborate wave-functions is more important than relativistic corrections up to Z approximately=14. For higher Z, relativistic corrections become somewhat more important.

Transition probabilities are evaluated for the 2s2p(³P₁⁰) to 2s²(¹S₀^e) transition in beryllium-like ions for Mg IX, Si XI, Ar XV, Ca XVII and Fe XXIII using configuration interaction wavefunctions.

The energies E of the ^1,3S autoionising states of Li⁺ below the n=2 and 3 ionisation thresholds of Li²⁺ have been calculated using time-stability theory. The trial wavefunction is of Kinoshita type (1957, 1959). The calculated energies are also compared with experimental and other theoretical results.

The Monte-Carlo classical trajectory method is used to obtain the probability for microwave ionisation of atoms in states n approximately=66 in agreement with the experiments of Bayfield and Koch (1974). In addition the method displays the time evolution of excitation and ionisation processes and shows that the latter may proceed through a great variety of intermediate excitation processes. The mechanism is discussed. The probability of ionisation at time t is given by P_ion(t)=(1-Q_T)(1-exp(- beta (t)t)) where Q_T is the probability of orbits which lie in invariant tori. Quantal effects are not significant for comparison with the Bayfield and Koch experiment, but they are discussed in connection with the implications of the results for multiphoton laser ionisation of low n states.

The theory of resonant and non-resonant three-photon ionisation of a multi-level atomic system illuminated by a not too intense source (in a sense which is defined precisely in the text) with a Lorentzian spectrum is presented. The following new results are obtained: (i) the authors find different expressions for the non-resonant ionisation probability under monochromatic and broad band excitation; (ii) in addition to the usual one- and two-photon resonances connecting the ground state to excited states, the resonance which occurs when the photon energy is equal to the difference in energy between two excited states is discussed; and (iii) the triple resonance (that is the resonance occurring when the conditions for a one-photon and two-photon resonance are simultaneously satisfied) is examined.

The pressure shift of some ammonia inversion lines has been measured by a modified double-modulation spectrometer. Due to some unique features, a substantial improvement in stability and frequency resolution has been obtained over previous measurements. The experimental results have been compared with values deduced from an extension of Anderson's theory. There is an order of magnitude agreement and possible causes of discrepancy are discussed.

Collisional excitation transfer between hydrogen isotopes through dipole-dipole interactions is investigated in a full quantal treatment. Results are presented for the total cross section for excitation transfer at thermal energies. Comparison with semiclassical calculations are given.

Transition probabilities for inner-shell electrons during the formation of quasi-molecular systems in heavy-ion collisions are considered. The effect of transitions to the continuum on discrete transition probabilities is given via a simple prescription satisfying unitarity. In this way, it is also shown that the direct ionisation probabilities will be modified by virtual transitions to the continuum. Adopting a scaling model and the authors' prescription, the effect of such virtual processes on the ionisation of 1s sigma electrons can be calculated analytically. As an example, the authors calculated it for the Pb-Pb quasi-molecular system. It is found that such virtual processes suppress the direct 1s sigma ionisation probability for large impact parameters and/or small relative ion velocities.

A novel and refined theory of charge transfer is introduced using multicomponent kets. The two-state non-resonant impact parameter version for fast collisions is developed using two-components kets, and is shown to have all the attributes of the refined orthogonal treatment of Bates (1958). An additional bonus is that the matrix elements are symmetrised, thus eliminating any post-prior discrepancy. The two-component ket ensures a complete set, viewed from the space of momentum translation factors, and at finite internuclear separations permits the correct assessment of the quantal change in translational kinetic energy of the electron being captured. The theory is also extended to cover more than two states and indeed the molecular treatment of two or more states at low velocities.

The peculiar scattering due to resonant charge transfer causes the ion velocity distribution function to be anisotropic and skewed, especially in strong electrostatic fields. A modified moment method is proposed for dealing with mobility in this case, and shown to be accurate by comparison with model calculations. The method is also applicable to other cases of distorted distribution functions.

Measurements are reported of the reactions of the ground (³P) and metastable electronically excited (¹D₂ and ¹S₀) states of Xe²⁺ ions with several atoms (Ar and Xe) and molecules (H₂, N₂, O₂ and CO₂) at 300K. The data were obtained using a selected-ion-flow-tube (SIFT) apparatus. State selection was achieved by introducing suitable 'chemical-filter' gases into the flow tube. The rate coefficients for the reactions with the molecules were large with the exception of some of the reactions of the ¹S₀ state. Conversely, the atom reactions were generally slow except for the ³P state reaction with Ar in which a favourable pseudo-energy resonance must occur. All the molecular reactions proceeded via single-charge transfer and in the ¹D₂ reactions with molecules, dissociation of the product ion was usually observed.

Elastic scattering of electrons by lithium atoms has been investigated by using an integral form of the adiabatic exchange approach. The effect of the polarisation potential has been included in the sense of Temkin's approach. Results for the differential cross sections at the incident electron energies 5, 10, 20 and 60 eV have been calculated and comparisons have been made with the recent experimental results of Williams et al. (1976). Results for integrated elastic cross sections have also been reported. Qualitative agreement between the present results and the measured values has been obtained.

The differential cross section and generalised oscillator strength for the 1s to (2s+2p) transition in atomic hydrogen has been measured for incident electron kinetic energies 136, 200 and 300 eV at scattering angles from 7 to 20 degrees . The data have been normalised to the Born approximation at 7 degrees and show excellent agreement with the Born curves. The present data are compared with the scattering cross section measurements of Williams and Willis at 20 degrees (1975). These latter data differ from the present work by 41, 30 and 31% for 136, 200 and 300 eV incident energies respectively.

Angular correlations between the scattered electrons resulting from the excitation of the 2¹P state of helium and the photons emitted in the de-excitation of the state have been measured using a coincidence technique for an incident electron energy of 81.2 eV. The measurements extend over the electron scattering angles 10 degrees to 130 degrees and are the first for large momentum transfer scattering. The angular correlations are analysed to yield the ratio of the differential cross sections for excitation of the degenerate magnetic sublevels of the state and the phase difference between the corresponding excitation amplitudes. The Fano-Macek orientation and alignment parameters are also derived from the data. The measurements provide a more sensitive test of theoretical models than previous small-angle scattering data and it is found that none of the published theoretical values agree with the authors' data.

A distorted-wave model is used to calculate electron impact excitation of the 2¹P and 2³S states of helium from the ground state at energies from 30 to 200 eV. Total and differential cross sections are calculated and compared with the results of other workers and with experiment. Similar comparisons are made with recent absolute measurements for lambda (2¹P) and mod chi (2¹P) mod . The discrepancy between theory and experiment for the total 1²S to 2³S cross section at energies above 100 eV is discussed and an explanation proposed.

Cross sections have been measured for detachment by electron impact from C^- and O^- ions for interaction energies between 10 and 1000 eV. For C^- the cross section attained a maximum value 1.85*10^-15 cm² at 18.2 eV whilst for O^- the maximum value was 6.48*10^-16 cm² at 28.1 eV. For both ions the Bethe asymptote was reached only at energies greater than about 300 times threshold. Structure was observed in the cross section for O^- at energies close to 18 eV which suggests the formation of a short-lived excited state of O^2-.

The two-potential eikonal approximation of Ishihara and Chen has been used to calculate differential and total cross sections for elastic scattering and 1s-2s excitation of atomic hydrogen by positrons in the energy range 50-200 eV. The authors' results are found to be very different from those obtained by Foster and Williamson (1976) in the full eikonal approximation and are close, in the elastic scattering case, to the eikonal-Born series results of Byron and Joachain (1977) (except in the large scattering angle region) and the modified Glauber approximation results of Gien (1977). The integrated cross sections are found to be close to those in the Born approximation for elastic scattering and the Glauber approximation for 1s-2s excitation.