Table of contents

The authors have calculated the excitation thresholds and oscillators strengths for the twelve lowest states of Si III by using the configuration interaction wavefunctions. The agreement for excitation thresholds with the experimental values is better than two per cent. The length and velocity values of oscillator strengths are close to each other and agree well with the values of other workers.

An expansion is presented for the absorption lineshape of a two-level system in a strong radiation field being perturbed by dephasing collisions, within the binary collision approximation. No dynamical assumptions regarding the time scale of the collisions (relative to the broadening) need to be made and the present theory interpolates smoothly between the impact and the quasistatic limits. The present solution reduces in weak radiation fields to the linear unified theory of spectral lineshapes and in the impact limit (fast collisions) the authors recover the Karplus-Schwinger formula. The only molecular information necessary for the exact evaluation of the absorption lineshape in a strong field is the same single-particle correlation function g( tau ) which appears in the linear (weak field) unified theory.

The conditions under which interference effects may appear in the decay spectrum of coherently excited overlapping initial states are considered. It is demonstrated that interference effects are excluded under such experimental conditions as those reported in a recent study of the autoionisation spectrum of sodium.

Electron-photon angular correlations have been measured for excitation of the 2P state of hydrogen at incident energies of 55 and 100 eV. The data presented extend the results of Weigold and co-workers (1980) to smaller scattering angles and reveal the existence of a deep minimum in the parameter lambda at theta _e=10 degrees at an incident electron energy of 100 eV.

Algebraic variational method close-coupling calculations for electron impact excitation of the n=2 to n=3 transition in atomic hydrogen, using a large basis, are presented at energies below the n=4 threshold. Near the n=3 threshold the cross section is almost a factor of two lower than that predicted by Johnson (1972). The results go some way towards resolving the discrepancy between Johnson's results and experiments on hydrogen plasmas. However, there is little effect on the rate coefficient at low temperatures ( approximately=0.8 eV) on the assumption of a Maxwellian distribution.

A slow positron beam time-of-flight system employing a localised scattering cell has been used to measure total cross sections for positrons in the energy range 15-600 eV in H₂, O₂, N₂, CO₂ and CH₄. The values obtained are higher than the measurements of Coleman et al. (1974, 1975 and 1976 private communication) at all energies. This confirms that those results were not fully corrected for forward scattering and are therefore superseded by the present data. There is now good agreement between the new data and those of Kauppila et al. (1977) for H₂, N₂ and CO₂ in the region where there is an energy overlap between the two experiments.

The operators acting within the lⁿ (any n>or=0) atomic shells are studied in the quasiparticle scheme of Armstrong and Judd (1970) Lie algebra generated by operators involving products of more than two creation or annihilation operators are obtained. The introduction of operators exchanging odd and even states of two quasiparticle spaces proves to be very useful in this study.

General expressions are obtained for the calculation of the matrix elements of the Coulomb repulsion between electrons in two-electron diatomic molecules using the adiabatic basis. Based on these formulae, the algorithm for calculating these matrix elements for a hydrogen molecule in a wide range of internuclear distances is realised. The authors' results are compared with the asymptotic calculations and the previous calculations (1979) in the vicinity of the equilibrium position.

The relative secondary emission intensity of the molecular ion Al₂^- sputtered from a pure aluminium target by ionic bombardment (primary ions: Ar⁺, 6.5 keV) is larger than that of Al^-, whereas this is not the case for the positive ions Al₂⁺ and Al⁺. An ab initio calculation (LCAO-STO-MO-SCF-CI) is carried out on the configurations ⁴ Sigma _g^- and ² Pi _u of Al₂^-, ² Sigma _g⁺ and ² Pi _u of Al₂⁺ and ³ Sigma _g^- and ¹ Sigma _g⁺ of Al₂ within first a minimal basis and then an extended one. After configuration interaction, it is found that the ground states of Al₂^-, Al₂⁺ and Al₂ are respectively ⁴ Sigma _g^- (R_e=5.0 Bohr, D_e^-=2.42 eV), ² Sigma _g⁺ (R_e=6.50 Bohr, D_e⁺=1.24 eV) and ³ Sigma _g^- (R_e=4.85 Bohr, D_e⁰=1.03 eV; experimental ground state of Al₂: ³ Sigma _g^-, R_e=4.85 Bohr, 1.2<or approximately=D₀<or approximately=2 eV). The binding energy of Al₂^- (⁴ Sigma _g^-), clearly larger than that of Al₂⁺ (² Sigma _g⁺, allows us to explain the special experimental result given by secondary ionic emission with a stability of Al₂^- stronger that than of Al₂⁺.

A high resolution study of 31 transitions in dysprosium in the wavelength regions 435-470 nm and 560-670 nm has been performed using CW dye lasers and a collimated atomic beam. Transitions from the ⁵I₈ ground state as well as from the metastable states at 4134 cm^-1 (⁵I₇) 7050 cm^-1 (⁵I₆) of the 4f¹⁰6s²⁵I multiplet were studied. The results of isotope shift measurements on transitions to the excited-state configurations 4f¹⁰6s6p, 4f⁹6s²5d, 4f⁹6s5d² are presented and values for the specific mass shift and atomic factor of the field shift for the various configurations are evaluated.

For pt.I see ibid., vol.13, p.2185 (1980). The hyperfine structure of 21 levels of the odd isotopes ¹⁶¹Dy and ¹⁶³Dy has been studied in laser-atomic-beam experiments. The magnetic dipole and electric quadrupole interaction constants have been analysed using the effective operator formalism and wavefunctions from fine-structure calculations. From this analysis the electronic quadrupole moments of ¹⁶¹Dy and ¹⁶³Dy, corrected for the Sternheimer effect, have been evaluated to be 2.42(27)*10² fm² and 2.55(29)*10² fm² respectively.

The authors derive an expression for the spectrum of fluorescence by a two-state atom in a perturber gas irradiated by nearly resonant monochromatic light of arbitrary intensity. The result is fully expressed in terms of the absorption profile that would be measured in a conventional low-intensity absorption experiment and of the rate of radiative and collisional transitions. The present result generalises previous work and the authors demonstrate that expressions obtained before in special cases are recovered by taking the appropriate limits.

A two-state atom in a perturber gas is irradiated by intense light with an arbitrary spectral profile. The authors derive a general expression for the fluorescence spectrum in terms of the conventional low-intensity absorption profile and the spectral density of the incident light. This result, which contains previous results as special limiting cases, describes the transition from the three-line fluorescence spectrum excited by intense monochromatic light and the power-broadened Lorentzian spectrum produced by broad-band excitation.

A time-of-flight technique for the determination of forbidden (ms time range) collision-free radiative lifetimes is presented. In this method, the time-of-flight spectrum of metastable atoms generated from a molecular beam source is compared with that for a non-radiating (ground or metastable) state, and the lifetime is derived from a semi-logarithmic plot of the ratio of intensities against flight time. The lifetimes of the calcium 4s4p ³P₁ and 4d3d ¹D states are determined to be 0.50+or-0.04 (2 sigma ) and 2.3+or-0.5 (2 sigma ) ms. The former result is in reasonable agreement with earlier determinations of this lifetime. One radiative decay pathway for the ¹D state is direct electric quadrupole radiation to the ground state. The transition rate for this process has been determined by a MCSCF/CI calculation.

A theory of resonant multiphoton ionisation by smooth pulses is developed. It is shown that the spectrum of the effect may exhibit structure associated with time domain correlations.

A transition accidentally observed at high dispersion in absorption spectra and at low resolution in emission spectra is shown to be due to the PN molecule. A vibrational classification is given. The transition involves the already known ground state X ¹ Sigma ⁺ and a new excited state which is thought to be a ¹ Sigma ⁺ state for which twelve vibrational levels are observed. Anomalous vibrational separations are detected for this state.

The L X-ray production and subshell ionisation cross sections of gold have been measured by proton, alpha particle, carbon, nitrogen and oxygen bombardment in the energy range 0.4-3.4 MeV. These data are compared with the predictions of the plane-wave Born approximation including the binding energy, Coulomb deflection and relativistic corrections. Satisfactory agreement has been obtained between theory and experiment except for the cross sections of the L₂-subshell ionisation induced by heavy ions (¹²C, ¹⁴N, ¹⁶O). The theory underestimates these cross sections by almost one order of magnitude.

A fast intersecting beam technique previously developed in this laboratory has been applied to study H⁺-Li⁺ collisions at centre-of-mass energies in the range 62-350 keV. Total cross sections for the production of Li²⁺ from the combined processes of charge transfer and ionisation have been determined. A coincidence technique has also been used to obtain the separate cross sections for these processes which are compared with current theoretical predictions.

It is shown that semiclassical theory accounts for q³ scaling in total cross sections for fast X^q++H electron-capture collisions. Semiclassical OBK, eikonal, continuum intermediate-state and continuum distorted-wave theories are all shown to lead to q³ scaling except possibly forq=1. It is also shown that a semiclassical continuum theory of Bohr and Lindhard (1954), suitably refined and partially based on dimensional arguments, gives cross sections of magnitude 8 pi q³v^-7a₀² where v is the impact velocity in atomic units. This is in very close agreement with the results of Goffe et al. (1979) for fully stripped ions and in reasonable agreement with Shah et al. for Li^q+ (1978). The one exception is the proton-hydrogen-atom cross section whose symmetric resonant nature is best described simply by the continuum intermediate-state description of Belkic (1979) or the continuum distorted-wave description of Cheshire (1964).

The authors use Wallace's second-order eikonal approximation, (WSOEA, 1973) to analyse the elastic scattering of electrons from helium at incident energies of 100, 200 and 500 eV. The predictions of the WSOEA are compared with those of the partial-wave calculation and the results obtained in Glauber's first-order eikonal approximation (1959).

The local free-electron-gas exchange potential is shown to be not only superior to the orthogonalisation approximation to exchange, but also to yield results in good agreement with those from an essentially exact treatment of exchange. The imposition of orthogonalisation in addition to the local exchange potential yields a further small improvement in the results. The use of these two approximations in combination appears to be a promising technique. A polarisation potential cut-off at about the position of the H atom yields results for the ² Sigma eigenphase sum that have resonant behaviour more or less where expected on the basis of measured vibrational excitation cross sections. The elastic differential cross section has a strong peak in the backward direction, with important implications for resonant vibrational excitation.

The R-matrix method calculations of Berrington et al. (1975) and Fon et al. (1979) are extended to the energy ranges 21.4<E<30 eV and 80<E<or=200 eV for the electron impact excitation of 2¹S and 2¹P state of helium. Differential and integrated cross sections for these transitions and the ( lambda , chi ) parameters for the 2¹P excitation are calculated and compared with the recent theoretical calculations and experimental measurements.

The differential vibrational excitation cross section v=0 to v=1 has been obtained for CO in a crossed molecular-beam electron-beam apparatus in the 1-30 eV energy range. Besides the well known pi shape resonance at 1.8 eV, a broad and low-intensity peak centred at 19.5 eV is observed and attributed to a sigma shape resonance. Angular distributions within the sigma resonance energy show the contribution of the f wave (l=3).