Table of contents

The transmission spectrum in the 2s region of sodium vapour contained in a heat pipe was obtained with a 3 m monochromator using synchrotron radiation from SURF II as a source. In the continuum at photon energies greater than the 2s threshold, a number of spectral lines were observed and attributed to doubly excited transitions. A tentative classification of these lines is presented that is in close correspondence with the Mg I spectrum. Previously observed resonances, due to single-electron excitation below the 2s threshold, are confirmed. In addition, structure attributed to higher members of the single-electron excitation series was observed.

A lifetime method based on measuring the fluorescence decay following pulsed laser excitation of sputtered metal vapours is described. The method is applicable to neutral and singly ionised atoms of a wide range of elements, including the highly refractory metals. New lifetime results are reported for some levels of Zr I and Zr II.

Absolute term values of Li(1s2snp)⁴P^o and (1s2pnp)⁴P states (n=3,4,5) and their corresponding transition probabilities for electric dipole decay are calculated by means of configuration interaction calculations approximately corrected for relativistic, radiative and mass polarisation effects. Only 1s2pnp to 1s2snp transitions (n=2,3,4,5) possess relatively high decay rates to be readily observed in the beam-foil source; however, the n=5 transition at lambda =5405.2(0.6) AA has not yet been detected. Unobserved weak and very weak lines are predicted. An earlier assignment of lambda =8504 AA to 1s2s3p is not confirmed ( lambda =8605.0(2.5) AA at very weak intensity but within the range of detectability). Comparison of theoretical E1 decay rates with experimental mean lifetimes suggests that autoionisation is by far the principal mode of decay for all these even-parity ⁴P states except 1s2p², thus explaining the scarce participation of ⁴P^o to or from ⁴P transitions in Li quarter spectra.

A semiclassical, internally consistent formulation of the dipole polarisabilities of atoms, of the shell model of ionic crystals, of the induced electric field effects on oscillator strengths in atoms and of the induced valence electron effects on the electronic energy levels was presented and applied to atomic lithium in earlier works and is applied to the remaining alkali atoms: sodium, potassium, rubidium and caesium. The results indicate that the simple induced dipole potential of the core due to the presence of the valence electron accounts for the major portion of the correlation effects to be added to a Hartree-Fock calculation in the alkali atoms. The model applies well to non-relativistic quantities. However, there are indications that the simple one-particle approximation is inaccurate for relativistic corrections.

Spin-forbidden transitions in atomic Ca are treated using ab initio MCSCF/CI line strengths and atomic spectral data. The lifetime of metastable Ca ¹D₂ is computed to be 1.8 ms, in good agreement with a previously determined experimental value of 2.3+or-0.5 ms, confirming the largely spin-forbidden dipole-allowed character of this decay. The implications for other radiative processes in Ca as well as Sr and Ba are discussed.

New data on sp² in Ga I, In I and Tl I obtained by photoabsorption with synchrotron radiation are presented. A discussion of ambiguities of assignment in the In I and Tl I isoelectronic sequences is presented. Several changes are suggested which modify recent analyses of photoelectron polarisation and photoabsorption cross section data. The changes are shown to yield a smoother transition from LS to jj coupling from Al I to Tl I and are also supported by Hartree-Fock calculations.

The spectrum of resonance fluorescence and scattered light is calculated, on the basis of the dressed-atom approach, in the case of joined S_1/2-P_1/2 and S_1/2-P_3/2 transitions. The dependence of the steady-state density matrix on detuning and field strength is investigated for linear polarisation of the laser beam. The influence of the polarisation of the laser beam and of the scattered light on spectral distribution is discussed.

Experimental data on depolarised intensities in the wings of the Raman vibrational Q branch of the hydrogen molecule are discussed for densities between 28 and 187 Amagat at room temperature. The low-density integrated depolarisation ratio as well as the far-wing spectral shape are in satisfactory agreement with the first-order dipole-induced dipole model for isotropic molecules. The permanent polarisability anisotropy of the molecule must be taken into account for the intensities in the centre of the Q branch.

Deactivation of the metastable 2s state of hydrogen in collisions with the noble gases Ne and Ar and with the molecules N₂ and CO₂ has been studied in the velocity range between 3*10⁵ cm s^-1 and 4*10⁶ cm s^-1. At these velocities the dominant process should be mixing of the 2s state with the adjacent 2p state, followed by radiative decay to the ground state. Appropriate construction of the target cell allows absolute total destruction cross sections and absolute radiative destruction cross sections to be measured simultaneously. In all cases the two cross sections turn out to be equal in velocity dependence and absolute value within experimental accuracy. This equality suggests that contributions of radiationless processes to the total destruction cross section are relatively unimportant.

The authors have observed changes in the mean state of L-shell ionisation when 300 MeV Kr²⁰⁺ ions traverse foil targets of varying thicknesses. The influence of related effects on K X-ray yield variation with target thickness is examined; in particular, K-hole lifetime and fluorescence yield modifications inside the target as well as a possible variation of the K-vacancy production cross section are considered. A new model shows how to extract K-shell ionisation and capture cross sections when these effects are taken into account. It is also seen that such experimental studies can lead to new information about the ionisation of the 2p pi MO during a collision.

The close-coupling equations are reformulated in a restricted inner domain of coordinate space, allowing continuum states to be included in the target expansion. Cross sections are obtained by matching to an unsymmetrised close-coupling solution in a restricted asymptotic region of coordinate space. The method is applied to model electron-hydrogen excitation using the simplest method of representing the continuum, that of sampling. The results are as accurate as pseudo-state calculations, and also display spurious resonances. Further methods of improving the theory, such as representing all continuum states by interpolation in energy, are discussed. It is expected that these improvements will cause a significant increase in accuracy.

A new experimental method is reported for the crossed beam measurement of absolute cross sections. It essentially consists in sweeping one of the beams across the other in a linear 'see-saw' motion. The method is applied to one case of electron-ion collisions. A description is given of an electron gun designed to realise a linear and distortionless motion of a ribbon electron beam. Measurements are reported regarding the electron impact ionisation of He⁺ in the energy range 55-74 eV.

The authors report on measurements of the electron impact ionisation cross section of metastable atomic hydrogen in the energy range 6.3-998.3 eV. Crossed electron and atom beams are used in a new method where the more usual intensity modulation is replaced by a 'see-saw' motion of the electron beam across the atom beam. The main advantage of the method is that it obviates the problem of the density distribution in the beams. Absolute results, with 10% accuracy, are obtained for the difference ( sigma _2s- sigma _1s) between the cross sections of the atom in the metastable and ground states. At high energies, the results are consistent with the Bethe formula, in contrast with earlier experimental data. At low energies large discrepancies subsist with all theoretical predictions.

The authors have calculated the collision strength for 43 semi-forbidden transitions among the states having configurations 1s²2s²2p², 1s²2s2p³ and 1s²2p⁴ in O III using the R-matrix method. Configuration interaction wavefunctions are used to represent the twelve target states which are included in the calculation. Collision strengths averaged over Maxwellian distribution of electrons are calculated for these transitions over a wide temperature range and the results are compared with other work.

Near-threshold electron impact spectra in Ne, demonstrably free of post-collision interaction (PCI) effects and negative-ion resonance structures, have been obtained as a function of scattered energy over the energy range from 41 to 52 eV. The author finds that single-electron transitions 2s²2p⁶ to 2s2p⁶nl dominate over two-electron transitions 2s²2p⁶ to 2s²2p⁴nln'l' and that states with terms ¹S and ³P are preferentially excited. Detailed comparisons with previous theoretical and experimental data are made, in particular with those obtained by threshold electron impact techniques where PCI and negative-ion resonances are often a complicating issue.

A general non-adiabatic theory of near-threshold resonant electron-molecule scattering in the presence of a long-range direct scattering potential is developed. As a special case, resonant s-wave scattering in the presence of a dipole potential is worked out in detail. An exactly solvable dynamical model for electron-HCl scattering is proposed and is shown to reproduce the essential features of the vibrational excitation functions of HCl measured by Rohr and Linder (1976). Particular emphasis is given to the analysis of the analytic properties of the S matrix and related quantities in the fixed-nuclei approximation. The passage of a resonance through threshold is shown to be a non-trivial phenomenon for polar molecules. The authors propose a re-interpretation of the HCl^- potential energy curves obtained by the stabilisation method.

Energy and angular distributions of H^-(D^-) ions formed by dissociative attachment in H₂O(D₂O) have been observed for the processes situated in the region of 6.5, 8.6 and 11.8 eV. The angular behaviour would indicate that the symmetries of the resonant states responsible for these processes are ²B₁, ²A₁ and ²B₂, respectively. The energy distributions of the H^-(D^-) ions for all three processes show that most of the dissociation energy is in the form of translation and furthermore the distributions for each process and isotope are similar. This similarity would indicate that the potential surfaces and the autodetachment widths of the H₂O^- states are also about the same which would support the proposals and calculations of Jungen, Vogt and Staemmler (1979) who predict that these three states are formed by two electrons in 3sa₁ orbitals bound to an H₂O⁺ core with a vacancy in the 1b₁, 3a₁ and 1b₂ orbitals, respectively.

The process of two-photon amplification of ultrashort pulses is analysed for the case where simultaneous single-photon ionisation occurs from the upper resonant level.