Table of contents

The doubly excited ¹S^e states of H^- and He are studied by using the hyperspherical coordinate representation and the adiabatic approximation. The adiabatic potentials below the N<or=6 hydrogenic thresholds are calculated by means of the trial functions which are constructed from the hyperspherical harmonics and the analytical channel functions proposed by Lin (1981, 1982). The lowest few adiabatic potentials belonging to the Nth threshold have close avoided crossings with higher ones belonging to the (N-1)th threshold. It is shown that the potentials can be connected diabatically at these crossing points. According to this prescription, the energy levels of the doubly excited states below the N<or=6 thresholds are calculated.

The Green's function obtained in the quantum defect scheme involves the contributions of core states when it is expanded over the complete atomic spectrum. The authors show that these contributions are negligible in the multiphoton ionisation of alkali metal atoms.

Reports new high-resolution measurements of the photoabsorption spectrum of C₂H₅I, in which prominent autoionising resonances have been observed. The spectrum is similar in some respects to the reported spectrum of CH₃I (Baig et al., 1981). Rydberg series are found to the ²E limits, arising from excitation of a lone-pair pi orbital. The authors have performed a graphical Lu-Fano analysis of the series converging to ²E_3/2 by comparison with Xe I and CH₃I. The authors are able to extract from their data a quasi-atomic ordering of Rydberg excitations. Above the ²E_3/2 threshold, the authors have applied the Dubau-Seaton formula to interpret the spectrum, and they can tentatively ascribe most of the observed width to autoionisation.

Astrophysical evidence favours solar empirical damping for the sodium D lines, in agreement with the latest experiment on D-line broadening at the lower end of the range of experimental uncertainty. Theoretical and experimental results for the D lines are briefly reviewed. It is suggested that the broadening of the D lines is controlled by intermediate-range interatomic forces that are best determined by exchange perturbation theory.

The cross sections sigma _c for charge transfer in H⁺-He⁺ collisions previously measured in this laboratory have been found to be subject to an arithmetical error of 25%. The error has now been confirmed by remeasurements of sigma _c in the CM energy range 49-141 keV. The revised values of sigma _c and the cross sections for ionisation sigma _i= sigma (He²⁺)- sigma _c derived from the previous measurements (Angel et al., 1978) of the total cross sections sigma (He²⁺) for He²⁺ formation are in good accord with other measurements. The experimental data now permit a useful assessment of the range of validity of a number of theoretical predictions.

Time-dependent Hartree-Fock calculations are performed for the two-electron system He⁺(1s)+He⁺(1s) in the centre-of-mass energy range 5-80 keV. One-electron capture cross sections are calculated for the M_s=0 and mod M_s mod =1 channels and compared with experimental results. The Pauli blocking for the n=2 shell is briefly discussed.

It is pointed out that, when considering electron capture in collisions of highly charged ions with atoms or molecules, a qualitative discussion in terms of a common 'reaction window' is valid only when fully stripped ions are involved. In the case of partly stripped ions with well separated subshell levels it becomes essential to define individual reaction windows for the final states of the captured electron. These reaction windows generally appear at smaller internuclear separations than in the corresponding bare-ion case. The shift is particularly pronounced for final states of high angular momentum. This observation is supported by available experimental data.

The total cross sections for 1-400 eV electrons and positrons colliding with H₂O molecules have been measured using a TOF method. The results for electron collisions were compared with positron collision data and with previous electron collision work. Data for electron scattering and positron scattering almost coincide with each other in the energy ranges above 200 eV and below 2 eV.

Reports vibrationally inelastic ( nu =0 to nu '=1 and 2) integral cross sections for positron-CO scattering below the positronium threshold. The fixed-nuclei approximation is employed in a single-centre approach. As expected, the final cross sections for the nu '=1 and 2 excitations are structureless with magnitudes of about (3-0.3)*10^-17 cm² and (4-0.15)*10^-20 cm² respectively, in the energy range from the vibrational excitation threshold up to the positronium formation threshold. The corresponding first Born results are in qualitative agreement throughout.

A Jacobi transformation has been made to the three-body Hamiltonian. The transformed equation has been applied to the three particle systems, e₃⁺, He, H₂⁺, and the triatomic systems H₂O, H₃⁺ and ArHCl.

Detailed numerical analysis of profiles of the calcium 422.7 nm resonance line perturbed by rare gases at low densities indicates the inadequacies of the conventional Voigt profile as a description of the lineshapes. Two independent contributions to the line asymmetry are identified: (i) a wing asymmetry whose magnitude is proportional to a typical collision time and which is the first-order correction to the impact approximation; and (ii) a core asymmetry arising from correlations between broadening rate and emitter velocity. An expression, based on classical Fourier analysis and correct velocity averaging, is given for the complete lineshape. This lineshape is found to describe the experimental profiles to a much greater degree of accuracy than the conventional expression, and yields accurate line parameters.

The analogy between the evolution of a quasi-closed (decaying) two-level quantum system and the relativistic spin-¹/₂ motion in electric and magnetic fields is shown. The non-stationary equations for the 4-vector of a pseudo-spin describing the two-level system evolution are derived. In contrast with the well known three-dimensional model, the description is valid for arbitrary relaxation constant relations. In some limits the four-dimensional equations reduce to the optical Bloch ones. The 'energetic space' Lorentz invariance allows one to describe all possible solutions. Simple approximate quasi-stationary solutions are given for dephasing collisions and the region of their validity is determined. The Lorentz invariance of a Schrodinger equation with a non-Hermitian Hamiltonian is found and a set of four-component orthogonal eigenstates of the Dirac-type is constructed. Several examples of the utilisation of the four-dimensional formalism in laser-induced autoionisation, optics of anisotropic media, magnetic phenomena and K₀ meson, decay are present.

Studies the coupling of N discrete levels to a common continuum. The discrete levels, which can include both genuine bound states and autoionisation states, are assumed to be coupled to the continuum and to each other in such a way as to permit a rotating wave approximation in the Schrodinger equation. A general solution for the model is derived with the aid of Laplace transforms and the Weisskopf-Wigner approximation. The author discusses general properties of the discrete level populations and the electron spectrum like population trapping, zeros, crossings, resonances and confluences. As examples the author treats cases where two or three discrete levels interact with a continuum.

Laser-induced fluorescence of the NaK molecule has been recorded at high resolution by Fourier-transform spectrometry in the region 4000-9500 cm^-1. Fourteen series excited by the Ar⁺ 4765, 4880 and 5287 AA lines have been assigned to the system D¹ Pi , d³ Pi to b³ Pi . The strongest transitions, at low values of J, are Omega =1 to Omega =1, but there are also series that arise from Omega =2 to Omega =2 and from Omega =0 to Omega =0. The lower state, the second excited triplet state of NaK, has not been observed before. Constants (in cm^-1) for b³ Pi ₁ are as follows: T_e=11562.18, omega _e=120.41, omega _ex_e=0.3425, B_e=0.095066 and alpha _e=3.28*10^-4. The spin-orbit coupling constant A is estimated to be 15.577 cm^-1 and the internuclear distance r_e is 3.502 AA. The results are in good agreement with pseudopotential and ab initio calculations.

For the description of ion-atom collisions in the semiclassical approximation, the time-dependent Hartree-Fock (TDHF) method is used. The TDHF equations are solved by expanding the electron wavefunctions within a basic set of Gauss-lobe functions. In this basis the matrix elements of the TDHF Hamiltonian can be obtained analytically, even if translational factors are included. As an application, the authors calculate the excitation cross sections of the (1s2s)¹S and (1s2p) ¹P states of He in proton-He collisions.

The K-shell X-ray production cross sections and K_beta /K_alpha ratios have been measured for thin targets of Ti, Rb, Zr, Nb, Mo, Pd, Ag and Sb by protons and for thin targets of Ti, Cr, Co, Cu, Se, Rb, Zr, Nb, Mo, Pd, Ag and Sn by ⁴He ions over the energy range of 1.5-3.8 MeV. The ionisation cross sections were calculated using atomic K-shell fluorescence yields and compared with the plane-wave Born approximation (PWBA) modified to included the effects of relativity, increasing target electron binding energy, target electron polarisation, Coulomb deflection of the incident particle and energy-loss effects in the ionisation process, resulting in the ECPSSR theory. The universal character of K-shell ionisation cross sections is discussed. The K_beta /K_alpha ratios are compared with theoretical predictions and with results reported by other authors.

The results of a study of electron detachment processes occurring in F^--inert-gas collisions in the 10 eV-4 keV laboratory collision energy range are reported. A differential time-of-flight energy loss study of scattered F atoms and an electron spectroscopy study were performed. The differential cross sections for the various processes observed are reported. As in other halogen negative-ion collisions, 'dynamic' effects in the direct detachment process were observed. The relative importance of excitation processes was determined. The production of a series of autodetaching and autoionising states is reported.

Penning ionisation electron spectroscopy (PIES) experiments are reported for acetonitrile (CH₃CN), acrylonitrile (CH₂CHCN) and polyacrylonitrile films. Self-consistent field/configuration interaction calculations on CH₃CN and CH₂CHCN using spatially selected localised orbitals show that the major feature of the PIES spectra is ionisation of the nitrogen lone pair, whose potential appears essentially constant in the series of CN-substituted molecules.

Experimental results for the integral alignment A₂₀ of H(2p) following charge-changing collisions of protons with helium and argon atoms are presented for incident energies of 1-5 keV and 35-300 keV. In addition, theoretical calculations in the continuum-distorted-wave approximation were performed. The observed discrepancies between experiment and theory are discussed.

Using the perturbed stationary state method, including electron translation factors, the authors have calculated electron capture cross sections for the helium-like ions Be²⁺ and B³⁺ incident on ground-state atomic hydrogen. The molecular structure calculation employs pseudo-potentials to represent the inactive K-shell electrons on the incident ions. The calculated cross sections vary rapidly in the 10⁷ cm s^-1 velocity range (E approximately 1 keV amu^-1). The calculations show preferential transfer to the Be⁺(2s) and the B²⁺(2p) product states. The calculations for B³⁺+H are in good agreement with experimental measurements of Cradall et al (1979). In addition, the calculations indicate the importance of including rotational coupling to ² Pi states when determining state-to-state cross sections.

Effective range theory, taking into account the polarisation interaction, is used for the extrapolation of the phaseshifts for the scattering of electrons by heavy alkali atoms to the region of thermal energies. The phaseshifts obtained are used for the calculation of impact broadening and the shift of Rydberg levels by alkali perturbers in the ground state. The parameters of the theory obtained from the ab initio calculations give a good description of the data on broadening and shift in the case of K and Rb. In the case of Cs the parameters have to be somewhat modified to give an agreement with experiment. The phaseshifts are also used for the calculation of the width and shift in the free-electron spin-resonance frequency due to collisions with heavy alkali atoms. The results of calculations are in good agreement with experimental data. The results for total and momentum transfer cross sections for the scattering of electrons with energies from 10^-4 to 10^-1 eV by K, Rb and Cs atoms are presented.

A fast reliable method is described for the calculation of the total di-electronic recombination rate coefficient alpha _d(i;tot) of a ground-state He-like ion i. The equivalent-electron frozen-core approximation of Badnell (1984) is used to describe the three-electron problem. Electron exchange in the radial function of the spectator electron is allowed for by using the CEDW radial equations of Badnell (1983). A very accurate result for the total di-electronic recombination rate coefficient for a highly charged ion can be obtained from a single-configuration LS-coupling calculation if A_a<<A_r and A_a<<A_r, where U:A_i to A_i, since this implies that alpha _d(i;tot) remains almost unchanged under the unitary transformation U to configuration mixing and intermediate coupling. For Fe²⁴⁺, di-electronic recombination via intermediate states of the form 1sn_cl_cnl has been calculated. Tables of a small set of results that depend only on n_c and n are presented. The contribution from higher states (n>5,2<or=n_c<or=5) is estimated. Results are presented for the total di-electronic recombination rate coefficient of the ground-state Fe²⁴⁺ ion.