Table of contents

The authors investigate the ground state energy of H^- using a variational function (M. Wu et al. 1985). Contrary to their conclusions, the authors find that their function produces results comparable with a previous calculation (R.E. Williamson, 1942). Furthermore, the explicit formulae given in the present paper can easily be applied to the helium isoelectronic series.

The authors have performed ab initio multiconfiguration Dirac-Fock (MCDF) calculations of the electron density at the nucleus for all states belonging to the four lowest-lying terms of the 4f¹¹5d6s² configuration in Er I. From a comparison with recent experimental optical isotope shift data for the two lowest lying terms within this manifold, the authors discuss the validity of the state-dependent ab initio MCDF approach to electron density calculations.

The XUV spectra of thorium in three different regimes of optical thickness have been observed in a laser-produced plasma. An intense 5d-5f transition array, the profile and extent of which are strongly sensitive to the thorium ion density, dominates the spectrum. A number of lines belonging to the 5d¹⁰6s²-5d⁹6s²5f, 6p and 5d¹⁰-5d⁹5f, 6p transition arrays of Th XI and Th XIII, respectively, are identified with the aid of Hartree-Fock and Dirac-Fock calculations.

Presents a qualitative interpretation of the area-dependent spectrum of pulse-induced resonance fluorescence predicted in recent theoretical papers. It is pointed out that the source integrals that determine the frequency distribution of the spontaneous emission resemble those that appear in the theory of diffraction, and the source functions become symmetric 'gratings' with finite numbers of 'slits' whenever the pulse area is a restoring eigenvalue. The author notes that these 'magic areas' are not invariably 2 pi n with n integral.

The multiphoton ionisation of xenon has been studied using a picosecond Nd:YAG laser and a time-of-flight electron spectrometer with a collection angle of 2 pi sr. In the absence of space charge effects, suppression of the first electron peak is observed at an intensity of 5*10¹² W cm^-2 in linear polarisation, while saturation occurs at 1.3*10¹³ W cm^-2. In circular polarisation, the same effect is observed at a slightly lower intensity. The intensity dependences of the first three peaks have been measured.

The angular distribution of electrons emitted after absorption of a large number of photons is strongly peaked in the direction of the light polarisation. The effect of space charge is to broaden the central lobe. It may also explain the vanishing of the first peaks in electron energy spectra observed with a small collection angle.

An ab initio calculation of the double photoionisation of molecular hydrogen in its equilibrium ground-state geometry is presented. The total cross section obtained in the dipole velocity formulation shows reasonable agreement with very recent experimental results. By use of the differential electron spectrum some conclusions are drawn concerning the kinetic energies of the epsilon s epsilon 'p and epsilon p epsilon 'd pairs of ejected electrons.

SCF-CI techniques are used to calculate electron capture cross sections for the symmetric two-electron He⁺He⁺ system using the measured energy range by Pearl et al. (1983). The radial coupling is obtained via a new formula for the CI numerical derivative, and the common translation factor is included to obtain convergence. The author finds good agreement with the experiment, but disagrees with other recent calculations.

Evidence for simultaneous electron excitation and capture is found in collisions between Ne⁷⁺(1s²2s) ions and He atoms. Population of the Ne⁶⁺(1s²2p3l) states is found to amount to up to 16% of the total electron capture cross section. Unambiguous identification of the transfer excitation channel has been obtained with a new technique where charge states of both projectile and target as well as energy gain and scattering angle are all simultaneously determined. The relative importance of 'correlated' electron processes is discussed in view of the presently available data.

The deviations from constancy in the density of energy partitioning are investigated. These are determined by the classical equations of the collinear motion of the outgoing electrons and change by less than 6% over the full region of ionisation. Some discrepancies with the results are discussed (F. Read, 1984).

A new forward-angle dispersion relation (DR) for electron-atom scattering is proposed. It is based on a subtraction of the static-exchange amplitude from the exact elastic scattering amplitude. Arguments are advanced to explain why this should obviate the difficulties associated with the Gerjuoy-Krall DR specifically with the exchange Born-amplitude. The new DR is tested in the elastic energy range for e-H scattering and compared with the GKDR.

An approximate expression is derived for the inelastic K-matrix elements for rotational excitation of molecules by electron impact. This expression incorporates a Born-Oppenheimer separation of the projectile and nuclear coordinates in the continuum wavefunction but allows for different energies in the entrance and exit channels. Hence it is particularly suited to near-threshold collisions. The method is applied to pure rotational excitation of H₂ by comparing approximate cross sections with those from laboratory frame close-coupling calculations.

Resonance energies and widths are calculated for s- and p-waves in some spherically symmetric short-range potentials, using the scattering matrix, the threshold-matrix and the confinement-in-a-box methods. Numerical results and comparisons of the three methods are presented and discussed.

For pt.IV see Physica, vol.131, A, p.363 (1985). The g-Hartree method is adapted to the direct ab initio calculation of transition energies in atoms. For the elements He (Z=2), Li (Z=3), Be (Z=4), Ne (Z=10), Mg (Z=12) and Ar (Z=18) all ionisation energies have been computed and found to agree with available experimental data within the computational errors. The systematic aspect (which is a truly second-order determination of the optimal mean field and contains no semi-phenomenological partial-summation prescriptions such as the self-energy summations in the Schwinger-Dyson approach) is emphasised and is shown to match experimental data significantly better than second-order (in the same strict sense) Brueckner-Hartree-Fock calculations. A detailed comparison of the method with other published approximation schemes shows a systematic improvement over the results obtained in conventional many body theories.

A theoretical treatment of the Stark effect based on the quantum-defect theory and the WKB method, which was recently developed for alkali atoms (D. Harmin, 1984) is generalised to include ion cores of any state. Also investigated is the effect of an electric field on electron-ion resonance scattering. The present theory serves also as an extension of the multichannel quantum-defect theory (M. Seaton, 1983) to the theory of an electron-ion system under the influence of an external electric field. Numerical calculations of some simple problems are performed.

The binary collision and complete binary collision approximations used in electronic Stark-broadening calculations have been analysed by means of a computer simulation technique for the Lyman alpha line in hydrogen. A simulation process has been developed in which three spectral profiles are simultaneously obtained, corresponding to the exact calculation and each of the two above approximations. Calculations have been performed under four different temperature and density conditions.

The Lehmberg-Agarwal master equation is used to study cooperative spontaneous emission by a system of three identical two-level atoms. A choice of basis states is described which allows an exact solution to be obtained for any geometrical configuration, any sample size, and any initial conditions of the atomic system, fully including both near-field and super-radiance effects.

The authors discuss the multiphoton resonant ionisation of atoms by smooth laser pulses. A number of interesting coherent phenomena, such as multipeak splitting of the photoelectron spectrum are observed and studied for different laser detunings, pulse durations and AC Stark shifts.

The coincidence between threshold electrons and ions from CO was used to separate the parent and fragment ions between 500 and 580 AA using synchrotron radiation as the light source. Most of the C and D state above the dissociation limit were confirmed to predissociate to C⁺+O.

The authors studied the elastic and inelastic processes for scattering of He⁺ ions by H₂ molecules between 2 and 30 keV in the angular range from 10' to 3 degrees by an energy loss technique. The analysis of elastic energy loss against E₀ theta ² shows that the incident ion is scattered by the entire molecule at low energies whereas at high energies it is scattered by only one atom in the target. Comparison of the relative probabilities of the various excitation processes with the isoelectronic system He⁺-He demonstrates the different behaviours of a molecular target in the excitation mechanism.

The impact parameter dependence of L-subshell ionisation probabilities I(L_i(b)) has been measured for 0.9 MeV u^-1 He on Pt (Z_ua=80), Ne on Pt (Z_ua=88) and Ne on Yb (Z_ua=80), and for 6 MeV p on Au. For the collision system 0.9 MeV u^-1 (I(b)<10^-3) the authors observed the crossing of I(L_I) and IL_III) at a very low value of the impact parameter, in good agreement with first-order perturbation theory; this crossing is due to relativistic effects in the electronic wavefunctions. For increasing perturbation (I(b) to 10^-2), however, the b dependence of measured L-subshell ionisation probabilities I(L_i(b)) differs strongly from the predicted values. These deviations can be explained by vacancy sharing mechanisms between the L subshells during the collision process.

The classical expressions for kinetic theory cross sections which are related to the Senftleben-Beenakker effect on viscosity, diffusion, conductivity and thermal diffusion are evaluated using the HFD1 surface (R. Fuchs et al. (1984)) in the temperature range 77.3-1100K. The depolarised Rayleigh scattering and rotational-relaxation cross sections are also obtained. Comparisons with experiment show that the authors' values for these cross sections are 10-80% larger than the measurements. These consistent discrepancies suggest that this potential surface is too anisotropic.

Recent experiments have shown oscillatory structures in angular differential cross sections for two-electron capture by C⁴⁺ from He at energies between 300 and 1500 eV. Using semiclassical and semiquantal methods, the authors have computed theoretical cross sections for this process and good agreement with the experiments is found. The oscillations are identified as Stueckelberg oscillations and special attention is drawn to a rainbow appearing in the semiclassical cross sections. The authors show that this rainbow is due to the definition of the semiclassical deflection functions and that it vanishes with the semiquantal method.

Experimental absolute differential cross sections are presented for elastic scattering of 20-200 eV electrons from the noble gases, with an angular resolution better than 0.25 degrees . A novel type of parallel-plate electrostatic energy analyser is used for the simultaneous determination of the angular distribution of the scattered particles. The use of microchannel plates and a multichannel detection technique yields an improvement in measuring efficiency of at least two orders of magnitude compared with the traditional angular-scan technique. The accumulated signals have been made absolute to within 7% by carefully determining all the relevant parameters such as primary-beam intensity, gas density and collision length, analyser transmission and channel plate efficiency. A comparison is made between the present results and those from various existing theoretical models. The best agreement was found with the R-matrix calculations of Fon et al. (1984). A hitherto unknown structure in helium was revealed when the impact energy was scanned in a narrow range around the threshold for ionisation.

The interaction potential of e^--molecule scattering is formulated in terms of the optical potential of many-body theory. In particular, dynamic polarisation effects directly follow from an RPA solution to the target linear response function. It is further pointed out that such a level of accuracy can most easily be obtained by simply working within the formalism of Hartree-Fock theory. An adiabatic approximation to the full optical potential is finally derived and explicitly applied to the e^--H₂ system, showing good agreement with alternative ab initio formulations of the literature.

Using a parametrised expression for the triply differential electron impact ionisation cross section derived in an earlier paper (P. Altick 1985), new fits are carried out on experimental data for e+He at 1, 2 and 4 eV. The data used for the fits include doubly differential cross sections as well as triply differential ones, and the inclusion of the former alters the parameters from those previously obtained. With the new parameters, non-coplanar plots of the cross section are constructed and displayed. Normalised triply differential cross sections are also derived from the absolute doubly differential measurements.

A new apparatus incorporating an electron transmission spectrometer and a free jet has been used to study temporary negative-ion formation in O₂. The resonant vibrational levels of the ground electronic state of O₂^- ( nu "=4-12) were observed as spin-orbit doublets, consistent with previous experimental results. The vibrational constants of O₂^-, an upper bound on the autodetachment width of the nu "=4 vibrational level and the relative electron attachment cross sections into the vibrational levels ( nu "=5-12) have been determined. A velocity broadening term, stemming from the use of an uncollimated free jet, has also been calculated. Measurements of resonances in Ar and He were used to verify the magnitude of this term and permitted the instrument function to be extracted as well.

Angular correlation parameters ( lambda , chi ,O_1-^c, A₁₊^c) and differential cross sections are calculated for the resonance excitation of lithium atoms by positron impact. A two-potential approach is used to obtain these parameters at energies of 20 and 60 eV. A comparison between positron and electron angular correlation parameters is also presented.