Table of contents

The recently proposed model potential for including correlation effects in relativistic Hartree-Fock calculations is applied to computations of oscillator strengths for the lowest transitions in Cu I, Ag I and Au I spectra. The present results remove the large discrepancies between the measured f_ik values for resonance transitions and the values from single-configuration calculations.

Spectra of Y XXXI and Mo XXXIV in the F I isoelectronic sequence have been obtained from laser-produced plasmas. 11 spectral lines for Y XXXI and 9 for Mo XXXIV have been identified as transitions between configurations 2p⁵-2p⁴3d.

The asymmetry parameter beta of photoelectrons ejected from the 2s shell of carbon have been calculated for three carbon ionic states, ⁴P^e, ²D^e and ²S^e. The large departures from the Cooper-Zare value of 2 in each case can be understood systematically in the angular momentum transfer theory of Dill and Fano.

Autler-Townes splitting of photoelectron energy distributions in resonant two-photon ionisation is predicted using standard Heitler-Ma techniques.

An expression for the N-photon ionisation probability for a many-level atom is obtained as a function of the laser pulse duration, t. It is shown that the leading term in the power series expansion is of order t^2N-1. The relevance of these calculations to some recent experiments by Lompre et al. (1976, 1977) is discussed.

A modified plane-wave approximation is used to calculate the differential cross section for 1s-2s excitation of atomic hydrogen by electron and positron impact. The results obtained are found to be in reasonable agreement with the other recent theoretical calculations.

A recent criticism by Jones and Ali, see ibid., vol.11, no.3, p.L87 (1978) of the paper 'Population inversions for soft X-ray transitions computed for rapidly cooling plasmas' of Talents, see ibid., vol.10, no.9, p.1769 (1977) is discussed.

Evaluation of the distribution function f(p₁₂), where p₁₂ is the momentum difference mod p₁-p₂ mod , enables the investigation of the concept of a Coulomb hole Delta f(p₁₂) in momentum space. Results are presented for the isoelectronic systems H^-, He and Li⁺. The electron correlation within each CI wavefunction was analysed into its radial and angular components so that the structure and composition of Delta f(p₁₂) could be assessed. The two-particle momentum radial density distribution and several two-particle expectation quantities are also examined. The findings indicate that, in momentum space, the radial components of correlation produce effects characteristic of total correlation in position space whereas, by contrast, angular correlation creates an opposite effect. Thus the shape and formation of Delta f(p₁₂) proves to be considerably more complex than that found for its counterpart in position space.

For pt.II see ibid., vol.11, no.12, p.2069 (1978). The authors have calculated the second-order perturbative corrections to the Hartree-Fock ground state energies of B, C, N, O and F. The basis used was sufficiently large to assure truncation errors less than 10^-3 au in all cases. They found E_B⁽²⁾=-0.1113 au, E_C⁽²⁾=-0.1476 au, E_N⁽²⁾=-0.1840 au, E_O⁽²⁾=-0.2540 au and E_F⁽²⁾=-0.3206 au which represent 89.2%, 94.3%, 97.6%, 98.5% and 99.6% respectively of the empirical correlation energies.

The 'relativistic pseudopotential approach' is applied to the calculation of electric-dipole transition probabilities in the valence shell of heavy atoms. An adequate transition operator is derived. The lowest spin-allowed and spin-forbidden transitions of the heavier group I and II elements and a series of higher transitions of Ca I and II, Cs, I, Ba I and II, Au I and Hg I are calculated. The influence of the core-valence correlation is investigated with Hameed's core-polarisation correction.

In a two-laser experiment the authors measure the broadening of the saturated absorption profile of the 3.51 mu m line of xenon, in the presence of helium. The pressure of xenon is fixed at 3 mTorr, and the helium pressure is varied between 0 and 550 mTorr. The computation of the profile includes the effect of elastic collisions without any adjustment of parameters. This is made possible since all the necessary parameters are known and a particularly simple description of the collisions is valid (Brownian motion, hard-sphere potential). The comparison with the experimental data shows that there is satisfactory agreement at high pressures of helium but an unexplained discrepancy exists at low pressure which could be attributed to inelastic processes.

A quantum mechanical treatment of a two-level atom interacting with a short laser pulse is presented. Rigorous conditions under which simulated processes dominate over spontaneous emission are established. Under these conditions an approximation is derived which may be viewed as a generalised form of the usual single-mode approximation provides a simple quantum mechanical way of treating interaction processes involving a coherent field.

The authors report an experimental investigation of the dependence of the selection rules for two-photon absorption as a function of the detuning from the real intermediate states of the laser light exciting the transition. The transition examined is the 3s-5s two-photon transition in atomic sodium. A clear wavelength dependence of the polarisation selection rules is observed, the range of which we attribute to saturation of the single-photon transition to the intermediate state.

He⁺-H collisions are studied for energies lower than the electron excitation threshold. Unpolarised cross sections having resonances and values greater than 4000 a₀² and spin-change cross sections showing high structures are obtained. Comparisons with the corresponding H-H collision cross sections are displayed. The sensitivity of the cross sections to the potential energy curves is discussed.

A time-resolved quantum beat technique has been employed in a beam-tilted-foil experiment to search for systematic changes in atomic orientation as the tilt angle of the foil is varied. The data obtained are in substantial agreement with existing theories relating to the production of such orientation, but indicate that further work is required in both the theoretical and experimental aspects of the problem.

The line shape of the (1s2s2p)⁴P (metastable) to (1s²)¹S+e^- Auger electron transition from heavy-ion impact is measured for targets containing carbon, nitrogen, oxygen and neon. The angular distribution of this transition is also measured for molecular gas targets containing carbon. Ar, Kr, Xe and U projectiles from the UNILAC were used with energy 1.4 MeV amu^-1. It is found that the line shape depends strongly on the initial molecular target. Kinematic effects such as projectile-induced target recoil do not account well for these changes. A simple model is proposed that assumes the creation of a highly ionised molecule. The line shape is due to Doppler broadening from the velocity acquired by the dissociation of the molecule. Reasonable agreement between this Coulomb explosion model and the experimental results is found, allowing dissociation velocities to be calculated from the experimental line shapes.

The elastic scattering of electrons by He⁺ ions is considered in a modified Coulomb-Glauber approximation in order to take better account of close-encounter collisions. It is found that in the ordinary Coulomb-Glauber case the ratio of the total differential cross section to that due to the Coulomb interaction alone is almost equal to unity near the forward and backward directions and shows a minimum at some intermediate angle. In the modified version this ratio increases beyond unity at large scattering angles. The minimum in the ratio moves towards smaller angles and becomes deeper as the energy increases.

Electron drift velocities, w, as a function of temperature were measured and on the basis of these data, Ramsauer-Townsend (RT) minima were found in the cross section for momentum transfer, sigma _m, for all these gases. The position of the RT minimum and the magnitude of the corresponding sigma _m at the energy where the minimum occurs are 0.12, 0.14, 0.14, and 0.25 eV and 1.2, 3.0, 4.6, and 5.8*10^-16 cm² for ethane, propane, butane, and neopentane, respectively. The density dependence of the w for neopentane was also investigated. This latter investigation and similar ones in the literature on methane, ethane, and propane are discussed in connection with the observed RT minima and the mobility of thermal electrons in the low- and high-pressure vapours and the liquids of these compounds.

A simplified electron-hydrogen excitation problem is solved for low and medium energies by neglecting all angular momentum. The integrals arising can be expressed in terms of hypergeometric functions, so that accurate solutions can be computed rapidly. This model situation still includes an effect that complicates more realistic problems, that of strong coupling between an infinite number of open channels, and the associated need for an adequate continuum representation. These results are therefore of importance in testing other methods which have been used, or are under development, for solving the full physical problem.

A distorted-wave model is developed, for the ionisation of helium by electron impact at intermediate energies, which takes into account polarisation and absorption in the initial state. The triple-differential cross sections are calculated in the energy range 80-250 eV and the results are compared with experimental data.

Absolute measurements are reported of the electron impact ionisation cross sections of ground-state atoms of helium, carbon, nitrogen and oxygen for electron energies ranging from 7 to 1000 eV. The method used was that of crossed beams in which fast beams of atoms are produced by charge capture of 2 to 4 keV ions in a gas target. The data for helium atoms above 100 eV are in excellent agreement (+or-5%) with the absolute data of Rapp and Englander-Golden (1965) who used a static gas target technique. However, the present data are smaller in magnitude at lower energies. Data for oxygen and nitrogen atoms are in adequate agreement with data from earlier crossed-beams experiments that employed thermal-energy atom beams.