Table of contents

A simple closed-form expression is obtained for all negative-order hydrogenic dipole sum rules. Together with an earlier theorem relating hydrogenic multipole sum rules to the dipole sum rules, one has a closed-form expression for all negative-order hydrogenic multiple sum rules.

The 6s to 7s shake-up relative to the 5s ionisation in Cs is calculated in the sudden approximation by a configuration interaction approach. A strong correlation is found for the 5s5p⁶7s excited state. A qualitative agreement is obtained between the computed values and a recent experiment.

The absolute photoionisation cross section of the 11³D Rydberg state of ⁴He has been measured over a range of frequencies very close to the ionisation threshold. The measurements are compared with a calculation of the cross section performed using the quantum defect method, and the agreement is found to be within the +or-20% uncertainty associated with the measurements.

The authors have calculated cross sections for the excitation of Na atoms by proton impact, employing a two-centre expansion in travelling atomic orbitals. They have compared the calculated cross section for the excitation of Na(3p) state with the recent absolute measurements of Jitschin et al. (1986) and Aumayr et al. (1987). The authors have also calculated the collisionally induced alignment parameter A₂₀ of the 3p state of the Na atom. The computed values are in good quantitative agreement with the experimental data.

Absolute measurements of the total cross section for electron scattering on H₂O molecules are presented. The energy range extends from 81 to 3000 eV. A comparison with previous experimental data is given.

The self-consistent multiconfiguration Dirac-Fock method is used to obtain total energies of helium-like ions. The authors first discuss foundations and gauge dependence of the model from the standpoint of quantum electrodynamics. Retardation contributions to energies are considered and relativistic correlation energies presented, which result in non-negligible shifts. Numerical data and scaling laws are given and interpreted.

For pt.I see ibid., vol.20, p.639-49 (1987). A new method of estimating self-energy corrections in two-electron systems is presented. Multiconfiguration Dirac-Fock results for helium-like ions, including radiative corrections, are discussed and compared with recent experimental data. Magnetic interaction is shown to induce level crossings between 1s2p fine-structure levels.

The extended average level version of the multiconfiguration Dirac-Fock method is used to calculate the fine-structure intervals of the 1s²2s2p configuration in the beryllium isoelectronic sequence. These results are compared with experimental data, theoretical results of Cheng, Kim and Desclaux (1979), and the semi-empirical results of Edlen (1981). The present results are in excellent agreement with experimental data.

A two-electron model together with a correlated orbital method are used to compute the Sigma , Pi and Delta potential curves of Na₂ up to the states dissociating into Na(3p)+Na(3p). Crossings of the 'adiabatic' curves with the ground-state potential curve of the Na₂⁺ ion occur for the 8 ³ Sigma _u⁺ and 1¹ Sigma _u^- states at the internuclear distances R=5 and 8.9 au respectively. The efficiency of the Pluvinage method for the treatment of excited states is discussed.

A revised and extended analysis of the optical spectrum of Ar III is presented. All the energy levels in the 3s²3p⁴, 3s3p⁵, 3s⁰3p⁶, 3s²3p³4s, 4p and 3d configurations are reported. Nearly all the singlet terms, which are of particular interest in Auger spectroscopy are new. An interpretation of the odd configurations based on a newly introduced orthogonal parameter set for p^Nd configurations is reported. The strong mixing between parent terms in the 3p³3d configuration is noted and some consequences for the spectroscopy of Arⁿ⁺+Ar collisions are discussed. A comparison with Auger results for the LMM transitions in Ar is given.

An alignment factor which expresses the degree of alignment induced in an excited level by two-step excitation with short, broad-line pulses with parallel linear polarisations, is evaluated for various sequences of angular momenta J₀ to J₁ to J₂. Unexpected zero alignments (in the short-pulse case) are found for two-step excitation involving the pairs of transitions J₀=4 to J₁=5 to J₂=5 and J₀=4 to J₁=4 to J₂=5. The results have application in the interpretation of Zeeman quantum beat signals, both direct and cascade beats, resulting from two-step excitation using pulsed laser sources.

Inter-channel mixing effects in Auger decay are computed from the asymptotic form of the free-electron wavefunctions. These wavefunctions are solutions of an integro-differential system and are similar to those solutions obtained in the close-coupling method used in scattering problems. The method is developed in a relativistic form. As an application, the K-LL Auger transitions of neon are calculated and the results show good agreement with other methods which take these inter-channel mixing effects into account.

The authors present results of an experiment in which isolated atoms of five rare gases were ionised by an extremely intense CO₂ laser pulse (10¹⁴ W cm^-2). The results are in the form of plots of ion numbers against laser intensity. These are shown for each gas and at two different arbitrary wavelengths. The authors observed up to triply charged ions, and analysis indicates that these are formed through a step-wise process, involving the absorption of at least 500 photons for energy conservation. They also show that the interactions are highly non-linear.

Width and shift parameters for the components of the 4p-3d triplet of Ca⁺ broadened by helium were calculated for temperatures of 500-3000 K. Fine-structure-changing rates for 3d_5/2 to 3d_3/2 transitions of Ca⁺ were also calculated and an analytical form for the temperature dependence is given. Ca⁺-He interaction energies obtained from one-electron model potential calculations and an ab initio core-core calculation were used. A comparison with experimental results proved less satisfactory than for a similar calculation carried out previously on Ca⁺ 4p-4s broadening. Calculated values were 16 and 35% larger in the case of the j=3/2-5/2 and j=1/2-3/2 lines respectively at the only temperature for which a measurement was available (650 K). The broadening and fine-structure results are discussed.

Helium atoms are excited to an n=3 level by pulsed laser light in a glow discharge plasma, and the subsequent temporal development of the upper-level populations and the electron density are determined from the emission line intensities and by the microwave cavity resonance method, respectively. Associative ionisation rate coefficients are obtained for 3¹P-, 3¹D- and 3³D-state atoms. When combined with the previous determination of the excitation transfer and total depopulation collision rate coefficients a consistent set of rate coefficients is obtained. A qualitative discussion on the detailed mechanism of these transitions is given on the basis of the diabetic molecular potential curve crossings.

A beam gas method is used to measure cross sections Q for ion pair production in collisions of Na-Na and Na*-Na in a projectile energy range from 1000 to 5500 eV. The Na*, which represents Na in the 3p²P resonance state, is generated by a laser. Six reactions arise from the collision, because negative sodium ions can be formed in the ground or an excited state and because an ionic product can originate from either Na or Na*. The most obvious experimental result is that all Q for Na*-Na collisions are at least a factor of 35 greater than the Q for Na-Na collisions. This is due to laser excitation of Na. Ion pair production for Na -Na involves a many-state, complex system for which there are no theoretical predictions. However, the authors have successfully used some simplistic approaches to explain qualitatively a few experimental observations.

The probability for capture at a given angle of deflection, defined as the ratio between the differential cross sections for capture and scattering at that angle, was measured as a function of scattering angle (22.5 degrees -90 degrees ) for 0.4-1.0 MeV protons on neon. The probability exhibits a broad structure, which may be interpreted in terms of specific double-scattering events predicted classically as well as in the second Born approximation.

The coupled state impact parameter model has been used to study collisions of alpha particles with sodium atoms. Straight-line trajectories are employed. The charge exchange and excitation cross sections are computed for impact energies in the range 10.0-270.0 keV. The calculated total charge exchange cross sections are found to be in good agreement with the recent experimental data of Dubois and Toburen (1985) in the energy range 10.0<or=E_lab<or=100.0 keV. It is predicted that above this energy electron capture from the L shell starts to compete with electron capture from the M shell which is considered. The authors find that, for impact energies below 30.0 keV, the electrons are preferentially captured in the He⁺ (3l) state. This leads to a distinct possibility of population inversion and emission of Lyman- alpha (304 AA) and Lyman- beta (256 AA) soft X-ray photons.

The free-free absorption coefficient of the negative ion of hydrogen is calculated within the framework of the R-matrix method. The 1s, 2s and 2p states of hydrogen together with three pseudostates have been employed in the eigenfunction expansion. The results have been found to be in good agreement with other calculations and are believed to be currently the most accurate available.

Dielectronic recombination (DR) processes under the influence of an electric field are investigated theoretically using a multichannel quantum-defect theory of the Stark effect which has recently been developed by the present author. Simple formulae are derived for calculating the DR cross sections so that the effect of an applied electric field is correctly taken into account. Some model calculations are performed for the DR probability averaged over the resonance profile. An enhancement of the DR probability is induced by an electric field, as is predicted by Jacobs et al. (1976) and LaGattuta and Hahn (1983). It is found that the method of Jacobs et al and LaGattuta and Hahn can be used to estimate roughly the effect of an electric field on the DR process.

Triple differential cross sections at incident electron energies of E₀=150 and 250 eV have been measured and compared with theoretical work performed in the Born and Glauber approximations. Post-collision interaction effects lead to an exchange of energy and angular momentum between the escaping electrons. These effects have been taken into account approximately and improve the results considerably.

Electron impact collision strengths for forbidden and allowed transitions in Cl VI from its ground state have been calculated using the R-matrix method. The authors have included the states 3s²¹S; 3s3p^1,3P^o; 3p²¹D, ³P, ¹S; 3s3d^1,3D and 3p3d^1,3P^o3D^o in the expansion of the total wavefunction. These target states are represented by configuration interaction wavefunctions modified from those of Baluja and Hibbert (1985). The contribution from higher partial waves for allowed transitions is obtained by a geometric series sum. The effective collision strengths were obtained over a wide temperature range by integrating the collision strengths over a Maxwellian distribution of electron energies. This is the first detailed calculation on this ion in which the effects of exchange, channel coupling and correlation have been taken into account.

Electron attachment cross sections are reported in the electron energy range 0-160 meV, and at energy resolutions of 7.0 and 7.5 meV (FWHM), for the molecules CF₂Cl₂ (dichlorodifluoromethane), c-C₄F₆ (perfluorocyclobutene), c-C₄F₈ (perfluorocyclobutane) and c-C₇F₁₄ (perfluoromethylcyclohexane). Use is made of the Kr photoionisation method. Measured attachment lineshapes are deconvoluted from the spectral slit function, and are converted to cross sections by normalisation through thermal attachment rate constants. Comparisons are made with attachment cross sections derived from several independent sets of swarm-measured rate constants, and with collisional ionisation (high-Rydberg attachment) data.

The temporal evolution of a non-equilibrium positron swarm undergoing thermalisation and annihilation in either helium or neon is studied with the appropriate Fokker-Planck equation. The solution of the Fokker-Planck equation is obtained with the expansion of the positron distribution function in the eigenfunctions of the Fokker-Planck operator. The distribution function and average values such as the number density, the average energy, the effective annihilation rate parameter and the drift velocity can be expressed as a sum of exponential terms, with each term characterised by a different eigenvalue of the Fokker-Planck equation. In particular, the lifetime spectrum can be written in the form A(t)=n₀ lambda ₀exp(- lambda ₀t)+ Sigma ^infinity _k=1n_k lambda _kexp(- lambda _kt) where the lambda _k are the eigenvalues of the Fokker-Planck equation and the coefficients n_k are determined from the initial positron distribution. The nature of the relaxation to equilibrium is then understood in terms of the eigenvalue spectrum of the Fokker-Planck operator. In particular, the shoulder region of the lifetime spectrum is described by the summation above, whereas the equilibrium region is given by the first exponential term in lambda ₀, which is the annihilation rate coefficient. The nature of this eigenvalue spectrum is also interpreted with the transformation to an equivalent Schrodinger eigenvalue problem.

The authors have analysed the density and temperature dependences of orthopositronium annihilation rates in ethane gas following the simple model of positronium-induced cavities. The results obtained for the potential barrier, size of the cavity and positronium binding energy show that a bound state of positronium does not exist in this cavity. This is further supported by calculations presented for the changes in free energy due to positronium trapping following the self-consistent field model. An analysis of these annihilation rate data following a simple model of density fluctuations provides a reasonably good description of the observations for relatively small density fluctuations.