Table of contents

The interaction between a shape or giant resonance and a Rydberg series of finer resonances is discussed. In the limiting case of an infinitely broad shape resonance, the Seaton-Dubau formula is recovered in the two-channel approximation. More generally, it is shown that a q reversal always occurs at about the energy of the giant resonance and that small values of q occur towards its centre.

The authors present new high-accuracy measurements of the triple differential cross sections for ionisation of helium by several keV electron impact, obtained in fully asymmetric conditions at small ejected electron energies and small and intermediate momentum transfer values. Comparison is made with two accurate calculations within the first Born approximation. It is shown that this approximation correctly describes the experimental findings and that higher-order effects (if any) are smaller than 10%.

A set of measured triple differential cross sections with symmetric kinematics is shown to converge for increasing incident energy to a finite limit identical to the first Born approximation. Experiments on helium for momentum transfer 0.5 au are compared with each other as a function of electron impact energies between 150 and 8000 eV. For all impact energies, for which deviations from the first Born approximation appear, the most prominent correction term is the post-collision interaction.

A procedure is described for using orthogonal operators, nu _k to reduce the residual discrepancies Delta _i that result when a least-squares fit to the levels of an atomic configuration is carried out. Linear combinations nu '_j of the members of a given set of nu _k for which the associated parameters p'_j are strictly uncorrelated are constructed. The theory is applied to FE VI 3d²4p. The mean error of 188 cm^-1 obtained by Ekberg with 9 parameters is reduced in successive steps to 77, 15, 11 and 10 cm^-1 with 12, 18, 19 and 21 parameters, respectively. The importance of a class of three-electron scalar operators is established. However, the nu _k that are based on group theory are connected to the nu '_j by a unitary matrix that possesses significant off-diagonal components.

Strong sd-p² interaction in the 3d⁵(4s4d+4p²) complex of doubly excited configurations in Fe II causes a significant mixing and perturbation of sextet levels. This expected strong interaction induces a spin-orbit mixing between a ⁶D and an ⁸P term, manifested in an anomalous fine structure. A theoretical treatment of this complex explains the anomalies and illustrates how sensitive the effect of the spin-orbit interaction is to the sharpness of an accidental coincidence of levels.

The authors present diabatic calculations of A₂²⁺ (A=Mg or Hg); electronic sigma states are shown to be very similar for these two systems. For A=Hg, spin-orbit coupling has been accounted for. Although initiated for the extraction of parameters of a model Hamiltonian for large charged clusters, these calculations can also be used for charge transfer collision studies with the help of electronic couplings.

For pt.I, see ibid., vol.17, p.2599 (1984). General expressions for the line profile produced by an emitting atom embedded in a sea of randomly distributed identical perturbers, on the assumption that the interaction between the emitter and a perturber is of the type 1/R^p, were presented in Paper I. The basic assumptions of the theory are that all emitter-perturber interactions are additive and that the trajectory of the perturber is a straight line. The profile is expressed in terms of a 'universal' function that does not depend explicitly on any parameters associated with the particular spectral line or perturbing gas considered. Explicit results are given for the cases p=6 and p=7; these are relevant to the analysis of the broadening of atomic and molecular lines by atoms and molecules. For the case of p=6, the sensitivity of the line parameters to the direction assumed for the quantisation axis of the emitter is examined. It is also shown that the degree of asymmetry depends significantly upon the thermal average being correctly evaluated, and good agreement with experiment is obtained for the asymmetries of the sodium resonance lines perturbed by the heavier rare gases.

The 2p⁴-2p³3s transitions in atomic oxygen were investigated by the MCHF method with Breit-Pauli corrections for relativistic effects. Particular attention was given to the spin-forbidden 2p⁴¹D-2p³3s ³S^o transition recently identified in the satellite dayglow spectrum. A somewhat smaller branching ratio for the decay of the ³S^o state to the ¹D state relative to ³P than that derived by Meier and Conway (1985) from modelling the ³P-³S^o transition was found.

The photoabsorption spectrum of atomic sulphur was obtained in the 1220-840 AA wavelength region on a 3m normal incidence spectrograph using the flash pyrolysis technique. The previously known photoionisation cross section in the 1200-940 AA range has been extended and confirmed on the relative scale. The new discrete line absorption data have enabled the authors to extend the principal series limiting on ⁴S to n=23. Of the seven Rydberg series limiting on ²D and ²P of S II reported recently, six could be identified on the plates. They have been revised and confirmed. Four additional Rydberg series limiting on ²D and ²P have been observed and interpreted. All lines below 1220 AA assigned to S I have been reported.

The elastic scattering cross section of the h⁰-H⁺ interaction is calculated in the energy range from 1 to 100 keV using the partial-wave method. Using this method, sharp angular dependences of the cross section are obtained. These angular dependences are then compared with the results of the Born approximation and Rutherford scattering. The cross section in the Born approximation is obtained by exchanging an electron with a proton in the known electron-H⁰ scattering model. Agreements are obtained between these cross sections at high energy or in the region of large scattering angles. The results of the born approximation fit especially well with the partial-wave method at almost all scattering angles in the high-energy range. When total cross sections are obtained, they are compared with a charge transfer cross section and the effects of symmetry are also discussed.

Rb⁺ and Rb₂⁺ formation in a laser-excited Rb vapour is extensively studied for s and d levels ranging from 5d to 11s levels. Collisional ionisation with a ground-state atom leads to Rb₂⁺ formation by Hornbeck-Molnar reaction with rate coefficients lying in the 10^-10 cm³ s^-1 range. This reaction is analysed theoretically. Penning ionisation occurs in collision with a resonant-state atom, Rb(5p), with a high efficiency (rate coefficient above 10^-8 cm³ s^-1). Associative ionisation between the 7s level and the 5p level is also observed (rate coefficient about 10^-10 cm³ s^-1). This reaction appears negligible for the neighbouring 5d level.

Associative and Penning ionisation cross sections, rho _AI and rho _PI are calculated for collisions of two excited atoms Rb(nl)+Rb(5p ²P₃2/) in the classical impact parameter formulation for nl=5d to 9d and 7s. It is shown that the very large Penning ionisation rate constants, measured by the authors and presented earlier, can be well explained by such a model in terms of direct and exchange autoionisation widths, as well as the associative ionisation contributions which have been observed to be quite different for ns and nd cases.

An ejected-electron spectroscopic study has been carried out on the potassium autoionising state created by charge transfer excitation in collisions of the potassium ion with the neon atom. The ion impact energy was varied between 2.1 and 7.2 keV. A detailed analysis has been performed for the spectral profiles of autoionisation of the potassium 3p⁵4s²²P_1/2 2 and ²P_3/2 states. Their complete alignment with respect to the internuclear axis has been observed.

An analysis has been performed on previously measured proton- and ⁴He-ion-induced X-ray production cross sections for Z=47-53 targets. A smoother behaviour in relation to Z than the one predicted by theory has been confirmed for the L₁ subshell fluorescence yields and Coster-Kronig rates in this region. In the case of ⁴He-ion-induced measurements, the relative importance of LM and LN multi-ionisation and their dependence on the incident energy have been extracted by comparing the cross section ratios with the ones obtained using protons as projectiles. The effects predicted on the L-shell fluorescence yields and Coster-Kronig rates, due to the NL multi-ionisation processes, have been confirmed experimentally to be negligible.

The symmetric eikonal model has been extended for electron ionisation in ion-atom collisions. An approximation to this model has been applied to describe double differential cross sections. Calculations for H⁺H(1s) to H⁺+H⁺+e^- are compared with other present theoretical results obtained using the continuum-distorted-wave-eikonal-initial-state approximation. A qualitative comparison is made with experimental data for the H⁺+H₂, collision system.

The authors propose the use of the complex coordinate method in conjunction with a local-density, X alpha static exchange plus polarisation model for the analysis of shape resonances in electron-atom scattering with possible extension to electron-molecule collisional processes. The basic model is discussed and the X alpha formalism is briefly reviewed. The authors investigate the inclusion of polarisation and exchange terms for the study of shape resonances in electron-atom scattering. By the use of the exterior complex scaling technique, complex resonance poles of the partial-wave S matrix can be directly evaluated in the positive lower half of the complex energy plane on the second Riemann sheet. The method is illustrated by presenting numerical investigations of the e^-Be and e^--Mg scattering systems. The behaviour of the partial-wave cross section and its dependence on the corresponding partial-wave S-matrix poles are discussed. Furthermore, the authors examine the appearance of asymmetries in the cross section in the vicinity of the poles.

The free-free absorption coefficient for Ar^- is calculated at a wide range of radiation wavelengths and temperatures. The calculation employs a multichannel theory based upon the R-matrix method for electron-atom collisions. Comparison is made with other theoretical methods and with experiment. Highly satisfactory agreement exists with the bremsstrahlung emissivity experimental data of Ranson et al. (1977).

The results of an analysis of the 2s-2p resonance transition of the Li atom as a result of electron impact in the Glauber and modified Glauber approximations are reported. The cross sections of the process in the energy range 20-400 eV are calculated, using a model potential approach. The analytic expressions of the relevant scattering terms which constitute the Glauber and modified Glauber amplitudes of the process are derived and then reduced to forms which are accessible to numerical computation. Numerical techniques are developed to help alleviate the difficulties encountered in the evaluation of these amplitudes. The theoretical differential and integrated cross sections obtained are compared with experimental data.

The absolute cross section for the ionisation process Mo⁺+e to Mo²⁺+2e has been measured at electron energies ranging from the ionisation threshold to 1000 eV using the crossed electron-ion beams technique. The total error at a 90% confidence level is estimated to be less than +or-4.5% for energies above 20 eV. There is evidence that the Mo⁺ beam contained some metastable ions. In comparison with the experimental data, the scaled Born calculation of McGuire (1977), taking into account the 5s, 4d and 4p electrons, underestimates the cross section, particularly at low energies. There is evidence that excitation-autoionisation processes enhance the cross section near the peak and this may account for part of the discrepancy.

The authors present total absolute cross sections for metastable production in the rare gases by electron impact excitation, from threshold to 140 eV. Detailed comparison is made with previous experimental and theoretical work. The effects of atomic recoil in the detection process are discussed.

The average static dipole polarisability of the ground state of scandium is calculated. The polarisability is expressed in terms of the first-order correction in the wavefunction, due to the static electric dipole perturbation, which is determined variationally as a configuration interaction expansion. The present investigation illustrates the importance of including the configurations 3d4p5p and 3d4d5p in the wavefunction expansions. The importance of including pseudo-orbitals specifically determined to describe the polarisation of 3d is also discussed. It is also shown that it is possible to obtain a value for the average static dipole polarisability of scandium using quite simple wavefunctions.

The authors present exact solutions of the standard quantum optical model of near-threshold photodetachment for two kinds of pulse shapes: the exponentially rising pulse f₁(t) =exp(t/ tau ) theta (-6) and the continuously switched-off exponential pulse f₂(t)=(exp(t/ tau )-exp(2t/ tau )) theta (-t). The method of solving the Schrodinger equations consists of power expansion of time-dependent functions in the variable z=exp(t/ tau ). The authors analyse phenomena related to the dependence of photodetachment probability on the shape of the laser pulse. In particular, the authors discuss the asymptotic behaviour of the photodetachment probability for large pulse durations and the splitting of photoelectron spectra. Finally, the authors find a general approximate solution of the model for the below-threshold region.