Table of contents

Atomic orbital collapse in homologous sequences of atoms and singly and doubly charged ions is investigated from published experimental data, using a novel graphical technique. It is shown that the concept of a homologous sequence can be extended for this purpose, so that the plots embrace more elements than would otherwise be included, and that the most appropriate ordering within the sequence is not the one implied by the periodic table of the elements.

The author reports self-consistent field (SCF) and fourth-order Moeller-Plesset (MP4) calculations for the quadrupole ( Theta ), octopole ( Omega ) and hexadecapole ( Phi ) moments of hydrogen chloride. Their estimate for the ground vibrational state value of Theta is 2.77 ea²₀, to be compared with the experimental value of 2.78+or-0.09 ea²₀ reported by DeLeeuw and Dymanus (1973). For Omega and Phi their estimates are 4.15 ea³₀ and 14.21 ea⁴₀ respectively.

The adaption of the cluster expansion method to the calculation of resonance states in atomic photoionization is discussed and the corresponding one-particle and two-particle equations are derived. Furthermore some of the difficulties to extend this approach for the calculation of the photoionization cross section and the implementation of a coupled cluster expansion are discussed.

Simple analytical expressions for the cross sections of nl to n'l' transitions from Rydberg-neutral inelastic collisions are obtained on the basis of the free electron model for electron-perturber scattering. The dependences of the electron-perturber scattering amplitude on the electron momentum and scattering angle are allowed for. The analytical results agree well with available numerical calculations. The analysis of the equations obtained yields a conclusion that the cross sections at l<or=l' depend weakly on l and l' for degenerate states and are proportional to the statistical weights of the final states for transitions with large energy defects.

The double electron capture process by He²⁺ ions in collisions with helium is studied in the high velocity regime with the measurement of total cross sections at 1.5, 4 and 6 MeV beam energy and angular differential cross sections for 1.5 MeV. The experimental results compare well with theoretical calculations which include correlation effects in the initial and final He ground states for the derivation of the capture amplitude in a continuum distorted-wave approximation.

To evaluate single and double K-shell inclusive charge transfer probabilities in ion-atom collisions the authors solve the time-dependent Dirac equation. By expanding the time-dependent wavefunction in a set of molecular basis states the time-dependent equation reduces to a set of coupled-channel equations. The energy eigenvalues and matrix elements are taken from self-consistent relativistic molecular many-electron Dirac-Fock-Slater calculations. They present many-electron inclusive probabilities for different final configurations as a function of impact parameter for single and double K-shell vacancy production in collisions of bare S on Ar.

Cross sections for charge transfer between ground state (⁴S) O⁺ ions in H₂ have been measured within the energy range 0.1-1.0 keV. A novel photoionization source which eliminates problems associated with metastable ions was used and the results resolve the large discrepancies between previous experiments.

The effect of internal energy on the cross section for electron scattering from SF₆ is examined. A triply crossed beam geometry is employed, in which a free jet of SF₆ is irradiated with infrared light from a CO₂ laser and a magnetically collimated electron beam crosses the jet downstream from the laser beam. The laser beam is modulated and the change in the attenuated electron beam current is detected synchronously. Electron scattering is most strongly altered below 2 eV and at electron impact energies associated with the formation of temporary negative ions.

The electrostatic energy displacement for a hydrogen atom inserted between two parallel, perfectly conducting plates is calculated by means of perturbation theory, starting from the free-space solution of the Dirac equation for the hydrogen atom. The correction to the Coulomb potential of an electric point charge due to the influence of the boundary conditions imposed by the plates is determined using different methods. The resulting level shift is calculated in first order of perturbation theory with results given in terms of multipole expansions. The leading terms reproduce results already known, giving rise to a level shift of roughly in approximately=-n⁴ 10^-6 Hz (1 mm/a)³ for high principal quantum number n.

A model is developed to calculate anisotropy coefficients for resonant Auger emissions and for normal Auger emissions on elements with one electron in an open subshell. By applying some simple restrictions to the model the author connects the required matrix elements with those for the case of closed subshells which are already analysed. For an intermediate state with J_f=0 the angular distribution becomes independent of the matrix elements. The angular distribution parameters are calculated for these cases.

The finite-temperature Thomas-Fermi model has been used for many years to describe the electronic structure of hot dense plasmas. The authors investigates a simple correction to this model which involves excluding a region near the nucleus from the calculation. Comparison is made with energies in the low-density low-temperature limit.

The interactions of an oxygen atom with lithium clusters in the BCC (100) and BCC (110) symmetries have been investigated from the point of view of many-body perturbation theory. Different approach positions of the oxygen atom, namely, on top, open and bridge positions have been considered and the variations of the chemisorption energies among the different approach positions have been studied in detail. Results have been compared with the previous work on hydrogen and oxygen interactions with lithium clusters. Stronger interaction is predicted for the oxygen atom with the (110) surface than with the (100) surface and penetration of the adatom into the surface is expected. Electronic charge distributions have been analysed using Mulliken population analysis and charge density plots.

Quantum beats arising from the hyperfine interaction were observed in the 7D_3/2 to 5P_1/2 transition in ⁸⁵Rb. Theoretical expressions for the beat signals were fit to the data to determine the magnetic dipole constant a=1.415+or-0.030 MHz, electric quadrupole constant b=0.31+or-0.06 MHz and radiative lifetime tau =346+or-25 ns of the ⁸⁵Rb 7D_3/2 state.

The low-lying core-excited quintet states of Mg I have been studied experimentally and theoretically. On the basis of beam-foil spectroscopy, the energies of nearly all the quintet levels belonging to the 2p⁵3s3p² and 2p⁵3s3p4s configurations have been established, while only ⁵F levels from the 2p⁵3s3p3d configuration could be identified. Multiconfigurational Hartree-Fock calculations, including the 2p⁵3s3p², 3s3p3d, 3p³ and 3s3p4s configurations, have been performed to support the experimental assignments and to test the potentiality of this type of calculations to treat systems with a 2p⁵ core and three valence electrons. Good agreement is obtained between theory and experiment for the quintet levels belonging to the 2p⁵3s3p² and 2p⁵3s3p4s configurations.

The fifth spectrum of thallium was photographed in the 800-1700 AA wavelength region using a sliding spark and a triggered spark source. Two hundred and ninety-five lines, of which 102 lines show HFS, were classified in this spectrum. All but one level in the 5d⁸6s and all but two levels in the 5d⁸6p configuration have been established. The new observations confirm the earlier analysis of the 5d⁸6p configuration of Tl V. Least-squares fitted parametric calculations, using Slater-Condon parameters, were carried out.

The R-matrix method is used to obtain the cross section for the photoionization of the ground state of atomic nitrogen together with oscillator strengths for transitions between symmetries ⁴S^o and ⁴P. Using these results the discrepancy between theory and experiment for the photoionization cross section near threshold can be resolved.

The pressure-broadening coefficient of the nu ₃:^QQ(1,1) transition of CH₃F is measured in the infrared region using a high resolution saturation spectroscopic technique. The experiment employed a CO₂ laser locked to a Stark-tunable transition giving a tunability over 70 MHz at a linewidth better than 50 kHz. The corresponding coefficient is measured for non-zero molecular velocities in the optical field direction by using an acousto-optic modulator to provide a shifted laser frequency. It is found that long-range dipole-dipole interaction is responsible for the broadening.

The PEPIPICO technique has been used in conjunction with a source of continuum (synchrotron radiation) in order to determine the thresholds for fragmentation of CF₄ into CF₃⁺+F⁺ (37.6 eV), CF₂⁺+F⁺ (42.4 eV), CF⁺+F⁺ (47.5 eV) and C⁺+F⁺ (62.0 eV). These thresholds are tentatively correlated with specific double-hole states of CF₄.

Density matrices for the excited H (n=2 and 3) atoms produced in the electron transfer collisions between protons and helium atoms have been calculated for impact energies between 15 and 100 keV. The multichannel semiclassical impact parameter model with straight-line trajectories and an expansion in travelling atomic orbitals was used. The helium atom was approximated by a single electron moving in an effective potential. The calculated density matrices, the electric dipole moment and the first moment of the electron current density distribution are compared with experimental data of Ashburn et al. (1990).

Cross sections for one-electron loss by Li⁺ and Li²⁺ in H, H₂ and He have been measured within the range 0.3-2.7 MeV. The results are considered in terms of recent descriptions of electron loss based on both screening and antiscreening effects involving the target electrons. Recent calculations based on the Born approximation are shown to provide improved agreement with experiment at high velocities when antiscreening effects are included. In all cases, cross sections attain maximum values at impact energies lower than those predicted. It is also shown that at high velocities, cross sections for one-electron loss by both Li⁺ and Li²⁺ in H approximate closely to one half the corresponding cross sections in H₂.

The second-order Oppenheimer-Brinkman-Kramers approximation is used to derive simple analytical formulae, evaluated to the lowest order in the fine-structure constant alpha in the numerator, for the differential cross section for electron capture to the continuum (ECC) by incident bare ions having velocity nu from target hydrogenic atomic systems. Relativistic as well as non-relativistic forms are derived. Comparison of the theory with the experimental data of Dahl (1985) and Andersen et al. (1986) for H⁺, He²⁺+He collisions is fairly satisfactory for energies >50 keV amu^-1. However, although the velocity dependence obtained by Andersen et al. is nu ^-11.3+or-0.2 in the range of impact energies 1-2.6 MeV amu^-1, this does not imply that the asymptotic nu ^-11 velocity dependence given by the non-relativistic second-order OBK cross section is almost attained. It is shown that this cannot happen until an energy >500 MeV amu^-1 is reached where allowance for relativity produces a considerable change in the energy fall off.

The probabilities for cusp and delta -electron production were measured as a function of the impact parameter and of the outgoing projectile charge state for collisions of 0.5 MeV u^-1 H⁺, He⁺, He²⁺ on Ne and He targets. The experimental results for H⁺ projectiles are presented here and compared with both quantum mechanical and classical calculations. The results give strong evidence that in collisions between fast light ions and atoms a post-collision Coulomb focusing effect of the outgoing projectile ionic charge on the ionized electrons is of great importance for capture to the continuum.

Absolute total cross section measurements and state-selective translational energy spectroscopy have been performed for single electron capture by 0.5-9 keV C²⁺ from H₂, He and Ar, for the C²⁺ ground state as well as the C²⁺ metastable state. The experimental results are compared with available data obtained with undefined metastable primary ion beam fractions. For the particular case of C²⁺-H₂ collisions, experimental results could be explained by close-coupling calculations involving empirically derived coupling matrix elements and Franck-Condon factors for H₂ ionization via electron capture.

The Dirac R-matrix method has been used to calculate collision strengths for the scattering of electrons with kinetic energy 100-300 Ryd by a neon-like selenium (Se²⁴⁺) target. In these calculations the 27 jj-coupled levels arising from the configurations 1s²2s²2p⁶, 1s²2s²2p⁵3s, 1s²2s²2p⁵3p and 1s²2s²20p⁵3d are included. Collision strengths converged to within 5% are obtained with twenty-four (N+1)-particle collision symmetries (partial waves). The rates for radiative transitions between the 27 levels have also been calculated. The authors' results are compared with previous calculations.

The first absolute cross section measurements for single and double electron impact ionization of sodium-like Ar⁷⁺ are reported. The animated crossed beams method has been employed in the energy range from threshold to 3000 eV. The measured cross sections for single ionization are higher than the theoretical and semi-empirical predictions by about 20-50%. This discrepancy has been associated with the contribution of the indirect ionization processes. The double ionization cross section is only 1% of the single one.

Electron-photon coincidence experiments are usually carried out by crossing an electron beam with a beam of atoms emerging from a capillary tube or array. Recently it has been found that the finite dimensions of the interaction region formed by the crossing beams can affect the coherence parameters that are measured in these experiments. The authors have developed models with which these effects and those related to the finite acceptance angles of the detectors can be simulated numerically. This article presents a description of the models and presents results which illustrate the possible magnitude of these effects in electron-rare gas scattering experiments. Other depolarizing effects such as internal atomic interactions are also discussed.

Electron impact excitation of the resonance levels of Ne, Ar, Kr and Xe has been studied for electron scattering angles up to 50' and impact energies between 30 and 80 eV. The P₁ and P₄ Stokes parameters have been measured in each case so that the influence of spin in the excitation process could be studied through evaluation of rho ₀₀, the relative spin-flip cross section. After careful account was taken of various depolarizing effects due particularly to the finite volume of the interaction region and, in the cases of Kr and Xe, to nuclear spin, very good agreement has been found with theoretical predictions thus resolving a previously reported discrepancy. No evidence has been found for spin-flip under the experimental parameters used in this study, even for the heaviest target studied.

The authors investigate rotationally elastic, inelastic and summed processes in the e-SiH₄ collisions at 0.001-20 eV. In these calculations, the electron exchange interaction is treated exactly in an iterative approach, while the target charge correlation and polarization effects are included using a parameter-free model polarization potential based on the second-order perturbation energy. The coupled integro-differential equations are set up in the one-centre-expansion scheme under the fixed-nuclei approximation. Results on the differential, integral, momentum transfer, viscosity and energy-loss cross sections are presented and compared with other available data (both theoretical and experimental). The value of the scattering length is also reported.

Cross sections for positron-rubidium scattering have been calculated in a five-state close-coupling approximation, with polarized frozen-core Hartree-Fock wavefunctions, for energies in the range 3.7-48.1 eV. In addition, the low-energy elastic scattering cross sections have been determined by a polarized-orbital method. These latter results exhibit a large shape resonance at very low energy. Comparison of the theoretical results is made with recent experimental measurements.

The afterglow resulting from the nitrogen atom recombination in a flowing Ar-N₂ microwave discharge (915 MHz and 100 W) is characterized by the first positive emission from N₂ (B,V'). This emission occurs with a vibrational enhancement which is shifted from V'=11 in pure nitrogen to V'=8 in an Ar-2%N₂ mixture. The N-N recombination rates producing N₂(B,V') in Ar-x%N₂ mixtures (x varies from 2-100%) were calculated from the measurement of the first positive emission intensity by spectrometry and the determination of the nitrogen atom concentration by a NO titration. Normalized values were obtained from previous published data in pure nitrogen.