Table of contents

A radiative correction formula to the 1s2p³P₁-1s2p¹P₁ transition energy is proposed, and is compared with the latest experiment by Sansonetti and Martin (1984). The theory is consistent with the experiment at the present level of experimental accuracy.

By extrapolating Wannier continuum wavefunctions for two-electron escape to energies just below threshold, the authors obtain a Rydberg series of resonances. By extrapolating the Wannier cross section to energies below threshold, they argue that the partial widths Gamma _n of the resonances vary as n^-5.254.

New theoretical results of the autoionisation widths of the 1s2s2p²P⁰ and 1s2p²²D states of Be⁺ show discrepancies from previous large calculations but are in agreement with recent accurate experimental values. The authors' results are (in meV): ²P⁰, 4.3 and ²D, 31.3; compared with the experimental results of Cederquist et al. (1982) ²P⁰, 4.6+or-0.1 and ²D, 30.1+or-1.1.

A convenient approximate prescription for studying the interplay between dielectric polarisation effects and the relaxation of outer-shell electrons towards a deep core hole is tested for the case of 3d photoabsorption in xenon. The authors adapt the methods of Amusia and co-workers (1980) to a newly developed relativistic time-dependent local density approximation (RTDLDA). The natural choice of a Slater transition-state potential for the RTDLDA calculation produces nearly quantitative agreement with recent absolute cross section data.

The photoelectron spectrum of CO₂ excited by polarised He I radiation is investigated in the binding energy region 18.0-18.7 eV, using a focusing VUV polariser at an energy resolution of 15 meV (FWHM). Comparison of spectra taken at different polariser settings makes it possible to give vibrational assignments and beta values for 18 bands belonging to the B² Sigma _u⁺ state and 6 bands from the A² Pi _u state. The v₃ energy in the B state is 230 meV.

The authors have studied the orbital angular momentum transfer (as a function of the electron scattering angle) in the excitation of the 2¹P state of helium by electrons at incident electron energies of 50, 60 and 80 eV. This was done in an electron-photon coincidence experiment. At 80 eV the orbital angular momentum transferred by the electron to the atom appears to change sign at a certain scattering angle. At lower energies of 50 and 60 eV no sign reversal is observed. These results can be explained qualitatively by the semiclassical model of Steph and Golden (1980).

Variational electron correlation techniques described earlier (by Wilets, Henley and Martensson, 1980) are developed and generalised to four-electron (i.e. Be-like) atoms. Both spin-dependent and momentum-dependent two-body correlations are considered; the latter prove to be the more significant and it is found that, with a very few parameters, it is possible to obtain the major part of the improvements of the correlation energy.

MRD-CI calculations have been carried out for the ground- and some excited-state potential curves of He+H, Ne+H and Ar+H. Deep minima are obtained for the excited states at internuclear distances similar to those of the corresponding cations. The B² Pi to A² Sigma ⁺ computed transition energies do not support a previous assignment to this transition in ArH, at 7670 AA.

Static dipole polarisabilities of neutral atoms are calculated using a modified version of the McWeeny density matrix method (1960). The change in the electronic charge density induced by an external electric field is determined by minimising the total energy as a function of the density matrix elements. Exchange correlation effects are approximated by a spin-dependent X_alpha expression. The polarisabilities obtained for the ground states of the first-row and second-row atoms Li to Ar, and additionally, for the noble-metal atoms Cu, Ag and Au, are presented and compared with experiments and other theoretical results.

Stark profiles of the H alpha lines of hydrogen are computed at low densities (from 10¹⁰ to 10¹³ cm^-3) in the 'impact' theory. By a comparison with quantal results the authors show that a simple semiclassical perturbational approach with appropriate cutoffs is sufficient to give accurate profiles in the line centre. Neglecting the natural broadening and the fine-structure effects, they prove that the electronic broadening is negligible and that the profile has a Lorentzian shape, and they give an analytical expression of the half width.

The absorption spectra of a pair of two-level atoms driven on resonance by a coherent laser field are considered analytically including the dipole-dipole interaction between the atoms. Analytical formulae for the absorption spectra are derived for four regions of the dipole-dipole interaction Omega ₁₂: Omega >> gamma >> mod Omega ₁₂ mod , Omega >> mod Omega ₁₂ mod >> gamma Omega approximately= mod Omega ₁₂ mod >> gamma , mod Omega ₁₂ mod >> gamma >> Omega (2 gamma being Einstein's coefficient A, and Omega the Rabi frequency). The formulae, containing both absorption-like and dispersion-like terms, reveal explicitly the resonant structure of the absorption spectra. At great intensities of the driving field the absorption is negative in a certain range of frequencies. The dipole-dipole interaction between the atoms is shown to modify considerably the form of this absorption spectrum. For sufficiently strong dipole-dipole interaction it exhibits dispersion-like behaviour in the vicinity of 2 Omega with a new region of negative absorption. The spectrum is asymmetric, albeit slightly. For strong dipole-dipole interaction and a weak driving field the spectrum consists of one absorption-like peak, located at omega - omega ₀= Omega ₁₂.

The experimental relative photoionisation cross section of atomic bromine from ionisation threshold to approximately 900 AA is presented. Two autoionising Rydberg series are identified converging to ³P₁, two to ³P₀, three to ¹D₂ and two to ¹S₀. The spectrum bears a strong resemblance to that of atomic chlorine, including the additional sharp series converging to ¹D₂ unaccounted for in ab initio calculations. Complexities emerge in the ³P region, prefiguring the complex structure in atomic iodine.

Ultraviolet lines of krypton-ion and argon-ion lasers have been used to excite infrared fluorescence in Na₂ which has been recorded at high resolution by Fourier transform spectroscopy. Detailed information about three new singlet gerade states has been obtained, one of which, (3s, 4s)¹ Sigma _g⁺, is known to be involved in optically pumped infrared lasing transitions. Constants for the states are given.

The differential cross section for the double photoionisation ( gamma , 2e) of the H₂ molecule is calculated at the photon energy up to 1 keV bearing in mind the double coincidence experiment. The final electrons are described by plane waves orthogonalised to the electron wavefunction of the H₂⁺ ion at a distance between protons which is equilibrium for the H₂ molecule. The differential cross sections are very sensitive to the choice of the wavefunction for the molecular ground state. The interplay of two mechanisms, namely, the 'instantaneous' change in the self-consistent field and a manifestation of the short-range pairwise electron-electron correlations, is discussed. This last mechanism dominates if two final electrons have antiparallel momenta. The experimental problems are outlined; in particular, the (e, 3e) experiment can reveal some essential details of the above mechanisms.

The partial photodissociation cross sections of the two-electron heteronuclear and heteropolar molecular ion HeH⁺ as a function of wavelength are calculated by using the general formulation for photodissociation due to electronic transitions Lambda to Lambda '. Transitions to the three excited electronic states A¹ Sigma ⁺, B¹ Sigma ⁺ and C¹ Pi from the ground state X¹ Sigma ⁺ have been studied for the vibrational levels from v=0-5 and for rotational level J=1 which is the most probable value of J at room temperature (300K). The gradual increase of the height of the successive maxima is somewhat perturbed. The various factors affecting the relative magnitude of the partial photodissociation cross sections are discussed.

Electron capture, electron loss and excitation cross sections have been determined for 2.5-25 keV H(1s) and H(2s) atoms and H⁺ and H^- ions in both O and O₂. An iridium tube furnace has been used to provide a target of highly dissociated oxygen. The cross section for the total destruction of H(2s) atoms in O and O₂ has also been determined. In general, the cross sections in atomic oxygen exhibit an energy dependence similar to that of the corresponding cross sections in O₂, but are smaller in absolute magnitude.

The possibility of a complete determination of the scattering amplitudes for a D state produced by a collisionally induced coherent S to D excitation is investigated. A polarisation analysis of the subsequent D to P dipole transition by the photon-particle coincidence method is not sufficient to completely determine the excitation amplitudes for the D state. Additional information can be obtained from collision experiments performed in the presence of symmetry-breaking electromagnetic fields. In the case of Li(3d²D), the combined action of an external magnetic field and the spin-orbit coupling has been utilised. The results of the first coincident level crossing experiment is reported, allowing an unambiguous determination of the excitation amplitudes for the Li(3d²D) state produced in Li-He collisions.

The authors have measured K-K transfer cross sections for 400 MeV bare Fe²⁶⁺ ions colliding with Ar, Kr, Zr, Ag and Sn targets in the intermediate velocity range (v/v_K^P approximately 0.6 and 0.4<v/v_K^T<1 where v is the projectile velocity and v_K^P, v_K^T are the K orbital velocities of the projectile and target respectively). Cross sections have been extracted from the analysis of single K and double K target vacancy production as a function of the projectile charge state. Experimental results have been compared with theoretical calculations based on a two-state expansion method: good agreement is found and in particular the data are consistent with the theoretical behaviour predicted round the symmetry region Z_P=Z_T.

Thin targets of Sb, Te, Ho, Ta, W, Pt and Bi have been employed and L X-ray yields have been measured as a function of proton energy from 1.5 to 3.8 MeV. From these measurements the individual and total X-ray production cross sections and their ratios have been calculated and compared with the experimental results reported by other authors and with the perturbed stationary-state plane-wave Born approximation with correction for Coulomb deflection, relativistic and energy-loss effects resulting in the ECPSSR theory of Brandt and Lapicki (1979, 1981).

Energy spectra of X-rays have been obtained for Lyman series transitions (np-1s; n=2, 3, 4 and 5) in 5.5 MeV amu^-1 H-like Ne⁹⁺ ions passing through gaseous targets (Ne and N₂). Relative intensities of the transitions are dependent on the target gas pressure, which is attributed to collisional quenching of the excited states of the ions. The value of the total quenching cross section for each state is estimated using the experimental data and is compared with that obtained by a perturbation calculation. The cross section is less dependent on the principal quantum number of the excited states than the geometrical cross section of the ion. This is the first observation of the collisional quenching of fast highly ionised heavy ions in excited states with lifetimes of about 10^-12 s in gas targets.

The cross section for the mutual neutralisation process: H⁺+H^- to H+H has been measured over the energy range 5-2000 eV, using merged beams and coincident detection of the products. The results obtained differ significantly from those published by other investigators. In particular, the energy dependence is free of structures and similar to that predicted by theory. The cross section is also in excellent agreement with recent calculations.

The authors present theoretical results for Li³⁺-H(2s) and Li³⁺-H(1s) total charge exchange cross sections at low energies (130 eV-18 keV and 500 eV-10 keV, respectively). Exact OEDM molecular energies and the Landau-Zener method are employed in the study of the first reaction. The error introduced in the cross section by the molecular curves in the context of the Landau-Zener approximation is estimated by error propagation analysis. The total cross section for the latter reaction obtained in previous works is extended to low energies. The results, which show quite a large increase with the collision energy, are compared with other theoretical data.

Cross sections for the combined processes of charge transfer and ionisation in Li⁺-Li⁺ collisions have been determined using an intersecting beam technique for CM energies in the range 53-240 keV. The results are compared with the previous experimental measurements of Peart et al. (1981) spanning the range 19-88 keV and with theoretical predictions. The latter form the basis of models used to describe collisions between much heavier ions of relevance to heavy-ion fusion.

The authors derive a new expression for the exchange potential applicable to the elastic scattering of electrons by atoms. The derivation is based on a recent study by Fritsche and Gollisch (1982). Distinctly different from Slater-type X_alpha potentials, the expression exhibits the correct long-range behaviour resulting from the electrostatic polarisation of the target atom in response to the incident electron. In order to facilitate a self-consistent calculation of the scattering potential (including the contributions of all target electrons and of the projectile as well) the authors derive a method which provides the localisation of the incident electron during the scattering process. The scattering potential obtained is parameter free and shows a noticeable energy dependence in the low-energy regime up to about 150 eV. Differential cross sections are calculated for Ne and Xe atoms and compared with recent experiments. Since the calculation includes relativistic effects, they obtain scattering-induced spin polarisation as well, yielding a relatively good description of the respective experiments by Klewer et al. (1979) and Berger et al. (1982).

The plane-wave impulse approximation is used to analyse the experimental cross section for the argon (e, 2e) reaction (using the non-coplanar symmetric geometry). A series of configuration interaction calculations for the structure of Ar I and Ar II are reported demonstrating that correlation effects have only a minor effect on the momentum distributions (which are well described by the target Hartree-Fock approximation). Fair agreement between the theoretical and experimental spectroscopic factors for the separation energy spectrum is obtained, and in particular the distribution of strength amongst the 3s²3p⁴nd²S^e satellite states specified by the largest calculations is in better agreement with data than that obtained with simple structure models.

A theory of stepwise excitation of atoms in which electron excitation is followed by single-mode laser excitation is presented. Theoretical expressions are developed using density matrix techniques for line polarisations, excitation cross sections and electron-photon coincidence signals applicable to both axial and planar symmetry stepwise excitation experiments.

A coupled-state method in which three rotational states (j=0, 2, 4) are retained has been employed to investigate e⁺-H₂ scattering at low incident energies. Two different effective potentials have been used to make comparative studies. The results for the total and rotational cross sections are reported. The present results for total cross sections are in good agreement with the recent measurements of Charlton et al. (1980, 1983).