Table of contents

A recent calculation of the interaction between 3s3p⁶ and the 3s²3p⁴md/ epsilon d series in Ar II by Mitroy et al. (1984) made use of a variationally determined d orbital to represent the complete d series in a fixed basis CI calculation. The authors show that this approach gives the correct wavefunction only for the particular state for which the d orbital was optimised. This result explains the differences between the calculations by Mitroy et al. (1984) and previous calculations by the authors.

The calculation of two-electron QED energy shifts in helium-like ions is discussed in order to correct a number of errors and misunderstandings which have appeared in the literature. New results are presented for the terms involving (r₁₂^-3) in the ground and low-lying excited states for ions up to Z=10. A detailed comparison is made with the high-precision measurement by Martin (1984) and a calculation by Hata (1984) for the 1s2p ¹P₁-1s2p ³P₁ transition of helium.

The autoionisation of the Rydberg series Ca I 3dnp ¹P₁ (n>or=5) was studied by a photoionisation experiment using the synchrotron radiation facility of the Berlin electron storage ring (BESSY) and the atomic beam technique. Special attention was given to the line shape of these double electron resonances for a critical comparison with recent ab initio calculations (R-matrix method and many-body perturbation theory).

The relationship between the cross section and impact velocity for the electron stripping process and that for the K-shell ionisation process by proton bombardment has been studied. In the case of Li⁺+H to Li²⁺+H( Sigma ), the relationship has been plotted and compared with that for K-shell ionisation of neutral atoms by proton and alpha particle bombardment. The observed similarity indicates that electron stripping occurs in a way similar to inner-shell ionisation by a nucleus and is expressed rather well by the plane-wave Born approximation with some corrections.

It has been demonstrated explicitly that a cascade correlation exists between the filling of a vacancy in the 1s sigma molecular orbital (MO) and the emission of a K X-ray of the heavier collision partner following vacancy sharing in a heavy-ion collision.

A comparison has been made of numerical methods with basis set expansion techniques in the evaluation of the sum-over-states expressions which arise in the diagrammatic perturbation expansion in second and higher orders. The model problem of a ground-state hydrogenic atom with charge Z perturbed by the potential -Z'/r is used. The energy corresponding to each diagrammatic component can be evaluated explicitly for this mode. Universal systematic sequences of even-tempered basis sets of exponential-type functions and Gaussian-type functions are employed and the convergence of the individual components of the energy expansion with increasing size of basis set is examined.

This work is concerned with the question of calculating transition oscillator strengths for atomic fine-structure levels that are described by strongly mixed superpositions of terms. Rather accurate non-relativistic calculations of wavefunctions followed by perturbation treatment of relativistic interactions can lead to very poor term mixing, hence poor oscillator strengths, in situations where there is near degeneracy of the non-relativistic energy eigenvalues. It is shown in a three-term mixing case how a corrected Hamiltonian can be constructed using observed level energies as known input. The example, involving the three J=2 levels of the 2p⁵3p configuration of neutral neon, shows a dramatic improvement for transition probabilities and Lande factors when the correction is made and the results compared with recent observations.

It is shown that the use of relativistic Hartree-Fock equations in an external field, Brueckner orbitals and many-body perturbation theory provides, as a rule, an accuracy to 1-3% in atomic calculations. To control the accuracy, the energy levels and the fine and hyperfine structure intervals as well as the E1 amplitudes for s-p transitions in Cs have been calculated. The parity-violating E1 amplitude for the 6s-7s transition has been found: (7s mod D_z mod 6s)=(0.88+or-0.03)*10^-11(-Q_w/N)(-iea_B), where Q_w is the weak charge of the nucleus and N is the number of neutrons. Previous experimental data make it possible to find the weak nuclear charge of ¹³³Cs, Q_W=-73.4+or-8.1+or-6, and the Weinberg angle sin² theta _w=0.237+or-0.036+or-0.03.

A method for obtaining the discrete component of resonance states in negative molecular ions is described. It is based on standard MRD-CI type calculations and especially designed for treating larger molecular systems. The resonance is thereby stabilised by increased nuclear charge. The method is applied to the calculation of N₂^- in its ² Pi _g( pi _u⁴ pi _g) and ² Pi _u( pi _u³Ss²) resonance states.

The authors have investigated the zero-field level crossing lineshape in resonance absorption. A single-mode HeNe laser beam at 633 nm is applied to an external neon-gas absorption cell in a longitudinal magnetic field. They compare the measured lineshapes with theoretical predictions. Two superposed nonlinear Lorentzian-shaped resonances, known as the population effect of the 2p₄-3s₂ transition and the Zeeman coherence effect of the lower 2p₄ level, appear on the background caused by the inhomogeneous broadening. Taking into account the background and the power broadening the authors obtain the linewidths at zero pressure gamma (2p₄,3s₂)=(7.7+or-2.1) MHz and gamma (2p₄)=(8.4+or-1.2) MHz. For the neon pressure broadening the authors get for pressures below 200 Pa (1.5 Torr) d gamma (2p₄,3s₂)/dp=0.20+or-0.02 MHz Pa^-1 and d(2p₄)/dp=0.040+or-0.007 MHz Pa^-1. Their values are in acceptable agreement with the values obtained with other spectroscopic methods.

Pair production cross sections have been determined for the 1.238-3.548 MeV energy range in germanium (Z=32) using a Ge(Li) gamma ray detector. The experimental results have been compared with the theoretical cross sections of Bethe and Heitler (1934), Tseng and Pratt (1981) and Overbo (1970) and experimental results of En'yo et al. (1980). The results of the present measurements agree with the Bethe-Heitler results down to 1.771 MeV. However, at 1.238 MeV the experimental results are higher than all the theories.

Fluorescence in the B ³ Pi (0⁺u)-X ¹ Sigma _g⁺ system of I₂ excited with 5145 AA radiation of an argon ion laser was recorded photoelectrically in the region 5200-8500 AA on a SPEX 0.85 m double monochromator. Using the measured intensities of P(17) rotational lines of thirty-eight ( nu '=43, nu ") bands and the Franck-Condon factors based on the recent molecular data of the B and X states, the variation of the square of the electron transition moment, mod M(R) mod ², with R centroid in the range R=2.65-3.09 AA was studied. It was noticed that the electronic transition strength has a maximum value at R=2.83 AA and is represented by the relation mod M(R) mod ²=constant(-500.358+377.125R-38.483R²-25.429 R³+4.9831R⁴). Present results are discussed in relation to those obtained from earlier studies.

A method is developed for the calculation, at internuclear distances R>5 au, of the potential curves of the NeHe* and NeNe* systems correlated to the levels of the 2p⁵3s and 2p⁵3p configurations of the neon atom. It is a generalisation to open shell cores of the model potential method of Valiron et al. (1979) for alkali-rare-gas systems. The neon atom is treated using the model of Feneuille et al. (1970). For the NeHe* system, radial coupling calculations are performed, and a two-state quantal calculation provides a temperature variation of the 2p₃ to 2p₄ and 2p₄ to 2p₅ population transfer cross sections in good agreement with experiment. The inclusion of u-g symmetry effects for the NeNe* system is discussed. The humps in the 1s₅0_u^- and 1_u states of Ne₂ are found to be close to the experimental determination of Colomb de Daunant et al. (1981) when the estimation of Cohen and Schneider (1974) for the Ne₂⁺ potentials is used. Such humps are sensitive to the estimation of the core-core interaction.

The H⁺-H collisional system is revisited in order to study the nature and the effects of the non-adiabatic 2p sigma _u-np sigma _u radial couplings in the range of internuclear distances 0<R<2 a₀. To investigate whether these couplings reflect pseudo-crossings caused by the 'super-promotion' of an exotic state the authors build alternatively diabatic representations that preserve the characteristics of the two-centre separated-atom wavefunctions or those of the single-centre united-atom wavefunctions. It is found that none of the considered approaches supports a curve-crossing view. They demonstrate that the common bell-shaped form of the (2p sigma _u mod delta / delta R mod np sigma _u) matrix elements is not an exclusive property of the 2p sigma _u state and the authors uncover a duplication property of all the (np sigma _u mod delta / delta R mod n'p sigma _u) non-adiabatic couplings. Finally, calculations of the probabilities of electron transitions due to these couplings show that a 10% leakage from the adiabatic 2p sigma _u state does occur at small impact parameters at collision energies above 1 keV.

The Na(3s to 3p) excitation in the Na+He collision, in the 30 eV<E_CM<10 keV energy range, is investigated theoretically along the same lines as those of a previous study on the Na+Ne collision by Courbin-Gaussorgues et al. (1983). Calculations are first performed on the curve-crossing mechanism (i) at low energy using a five-state molecular basis that involves the diabatic SCF orbitals obtained by Courbin-Gaussorgues et al. Contrary to the Na+Ne case the threshold of the experimental Na (3s-3p) cross section and its magnitude are not reproduced by this calculation. On the other hand the direct 3s to 3p transition mechanism (ii), treated in an eight-state basis that involves orthogonalised frozen atomic orbitals, accounts for the gross features of the experimental data in the whole energy range. Finally an eight-state model that combines both mechanisms (i)+(ii) is proposed and its results are discussed and compared with the available total and differential experimental data.

L X-ray production cross sections for protons impinging on thin targets of Cd, Sb and Te have been measured in the energy range 0.3-2.0 MeV. L-subshell ionisation cross sections have been extracted and compared with existing data and with the predictions of the perturbed stationary-state theory including corrections for energy loss, Coulomb deflection and relativistic effects (ECPSSR), the binary encounter approximation taking into account the binding effect and Coulomb retardation (BEABC) and the semiclassical approximation with binding, energy loss, Coulomb deflection and relativistic corrections included (SCABCR). Generally, good agreement has been found, slightly favouring the ECPSSR theory over the others. A better insight can be achieved by looking at the ratios of the subshell ionisation cross sections, owing to the nodal structure of the 2s_1/2 wavefunction. These results are in good agreement with analogous measurements on neighbouring elements and confirm the above conclusions. A semiempirical value for the omega ₁ fluorescence yield of Cd is suggested.

Electron detachment in H^--molecule (N₂, O₂, CO, D₂ and CO₂) collisions in the keV energy range is studied by a differential time of flight analysis of the neutral. Detachment occurs via three different processes: direct detachment, charge exchange to a shape resonance of the target molecule and detachment accompanied by excitation. The charge exchange process is theoretically analysed by the means of a simple model that considers the H^- ion colliding with a molecule in the keV energy range as a wavepacket of free electrons impinging on the target molecule. It is thus possible to describe the H^--molecule collisions in terms of the electron-molecule scattering amplitudes.

By means of VUV photon spectroscopy absolute (n, l)-subshell-selective electron capture cross sections sigma _nl have been determined for collisions of C⁶⁺, N⁶⁺, O⁶⁺, Ne⁶⁺ with He; C⁶⁺, N⁶⁺, O⁶⁺ with H₂ and O⁶⁺ with Ar, in the velocity range 0.15-0.5 au. These cross sections depend strongly on the collision velocity, in contrast with total cross sections for electron capture and cross sections for capture into different n shells. One n shell is predominantly populated. The comparison of sigma _nl( nu ) for different ions with the same ionic charge suggests that the structure of the ionic core is not very important for the l distribution, as long as the energy levels of the ionic species under consideration are almost hydrogen-like. The comparison of results for O⁶⁺-H₂, Ar shows that the binding energy of the target is much more important than its internal structure for the resultant shape of sigma _nl( nu ).

The strong potential Born theory for charge transfer in fast, asymmetric ion-atom collisions has been extended to allow for nuclear resonant scattering using distorted nuclear waves. In the absence of a nuclear resonance, the semiclassical result is recovered. A large variation in the capture probability is found when the projectile energy passes through the resonance. As examples, the authors present results for the capture from the target K shell in the collisions of protons with ²²Ne and ²⁸Si, as well as for capture from the K and L shells of ⁵⁸Ni in collisions with protons, and of ¹⁶O, ²⁰Ne and ²⁸Si by He²⁺ impact.

The ejected-electron spectrum of aluminium vapour atoms has been observed at 75 degrees with respect to an incident electron beam of 20-500 eV kinetic energy. A total of 48 lines, in the region 0-6 eV, are reported corresponding to autoionisation processes in Al I. Where possible, assignments has been made based on available experimental data and hartree-Fock-type estimates. All the prominent features in the spectrum have been accounted for. In addition, the authors have tentatively identified the ²D degrees and ²P degrees terms of the 3p³ configuration.

The electronic collisions of ground-state H₂⁺ followed by capture into the (2p sigma _u)(nlm) state (n>or=2) are investigated theoretically by means of a configuration interaction theory. Emphasis is put on the investigation of the domain of very low energy. The mean energy in the chosen mix of molecular-ion-hydrogen configurations is evaluated and, consequently, the corresponding capture radius, which appears to depend approximately linearly on the energy. Finally the cross section is evaluated. It diverges around zero, as expected. For capture into the n=2 states, it has an approximately E^-1 dependence for a large range of energies, having values of above 10^-16 cm² for less than 2 eV, in good agreement with experimental results. For capture into the 3s states, the authors give evidence of a dip of 2*10^-20 cm² at 0.2 eV and a peak of 1.4*10^-17 cm² at 6.6 eV, in good agreement with previous calculations.

An electrostatic impact spectrometer is used to study the formation of H^- ions by dissociative attachment in HI between 3 and 9 eV. Three repulsive electronic resonance states of HI^- are found to dissociate adiabatically to H^-(¹S)+I(²P_3/2) for the lowest 1 Omega _1/2 and the following Omega _3/2 states, and to H^-(¹S)+I(²P_1/2) for the highest 2 Omega _1/2 state. The dominant Sigma symmetry of the experimental angular distributions of H^- ions dissociating through the two Omega =¹/₂ states is significant of strong spin-orbit coupling between these two resonance states of HI^-. The lack of Pi -type contributions in these angular distributions, as well as the negligible value of the cross section for H^- formation via the Omega =³/₂ resonance state, may mean that an electron cannot be bound in the expected Pi symmetry resonance states of HI^-. Above an energy of 6.5 eV, both the Omega _3/2 and the 2 Omega _1/2 states interact with bound Feshbach resonances associated with Rydberg states of HI; the interaction can be described as a predissociation by electronic coupling.

Positron-hydrogen scattering to 1s, 2s and 2p states has been calculated at 54.42, 100 and 200 eV. Close coupling within the n=1, 2 and 3 channel space is used. Coupling potentials include polarisation terms calculated explicitly for the continuum and for relevant discrete excitations. The discrete polarisation potentials are checked at 100 eV by including 4s and 4p channels in the coupled space.

For the first time a theory of the multiple scattering effects in the disordered medium of dipole force centres has been developed, which makes it possible to explain a considerable discrepancy, recently discovered in dense water and ammonia vapours, between measured electronic mobility values and those calculated from the Lorentz formula.

The vibrational population of the excited N₂(B³ Pi _g) state has been experimentally determined by measuring the first positive band intensities in a N₂ flowing glow discharge. The measured N₂(B, nu ') distributions are compared with the calculated population assuming: (1) excitation by direct electron collisions from the N₂(X¹ Sigma _g⁺) ground state and by radiative cascade from the N₂(C³ Pi _u); (ii) destruction by radiative emission and quenching. It is observed that the N₂(B, nu ') vibrational excitation increases with time following the temporal evolution of the vibrational distribution of the N₂(X, nu ) ground state and the electron distribution function.