Table of contents

Satellite lines are identified in the X-ray spectrum of foil-excited fast silicon ions. These lines result from transitions from levels of principal quantum number n: up to n=6 in He-like Si XIII, up to n=5 in Li-like Si XII, and up to n=3 in Si XI, Si X, Si IX and Si VIII. The identifications are made with the aid of theoretical 'Hartree-XR' atomic-structure calculations of energy levels and oscillator strengths.

A simple description of the time evolution of interference effects in resonant multiphoton absorption is presented which draws together recent studies of transients and laser coherence in two-photon ionisation.

Using synchrotron radiation, the production of Ly- alpha and Balmer atomic fluorescence from photodissociation of H₂ has been investigated from 620 to 350 AA. Upper limits for the corresponding cross sections were measured, at 425 AA, and found to be 1*10^-20 cm² and 2*10^-21 cm², respectively.

Relative populations of excited states of He and H are determined for the back-scattering of He⁺ and H⁺ ions from a metal surface. The calculations are based on a theoretical model of the dynamic atom- and ion-surface interactions.

The energy dependence of the cross section for the process K(4P₁2/)+He to K(4P₃2/)+He has been measured in a crossed-beam experiment over the energy range 0.07-0.4 eV. Partial disagreement with previous results of Anderson et al., (1976), is observed. The variation of the cross section with energy is similar to that predicted by recent theoretical work. In particular the cross section has a nearly constant value for energies above 0.18 eV.

Differential cross sections and angular distributions of H^- ions in HCl have been obtained in a crossed-beam electron impact spectrometer in the 6-12 eV energy range. The H^- ions yield exhibits two peaks: the 7.1 eV peak has an angular distribution characteristic of a d sigma wave with a maximum at 90 degrees and a minimum at 55 degrees ; the 9.3 eV peak shows superimposed fine structures on the high energy tail and has an angular distribution which has a minimum at 90 degrees and is characteristic of a d pi wave. These results are confronted with recent configuration; interaction calculations on the resonance states of HCl^-.

Ab initio calculations are presented which point out the similarities between the energy levels of positron-anion complexes and Rydberg series. In order to illustrate these similarities, the energy levels and charge densities of (F^-, e⁺) and (CN^-; e⁺) are compared with those of the Rydberg states of Na and NO, respectively.

The ejected-electron spectra of highly excited autoionising levels of Li, Be, B and C have been studied by using the projectile electron spectroscopy method. Singly and doubly core-excited states are strongly populated in 100 to 500 keV Li⁺, Be⁺, B⁺ and C⁺ single collisions with He and CH₄. High resolution is obtained by selecting a small observation angle (6.4 degrees ) with respect to the beam axis. Numerous states, previously not seen, have been resolved and identified by comparison with existing theoretical data. For a large number of core-excited states, absolute Auger transition and excitation energies are given. In particular, isoelectronic sequences for singly core-excited lithium-like (Z=2-16) and doubly core-excited helium-like (Z=1-14) autoionising states are studied.

Photoionisation of the nitrogen ground state N(2s²2p³⁴S⁰) has been extensively studied starting from an accurate physical model. The initial bound state N(⁴S⁰) and the final continuum state N⁺+e^-(⁴P^e) are described consistently by collisional R-matrix-type expansions over the five parent ionic states N⁺(2s²2p²³P^e; 2s2p³⁵S⁰, ³D⁰, ³P⁰, ³S⁰), represented in turn by accurate configuration interaction wavefunctions. An exact account of the couplings between the photoejection of the 2p and 2s valence electrons is essential in the low-energy spectrum, dominated by wide resonant structures. Close agreement with a recent high-resolution measurement of the strongest resonances contributes towards an assessment of the accuracy of the calculation. Detailed graphs and tables provide all the relevant information-total and partial cross sections, angular distribution of photoelectrons, analysis and fitting of prominent resonant structures-from the threshold at 852 AA ( approximately=1 Ryd) to 300 AA ( approximately=3 Ryd).

A time-of-flight spectrometer has been coupled with a source of synchrotron radiation to measure the photoionisation cross sections for multiple ionisation of the rare gases He to Xe in the photon energy range 50-280 eV. Comparison is made with theoretical calculations both in the case of double ionisation of He, Ne and Ar and in the case of single ionisation of Kr and Xe, where electron correlation effects are supposed to enhance the various cross sections. The importance of an accurate knowledge of monochromator scattered light in such experiments is emphasised.

The photodissociation of the hydrogen molecule near the dissociation threshold into H(1s)+H(2s,2p) states is considered. The experimentally observed formation of H(2p) can only be explained by the interaction of the covalent and ionic electronic states of the H₂ molecule at large internuclear distances in the vicinity of the avoided crossing of the potential curves. The theory is based on the Landau-Zener model with a quantum description of nuclear motion. The calculated relative yields of H(2s) and H(2p) atoms are 59.8% and 40.2% compared with 57% and 43% obtained in a recent experiment (Mental and Guyon (1977)).

Results of model-potential calculations for the ground-state and the first nine excited-state molecular curves of the Na-Ne system are presented. A two-parameter model potential for the electron-neon interaction can be fitted to scattering data. The molecular curves are not affected by the uncertainty of the data. The introduction of a non-local orthogonality condition to the neon core orbitals is discussed. For internuclear distances R>or=5 au, the authors' results agree within 5% with the laser spectroscopy determination of the well in the X² Sigma and A² Pi potential curves, and with the far wing profile determined by York et al., (1975). A blue satellite is predicted at 1224 cm^-1. The accuracy at smaller internuclear distances is limited by the lack of precision in the NaNe⁺ core-core repulsion. In contrast with pseudopotential calculations, a 450 cm^-1 well is obtained in the delta 3d curve at 4.5 au. The sigma 4s, sigma 3d and sigma 4p states present a barrier at 10 au and a well at 4.5 au.

A theoretical model for the collision of atoms in high Rydberg states (nl) with neutral atoms or molecules is proposed, based on the assumption that the electron-perturber interaction is of very short range compared with the mean radius of the Rydberg atom. A semiclassical method is used to derive a compact expression for the collision probability. The calculations need total cross sections for the scattering of a free and low-energy electron by the perturber. The model is applied to the calculation of a free and low-energy electron by the perturber. The model is applied to the calculation of cross sections for the quenching of n²D or n²F Rydberg states of Na and Rb by rare-gas atoms of N₂. On the whole the calculated cross sections are in reasonable agreement with experimental data for n>10 approximately. The model and the results of the calculations are discussed and compared with other theoretical works.

The impact-parameter dependent probabilities for Na(3s from 3p) emission in 1.5-10 keV Na-He, Ne and Ar collisions have been determined experimentally in the range from 0.4 to 3.0 au by means of scattered-particle, photon coincidence measurements. Two mechanisms are proposed to account for the observed emission. At low collision energies, the Na(3s to 3p) excitation takes place at molecular curve crossings, whereas direct excitation dominates at higher energies, around the maximum in the total emission cross section. The direct mechanism is essentially the same for all three collision systems, whereas the quasi-molecular mechanism depends strongly on the asymmetry of the collision.

For pt.I see ibid., vol.12, no.15, p.2529 (1979). The impact-parameter dependence of the Mg II 3²S-3²P emission probability and polarisation has been measured for 3-15 keV Mg⁺-He, Ne and Ar collisions by detection of the photons emitted perpendicular to the scattering plane in coincidence with ions scattered over a selected angle theta . From the four measured Stokes parameters, the emission probability and the degree of polarisation P have been determined. The two parameters lambda and chi describing the relative size and phase of the quantal excitation amplitudes for the magnetic sublevels were evaluated in the cases where the excitation is coherent. The degree of polarisation is high in all cases except for the Mg⁺-Ne system for distances of closest approach below 1.3 au, where molecular curve crossings due to promotion of neon 2p electrons cause a breakdown of the quasi-one-electron description. The parameters lambda and chi exhibit a strong dependence on the impact parameter, beam energy and target gas.

The C⁺-Ne collision has been studied in the 500-2000 eV laboratory energy range. Using the energy-loss technique, the various inelastic channels are identified and the relative excitation probability is determined as a function of the reduced scattering angle. Excitation of C⁺ and electron capture into the ground-state configuration are the dominant processes, contrasting with the weak excitation of the neon atom. A quasimolecular description of the collision mechanisms is attempted to provide a qualitative basis for the interpretation.

In 1927 Thomas suggested a classical mechanism of charge transfer whereby the attachment of a target electron to an incident ion is facilitated by the recoil of a second target electron. This second electron recoils with the speed of the incident ion in a direction perpendicular to the ion direction. A quantum-mechanical description of this process is given. The cross section for charge transfer, differential in the electron recoil momentum, is shown to peak when the classical Thomas scattering conditions are satisfied. At asymptotically high collision velocities nu , the total cross section for charge transfer via this double-scattering mechanism exhibits the same nu ^-11 decrease as the classical cross section of Thomas.

The excitation of the (2p²)³P doubly excited state of helium by electron impact has been studied in the energy range from threshold to 150 eV. This was done by observing the XUV radiation (32 nm) emitted in the decay of that state to the (1s2p)³P singly excited state. The shape of the measured excitation curve deviates appreciably from all existing theoretical results. In the threshold region clear evidence is found for the occurrence of negative-ion resonances.

Absolute (e,2e) five times differential cross sections, measured in coplanar symmetric conditions, for the outermost 2s and 2p orbitals of neon are reported. The incident energy is varied from 200 to 3600 eV in order to test different theoretical models. The averaged eikonal distorted-wave impulse approximation is a reasonable approximation at incident energies larger than 200 eV and recoil momenta less than 1 au, provided the effective two-body operator is the t matrix and not the nu matrix. The imaginary part of the averaged eikonal potential is non-zero only for the 2s orbital. Previous indications that the impulse approximation breaks down for small relative momenta between the outgoing electrons are confirmed by these measurements.