Table of contents

By applying a many-electron approach to the calculation of autoionising states, which takes electron correlation and non-orthonormality into account, the authors have obtained the autoionisation rates of the Li 1s2p²²D and (1s(2s2p) ³P⁰) ²P⁰ states as 15.2*10¹² s^-1 for ²D and 5.2*10¹² s^-1 for ²P⁰. Contrary to previous theory, these results agree with the recent experimental values of Cederquist and Mannervik (1982) which are 15.8+or-0.4*10¹² s^-1 for ²D and 4.0+or-0.2*10¹² s^-1 for ²P⁰.

A new and detailed interpretation of the discrete structure in the Ba²⁺ spectrum based on a standard application of quantum defect theory (QDT) is presented, and the method is shown to yield realistic relative intensities in the discrete spectrum. It is argued that any collective oscillations are damped within one period and can give rise to no harmonics, while any ab initio theory capable of prediction the giant resonance in the continuum correctly is also suitable for the discrete structure.

The high resolution spectrum of gaseous neon in the 1s region was obtained by using synchrotron radiation. In addition to previously observed single-electron excitation lines, the authors observed two new sharp lines of comparable intensities in the continuum at 35.1 and 38.1 eV above the 1s to 3p resonance line. On the basis of the known Ne 1s XPS shake-up lines the authors attribute them to transitions to ¹P⁰ levels of the 1s2s²2P⁵3p² final-state configuration.

Photoionisation cross sections for the 4s, 3d and 3p subshells and the 3d angular distribution have been calculated for atomic Mn by many-body perturbation theory. Spin-orbit effects in the 3p to 3d resonance and resonances due to 3p to ns,nd (n>3) excitations have been computed. Comparison is made with recent experiments.

The united-atom ionisation model of Briggs (1975) is applied to direct ionisation of the highly promoted 4f sigma molecular orbital. The coherence of the amplitudes corresponding to projectile and target nucleus is fully taken into account. For cases in which a first-order treatment is inadequate, a simple non-perturbative generalisation of the Briggs model is proposed and applied. Results of numerical calculations for Ag+Ag and I+Ag collisions are discussed in the light of recent experiments.

The authors propose a new kind of model for absorption of flux from the elastic scattering channel in electron-atom or electron-molecule collisions. It is a non-empirical model so it predicts the absorption cross section as well as the elastic scattering wavefunction and elastic differential cross sections. They have tested the model for electron scattering by He, Ne and Ar at impact energies 30-400 eV. Agreement with experiment is very good for both the absorption cross sections and elastic differential cross sections.

When radiative decay competes with autoionisation, below an excitation threshold, the resonance contribution to excitation in highly-charged ions can be considerably reduced. Just above threshold, radiative decay of the ion may also occur before excitation, and the excitation cross section is reduced. The importance of this effect is studied in the case of resonance transitions in helium-like ions.

The importance of diffusion relative to drift in ion-ion recombination is a decreasing function of the separation between the ions (contrary to a common supposition). Langevin's formula for the recombination coefficient is obtained very simply using a physical model which allows for both diffusion and drift.

A time-resolved spectroscopic study of the perturbation of a capillary glow discharge in helium by radiation from a TEA CO₂ laser (I_L=6 MW cm^-2) is reported. The one- and two-photon ionisation processes induced a lowering of the p>or=8 excited-state population. However, due to collisional and radiative processes the states p<or=7 which do not undergo photoionisation are also strongly depopulated. From a comparison of a kinetic model with experiment, total recombination and quenching coefficients for p<or=7 levels are obtained.

The absorption spectrum of mercury vapour has been recorded at high resolution (+or-0.003 AA) in the 1150-1350 AA region with synchrotron radiation as the background source of the continuum. Observations comprise the principal series: 6s²¹S₀ to 6snp ¹P₁⁰ (8<n<or=37) and the intercombination series: 6s²¹S₀ to 6snp ³P₁⁰ (8<n<or=48). An intensity anomaly occurs at high n values with ³P₁⁰ gaining most of the oscillator strength at the expense of the ¹P₁⁰ transitions. Simple intermediate coupling calculations show that this is due to a change from LS to jj coupling with increasing n values. A two-dimensional Lu-Fano plot it is also presented and this exhibits interchannel interactions between the ¹P₁⁰ channels. As a result of the new measurements, the previously somewhat uncertain determination of the first ionisation potential is reliably confirmed as 84184.10+or-0.20 cm^-1.

The Stark shifts of Pb II, P II, Sn II and Sb II non-resonance lines have been measured in an electromagnetic T-tube plasma. The electron temperature was varied in a range from 16000 to 20000K and the electron number density from 0.3*10²³ to 1.4*10²³ m^-3. On the basis of the some established Stark shift regularities and systematic trends, predictions for the same lines were made. The experimental data and complete data set obtained in this way were used to demonstrate the existence of the other types of common regularities for the lines originating from the same type of transition.

The N_4,5N_6,7X (X=N_6,7, O_2,3, O_4,5) Coster-Kronig-type Auger spectra of free mercury atoms have been measured and analysed by comparing the experimental results with the calculated profiles. The spectra have been measured with the use of electron beam excitation. The kinetic energies of the Auger lines have been calculated by the Delta SCF method using a relativistic Dirac-Fock program. The relative energies of the line components have also been computed non-relativistically and a comparison has been made between the non-relativistic and the relativistic results. The intensities of the lines have been calculated in the mixed coupling scheme applying jj coupling to the initial states and intermediate coupling to the final states. The calculated profile reproduces the fine structure in the experimental spectrum rather well. The experimental intensity ratios between different line groups are between the experimental results of Matthew et al. (1980) and Larkins et al. (1977) for solid Au, and in obvious discrepancy with the calculated group intensity ratios based on McGuire's results (1974).

The lack of a plane of symmetry in the structure of optically active molecules leads, even in the dipole approximation, to the appearance of five new terms in the angular distribution of photoelectrons with defined spin orientation ejected from unoriented molecules by circularly polarised light. The vector structure of these terms is presented, and the possibility of their detection is discussed. Photoionisation of oriented chiral molecules is also discussed briefly.

By high-resolution photoelectron spectrometry using a cooled H₂( nu "=0, J"=0,1) target beam, the authors have determined the relative intensities of the Q and S rotational branches for several H₂⁺( nu ') vibrational states at photon wavelengths of 58.4, 73.6 and 74.4 nm. From results measured at different angles between the direction of the unpolarised photon beam and the momentum of the ejected electrons, they have determined the angular anisotropy parameter beta _s for S-branch electrons ( Delta J=2 transitions), and find (e.g. for nu '=0) beta _s(58.4 nm)=0.81(17), beta _s(73.6 nm)=0.54(16) and beta _s(74.4 nm)=0.76(27). These data show that at photoelectron energies of 5 eV and 1 eV the contribution due to f-electron waves cannot be ignored. A search for Delta J=4 transitions at 58.4 nm has yielded an upper limit of 0.3% with respect to the Delta J=0 partial cross section.

For pt.II see ibid., vol.14, p.2635, 1981. A systematic analysis of experimental data on the alignment of the L₃ vacancy in light atoms formed in collisions with fast ions has been made. Two vacancy production mechanisms were considered: direct Coulomb ionisation and electron capture. The first Born approximation and the Oppenheimer-Brinkman-Kramers approximation, both employing realistic atomic wavefunctions, were used for the calculation of the alignment due to direct ionisation and electron capture, respectively. The results of the calculations are in good agreement with experimental data.

Impact parameter dependences and total cross sections for K and L X-ray production have been measured near K-L level matching conditions at 1.4 MeV/N projectile energy. Comparing different collision systems (Z₁>Z₂ and Z₁<Z₂ with Z₁+Z₂>or approximately=100) the features of those processes which contribute to the excitation are discussed within existing models.

A modified two-centre atomic orbital expansion is used in an investigation of electron capture in collisions of H⁺ and He²⁺ ions with Li atoms. The calculated total and partial transfer cross sections constitute the first published origin-independent results in the energy range 0.5-20 keV for H⁺+Li and in the range 0.1-2.0 keV amu^-1 for He²⁺+Li collisions. Total capture cross sections agree well with experimental data wherever the measurements have led to a consistent picture among themselves. For He²⁺+Li collisions, low-energy capture cross sections are predicted to be much larger than is indicated by results from two-state molecular orbital calculations without translational factors.

The bremsstrahlung cross section (BCS) for electrons scattering on a short-range model potential is expressed in terms of the partial elastic scattering cross sections, sigma _t(E), for the initial (E_i) and final (E_f) electron energies without the use of the low-frequency approximation. The relation obtained allows a description of the behaviour of BCS close to the scattering resonances. Resonances in the high-frequency limit of bremsstrahlung and 'two-state resonances' (free-free transitions connecting two scattering resonances) are considered. It is shown that in the vicinity of the 'two-state resonance' BCS (considered as a function of the two variables E_i and E_f) behaves as the product of the corresponding partial resonant scattering cross sections sigma _l1(E_i) sigma _l2(E_f). Resonances in the soft X-ray emission from lanthanum and cerium are discussed.

The coupled integral equations of the momentum representation are solved for the n=1,2 and 3 channels in the e-H system at 54.42 eV. The coupled potential includes polarisation potentials for exciting higher channels from the n=1 and 2 channels. Differential cross sections are given for all channels together with angular correlation parameters for the decay of the 2p state. Different approximations are examined for including n=3 channels in calculations of scattering data for n=1 and 2.

In a scattered-electron cascaded-photon coincidence experiment the authors have studied the electron impact excitation of the in=3 singlet states of helium for incident electron energies between 28 and 46 eV and for a scattering angle of 35 degrees . This was done by measuring the angular distribution of the 3¹P to 1¹S photons (the later resulting from the 3¹D(3¹S) to 2¹P to 1¹S cascade) in coincidence with the n=3 scattered electrons, both in the scattering plane and in the plane perpendicular to the direction of the incoming electrons. At an incident electron energy of 45.6 eV they found a small 3¹D contribution to the coincidence peak. The 3¹P results at this energy are in perfect agreement with previously reported experimental data. At an energy of 34.6 eV this is not the case. At this and lower energies they observe, apart from the dominant 3¹P contribution, a significant 3¹D contribution to the coincidence signal. The authors present new experimental data for the orientation and alignment of both excitations at 34.6, 31.5 and 28.5 eV and for the ratios of the cross sections at these energies.

An apparatus for the measurement of positron and electron total cross sections on gases is described. The low-energy positrons are produced by moderating high-energy positrons from beta -decay sources. The authors used a ¹¹C positron source, produced at a proton accelerator and, alternatively, a ²²Na source, combined with various moderators. For electron measurements the secondary electrons originating from the moderator are used. The beam transport system consists of electrostatic lenses and a weak magnetic lens located in the middle of the gas cell. The discrimination against forward scattering is characterised by a cut-off angle of about 6 degrees . The authors' e⁺-H₂ and e^--H₂ data are compared with the results of recent measurements of similar accuracy performed at University College, London, and Wayne State University (Detroit), and with other e^--H₂ data. The agreement is satisfactory.

The radial density distribution of Ar I and O I excited atoms in a radiative state and Ar(³P₂) metastable atoms in an Ar-O₂ cylindrical plasma column produced by an electromagnetic surface wave is examined experimentally. The radial density distributions of the Ar(³P₂) metastable atoms and that of the O I radiative atoms are nearly flat, whereas the radial variation of Ar I radiative atoms can have a deep minimum on the plasma column axis. The difference in radial profiles leads to the conclusion that the atomic number density in the O(3³P) level is the result of a two-step process, viz. dissociation of the O₂ molecule and excitation of the ground-state atomic oxygen, this process being caused by Ar(³P₂) metastable atoms (rather than by electrons) successively colliding with O₂ and O. The results in Ar-OI₂ are compared with observations made in Ne-O₂.