Table of contents

We use the global theory of the Stark electronic broadening to calculate the global and wing profiles of the hydrogen Paschen- alpha line. The results obtained for the wings agree with the H_beta wings and those for the global profile are in good agreement with the semiclassical theory.

It was earlier found that the size of the binary encounter peak does not show a q² scaling (q is the ionic charge) for partially stripped ions, but instead increases with the number of electrons carried by the projectile. Measurements of this enhancement of the binary encounter peak for collisions of fast C^q+ and O^q+ on H₂ have for the first time been extended to the lowest ionic charge state. The authors get the surprising result that the enhancement for q=1 is smaller than for q=2.

For pt. I see Malinowski and Jankowski, ibid., vol. 26, p. 3035 (1993). The recently formulated atomically oriented form of the valence-universal coupled-cluster theory accounting for one- and two-electron excitations (VU-CCSD/R) is applied to several states or Be. The complete model space (CMS) is employed. Various approximate forms of this method including the lowest-order perturbation theory are also considered. The standard single-reference (SR) formulation of the CCSD, method, treated as a special case of the VU-CCSD/R approach, is applied to the ground state. The calculations for the 9s9p7d7f5g STO basis set are the most accurate ones performed so far for any SR- and VU-CCSD method formulated in terms of analytical orbitals. Proceeding from the SR- to the VU-formulation increased the magnitude of the ground-state correlation energy by 1.1%. Special attention is paid to the well known intruder state problem for the pair of 2s²¹S and 2p²¹S states. It is demonstrated that, despite earlier findings, the CMS based VU-CCSD, methods can be safely used in the presence of intruder states. The impact of the electronic structure of the series of systems considered in VU-CCSD calculations on the accuracy of results is also addressed.

Fano- and Lorentz-type profile formulae are tested for Stark resonances against exact quantum results. If background terms are included, both work very satisfactorily for a wide range of resonances.

The dipole polarizabilities of the lithium-like ground states, 1s²2s ²S, are calculated with the full-core plus correlation (FCPC) wavefunctions. The non-relativistic polarizabilities are obtained by using a variation-perturbation method. Based on the relativistic and QED corrections to the energy, the dipole polarizabilities are corrected using an oscillator strength formula. The results indicate that relativistic and QED effects on the dipole polarizability become very important for Z>or=15. The result for Li I, 164.08 a₀³, is in good agreement with a recent CI-Hylleraas calculation, 164.1 a₀³ and with the experiment, 164.0+or-3.4 a₀³. The calculated results of other lithium-like systems are compared with previous theoretical results in the literature.

The authors obtain the accurate wavefunctions and energies for n^1,3S and n^1,3P states of He for n<or=5 by variational configuration calculation with B-spline basis functions. The accurate wavefunctions have been used to calculate oscillator strengths for the transitions mS-nP, m, n<or=5 in He. The results of f values are accurate to within 0.01 per cent or better. The error estimate is based on the numerical convergence as the number of the angular partial waves of the employed wavefunctions is increased, and on the agreement between the results obtained using the dipole length and velocity formula. The accuracy of the results does not deteriorate with increasing principal quantum number and results is better than the previous highly accurate calculated results of Schiff et al. (1971) and Kono and Hattori (1984-6). The results also agree with some recent accurate experimental results. Therefore, the results can be regarded as being as reliable as those of Kono and Hattori at least, which are likely to be the most accurate theoretical results. Some radial expectation values are also calculated to give some indication of the accuracy of the wavefunctions.

The paper reports lifetime measurements of quartet levels in the 2s2p3p and 2s2p3d configurations in B-like oxygen, O IV, by means of the beam-foil technique. Reliable decay-times are obtained by explicitly including the dominant cascade repopulation in the decay curve analyses through the non-linear ANDC method. Different fine-structure levels in the 3p ⁴D and 3d ⁴F terms are found to have significantly different lifetimes due to the occurrence of spin-forbidden and Delta L=2 combinations, respectively. A comparison of the experimental 3p ⁴D and 3d ⁴F lifetimes in N III, O IV and F V with recently published theoretical results shows an overall agreement but also some surprising discrepancies.

Precision photoionization cross sections for He have been measured from threshold to 120 eV with an accuracy ranging from 1 to 2%. In addition, a critical review of the published data has been made and recommended photoionization cross sections from the ionization threshold to 8 keV are presented with an estimated uncertainty of +or-10% above 500 eV. The effect of coherent (Rayleigh) and incoherent (Compton) scattering is discussed.

Photoionization spectra from excited Cu I atoms have been obtained in the dark space of a hollow cathode discharge. Electrons generated by ionization of sputtered copper atoms with dye laser pulses are detected as a fast optogalvanic signal. By scanning a pulsed dye laser over a wavelength range From 415 to 530 nm a variety of autoionizing resonances have been found indicating that the dark space atoms are excited to high levels. Feasibility and restrictions of the method are discussed in order to achieve absolute photoionization cross sections.

The kinetic energy distribution of a cloud of ions stored in a RF trap has been investigated by use of two methods based on phase-stopped ejection. The oscillation of the RF voltage is stopped at a phase corresponding to zero field Value inside the trap. After their subsequent ejection from the trap, the ions are detected by an electron multiplier. The instantaneous ion cloud temperature value corresponding to the phase-stopping time can be deduced from the recorded profile of the ion signal. An alternative method consists in submitting the ejected ions to a repulsive field, applied before the detector, which allows us to discriminate the velocity classes. In both cases, the experiments are interpreted thanks to a simple theoretical description of a cloud of ions at thermal equilibrium in a RF trap and to a precise calculation of the ejection process. Both methods are compared, for the various trap working points. In all cases, the ion cloud average temperature is of the order of a tenth of the potential well depth.

The multiple-photon absorption cross section of CF₃D in CF₃H buffer gas has been examined in the 10 mu m R and P branches of the CO₂ laser. Absorbed energies were measured by the photoacoustic method. To allow probing of the multiphoton absorption cross section in excited states, two laser pulses were used, allowing excitation at one laser wavelength followed by probing of the absorption cross section at a second laser wavelength. The onset of non-linearity in absorption was determined to coincide with the onset of the quasi-continuum, as there was a distinct change in the spectral distribution of the absorption cross section when the molecules were pre-excited into the non-linear region. The time dependence of the results showed that the overall time for vibrational energy transfer from highly excited CF₃D to surrounding CF₃H was of the order of 200 ns.

Measurements of the cross sections for single and double ionization of helium as well as for the creation of H₂⁺ and H⁺ ions from H₂ for impact of antiprotons in the energy range 13-500 keV are presented. The results are compared with our earlier, less accurate data and with data for equivelocity proton impact. The single ionization cross section of helium agrees remarkably well with a continuum distorted wave (CDW-EIS) calculation. The ratio between the double and the single ionization cross sections of helium increases dramatically with decreasing projectile energy. New theoretical calculations are called for.

The classical trajectory Monte Carlo method is used to calculate coherence parameters for electron capture to H(n=2, 3 and 4) in 30 to 250 keV H⁺+He collisions. The parameters evaluated are the x- and z-components of the dipole moment (D) and the z-component of the perihelion velocity of the captured election (L*A). The calculated integral values are found to be in reasonable agreement with available experimental results. Components of the dipole moment and (L*A) were also evaluated as a function of projectile scattering angle. Here, rapid variations of the parameters are observed near the Thomas angle that indicate the transition from the hard, large angle collisions between the projectile and target nucleus to the soft, small angle scattering between the projectile and active electron.

Generalized oscillator strengths (GOS) for ionization of the 2s subshell of several atoms are reported. The atomic wavefunctions used in the work have been obtained from a relativistic local density potential method known as the RC Xi E method. The GOS has been obtained using the Born approximation for 2s subshell ionization of Be, C, O, Ne, Al, Ar, Fe and Zn. The GOS behaviour has been studied both as a function of momentum transfer and energy transfer to understand the Bethe surface properties. Besides the RC Xi potential, the non-relativistic Hartree-Slater (HS) potential has also been used in a separate calculation for comparison. There is in general reasonably close agreement between the results of the RC Xi and the Ha calculations. The main feature of the GOS for 2s ionization is that in the case of small-Z atoms the GOS at threshold has its maximum at a finite value of K, away from the optical limit (K to O). At the position of the peak, significant contribution from optically forbidden partial wave channels has been observed. The 'Bethe ridge' is found to occur when delta approximately=(Ka₀)² at energies above the ionization threshold. At the ionization threshold itself, a ridge on the Bethe surface has been found to occur due to contributions from non-optical channels.

Excitation of the 4p⁵(²P₃2/)5s ³P₁ state of krypton by 15 eV incident electrons is studied using the electron-photon double angular correlation technique. Measurements are reported for electron scattering angles in the range 20-120 degrees , corrected for the influence of hyperfine depolarization. Although the incident electron energy is within 5 eV of threshold, the data show substantial qualitative agreement with a distorted wave Born approximation calculation.

An electrostatic impact spectrometer is used to measure, between 3 and 10 eV, the kinetic energy, the angular distributions and the yield of negative fragment ions formed by dissociative attachment in CF₄. The energy range and the symmetry of the resonances are determined as well as the dissociation processes and their dynamics. The first resonance state, observed around 6.8 eV, is attributed to the CF₄^-3T₂ shape resonance adiabatically dissociating in the two channels F^-+CF₃ and CF₃^-+F. The excess energy is shared between the fragment kinetic energies and the internal (vibrational) excitation of the trifluoromethyl fragment. The next resonance, located around 8 eV, is assumed to be of T₁ symmetry. The authors suggest that this Feshbach resonance dissociates in F^- ions by the sequential breaking of two C-F bonds through an intermediate CF₃^-* electronically excited negative molecular ion. The ²A₁ shape resonance has not been observed in the dissociative channel.

Two-dimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in low-density helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values. The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously determined in lifetime measurements; the widths of the Ps component in helium, neon and argon are consistent with a Ps-atom scattering cross section below 6.8 eV in all three gases of approximately (9-0.5 E) pi a₀², where E is the Ps energy in eV.

The recently developed relativistic Hamiltonian for the atomic bound states of one spin- 1/2 and one spinless particle is issued for calculations of the relativistic Zeeman splitting of such atoms. Solutions are applied to the pi - mu atom.

The transition matrix elements and differential scattering cross sections for stimulated Raman scattering (SRS) from excited states of H₂ are calculated by means of the group theory approach. Some related experiment results are discussed.

Observations of H-like Ne x and He-like Ne IX soft X-ray emission line spectra have been made on the JET (Joint European Torus) tokamak. Recently calculated electron impact excitation rates for He-like neon are used in the derivation of the electron temperature sensitive line ratios G=((x+y+z)/w), R₁=(1s3p¹P₁ to 1s²¹S₀/w Ly_alpha /w, and the electron density sensitive ratio R=(z/(x+y)), where w, x, y and z are the resonance line (1s2p ¹P₁ to 1s²¹S₀), intercombination (1s2p³P_2,1 to 1s²¹S₀) and forbidden lines (1s2s³S₁ to 1s²¹S₀) respectively and Ly_alpha is the H-like Lyman_alpha , transition. Extensive modelling is carried out to allow for non-coronal conditions due to diffusion effects in these calculations, which are compared with experimental ratios from JET, where independent temperature and density measurements are available. Departures from equilibrium are found for both ohmic and additionally heated plasmas, resulting in significant changes to the G and Ly_alpha /w ratios. The R ratio is approaching its upper density limit and is relatively unaffected by non-coronal conditions. Good agreement between the experimental and calculated ratios is found, giving an indication of the accuracy of the combination of the atomic data and transport employed in the calculations.