Table of contents

Ab initio calculations of both the radiative and autoionisation lifetimes of levels of the 5d⁹6s²6p configuration suggest that the recent interpretation of a new metastable level as 5d⁹(²D₃/ ₂)6s²6p_1/2 J=2 in Hg is in error. The authors suggest an alternative relatively long-lived level, 5d⁹(²D_5/2)6s²6p_3/2 J=3, may account for the observations.

Experimental results for the collisionally broadened Balmer alpha line in dense argon plasmas have recently been reported which deviate significantly from the earlier theoretical results. The authors have performed ab initio theoretical calculations at the experimental plasma conditions. The inclusion of time ordering in the electron operator (neglected in previous calculations) brings the theoretical and experimental results into agreement. The authors comment on the procedure adopted by Vitel (1987) for the extrapolation of theoretical results from lower electron densities to the experimental values.

The authors show that spontaneous decay of atoms interacting with reservoirs, which have a frequency-dependent photon mode density, not necessarily concentrated over a narrow frequency range, may become non-exponential.

The authors correct earlier calculations of associative ionisation rate coefficients in which velocity distributions appropriate to cell experiments were incorrectly applied to cross-beam experiments. In addition, they have derived the velocity distribution for collisions in a single beam, and they apply this distribution to the results of earlier single-beam experiments.

Single- and double-electron capture in He²⁺+He collisions and single-electron capture in He⁺+He⁺ collisions are studied systematically using a travelling molecular orbital (MO) method within a semiclassical formalism. Present results identify the origin of the discrepancy seen between theoretical results previously obtained for He⁺+He⁺ collisions and give a theoretical rationale to the low-energy behaviour of cross sections for the single-electron capture process in He²⁺+He collisions previously observed experimentally.

The author investigates the expected behaviour of the small-angle differential cross section for inelastic e^--atom and e^--ion collisions. The presence of the Coulomb phaseshift in the expression for the scattering amplitude for e ^--ion collisions leads to destructive interference between the individual partial-wave components at small angles. As a result, forward-angle differential cross sections for ions will be much smaller than for atoms. One consequence of this is that the limiting value of generalised oscillator strength for a dipole transition at zero momentum transfer is not the optical oscillator strength. Calculations for the Be⁺ 2s⁺2p transition show a large reduction in the forward-angle differential cross section, and demonstrate that this interference effect is so pronounced that it should be experimentally observable.

An ab initio theoretical investigation is carried out mainly on the ³ Pi _g state of the fluorine molecule (F₂). Results support the assignment of the new triplet state observed by Hoshiba and co-workers (1985) as ³ Pi _g. A possibility concerning the laser transition of the ³ Pi _u from ³ Pi _g system is discussed.

The resonance fluorescence of a two-level atom interacting with a laser pulse is studied, following the short-pulse approximation. The authors demonstrate that an explicit expression of the energy spectrum in terms of atomic-state amplitudes holds also for an off-resonance excitation. Some analytically solvable cases are studied in detail, showing that the main spectral properties of the fluorescence emitted under adiabatic and non-adiabatic pulsed excitation can be easily described. For the first time the continuous dependence of the fluorescence spectrum on laser detuning is reported for a smooth pulse.

Spectra profiles of the helium lines lambda =501.5, 667.8 and 728.1 nm have been studied in emission under high resolution by means of a scanning Fabry-Perot interferometer for a wide range of discharge temperatures and pressures. The collisional-broadening constants determined from the Lorentzian component of the spectral profiles vary significantly with temperature, especially for the case of lambda =501.5 nm. These departures from the behaviour of pure-resonance broadening suggest that the profiles are distorted as a result of excitation mechanisms and, for the lambda =501.5 nm line, indicate the presence of higher-order terms in the interaction potential.

Photoionisation cross sections of the ground states of neutral Si, P and S are calculated in the near-threshold region using the close-coupling approximation including configuration interaction in the target. Results are compared with recent measurements of the photoionisation spectrum of P and S and good agreement is found for the resonance positions, but there is a significant discrepancy with the absolute value scale adopted in one of the experiments, on S. Bound states and oscillator strengths for Si and S are calculated in an attempt to obtain a more precise estimate of the accuracy of the cross sections.

The angular distribution and spin polarisation of Auger electrons of free atoms are discussed. In the (LS) J-coupling approximation the anisotropy coefficient and polarisation are expressed in terms of Auger-decay matrix elements in the particular case of the d_3/2 and d_5/2 subshell vacancies. For some Auger transitions in Ar, Kr and Xe atoms the anisotropy coefficient and polarisation have been calculated. The Herman-Skillman atomic model was used to calculate the Auger-decay amplitudes. In all cases considered the polarisation is small (less than 20%). However, the anisotropy is large and its measurement may be useful for studying the Auger process.

Ab initio calculations for the energy and absolute transition rates of X-ray transition processes associated with the hydrogen chloride molecule have been undertaken at the relaxed Hartree-Fock level. Data for K-, L₁- and L₂₃-shell X-ray emission spectra are presented. Comparison with atomic calculations is used to show that the molecular effect is manifested most clearly in transitions involving the 5 sigma bonding molecular orbital. The spectra are, however, dominated by intra-atomic effects. Participator and spectator satellite transitions associated with an initial K-shell hole and a 3 pi excited valence electron are also considered. The energy and transition rate results for spectator satellites associated with the (1 sigma )3 pi resonant excited neutral state are close to those determined for the normal K_alpha and K_beta processes. The 3 pi participator satellite is predicted to be at higher photon energy with 10% of the main K_beta spectator line intensity.

The continuum-distorted-wave-eikonal-initial-state model is extended to describe single-electron ionisation by impact of bare projectiles on multielectronic targets. Applications are given for collisions between multicharged ions and helium. Double differential, single differential and total cross sections are calculated. Experimental data and present theoretical results show deviations from the square of the projectile charge dependence predicted by the first Born approximation.

The ionisation yields of the optically excited caesium atoms (9.4<or=n*<or approximately=29) are measured in a vapour cell at a caesium vapour density of about 10¹⁴ cm^-3. Direct ionisation rate constants are given for Rydberg atoms with 9.4<or=n*<or=21.4. Collisions with caesium ground-state atoms and black-body radiation strongly modify the population of Rydberg levels with n*>or approximately=25 so the concept of direct collisional ionisation cannot be applied to those levels. The diffusional ionisation mechanism of Rydberg atoms in vapour cells, atomic beams or buffer gases is discussed.

State-selective single-electron capture in low-energy collisions of Ne²⁺, Ar²⁺ with Li(2s) has been investigated by means of translational-energy spectroscopy. Both collision systems involve production of final projectile states with parents corresponding either to the unchanged or changed primary-ion states. For Ar²⁺ impact, final states with a mixture of ³P and ¹D parents could also be identified. The significantly different impact-energy dependences for the various reaction channels are related to different contributions from electron correlation effects connected with the single-electron capture transitions.

The detailed electronic structures of the fragments of a possible charge transfer reaction: BF+H⁺ to BF⁺+H, have been computed using a double-zeta quality CGTO basis augmented by one d orbital on each atom (DZP basis), and employing the MRD-C method for treating the configuration interaction effects. Reliable potential energy curves have been obtained for BF (X ¹ Sigma ⁺, A ¹ Pi ) and for BF⁺ (X ² Sigma ⁺, A ² Pi ) over a wide range of internuclear distances. Despite the relatively small basis set, the computed curves compare well with available experimental data and with earlier calculations.

Calculations, using the CRAY X-MP/48 at the SERC Rutherford Appleton Laboratory, are presented for electron impact excitation of Fe⁺, in which the sixteen fine-structure levels arising from the four lowest states 3d⁶4s ⁶D, 3d⁷⁴F, 3d⁶4s ⁴D and 3d⁷⁴P are included. The R-matrix method is used in which terms of the Breit-Pauli Hamiltonian are explicitly included in both the target and the scattering descriptions. Fine-structure collision strengths are thus calculated and comparisons made with the four-state calculations of Nussbaumer and Storey (1980), and with Baluja et al (1986), who used the R-matrix method in LS coupling. The role of the 4p orbital, included in the L² basis of the Baluja et al calculation, is investigated: it is concluded that their approach is inconsistent unless the corresponding collision channels are also included. It is also inferred that the excitation of the third state, 3d⁶4s ⁴D, could be strongly affected by coupling to the 3d⁶4p odd parity states, which are not included in any calculation. Excitation rates are therefore presented only for fine-structure transitions between the two lowest states (⁶D and ⁴F).

For the excitation of mesic atoms ( mu p) by electrons the threshold behaviour changes its type on the threshold when the mesic atom principal quantum number reaches N=9. For N<9, the excitation cross sections start from zero on the threshold and increase with energy following a power law. If N>or=9, the excitation cross sections take finite values immediately above the threshold while below the threshold there exist a resonance series distributed according to the geometrical progression law.

Electron scattering from acetylene has been studied at low energies by means of an electron impact spectrometer. The 2.5 eV Pi _g resonance was seen to lead to appreciable excitation of the nu ₄ bending mode and the observed angular dependences of the scattered electrons could only be reconciled with angular correlation theory by assuming a bent geometry for this resonance and consequently a lowering of the molecular symmetry. A ² Sigma ⁺_g resonance was located at 6 eV and is the analogue of that observed in isoelectronic N₂ at 8 eV. Further information has been obtained on the Feshbach resonance series and assignments have been proposed based on the grandparent model.

The reaction rate constants in aqueous solutions of some inorganic compounds with the orthopositronium atom (o-Ps) were measured or checked in order to compare the o-Ps rate constants, K(Ps), with those of the muonium atom. K(Mu). It was confirmed on a larger experimental background that the ratio r between K(Mu) and K(Ps) can vary by factors of 10 and evidence was found that its value depends on the reaction types. In particular r seems to be <2 for redox reactions and >2 for spin exchange and addition reactions.

Dynamic second hyperpolarisabilities ( gamma ) have been calculated for the two-electron cations Li⁺ through Ne⁸⁺. Four specific cases have been studied: degenerate four-wave mixing, Kerr effect, electric-field-induced second harmonic generation and third harmonic generation; the third-order non-linear susceptibilities for these processes for a range of frequencies are tabulated. Variational-perturbation theory and explicitly electron-correlated wavefunctions were used for the calculations. The authors believe that the final results, which in nearly all cases are new, are very accurate and should serve as a benchmark for future experiments and calculations. The results have been fitted to a general equation which unites the dispersion of all four cases to a single expression. Finally an expression for the anisotropy in these processes has been determined.