Table of contents

The quadrupole moment of the atomic hydrogen ground state is calculated. It arises due to a hyperfine interaction. Its true value is 20 times larger than the result obtained only taking into account the hyperfine mixing with the lowest d level.

Use of relativistic many-body perturbation theory is made to calculate the hyperfine splitting of the ground 6²S_1/2 state in ¹⁹⁷Au (I=³/₂). The calculated splitting of 6068 MHz is in good agreement with the experimental value of 6099.320184(13) MHz determined by an atomic beam magnetic resonance technique. Electron charge densities at the nuclear surface corresponding to various configurations of ¹⁹⁷Au are calculated in an ab initio way by numerically solving the relativistic Dirac-Fock equations incorporating the nuclear charge distribution. The electronic factor relevant to the volume isotope shift for the resonance transition 6s²S_1/2-6p²P_1/2 in gold is also calculated and compared with other available results.

Single, double and triple ionisation of calcium atoms is observed with picosecond pulses at 1064, 532 and 560-568 nm. A resonance is detected in the Ca²⁺ signal at 564.8 nm.

A retarding-potential method has been used to analyse the energy of electrons produced in the process of generating singly and doubly charged ions in Xe and Ne at 532 and 1064 nm. Energetic electrons observed in Xe at 1064 nm imply that up to 30 photons have been absorbed in the ionisation process, while only 11 photons are necessary to release one electron. The laser intensity dependence of energetic electrons emphasises the validity of perturbation theory. The high absorption rate of such large number of photons can be explained in terms of an interaction of the laser field with several electrons of the atomic shell. A tentative picture is given.

By a simple technique it is possible to discriminate experimentally between the self-shift and foreign-gas-induced shift of Rydberg levels excited in a heat-pipe system. Illustrative results for two particular rydberg levels of the potassium dimer K₂ are presented. The results concern the shift induced by the alkali itself and the shift induced by Ar and He. Earlier reported discrepancies are removed.

The authors present the results of further calculations of cross sections for the vibrational relaxation process CO(n=1)+para-H₂ to CO(n=0)+para-H₂ in which: (i) an error in the long-range form of the potential of Poulsen has been corrected; (ii) the influence of extending the size of the para-H₂ rotational bases set is more fully investigated; and (iii) the effect of varying the magnitude of the energy defect, Delta E, of the near-resonance process CO(n=1)+H₂(j=2) to CO(n=0)+H₂(J=6)+ Delta E is assessed. Conclusions are drawn regarding the accuracy currently attainable in calculations of the rovibrational relaxation cross sections.

Striking discrepancies have been noted recently in Na⁺-Na and Li⁺-Li inelastic differential scattering between experiment and classical calculations on the one hand and some semiclassical results on the other. The phase factors occurring upon integration of the coupled semiclassical impact parameter equations are discussed. A spurious angular shift of the inelastic cross section is shown to arise when these are not handled in a consistent way. A revised calculation of Na(3p) collisional excitation shows good agreement with experiment.

With the help of newly recorded spectrograms, the seven hitherto unknown levels of the 4p²4d configuration of Y VII and Mo X have been determined, and three previously reported level values of Y VII are improved upon. Eight new levels of 4s4p³ of Y VIII are added to the only two of this configuration which were already known. This completes the structural knowledge of the four lower configurations of Y VIII, namely 4s²4p², 4s4p³, 4s²4p4d and 4s²4p5s.

The exchange of momentum between atoms and photons in a deflection experiment is usually described by different formalisms depending on whether the interaction time T is shown or long compared with the radiative lifetime tau _R. The authors present a new approach to this problem leading to a single theoretical expression valid in both limits and therefore allowing the transition between them to be studied. They interpret in this way the resonant Kapitza-Dirac effect and the optical Stern and Gerlach effect appearing in the short-time limit (T<< tau _R) as well as the deflection profiles usually deduced from a Fokker-Planck equation in the long-time limit (T>> tau _R). The transition between these two regimes is interpreted in terms of momentum transfer due to absorption and stimulated emission of laser photons, convoluted by the distribution of recoil due to spontaneously emitted photons.

The resonant two-photon ionisation of alkali-metal atoms is analysed in the dressed-atom picture using the resolvent formalism. The authors define a ladder approximation which is explained with the help of a diagrammatic representation. They show that it is then possible to perform the infinite summations related to perturbation theory which are involved in the matrix elements of the shift operator. The theoretical framework used is the Green's function formalism extended to the spin-orbit scheme adopting the quantum defect method for the atomic model. A numerical example of such a procedure is presented in the case of the caesium atom.

For pt.I see ibid., vol. 17, p.4643 (1984). The authors apply the theoretical results obtained in a previous paper in order to study the two-photon resonant ionisation of atomic caesium in the neighbourhood of the fine-structure doublet 7P_1/2-7P_3/2 when the incident light is right-hand circularly polarised. This formalism allows one to consider these two resonant states on an equal footing and it includes the effects of the infinite summations involved in higher-order perturbation theory. The numerical results show that the two-photon resonant ionisation of caesium takes place through an anticrossing of levels, at least in the intensity range studied. The temporal behaviour of the probability exhibits two distinct temporal regimes: linear for long interaction times and cubic for the shorter ones. The intermediate regime is characterised by quantum beats. At high intensity values, it is found that the induced light shift of the resonances is time dependent. It is also shown that the contribution of the higher-order effects may explain the slight departures between the theoretical and experimental results in the off-resonant two-photon ionisation of atomic caesium.

Associative and Penning ionisation cross sections sigma _A4dI and sigma _PI and their sum sigma _TI= sigma _AI+ sigma _PI are calculated for collisions of Ne with radiative excited He^*(3¹P) in the energy range 0.025-0.21 eV, in the classical impact parameter formulation. It is shown that the energy dependence of the sigma _Al cross section previously measured in a crossed-beam apparatus in the authors; laboratory and the velocity-average quenching cross sections sigma _TI obtained in discharges can be explained in terms of an autoionisation width for (HeNe)* which includes simultaneous direct and exchange mechanisms and is saturated in the small-R region.

Different collision mechanisms, which create continuum electrons that are slow in the rest frame of the ion, have been investigated by zero-degree electron spectroscopy. For the four-electron ions C²⁺, O⁴⁺ and F⁵⁺, (1s²s²) colliding with Ar autoionisation occurs after double-core excitation at high impact velocities. The combination of transfer and excitation, however, becomes an important mechanism, leading to autoionisation at lower impact velocities. For the three-electron ion F⁶⁺, (1s²2s) autoionisation lines were observed only as a result of transfer and excitation in the velocity interval from 3 nu ₀ to 8 nu ₀ considered. For 20 MeV Au^q+ on H₂, He, and Ar (5<or=q<or=19), zero-degree electron energy spectra were measured in coincidence with charged-state-selected exit ions. The observed autoionisation lines are due to transfer and excitation. The cusp electrons originate from several processes: electron loss to the continuum (q low), electron capture to the continuum, and transfer ionisation. Cross sections for the different collision processes, giving rise to continuum electrons, were obtained.

The alignment of the M₃, M₄ and M₅ subshells of thorium was studied for proton impact ionisation by measuring the polarisation of the emitted M X-rays. The measurements were performed with a 5 in Johansson crystal spectrometer in an impact energy region of 0.15-4.0 MeV. The experimental data were found to be in fair agreement with PWBA calculations.

The population of highly excited n, l states in single-electron capture by Au ions of charge state 12-18 and 2 au velocity from molecular hydrogen has been investigated experimentally for 9<or=n<or=16. Cross sections sigma _nl were extracted from measurements of 2000-6000 AA photons emitted after capture. A refocusing technique made possible sufficient resolution for the observation of the different l components of the n to n-1 transitions. A comparison with UDWA and CTMC theoretical calculations for point-like ions colliding with atomic hydrogen shows one of their main conclusions to be true: the preferentially populated states have a high angular momentum. The CTMC l distribution agrees better with the experimental data than does the UDWA result. The validity of the comparison is discussed in detail. As a by-product of the measurement, the dipole polarisabilities of the ions in question were obtained.

Phaseshifts, differential, total and momentum transfer cross sections are calculated using an R-matrix approach for the elastic scattering of electrons by argon atoms in the impact energy range 0-19 eV. The coupled-state calculation is based upon a single-configuration atomic ground-state wavefunction coupled to a ¹P pseudostate. A critical assessment of earlier theoretical and experimental data is made and the conclusion is reached that the present results are the most satisfactory over the entire energy range considered.

The semiclassical approach is developed to calculate the cross sections of vibrational excitation and dissociative attachment for diatomic molecules within the framework of the 'boomerang model'. The cross sections are shown to factorise in this approximation. This feature results from the fact that processes proceed via two stages-capture of the incident electron into an intermediate state and the decay of this state. The formulae obtained reveal the energy dependence of the cross sections on the parameters of the system. Numerical calculations for N₂, CO, H₂, HD and D₂ confirm the high accuracy of the method.

Positronium formation cross sections for the n=2 level in e⁺-He scattering have been calculated using a distorted-wave model. Total positronium formation cross sections ( sigma _1s+ sigma _2s+ sigma _2p) have been compared with the most recent measurements of Fornari et al. (1983) and Charlton et al. (1983).

The mechanism of anomalous (both positive and negative) density effects in the electronic mobility in simple fluids is analysed within the framework of the unified theory.