Table of contents

The phase relations between connected states of configurations in the first and second half of a subshell are examined, with particular reference to the jj-LS transformation coefficients. It is shown that the phase conventions adopted must define states for both halves of the subshell in terms of particles, as is done in the quasi-spin representation, and not define states in the second half in terms of holes. Otherwise inconsistencies arise in the calculation of the jj-LS transformation coefficients.

Pseudo-relativistic Hartree-Fock calculations have been made for the configurations 4f⁷6s², 4f⁷5d6s and 4f⁷5d6p of Eu I, and for 4f⁷6s and 4f⁷5d of Eu II. These calculations have been used to provide ab initio estimates of screening effects and specific mass isotope shifts. Such estimates are in quite good accord with recent experimental results.

A relativistic many-body investigation has been carried out concerning the hyperfine field in the francium atom which is combined recently measured hyperfine constants to obtain accurate values of the magnetic moments of the isotopes ^208-213Fr. A comparison of the francium results with other alkali atoms shows trends in the contributions from exchange polarisation and correlation effects over the alkali atom series, providing valuable insights into these mechanisms.

The authors have performed Doppler-free two-photon spectroscopy to measure the wavelength of the 2³S to n³S (n=4-6) and 2³S to n³D (n=3-6) transitions in helium. The accuracy of their determinations permits them to do an interesting comparison with theory and to show very small Lamb shifts in the excited levels.

Recent progress in the description of electron processes in slow ion-atom collisions with atomic expansion models has stimulated interest in trajectory effects in those studies. The author discusses a reformulation of the atomic expansion method, explicitly taking into account curved-line internuclear trajectories. As a first application, total charge transfer cross sections in C⁶⁺+H collisions for energies E=0.1-1.0 keV amu^-1 are found to be considerably reduced in curved-line trajectory calculations from those derived with straight-line trajectories.

Recently, Stahler and Presser (1981) published target thickness dependent inner-shell vacancy production probability measurements on Cu-Fe collisions. They concluded that direct excitation cannot contribute appreciably to this probability. In the present letter their data are reanalysed assuming direct excitation to be the only relevant vacancy production mechanism besides rotational coupling. It is found that this analysis yields perhaps better agreement with the data than the conclusion of the original paper.

The results of an explicit optical potential calculation of electron-hydrogen entrance channel data at 50, 100 and 200 eV are compared with experiment, with recent optical potential results using different approximations, and with a close-coupling calculation containing pseudostates designed to mimic the effects of higher channels.

The existence of additional (more general than geometrical) symmetry is established for two important physical systems. The first of them is the excited hydrogen atom in perpendicular electric and magnetic fields when only the terms linear in the field strength are taken into account (corresponding to first-order perturbation theory). In particular, one of the fields can be absent, some additional discrete symmetry transformations appear. The second system is the doubly excited two-electron atom with the dipole approximation for the electron-electron interaction. The additional symmetry of these systems can be considered as a residual one related to the high symmetry of the unperturbed hydrogen-type system (the hydrogen atom and the two-electron atom with non-interacting electrons). The lowering of this initial symmetry, depending on the form of the symmetry-breaking interaction, is analysed.

The authors have measured level crossings in the Zeeman pattern of the two doublets d₄d₄' and d₁'d₁" of the 2_p⁵nd configurations of neon (n=3,4,5). The experimental accuracy (10^-4 or better), which considerably exceeds that of previous measurements (principally optical measurements by Green and Peoples), leads to new determinations of the Lande factors and energy splittings of the observed levels. For the 2p⁵3d configuration, the agreement with the present state of the theory is satisfactory. The study of the Zeeman pattern of the 2p⁵4d configuration shows an unexpectedly large and presently unexplained 'anomalous behaviour'. The authors have some evidence of similar anomalous behaviour in the 2p⁵5d configuration.

The time dependence of the two-photon absorption process is experimentally investigated in the low intensity regime as a function of laser linewidth, Doppler width, polarisation and delay between pulses, using 5 ns pulses at two different frequencies. Excellent agreement is found between experiment and the theoretical predictions of Allen and Stroud. The experiment arguments and expands earlier work on the dependence of linewidth in the two-photon process. The lineshape of the Hansch-type laser producing the pulses is shown to be best described by a Gaussian rather than a Lorentzian or as the result of an Ornstein-Uhlenbeck process.

Ionisation spectra of strontium have been measured using tunable laser radiation between 555 and 577 nm. The influence of bound two-photon resonances and autoionising three-photon resonances on the energy spectra and the angular distribution of the photoelectrons is studied for selected wavelengths. Four-photon ionisation via three-photon autoionising resonances is observed.

Multiphoton ionisation of strontium atoms to Sr⁺ and Sr²⁺ has been investigated with a Nd-YAG laser ( lambda =10642 AA), its second, third and fourth harmonics and a tunable dye laser in a wavelength region around 5321 AA. The power dependence of the Sr⁺ and Sr²⁺ production has been obtained. The creation of Sr²⁺ is discussed with respect to two models: the simultaneous excitation and elimination of two electrons and a stepwise ionisation via Sr⁺.

Multiple photodissociation rates of dichlorofluoromethane were measured from the dissociated products and/or the residual gases which escaped dissociation. The relationship between the separation factor for the hydrogen isotopes in the residual gases and that in the products is in good agreement with theory. Hydrogen isotope separation is influenced by the chlorine isotopes contained in the molecules. The experimental data obtained are compared with the theoretical dissociation model including two sets of kinetic equations in which energy exchanges between two kinds of isotopic molecules are considered. This gives the authors a good explanation for the dependence of the dissociation rate on the laser intensity and pressure, and the pressure dependence of the separation factor.

A detailed investigation of the energy dependence of Regge pole positions and residues has been carried out for r^-12 and Lennard-Jones (12,6) singular interatomic potentials. Semiclassical calculations are used to show that the pole positions can vary relatively slowly with wavenumber, whereas the residues can show a much faster variation. In addition, the semiclassical results are compared with the predictions of some simple formulae due to Brander (1966) and Tiktopoulos (1969) and one that arises from a complex harmonic oscillator formalism. It is found that these formulae, together with the criteria for their validity, are a useful way of understanding and predicting the energy dependence of pole positions and residues for singular interatomic potentials.

Recently, a theoretical calculation has been presented assuming that coherences can be produced in a radiatively assisted inelastic collision. The author reports the observation of final-state coherences in the simple case of dipole-dipole interaction in a strontium-europium mixture illuminated by a pi -polarised laser field. The author has measured the polarisation ratio P of the fluorescence emitted from the final degenerate Zeeman states of strontium. The result P=0.43+or-0.05 is in good agreement with theoretical predictions P=0.45.

The authors have performed close-coupling calculations of cross sections for rovibrational excitation of para-H₂ by He at energies extending up to 1.5 eV above the first vibrational threshold. The potential of Gordon and Secrest (1970) has been used. The n=0 to 1 vibrationally inelastic cross sections are found to differ from those of Lin (1981), who used the same potential and basis set, by factors attaining four orders of magnitude close to the n=1 excitation threshold. Structure in the variation of the vibrationally inelastic cross sections with energy, reported by Lin, is absent in the results of the authors' calculations. A comparison is made with coupled states (Alexander and McGuire, 1976) and with their own infinite-order sudden calculations. They also compare their results with measurements of the n=1 to 0 vibrational relaxation coefficient at low temperatures (Audibert et al., 1976).

Differential cross sections for vibrational excitation have been measured for individual states up to nu =5, for the collision He⁺ on H₂, in the mean energy range (100-3000 eV), in an ion-molecule crossed-beam experiment. The experimental requirements (high-energy resolution as well as very good angular resolution) are fulfilled thanks to the properties of the highly selected ion beam and of the extremely well cooled molecular beam used as a gas target. An energy spread of less than 200 meV is achieved together with an angular spread of approximately +or-0.025 degrees , both independent of the collision energy. At an impact energy of 510 eV, the cross sections are found to be peaked strongly at small angles. It is also observed that the cross sections corresponding to the different vibrational excitations ( Delta nu =1,2, . . . ,5) are of the same order of magnitude, and nearly equal for the highest excitations ( Delta nu >or=2) and for angles above 0.5 degrees in the laboratory frame. A semiclassical calculation, based on a schematic potential surface, yields a correct interpretation of the experimental data, at least for the Delta nu =0,1 processes investigated. This simple description also gives a detailed insight of the triatomic-system behaviour relative to its geometry.

Semiclassical coupled-states calculations have been carried out for collisions involving structureless positively and negatively charged projectiles to explore the dependence of the excitation and ionisation cross sections on the sign of the projectile charge. Both asymmetric collisions, where the magnitude of the projectile charge, Z_p, is much less than the target nuclear charge, Z_T, so that the perturbation of the target by the projectile is weak, and symmetric collisions, where mod Z_P mod approximately=Z_T, have been considered. The Coulomb deflection of muons is found to have an appreciable effect on the K-shell ionisation cross section. For the symmetric collision case, the authors' considered excitation and ionisation of atomic hydrogen targets both by protons and antiprotons and by electrons. For protons and antiprotons the ratios of the coupled-state cross sections to the corresponding first Born cross sections do not show the same relation to the first Born that is exhibited by the copper ionisation results, and are in fact remarkably the same for Z_P=-1 as for Z_P=+1. These results demonstrate the projectile charge asymmetry of the collision, that is due to the presence of the charge transfer channel for the positive projectiles, but its absence for the negative projectiles. For e^-+H collisions coupled-state corrections to the first Born approximation are found to be appreciable, and overall good agreement with experiment is found.

Experimental deuteron- and alpha-induced ionisation cross sections and cross section ratios for elements Ca to Cu (Z=20-29) in the incident energy range 0.2-0.45 MeV u^-1 are presented. The experimental ratios R= sigma _alpha /4 sigma _d are larger than the corresponding theoretical values and do not show the same dependence on incident projectile velocity. It is argued that this discrepancy is mainly due to the way in which the binding effect is accounted for because other effects important in the low incident velocity region approximately cancel out in the cross section ratios. The absolute values of the deuteron-and alpha-induced cross sections are also compared with theory using the procedures recommended by Brandt and Lapicki (1981) to correct for the relativistic, binding Coulomb and energy-loss effects. There is fair agreement for deuterons while the theoretical alpha-induced cross sections decrease too fast with decreasing incident velocity.

The process of H formation as a result of electron capture by protons from some alkali-like ions is investigated using the Coulomb-Born approximation with the full interaction potential. These ions are visualised as one-electron species to reduce the complexity of dealing with many-electron ions. In order to take the effect of the core electrons into account, the lone valence electron is assumed to move in the field of a model potential. Results for ground-state capture cross sections from Mg⁺, Ca⁺, Sr⁺ and Ba⁺ target ions are presented for several incident proton energies in the range 10-1000 keV retaining both the post and the prior forms of the interaction potential. The results for capture cross sections in the ground state in H⁺+Mg⁺ ion collisions are compared with the available experimental findings.

Charge transfer between H₂⁺ and Ar, O₂⁺ and Xe has been investigated as a function of the vibrational quantum number of the molecular ion within the energy range 20-1000 eV. The reverse reactions have also been studied and the ratio of the cross sections for the two spin-orbit levels of the ground-state rare-gas ion, sigma (²P₃2/)/ sigma (²P₁2/) determined. The results are interpreted in terms of a simple theory which incorporates the appropriate energy defects and Franck-Condon factors.

First-order many-body theory has been used to calculate correlation and coherence parameters for the electron impact excitation of the 3¹P₁ and 3³P₁ and states of neon. Both LS-coupled and spin-orbit-coupled results are reported for the 3¹P₁ state. Calculations were completed for E=20, 30, 40, 50, 80, 100 and 120 eV electron impact energies. Theoretical results are compared with those that were obtained from electron-photon coincidence measurements.

By using low-energy electron impact excitation the authors have observed the first example of an inner-shell single-to-triplet transition, namely the transition from the ground state to the (1s)^-1( pi 2p) ³ Pi state of N₂. The parameters omega _e and R_e of the triplet state have been deduced from the observed vibrational structure of the transition. The parity-forbidden transition to the (2p₃2/)^-14p state of Ar has also been studied, and the intensity of this transition relative to that of the electric-dipole-allowed transition to the (2p₃2/)^-14s state has been measured as a function of the incident electron energy.