Table of contents

The fourth-order quantum electrodynamic corrections of order α⁵mc² are made for the transition energies between excited states in neutral helium. It is found that the author's theory is consistent with the latest experiment by Sansonetti and Martin (1984).

The authors present a formalism based on the non-invariance algebra for the Coulomb problems that allows one to deduce an effective Hamiltonian for a wide variety of perturbing potentials. Applications to the problem of the hydrogen atom in magnetic field are performed. They especially derive the exact first- and second-order expressions of the effective diamagnetic Hamiltonian under a general operator form. Some of the consequences and further developments are briefly indicated.

The autoionising levels of the sp² configuration of Ga I, In I and Tl I were investigated by photoionisation experiments using the monochromatised synchrotron radiation of the 800 MeV electron storage ring in Berlin (BESSY) and the atomic-beam technique. The widths of the autoionising lines are discussed with respect to the wavefunctions of the corresponding levels.

Measurements of the photoionisation of Ba⁺ ions by VUV radiation have indicated cross sections in excess of 10^-16 cm². This is two or three orders of magnitude greater than would be expected from direct, outer-shell ionisation and it suggests that autoionisation can be extremely important. This view is supported by the results of preliminary calculations.

The ionisation potential of an atom in a laser field is known to be intensity dependent. Previous theories apply only to a non-resonant field. The result is extended to the case of the N_r photon resonance where N_r is any positive integer. The new result contains several new features. Rapid frequency dependence is one and a dependence upon the conditions in which the atom enters the field is another.

A statistical model is used to describe multiphoton ionisation of noble gases. The energy spectrum of emitted electrons consists of a series of peaks corresponding to the absorption of various numbers of photons. The intensity dependence of the peaks is analysed and compared with experimental results for xenon and helium atoms.

Total charge transfer cross sections for Be⁴⁺+H(1s) collisions have been calculated for Be⁴⁺ impact energies E=0.1-25.0 keV amu^-1 using a molecular orbital close-coupling method. Calculated results are compared with other theoretical results and with the experimental data for C⁴⁺+H(1s) collisions.

The R-matrix method is used to study the elastic scattering of electrons by F₂. Polarisation effects are included, without parameterisation, via an optical potential. Total elastic cross sections are calculated at the equilibrium internuclear separation of the target. The potential energy curve of the lowest ² Sigma _u⁺ state of F₂^- is obtained at internuclear separations 2.2<or=R<or=3.0 au. The authors find that this state is stable against autodetachment at the equilibrium internuclear separation of the neutral molecule.

The phaseshift corresponding to the lowest partial wave and the associated approximation to the total cross section are calculated for low-energy positron-hydrogen-molecule scattering using a generalisation of the Kohn variational method. The trial wavefunction is expressed in terms of confocal elliptical coordinates and is a generalisation of the wavefunction used by Massey and Ridley (1956) in their treatment of low-energy electron-hydrogen-molecule scattering. The most accurate target wavefunction used takes into account 50% of the electronic correlation energy. Up to 31 short-range correlation functions, made up of products of one-particle functions of sigma symmetry, are included in the trial wavefunction. Except at incident positron energies below about 2 eV, reasonable agreement with experiment is obtained below the positronium formation threshold at 8.63 eV.

Using second-order Moller-Plesset perturbation theory, the authors have calculated or =estimated (est) the geometries (the authors allow for Jahn-Teller distortion) and excitation energies, Delta E, of the five lowest triplets all associated with the 2p to 3s transition of CH₄ to be: C_2v, D_2d, C_3v, D_4h, T_d (unrelaxed) with Delta E=8.74, 8.95, 9.30 (est), 9.89 (est) and 10.41 eV. The C_2v and T_d singlets are found to be 0.33 and 0.22 eV above the respective triplets. Based on these results, two experimental singlet features at 9.65 eV and 10.33 eV are assigned to the D_4h and T_d geometries.

The XUV spectra of uranium in different regimes of conical thickness and in different ion stages have been observed in a laser-produced plasma. The spectra are dominated by 5d-5f transitions which produce an intense compact transition array in the 100 AA region. The profile and extent of this array is strongly sensitive to the density of uranium ions present in the plasma. A number of lines arising from 5d¹⁰ to 5d⁹nl transitions in U VIII and U XV are identified with the aid of Hartree-Fock and Dirac-Fock calculations. The similarities between the spectrum of uranium and those of the lanthanides observed under similar conditions are stressed.

The transition rate for the two-electron one-photon and one-electron one-photon decaying processes in atoms bearing initially two K-shell vacancies were evaluated for Ne and to Zr. The two-electron one-photon decay process is considered to be the result of the interaction between the jumping electrons and their interaction with the radiation field. The calculation is performed in second-order perturbation theory and the many-particle states are constructed from single-particle solutions. The present approach allows one to discuss several aspects of the decaying process. The results obtained for the branching ratio between the two processes reproduce reasonably well available experimental data and show an almost linear dependence on the second power of the atomic number. A comparison with other theoretical predictions is also presented for the two decaying processes and the strong dependence of the branching ratio on the initial configuration of the decaying atom is pointed out.

The radiative lifetimes of some Xe II states belonging to the 6d, 6d' and 7s electron configurations were determined in the present work. A multichannel delayed-coincidence method with pulsed electron excitation was used. A comparison was made with other already reported theoretical and experimental results.

Improved measurements of isotope shifts in the 4s²¹S₀-4s5s ¹S₀ transition of calcium are reported for the stable isotopes. A comparison with isotope shift measurements in other transitions by means of a King plot shows satisfactory agreement. Values of the changes in mean-square nuclear charge radius delta (r²) from a combined analysis of muonic isotope shifts and electron scattering data are used to separate the mass and field shifts in the optical lines. This procedure leads to values of delta (r²) for the calcium isotopes from ⁴⁰Ca to ⁴⁸Ca using all available high-precision data. The results for delta (r²)^A,40 are 3.2(2.5), 215.3 (4.9), 125.4 (3.2), 283.2 (6.4), 118.8 (5.9), 124.2 (5.0), 5 (13) and -4.4(6.0)*10^-3 fm² for A=41 to 48 respectively. Values of the electronic factors relating the observed shifts of delta (r²) are deduced, and discussed in terms of configuration mixing in calcium.

When a Rydberg series interacts with a broad underlying state, the Fano parameters of the Rydberg states vary in a systematic way with detuning from the centre of the broad state. The authors derive the general theoretical form of the variation, and show that the asymmetry parameter q may change sign up to four times as a function of detuning. Relevant data are discussed.

The first N_6,7O_4,5O_4,5 Auger electron spectrum of atomic Au has been measured from high-temperature vapour using a new spectrometer system with an inductively heated oven. The spectrum has been interpreted by comparison with the optical double-hole final-state energies. With the aid of optical energy levels as well, the 4f binding energies for free gold atoms have been determined to be 91.60+or-0.20 and 95.25+or-0.20 eV for the 4f_7/2 and 4f_5/2 levels, respectively. Furthermore, the values of 2.5+or-0.2 and 10.5+or-1.0 eV have been obtained for the free atom-solid binding and Auger energy shifts, respectively. The experimental free-atom Auger and binding energies are also compared with the results of Dirac-Fock calculations.

When two-level atoms interact with laser light which is fully amplitude modulated at frequency omega , it is found that if omega is a particular fraction of the Rabi frequency then the coherence associated with the density matrix element rho ₁₂, or dipole, can be suppressed, along with the modulation in the inversion, provided omega also greatly exceeds the longitudinal decay rate Gamma . Increasing Gamma , by random collisions for example, induces a non-zero oscillating dipole and a modulation of the inversion, the enhancements of which are directly proportional to Gamma , giving an example of a coherent effect produced by an incoherent perturbation of a random nature.

The problem of optical spectra of a molecule in a strong monochromatic field is considered within the framework of a simple numerically solvable quantum mechanical model. In this model two nondegenerate electronic states and one vibrational mode with different equilibrium positions in different electronic states are taken into account. The calculated spectra show a new vibrational structure, with the quantum exceeding that of a free molecule, and also weak satellites of strong lines. Considerable changes of spectra with a small variation of laser frequency in the region of resonances with vibronic transitions and some other effects are also found and discussed. It is shown that orientational averaging does not lead to the disappearance of peculiarities in the spectrum.

The semiempirical pseudopotential method of Baylis (1969) has been used with some modifications in calculations of the molecular terms of diatomic systems composed of a group II atom and a ground-state rare-gas atom. The calculations have been carried out for Cd-heavy-rare-gas systems. The adiabatic potentials and oscillator strengths associated with several lowest terms of the cadmium atom are demonstrated and discussed.

Close-coupled calculations are reported for intramultiplet transitions in collisions of Mg(3s3p ³P) with He, based on a model potential description of the relevant electrostatic potential curves. An oscillatory structure is found in the energy dependence of the degeneracy averaged j to j' cross sections. A new selection rule is derived which prohibits j=1, m=0 to j'=0 (or 2), m'=0 transitions for quantisation in the collision frame.

Electron detachment processes have been measured for the (quasi-) two-electron negative ions with ns² structures (n=1, 2, 3, 4), namely H^-, Li^-, Na^- and K^- in collisions with He, Ne and Ar at energies of 0.5-100 keV amu^-1. Total detachment cross sections have been determined for n=1-4. Detachment with simultaneous ns-np excitation has been measured for n=2 and 3. Finally, cross sections for target excitation and/or ionisation have been estimated for 50 keV. For energies above 1 keV amu^-1, the total detachment cross sections are found to scale with the cross section for H^- at the same velocity as the inverse square of the detachment energies. Simultaneous ns-nsp excitation is found to be important ( approximately 30%) and exhibits a considerable alignment. Target excitation at these energies is relatively small in all cases except for Na^--Ne, due to strong quasi-molecular effects in this system.

Cross sections for single electron capture and ionisation, as well as for simultaneous capture and ionisation ('transfer ionisation') are calculated for collisions of partially stripped Au^q+ (q=6-25) ions with He atoms at collision energies between 20 and 300 keV amu^-1 by using the classical-trajectory Monte Carlo method. The helium ion core has been represented by an effective charge Z_eff(He)=1.69, while the electron interaction with the projectile ion has been described by a variable-charge potential. The calculated capture cross sections are in close agreement with the available experimental data, while the ionisation cross sections are consistent with them except for 10<or=q<or=16, where the authors' results at 100 keV amu^-1 are too small by about a factor of two. The authors give the calculated energy dependence of the total cross sections and for electron capture the distribution of captured electrons among final n states, as well as their impact parameter distribution.

Proton-induced K-shell ionisation cross sections for W, Au and U by low-velocity protons were obtained from thick-target measurements. For the first time the lowest incident energy reached a value less than ten times the binding energy of the K-shell electron (less than nine times in the case of Au). Possible sources of error are thoroughly examined and a comparison with other available experimental results and theoretical values is presented and discussed.

The impact-parameter-dependent K-shell to K-shell vacancy transfer probability has been investigated for 4.7, 7.9, 16, 32 and 90 MeV S¹⁵⁺-Ar collisions by measuring S and Ar K X-ray particle coincidences. With low-energy H-like S beams (obtained by poststripping S at 108 MeV and decelerating with the Heidelberg RF linac), oscillating Ar K vacancy probabilities as a function of impact parameter were measured. This structure can be interpreted as constructive and destructive interference of the transfer amplitudes on incoming and outgoing parts of the trajectory. Thus measurement of the height as well as the phase of the coupling amplitude allows for a very sensitive test of different coupling models. The interference maxima and minima could be related to values of the phase integral over the 2 p sigma -1 s sigma energy difference from which information about this energy difference could be obtained.

The 1200 eV non-coplanar symmetric (e, 2e) cross sections for helium leading to the n=1, 2 and 3 ion states were measured with a modified coincidence spectrometer using position-sensitive microchannel plate detectors. The ground-state momentum profiles (cross sections) are compared with the momentum distribution given by the Hartree-Fock wavefunction, the accurate correlated wavefunction of Joachain and Vanderpoorten (1970), and the momentum distribution derived from accurate Compton profile measurements by Lee (1977). Good agreement is obtained between theory and the (e, 2e) measurements, but the Compton momentum distribution is seriously in error at low momentum. The small discrepancy at high momenta between the present measurements and the plane-wave impulse approximation is completely explained by allowing for distortion of the continuum electron waves by the helium and ion potentials. The n=2 and 3 cross sections cannot be explained using the helium HF wavefunctions, but are well described by the accurate correlated JV wavefunction. The measured n=2 to n=1 cross section ratio discriminates between several accurate correlated helium ground-state wavefunctions.

Detailed investigations are made of the convergence of the K-matrix elements and the cross sections for elastic scattering and positronium formation in the s-wave scattering of positrons by atomic hydrogen in the energy range 6.8-10.2 eV. The Kohn and inverse Kohn variational methods are used with trial functions containing many linear variational parameters and associated short-range correlation terms, and results are obtained for a set of systematically improved trial functions. The diagonal elements of the K matrix and the eigenphase shifts obtained with these trial functions can be fitted very accurately to a convergence formula which permits extrapolation to the essentially exact results corresponding to an infinite number of short-range correlation functions. No such convergence formula applies to the off-diagonal matrix element K₁₂, nor to the cross sections, but the very slight changes in the results as the trial functions undergo their final improvements strongly suggest that the most accurate values obtained here for K₁₂ and the cross sections for positronium formation and elastic scattering are in error by no more than 5%, 10% and 5% respectively.

Recombination in a helium afterglow in which the ions are mainly He₂⁺ is not entirely of the collisional-radiative type as is evident from the enhancement of the rate coefficient by the ambient gas and from the emission of the 2³P to 2³S He line. Letting (p, 1) be the principal and azimuthal quantum numbers of the outer electron of a molecular state the channels normally taken into account in collisional-radiative recombination are augmented by He₂(p, 1)+He to or from He₃(p, 1) followed by the Rydberg electron analogue of dissociative recombination: He₃(p, 1) to He(2³P)+2He. The theory of the augmentation is formulated. Computations are done covering the range of afterglows studied. There is only a minor contribution to the electron loss rate from He₃⁺ dissociative recombination. The agreement with experiment is satisfactory.