Table of contents

The results of the quadrupole moment Q calculations in those atoms where the total moment of all the electron shells j=1/2 are presented. It is shown that the non-zero quadrupole moment arises due to the hyperfine interaction of the electron shell with the nucleus. The Q values turn out to be especially large in cases when the atoms have the term ²P_1/2.

An RF-trap ion source has been used to produce H₃⁺ with low vibrational energy. The internal energy of the ions has been measured by collision-induced dissociation to be less than 0.5 eV. The implication of this to studies of dissociative recombination is discussed.

The second-order correlation energy of the ground state of the argon atoms is calculated using basis sets of Gaussian-type functions. It is demonstrated that accurate second-order correlation energies can be obtained by employing systematic sequences of even-tempered basis sets of Gaussian-type functions. The pair correlation energies are compared with those obtained by Jankowski et al. (1979) using basis sets of exponential-type functions.

The authors apply the recently developed theory of multiplicative stochastic correlation functions with Markovian random jumps to the problem of resonance fluorescence from a two-level atom in a strong laser field. The finite bandwidth of a single-mode laser or the appearance of distinct lines in a multimode laser is considered to be brought about by a random mode-switching process. The two-time dipole correlation functions, which determine the spectral distribution of the fluorescence and the temporal correlations between the emitted photons are averaged exactly over the stochastics of the driving field. The authors' explicit expressions only involve single-time averages, which can be easily evaluated for a given probability distribution. These results generalise and modify earlier results. The authors obtain closed expressions, rather than implicit differential equations, which can handle the continuous case as well, and take into account the initial correlations, which always arise when the laser coherence time is finite. They illustrate the dependence on the stochastics of the laser model by comparing the multimode description with the phase-diffusion process and the Lorentz wave, which all give rise to the same laser profile, but to a different fluorescence spectrum.

A weak magnetic field has been found to have a pronounced effect on the selective field ionisation (SFI) of Rydberg atoms. Field ionisation quantum beats have been observed from coherently excited sodium 30d²D_3/2 Zeeman levels. Both pi laser excitation and SFI took place in a magnetic field of approximately 0.3 G. The time evolution of the Zeeman levels leads to the observation of SFI traces that range between those seen for pi and sigma laser excitation in zero magnetic field. Good agreement is found between theory and experiment, except for the depth of modulation of the quantum beats, which is less than the theoretical value.

Expressions for the electron shift and broadening of spectral lines in dense plasmas are derived on the basis of a recently given Green's function approach to the dielectric function. It is shown that in the case of not too high densities these expressions are equivalent to those following from the semiclassical impact approximation. Estimates are given for the shift and broadening of some argon lines.

The two-electron wavefunction for negative alkali ions is analysed. It follows that the states of weakly bound and external optical electrons are not equivalent. For a description of the nonlinear interaction of negative alkali ions with a light field this fact makes it possible to apply a quasi-classical approximation, from which the analytic expressions for the dynamic polarisability of the negative ion and for the multiphoton photodetachment cross sections are obtained.

The total and partial cross sections for the photoionisation of the Ba(6s²)¹S₀ ground state are calculated in the wavelength range 1200-2380 AA using a relativistic R-matrix approach based on the Breit-Pauli Hamiltonian. Good agreement with the experimental resonance positions is obtained; the magnitude of the cross section is found to be between five and ten times larger than that measured by Hudson et al. (1970). Calculations of the branching ratio in the wavelength range 1600-1200 AA (where the 6s_1/2, 5d_3/2,5/2 and 6p_1/2,3/2 channels are open) indicate that the 5d states of Ba⁺ are more highly populated than the 6s ground state or the 6p states in this region.

A consistent classical theory of atomic ionisation by charged particles is presented. The theory is consistent in the sense that not only individual collisions between the charged projectile and the atomic electron are described in a classical way (the theory is based on the binary-encounter approximation corrected for the collective field of the atom) but the entire atomic system considered as a classical structure with well defined electron orbits. The developed theory is applied to the analysis of atomic ionisation by heavy charged particles. The theoretical results appear to be sensitive to the atomic model used; in the case of the free-fall atomic model, they are found to be in good agreement with the experimental data.

The density matrix describing excitation of the n=2 and n=3 levels of atomic hydrogen under proton impact is calculated, both in the first Born approximation and in a close-coupling expansion in target eigenstates. Various ways in which the properties of the excited state can be characterized in terms of certain linear combinations of density matrix elements are discussed. A new characterisation, based on an extrapolation below threshold of parameters describing the angular distribution of low-velocity ionised electrons, is suggested.

The L₃-subshell alignment parameter for ¹H, ²H, ⁴He, and ¹⁴N impact ionisation of gold has been calculated in the energy range of 0.15-2 MeV amu^-1 using an improved version of the recent second-order L-shell ionisation model of Sarkadi and Mukoyama (1980). The calculations show that the second-order contribution is significant even for light ions, and that its inclusion brings the theory into better agreement with experiment. For nitrogen-ion impact, in accordance with the observations of the heavy-ion experiments, the present model predicts sign reversal and large positive values for the alignment parameter at low collision velocities. To get further improvements in the description of the collision-induced L₃-subshell alignment the need for coupled-channel calculations is emphasised.

The authors report the observation of highly stripped silicon-ion spectra. These spectra have been obtained after excitation of a 44 MeV silicon-ion beam travelling through a thin carbon foil. 2p to 1s transitions have been identified in H-, He-, Li- and Be-like ions. Transitions in the presence of one M-shell spectator electron are also identified. Experimental and theoretical results are compared with other measurements and calculations.

A time-of-flight electron spectrometer has been used to measure absolute total collision cross sections for electrons scattered from helium and argon over the energy range 0.1-20 eV. The measured cross sections are in good agreement with other recent beam experiments and theoretical calculations. At energies below 1 eV modified effective-range theory has been used to enable a comparison of the present work with swarm-derived momentum transfer cross sections. Some limitations of the applicability of the MERT analysis are discussed.

A low-energy time-of-flight electron spectrometer has been used to measure total collision cross sections for methane in the energy range 0.1-20 eV. Agreement with two recent experiments and with the pioneering work of Ramsauer and co-workers (1930) from the 1930s is excellent.

A merging procedure and a direct two-state approach have been applied to determine the molecular constants of GeCl from the B² Sigma ⁺-X² Pi _1/2 0-0, 0-2 and B² Sigma ⁺-X² Pi _3/2 0-0, 0-1, 0-2 sub-bands. The rotational constants for the X² Pi nu =0, 2 states of GeCl have been calculated for the first time using the appropriate Hamiltonians in a direct approach. The centrifugal distortion in the spin-orbit constant A_D, and the spin-rotation coupling constant gamma ^pi have been determined and are discussed.

Binding energies and formation rates of p mu He molecules, determining the rate of muon transfer from unexcited mesic hydrogen to helium, are calculated. The screening of nuclear charge by the atomic electrons is taken into account, as well as the electron form factor of the remainder, in the field of which the outgoing electron moves. The results obtained in a one-level approximation as well as in a 'simple-approach approximation' are compared with the experimental data.

The authors study the tunnelling of electrons through an inverted parabolic barrier in the presence of a laser field. Using wavepackets, they derive analytic expressions for the tunnelling current and probability. Special attention is paid to the initial phase of the electric field. The distribution of tunnelling times is obtained from an analysis of the tunnelling current.