Table of contents

The authors study the spontaneous emission of an atom described by an extended wavepacket. They show that the shape of the emitted line is affected by the Doppler shifts and carries the information of the initial wavefunction. The total intensity is found to exhibit a standard dipole pattern.

Detachment of two electrons from S₂^- ions in 3 keV collisions with N₂ has been studied by means of ion translational energy spectrometry. The vertical electron affinity of S₂^- is measured to be 1.6+or-0.2 eV. In addition to S₂⁺ formation, dissociative double detachment is observed for the first time. Cross sections for non-dissociative and dissociative double detachment are determined to be 4.4+or-0.1*10^-17 cm² and 3.7+or-0.3*10^-17 cm², respectively.

Spin-dependent momentum-space distributions are introduced within the density-matrix formalism and the impulse approximation. The m_s components of the spherically averaged momentum densities and isotropic Compton profiles are calculated for the lithium atom and members of its isoelectronic series by using full configuration-interaction wavefunctions that include over 96% of the correlation energy and represent the final stage of a convergent sequence, both in the charge and spin densities in position space. The effects of electron correlation on momentum-space properties of the lithium atom are assessed by comparisons to those obtained from an unrestricted Hartree-Fock wavefunction. Results for the ions B²⁺, N⁴⁺ and F⁶⁺ do not support the observation that the sum of information entropies in momentum and position spaces enhances with the quality of the wavefunction. The conjecture that J_HF(0)>J_corr( phi ) is in complete agreement with the results of this study.

Analytical expressions describing the light-induced drift (LID) of alkali metal vapours reasonably well both under conditions of ground state optical hyperfine pumping and in its absence are obtained. It is shown that under the conditions of optical pumping the LID effect of the particles arises under the action of its own fluorescent radiation. The drift velocity for sodium atoms in a argon atmosphere reaches a value of 1 cm s^-1 for fluorescent radiation of intensity 20 mW cm^-2.

A system formed of one electron and three nuclei with charges Z_a=Z_b=1 and Z_c=Z=1, 2, 3 . . . is analysed. The asymptotic expansions of the solutions for the systems (Z+H⁺₂) and (p+p+A^+(Z-1)) are found when the distances between the particles are large. The quadrupole moments and the polarizability of the hydrogen molecular ion H₂⁺ are calculated for the equilibrium internuclear distance R=2 a₀. The asymptotic formula obtained for the energy terms of the system formed from the hydrogen-like ion A^+(Z-1) and two protons transforms into the well known formula for two Coulomb centres when R=0.

A trajectory method for the solution of the time-dependent Thomas-Fermi-Dirac equation is applied to atomic scattering processes. The time evolution of the electron density is described by trajectories of test particles. This method is studied in the case of proton-impact ionization of an argon atom. Various cross sections for electron production, also differential in the emission angle and energy, are calculated and compared with experimental data.

Model complex quasimolecular potential curves are used to reproduce characteristic features in the energy spectra of electrons ejected in slow atomic collisions. The parabolic approximation for the time dependence of the potential curves (and its modifications) allows one to describe the contribution to the spectrum of the electrons ejected, when the atomic nuclei are near the position of closest approach. The new model proposed includes both united-atom and separated-atom limits. It accounts also for the possibility of steep variation of the quasimolecular state width with internuclear distance. The angular distribution of the ejected electrons is shown to be important when the interference oscillations in the spectrum are analysed.

Energy discriminated mass spectra have been measured for recoil ions produced in 100 MeV collisions of Si⁸⁺ with linear triatomics CS₂ and CO₂ using a cross-beams apparatus incorporating a quadrupole mass spectrometer and a retarding potential energy analyser. Fragment atomic and diatomic ions are observed in the mass spectra whose precursors are very highly charged molecular ions which undergo rapid dissociation. Kinetic energy measurements of atomic recoil ions indicate that the peripheral S ions (in the case of CS₂ target) and O ions (in the case of CO₂ target) possess larger energies than the ions of the central C atom; the relatively large energies imparted to C ions are explained in terms of a sequential dissociation model involving formation of an intermediate molecular ion with inner shell vacancies.

Cross sections for single and multiple electron capture in collisions of C⁴⁺ with H₂, N₂ and O₂ have been measured within the energy range 0.33-666 eV amu^-1. Where comparison is possible, the results are in good agreement with previous measurements at higher energies. Below 100 eV amu^-1 there are significant differences between experiment and the currently available theories.

Formulae for the electron capture differential cross section given as a function of the momentum change by the first and second order OBK approximations as well as the eikonal approximation, are compared using a relativistic analysis and expanding in the fine structure constant alpha . This analysis is extended to the total electron capture cross section. The spin-flip as well as the non-spin-flip cases are examined. It is emphasized that the second order OBK approximation must be evaluated with the inclusion of only the leading order terms in alpha for satisfactory results to be obtained. The non-relativistic limiting case is also discussed with particular reference to proton-helium scattering.

Results are presented of (e, 2e) coincidence measurements on helium in the perpendicular plane, defined as the plane orthogonal to the incident electron trajectory. The incident energy is varied in the range from 10 to 80 eV above the ionization threshold and results are presented for both symmetric energy sharing, where the outgoing electrons have the same energy, and for non-symmetric sharing. The experimental apparatus used to collect these results is a fully computer controlled and computer optimized spectrometer able to access a wide range of geometries from the perpendicular to the coplanar geometry.

The random phase approximation formulation of the many-body Green function formalism is applied to inelastic transitions, in e⁺-atom scattering. Explicit numerical calculations are performed for excitation to the n=2 and n=3 manifolds of an He target. Theoretical results for excitation to the 2¹P state are found to be in very good agreement with available experimental values of the literature, generally attributed to excitation of the 2¹S level of He. It is further pointed out that the cross section for excitation of the 3¹P level is, generally, bigger than that leading to the 2¹S state and, possibly, within reach of present experimental sensitivity.

The results of highly accurate variational calculations of low-lying bound P (L=1) and D (L=2) pre-threshold states in a Coulomb three-body system with unit charges (X⁺Y⁺Z^-) are presented. The pre-threshold behaviour of the binding energy is studied. The presence of quasi-stationary states is briefly discussed.

For pt. I. see ibid. vol.25, no.13, p.3049-58 (1992). The results of highly accurate variational calculations of some lower-lying bound excited prethreshold S (L=0)-, P (L=1)- and D (L=2)-states in a Coulomb three-body symmetric system with unit charges (X⁺X⁺Z^-) are presented. The positions of the thresholds for a number of states in such a system have been determined. The dependence of the threshold position as a function of angular momentum L is studied for the ground and excited states.

Energy considerations prove that dissociative recombination of the proton-bridge ions, H₂O.H⁺.H₂O and NH₃.H⁺.NH₃, can take place only through single-electron transitions. Any possibility that the recombination coefficients alpha (H₅O⁺) and alpha (H₇N⁺) are large by virtue of the many vibrational modes of the ions is excluded by a comparison with the channel specific dissociative recombination coefficients per equivalent bond for divers polyatomic ions. The nature of a single-electron transition leading to dissociative recombination is described. Approximate calculations show that the rate of the electronic part of the transition can indeed be extremely fast. Dissociative recombination of bigger cluster ions is discussed.

The emission spectrum of a pair of two-level atoms interacting with a single mode of radiation field in an ideal cavity is investigated, where the interaction of each atom with the field may be different. A general expression for the emission spectrum is presented and the spectrum is studied numerically when the radiation field is initially in the vacuum state, pure number state and squeezed-vacuum state, respectively. The effects of the ratio of two coupling constants (R=g₂/g₁) on the emission spectrum are discussed. In contrast with the spectrum of a one-atom system, the spectrum of a two-atom system is much more complicated. Generally, the emission spectrum shows a 12-peak structure for the vacuum state and 16-peak structure for the pure number state. For the squeezed-vacuum state, it is found that the Rabi splitting is most apparent when R=1.