Table of contents

The authors demonstrate experimentally that Xe atoms are ionised by a tunnelling process by short CO₂ laser pulses at the 10¹⁴ W cm^-2 intensity level. Two different wavelengths were used and up to triply charged ions were observed.

Rate coefficients for the collisional ionisation between Na(nP) atoms and Na(3S) ground-state atoms for the principal quantum numbers 5<or=n<or=21 have been measured in a single atomic beam. The maximum value of the rate coefficient has been found to be approximately 1*10^-9 cm³ s^-1 for n=10-11.

The collisional excitation transfer fine-structure mixing cross sections for nD states of Cs (n=11-13) are measured by the technique of selective two-photon excitation with pulses of radiation from a N₂ laser-pumped dye laser and observation of collisionally induced fluorescence.

In order to explain the disagreement that exists between L-shell ionisation data in slow asymmetric collisions and first-order direct ionisation theories, the authors developed a relativistic molecular model of the collisional intra-shell coupling. As a consequence of the rotational coupling among the molecular orbitals correlating to the L₃ magnetic substrates, strong dealignment relative to the primary ionisation and even sign reversal of the alignment parameter is predicted. This behaviour is in agreement with the trend in experimental data at low projectile velocities.

Free-free transitions have been observed in electron-argon scattering in the presence of the radiative field of a CO laser. The ratio of the free-free and elastic differential cross sections has been determined at a scattering angle of theta =160 degrees and an electron energy of E_i=11.06 eV. Good agreement was found with the soft-photon approximation at this energy.

A model potential relativistic method is used to calculate the energies of the lower levels with configurations 4s4p, 4p², 4s4d, 4p4d in the Zn I isoelectronic series for nuclear charges Z=36-50. The calculation is made in the first order of perturbation theory with the effective inclusion of higher corrections. The role of various correlation effects is evaluated: core polarisation due to outer electrons, superposition of configurations, mutual screening of the outer electrons. As a zeroth approximation the authors obtain the wavefunctions of the outer electron in the field of the core model potential; the model potential parameter being determined from the empirical energy levels of Cu-like ions. An extrapolation procedure for model potential parameters is used for the calculation of energy levels of Cu-like ions. Energy levels 4lj and 5lj of Cu-like ions for Z=36-60 are given with an accuracy of about 0.1%. The accuracy of the energies of Zn-like ions is within 1%.

Ultraviolet transitions in foil-excited beams of fluorine, magnesium and aluminium have been observed using grating spectroscopy. The helium-like 1s2s³S-1s2p³P_2,1,0 transitions were observed in all three elements, and their wavelengths were measured with an accuracy of between 50 and 100 p.p.m. The results are in fair agreement with recent relativistic and QED atomic structure calculations.

Radiative decay of autoionising states in the presence of a strong electromagnetic field has been considered. The photon spectrum exhibits two peaks, the widths of which are much larger than the radiative ones.

The decay of an autoionising state (AS) is considered in the presence of a resonance electromagnetic field (EMF) connecting the AS with some other level. It is shown that if the EMF is switched on suddenly the process of decay can be characterised by two constants which depend on frequency, intensity and polarisation of the EMF. One of these constants is always more of about the order of the field-free autoionising width of the AS. However, if the EMF parameters obey some conditions the other decay constant can be much smaller than the field-free AS width. In this case the lifetime of the AS in the EMF as a whole can be much larger than the field-free lifetime of the AS, this means that there is a partial stabilisation of the AS. An increase in the AS lifetime is connected with a destructive interference between the autoionising and ionising transitions. However, there are always some 'non-interfering' transitions due to which stabilisation can never be absolute. The long living part of the AS population is shown to be rather small. If the EMF is switched on adiabatically there is only a simple one-exponential decay with only the characteristic decay time and there is no field-induced stabilisation.

The hyperfine structure and lifetime of the 6²D_3/2 and 6²D_5/2 states in ³⁹K are studied experimentally. The states were excited from the ground state by two-photon absorption of the laser light pulses. The hyperfine-structure intervals, and hence the hyperfine-structure interaction constants, were obtained from the numerical analysis of the zero-field quantum beat signals. The unmodulated component of the fluorescence decay curve served to determine the lifetime. In a separate experiment a magnetic field was applied to induce partial decoupling of the electronic and nuclear angular momenta in the 6²D states. The sign of the magnetic dipole interaction constants was deduced from the resulting difference between the intensities of sigma ⁺ and sigma ^- polarised fluorescence.

Theoretical studies of the lowest ¹S, ^1,3P and ^1,3D states of calcium and strontium are presented. The excitation energies, dipole-allowed transition moments and the ¹D-¹S quadrupole moment are studied as a function of the one-particle and molecular orbital bases and level of correlation treatment. Including core-valence correlation significantly reduces the magnitude of the dipole and quadrupole transition moments, producing oscillator strengths that lie within the experimental error bars. On the basis of relativistic effective-core potential calculations, the authors find that, unlike barium, the strontium transition moments are not significantly changed when relativistic effects are included. Spin-forbidden transitions were assumed to occur entirely as a result of the breakdown of LS coupling. The magnitude of the singlet-triplet mixing was determined both by ab initio calculation and from the observed deviations from the Lande interval rule. The calculated ¹D₂ state lifetime for Ca is 3.1+or-0.3 ms, which is in reasonable agreement with the experimental value of 2.3+or-0.5 ms (with an absolute upper bound of 4 ms). The dominant decay mechanism is spin-forbidden dipole-allowed transitions, with only 12% arising from quadrupole transitions. Similarly for the ¹D₂ state of Sr, the authors compute a radiative lifetime of 0.49+or-0.04 ms with only 2% of the decay from quadrupole transitions.

Relativistic excitation energies and oscillator strengths are computed for both the resonance transition 6s²¹S₀-6s6p ¹P₁ and the intercombination line 6s²¹S₀-6s6p ³P₁ in the mercury isoelectronic sequence. In the authors' approach to the correlation problem, limited relativistic configuration mixing which represents, intravalence correlation is combined with a polarisation model which accounts for valence-core correlation. Systematic trends along the sequence are discussed both the oscillator strengths and for contributions of intravalence and valence-core electron correlation. Theoretical ionisation potentials, corrected for relativistic and core polarisation effects, are predicted for ¹S₀, ³P₁ and ¹P₁ states in the Po V, At VI and Rn VII spectra of the mercury sequence as well as for the ²S_1/2 ground states in the Po VI, At VII and Rn VIII spectra of the gold isoelectronic sequence.

For pt.II see ibid., vol.14, p.4191 (1981). The author derives a Fokker-Planck equation for the Wigner function describing the centre-of-mass motion of a harmonically trapped ion in a travelling wave of light, using an expansion in the recoil velocity around the exact time-dependent solution of the recoilless case. The practical calculations are reduced to the scheme presented in the earlier papers of this series. However, the final energy of the cooling differs from his earlier results, and agrees with more recent calculations employing for the centre of mass the representation in the harmonic oscillator states.

The ESCA spectrum of the 1s line from carbon in methane has been recorded and calibrated against the 2p line of argon using a new high-resolution multipurpose ESCA instrument with monochromatised Al K_alpha radiation. For the first time vibrational structure in a core-electron ESCA line is resolved. The energies, relative intensities and inherent widths of the resolved components of the core line have been accurately determined. Inclusion of the Doppler broadening effect is shown to be essential for the natural linewidth determinations. Energies and relative intensities have also been calculated using a contracted configuration interaction scheme (CCI) and a complete active space self-consistent field (CASSCF) program. Although the calculations reached well beyond the Hartree-Fock level, incorporating a large part of the correlation energy, the shape and position of the potential curves were still found to be non-stable. It was not possible to simultaneously accurately reproduce the experimental ionisation energy, vibrational splittings and relative vibrational intensities. The present kind of highly resolved experimental data therefore represents a critical test for advanced molecular CI calculations on deep core-hole states.

Photoelectron asymmetry parameters ( beta ) have been measured for ionisation from the valence orbitals of the fluoromethanes CF₄ and CHF₃. Measurements were made in the energy range from 19 to 80 eV using radiation from the SERC Daresbury SRS. The variation of beta with photoelectron energy was very close to that previously observed for Ne 2p^-1 ionisation for most valence orbitals. For the 4t₂^-1 ionisation of CF₄, however, a shape resonance was observed at an energy of 15 eV.

In the process of dielectronic recombination (DR), radiative capture of an electron by a positive ion occurs via compound resonance states of the electron+ion system (states with two or more electrons excited). These states can decay by autoionisation, with probability A, or by radiative stabilisation, with probability R. In general, these are competing processes: one may have A>R or A<R. It is assumed that wavefunctions have been obtained for the electron+ion system, allowing for resonances but neglecting interactions with the radiation field. The interactions are then handled using the general formulation of the problem given by Davies and Seaton (1969). A scattering matrix S is calculated allowing for radiative channels. It has submatrices S_ee, which describes electron-electron scattering allowing for radiative decays, and S_pe, which describes photon emission following electron capture. Using the unitarity of S, the total DR rate can be expressed in terms of S_ee.

For pt.I see ibid., vol.18, p.1589 (1985). Formulae obtained by Bell and Seaton in the first paper of the present series are used to calculate collision strengths for the 1 ¹S-2¹S transitions in O⁶⁺ at energies below the 2¹P threshold. Radiative decays lead to modifications of autoionising resonance profiles and substantial reductions in collision strengths averaged over resonances.

The eikonal-Born series approximation is unitarised by using an improved Wallace-type amplitude to construct the direct scattering amplitude. The new unitarised eikonal-Born series (UEBS) direct amplitude obtained in this way is first tested successfully in potential scattering. It is then generalised to electron- and positron-atom collisions, and applied to elastic (1s-1s), 1s-2s and 1s-2p transitions in atomic hydrogen. The large momentum transfer properties of the UEBS direct amplitude are analysed for these three transitions, and the structure of the third term of the Born series is investigated by using the unitarity relations. Exchange effects are also discussed. The theoretical results are compared with those obtained from other theories and with recent measurements, for impact energies ranging from 54.4 to 400 eV.

The quantum kinetic equation describing slow atomic motion in laser light is derived by an operational method which provides mathematical expressions with a transparent physical structure. The authors prove in a general way that the coefficients appearing in this equation, which is of a Fokker-Planck type, are simply related to the mean value and to the correlation functions of the Heisenberg radiative force of the semiclassical approach, where the atomic position is treated classically. They derive in particular a new theoretical expression for the damping force responsible for radiative cooling and they interpret it in terms of linear response theory. The authors also obtain a new crossed r-p derivative term, which does not appear in semiclassical treatments, but which they find to be very small in most situations. Finally, all the theoretical expressions derived are valid for any J_g to J_e transitions and are not restricted to two-level atoms.