Table of contents

Multiconfiguration Dirac-Fock calculations of E1, E2, M1 and M2 transition probabilities and lifetimes for the low-lying levels of neutral ytterbium are reported. For E1 transitions the valence-core electron correlations are also included in the core-polarization model. In particular, the lifetimes of the 6s6p¹P₁ and ³P₁ states, when corrected for core polarization, are 4.78 ns and 1294 ns, respectively. The lifetime of the metastable 6s6p ³P₂ state is calculated to be 14.5 s.

Electron capture cross sections in collisions between H⁺ and He²⁺ projectiles and ions He⁺, Li²⁺, Be³⁺, B⁴⁺, C⁵⁺, N⁶⁺ and O⁷⁺ are obtained by using the modified two orthogonal-state expansion method developed previously. Projectile energies are in the range 50-2000 keV amu^-1. A comparison with other theoretical results is presented and a good agreement is obtained. Scaling rules for these cross sections are examined.

Non-resonance multiphoton atom ionization by a linearly polarized electromagnetic field is considered. An expression for the ionization probability depending on the direction of a photoelectron emission has been obtained. The spectrum of above-threshold photoelectrons has been investigated with the laser focus inhomogeneity taken into account.

The rates of decay, both radiative and non-radiative, have been calculated for the valence shake-up states (vs, vp)nl of Ne(v = 2) and Ar(v = 3) for the five lowest n values. The various decay channels are discussed and trends for large n are examined. The calculations are performed at the single-configuration non-relativistic Hartree-Fock level. Inner-valence and valence-multiplet Auger channels are shown to be completely dominant, with maximum fluorescence yields of 2.7% in Ne and 0.2% in Ar. The total decay rates can be larger than the corresponding atomic K-shell rates for certain states, an extreme example being the Ar (3s, 3p)(¹P)3d(²P) rate of 0.148 au. The calculated widths are shown to be consistent with the experimental results of Svensson et al (1988). The valence-multiplet participator Auger transition is a mode of decay involving a change of multiplet state for the same initial and final two-hole valence configuration, and has been shown to account for the difference in width between certain photoelectron satellites in Ne. The experimental spectrum of Becker et al (1989) is discussed in the light of present results. The valence decay mechanisms identified are expected to have general applicability to light atoms and to molecules containing first or second row atoms.

The authors propose a unified R-matrix-Floquet method which can be used to analyse both the multiphoton ionization of atoms and laser-assisted electron–atom collisions. Their treatment is non-perturbative and can be applied to an arbitrary atom. It takes advantage of the R-matrix division of configuration space into an internal and an external region to treat the interaction between the laser field and the atomic system by using the most appropriate form of the interaction Hamiltonian in each region by means of unitary transformations. It also allows the use of the standard Hermitian Floquet theory in the internal region and of existing techniques developed to study field-free electron-atom collisions and photoionization. The authors' method is illustrated by analysing the multiphoton ionization of a charged particle initially bound in a one-dimensional well.

The coincidence between threshold electrons and ions from Ar₂⁺ was taken to separate the monomer ions and the dimer ions between 790 and 855 AA using synchrotron radiation as a light source. The vibrational structure of the ground state of the Ar₂⁺ ion was clearly observed. The energy of the Ar₂⁺ ionic state is determined to be 116 432+317.2( nu +1/2)-2.5( nu +1/2)²(cm^-1) from the zero vibrational state of Ar₂.

Resonances in rotational collisions of the highly anisotropic He-Li₂ ((A)¹ Sigma _u⁺) system are analysed. Energy transfer in this system is efficient, thus precluding standard methods of analysis. Several resonances are found at low energies.

Two-dimensional classical analysis of long-lived state in collisions of He-Li₂ ((A)¹ Sigma _u⁺) is reported. Multiple collisions are found at a range of collisional energies and result in long-lived states having lifetimes in the range 50*10^-12 to 600*10^-12 s. Total cross sections are calculated.

Absolute differential, integral and momentum transfer cross sections for electron-methane collisions were determined using a crossed beam spectrometer for impact energies from 1.5-100 eV and angles from 10'-130'. Comparison with published total cross sections shows good agreement. At the shape resonance (7.5 eV) excitation measurements of the two vibrational peaks were taken and the angular distribution of their DCS was determined. The elastic DCS are compared with recent measurements and theoretical calculations.

This paper includes a summary of R-matrix theory (in the inner region) and a discussion of how to solve the close-coupling equations exactly when calculating an orthonormal set of outer-region solutions by inward integration when all channels are closed. Techniques are described for obtaining these solutions, satisfying three necessary conditions for a correct bound state calculation: solution columns should: (i) have well defined asymptotic forms; (ii) have good linear independence at the matching point between the inner and outer region; and (iii) allow for accurate linear interpolation in the bound state energy E_k using scanning techniques which do not require estimates of such energies E_k to be provided. Test results for the bound state calculations for an ionic core, C²⁺+e C⁺, and for a neutral core H+e H^-.