Table of contents

The configuration-interaction Dirac-Hartree-Fock method is developed and applied to the elastic scattering of electrons from xenon atoms. The phase shifts obtained by this method are used to calculate spin polarization at 2, 4, 6 and 10 eV.

We present an experimental investigation of the effect of incoherent processes on a laser-induced continuum structure. Accurate measurements of the resonance profile, performed in a weak-field regime by the polarization technique, have allowed a quantitative study of the effect of a finite laser bandwidth and of the collisional self-broadening on the line shape.

A simple scaling relation is derived for the partial and total multiple-electron capture cross sections in slow collisions of highly charged ions with atoms based on the extended classical over-barrier model. It is shown that the currently available experimental cross sections are reproduced quite satisfactorily by this relation.

A simple variational-type model for the ground state of two-electron atoms is here proposed. It is based on a single-particle wavefunction with two free parameters generalizing the screened hydrogenic one, and which holds the Kato cusp condition exactly. The results for the expectation values of different properties including the energy are close to different Hartree-Fock ones reported in the literature.

The existence conditions of Lagrange meshes, i.e. meshes leading to very accurate results without analytical calculation of potential matrix elements, are studied in relation with sine and cosine bases on a finite interval. For each one of three different types of boundary conditions, three Lagrange meshes are found. A numerical example shows that (i) the Lagrange-mesh calculations nearly reach the same accuracy as a variational calculation involving the same basis in spite of the approximation on the potential matrix elements and (ii) they are much more accurate than mesh calculations which do not satisfy the existence conditions. The striking accuracy of the Gauss quadrature employed in the mesh approximation for the significant eigenvalues remains unexplained in view of the limited accuracy of the same quadrature for individual potential matrix elements.

We calculate the total direct double photoionization rates in the strong-field approximation for a one-dimensional model. We study the role of the electron-electron repulsion in the final state for different frequencies and intensities of the applied laser field.

A recently found additional hyperfine interaction in the Dirac equation extends the validity of this equation to comparable masses of its two spin- 1/2 particles. For equal masses as in positronium, it ensures CP conservation. The new operator simplifies the hyperfine mixing of states with different j. Anomalous magnetic moments are incorporated, and an application to helium-like Rydberg states is mentioned. CPT invariance is demonstrated and discussed, including the extension to n-electron atoms.

In the asymmetric doubly excited states of the helium atom one electron is excited much more than the other. We consider the special type of such states with both electrons located on one side of the atomic nucleus. The new version of the classical adiabatic theory is developed using approximate separation of rapid and slow motions. It is based on the formalism of adiabatic invariants. The theory takes into account two important effects simultaneously: (i) exchange by the orbital momentum between the electrons; (ii) exchange by the energy. An effective Hamiltonian is constructed for the description of the slow motions. The semiclassical quantization is carried out. The energy spectrum is compared with the calculations by other methods. The present adiabatic scheme is shown to be superior to the previous version where the effective Hamiltonian for the slow motion is found as an average of the full Hamiltonian over the rapid phase. The relation between the two approaches is elucidated.

A quantum close-coupled calculation of the collisional self-broadening of neon lines based on the quantum-mechanical impact theory of Baranger is presented in which the interatomic interaction is represented by adiabatic molecular potentials calculated using model potentials for the electron-atom and atomic core-core interactions. The close-coupled equations are solved using an R-matrix method and results obtained for seven lines involving transitions between the 3s'( 1/2 )₀, 3s(3/2)_1,2, 3p'( 1/2 )₁ and 3p'(3/2)_1,2 levels at temperatures of 77 and 273 K. The calculated linewidths agree well with recent measurements, particularly at 273 K, whereas for the shifts the agreement is generally good but there are discrepancies of order 20-30% with experiment for the 3s(3/2)_1,2-3p'(3/2)₂ and 3s(3/2)₁-3p'(3/2)₁ transitions.

We examine the excitation dynamics of a model atom excited by a two-colour laser field. Here the two colours or frequencies are the fundamental and its third harmonic. The atomic model consists of two bound states which are near-resonantly coupled by a three-photon excitation by the fundamental and a one-photon excitation by the third harmonic, and a structureless continuum. We investigate how the laser parameters (intensity and frequency) and the relative phase between the two colours affects the degree of phase modulation in the ionization signal. We compare our findings from this simple model atom with those from more realistic atomic models by carrying out numerical integration of the hydrogenic Schrodinger equation in both one and three dimensions.

Using the recently proposed R-matrix Floquet theory we have performed ab initio calculations for single- and multiphoton detachment of the negative hydrogen ion, H^-. Our method is non-perturbative in the field strength and is a multielectron theory allowing for full correlation between the two electrons. We present results depending on frequency up to nonperturbative field intensities for H^- in a single-frequency linearly polarized laser. We investigate a wide range of frequencies and intensities from the multiphoton detachment regime to the high-frequency regime. Our results include total detachment rates, shifts of the attachment energy in the field and partial rates into the various excess photon absorption and core excitation partial channels. We compare our results with other theoretical calculations and with experimental results.

The cross section for photodetachment of the ³P bound state of the negative ion of neutral boron is calculated from threshold to a photon energy of 0.6 Ryd. The calculation employs a multichannel theory based upon the R-matrix method for electron-atom collisions. The cross section shows a large dip prior to the 2s2p²⁴P threshold followed thereafter by a sharp rise. The sharp rise and following maximum are due to quasibound states in the ³P^o and ³D⁰ final symmetries.

The de-excitation of the resonantly excited Xe 4d_3/2,5/2^-17p and 4d_3/2,5/2^-18p states is studied with very high photon and electron energy resolution using the new Finnish beamline at MAX-laboratory, Lund, Sweden. The fine structure of the spectra is resolved with much higher resolution than before. Effects due to shake transitions, initial- and final-state configuration interaction and interference of different decay channels are discussed.

New measurements of previously determined natural widths of the Xe 4d_5/2^-1 np (n=6, 7, 8) and Xe 4d_3/2^-1 np (n 6, 7, 8, 9) resonances, and a new value for the 9p resonance in the former are reported. In general, our measured widths are nearly constant, are narrower than found in earlier experiments, are with one exception in better agreement with the older of the two, and have associated error bars appreciably smaller than either. An anticipated slight increase in the 7p, 8p, 9p widths in the 4d_3/2^-1 series arising from core-multiplet (j-changing) Auger decays was searched for, but was found to be too small to be definitively confirmed.

The optical limiting action of C₆₀ in toluene solution is mainly due to reverse saturable absorption (RSA). It is shown that the formalism describing nonlinear optical response due to instantaneous two-photon absorption can be used in the case of sequential two-photon absorption, yielding effective values of the relevant parameters of optical nonlinearity due to RSA. The effective two-photon absorption parameter beta _eff and the effective nonlinear refractive index parameter gamma _eff', which are related respectively to the imaginary and real parts of the effective third-order susceptibility chi _eff⁽³) were measured by the z-scan technique as a function of fullerene concentration and of incident laser intensity and wavelength over the 420-640 nm region. The concentration dependence of these parameters indicates that the solution is optically thin as far as the ground state of C₆₀ is concerned, whereas the wavelength dependence confirms the applicability of the formalism used in the sequential two-photon absorption model. Comparisons are made with other z-scan results on C₆₀.

The accuracy of the total energies obtained for the series of 14 electron molecules N₂, CO, BF, NO⁺ and CN^- by means of matrix Hartree-Fock calculations using a universal basis set of Gaussian-type functions is reaccessed in the light of improved finite difference calculations.