Table of contents

A study has been made on the oriented ground state 5²S_1/2 Rb atoms produced in molecular photodissociation. Low-pressure RbI vapours have been photolysed by a beam of circularly polarized impulse laser radiation at 266 nm. The resulting Rb atoms were probed by a resonance atomic line at 794.8 nm from a discharge Rb lamp in two different experimental geometries. A free precession of the oriented atomic spins in an external magnetic field was detected by a dichroism of the atomic vapours. The initial degree of spin orientation was found to be P_c = (20 +or- 2)%.

The double-electron excitation cross section of the 1s²2p²¹D stationary discrete state of the beryllium atom by proton and antiproton impact is calculated within the expansion of scattering amplitude up to second order. We discuss the role of electron correlations in the two-electron excitation of the beryllium atom.

Final state nlm distributions are studied for electron capture from orientated circular Rydberg atoms. Strong orientation effects are found for all quantum numbers at collision speeds comparable to the electron orbital speed. Predominant population of large m states is observed when the plane of the circular orbit is nearly perpendicular to the incident direction of the projectile. The lm distributions show structures due to quasi-Thomas scattering even at low relative collision velocities.

The integral: J(k,l,k',l',p,n,)= integral ₀^infinity r²j_l(kr)j_l'(k'r)j_n( rho r)dr where mod l-l' mod <or=n<or=l+l' is considered and expressed in terms of well known functions. These integrals occur in many different problems in scattering theory, in particular they are needed in the evaluation of partial wave integrals occurring in the theory of relativistic (e, 2e) collisions, as well as in pion nucleon problems, for example in the calculation of products of nucleon propagators in spherically symmetric medium.

The reduced IERM approach is applied to electron scattering from hydrogen atoms to obtain accurate integrated cross sections for n = 1 to n = 3 transitions at intermediate energies. The results are in good agreement with recent coupled pseudostate calculations and with experiment but show that earlier work including 15 bound states in a close coupling expansion considerably overestimates these cross sections.

We apply the convergent close-coupling method to the calculation of electron scattering on the ground state of He⁺. It is shown that the inclusion of the treatment of the continuum, even below the ionization threshold, significantly reduces the calculated 2S cross section. We generally find good agreement with the measurements of the 2S excitation cross section, though in the vicinity of a few eV near threshold our results are characteristically higher than the experiment. Complete quantitative agreement is obtained with the measurement of the total ionization cross section from threshold to 700 eV.

A metastable, triply charged, diatomic molecular ion, CS³⁺, has been discovered in collisions of O^- and Si^- with CS₂ at an impact energy of 40-70 keV. Concomitant configuration interaction calculations of potential energy curves and electron density distributions have been carried out which indicate that the lowest-energy ² Sigma _g state of this molecular ion is metastable for at least 3 mu s. Predictions are made of the values of the kinetic energies released upon dissociation of the lowest-energy ^2,4,6 Sigma _g and ^2,4 Pi _g states of CS³⁺.

The contact processes between positive particles from the ground bound states of Coulomb three-body systems with unit charges such as X⁺Y⁺Z^$are considered. The probability amplitude for the contact processes is proportional to the expectation value of the respective Dirac delta function, e.g., ( delta (r₊₊) It is shown that for a large number of similar systems the contact (X⁺, Y⁺) process (or (X⁺, Y⁺) reaction) from the ground bound state of the three-body system X⁺Y⁺Z^- proceeds through the three-particle collision.

Total as well as partial widths for autoionization and radiative decay of multiply charged helium-like ions (Z>or=8) are investigated along all spectral series for doubly excited ridge states (N=n). The propensity rules derived from the structure of the two-particle wavefunctions are discussed in a detailed numerical study within the framework of configurational interaction. The dependence of the total widths on the relevant quantum numbers is analysed, too, but scaling laws are found not to be universal in the sense that they depend on the actual series of states under consideration. Our results are compared with available quantum and classical calculations as well.

Ab initio MRD-CI calculations have been carried out on 19 electronic states of NaHe and on the ground state of NaHe⁺. Potential energy curves, radiative transition probabilities and lifetimes of the excited states of NaHe are presented. Most of the excited states of NaHe have typically Rydberg potentials, resembling that of the ground state of NaHe⁺. The results show several possible Rydberg transitions which might be observed while pre-dissociation by rotational-electronic coupling is negligible.

The 1s X-ray absorption spectra of atomic calcium, chromium, manganese and copper were determined experimentally and calculated theoretically. The spectra are dominated by a prominent line at threshold which precedes a smooth continuum extending to higher energies. Hartree-Fock calculations taking relativistic effects, relaxation and term dependence into account describe the discrete atomic resonance at threshold very well. The resonances at threshold have no counterpart in the spectra of the metals. The experimental atom to metal core level binding energy shifts agree well with the predictions of the thermodynamical model by Johansson and Martensson (1980).

Photodetachment cross sections are evaluated for the 1s²2s²2p³⁴S^o and 1s²2s²2p³²D^o bound states of the negative carbon ion over a wide range of initial photon energies. The calculation employs a multichannel theory based upon the R-matrix method for electron-atom collisions. A prominent 2s2p⁴⁴P^e resonance is found to exist in the low-energy region of the C^-(⁴S^o) bound state photodetachment cross section, while a ²D^e shape resonance is the dominant feature in the cross section for the C^-(²D^o) bound state. Comparisons are made with previous experimental data and theoretical calculations, and significant differences are found to occur.

As a contribution to the International Opacity Project, large numbers of multiplet term energies, f-values and photoionization cross sections have been calculated for those Mg-like ions of astrophysical interest: Mg I, Al II, Si III, S V, Ar VII, Ca IX and Fe XV. The present atomic data have been obtained in the close-coupling approximation with a modified version of the RMATRX computer program and asymptotic codes. We briefly discuss the new results, and representative comparisons with previous experimental and theoretical work are carried out. Emphasis is given to the large number of photoionization cross sections reported for the first time. We conclude that the statistical accuracy of the Opacity Project datasets for the Mg isoelectronic sequence is at least as high as that of any other set of data produced with the most elaborate methods available to date.

The Auger electron yield from xenon has been measured in the vicinity of the 4d inner-shell ionization threshold. Just above this ionization threshold the Auger line profile is modified by the post-collision interaction effect involving a slow photoelectron. The process is followed through threshold into a region where the photoelectron is recaptured and either shaken up/down or remains in its Rydberg orbital during the Auger decay. By measuring the electron yield as a function of both incident photon energy and electron kinetic energy a comprehensive study of these processes is made.

Model photodetachment spectra are calculated analytically for a system including an infinite number of circular states. Characteristic features of the ATD spectra are derived and interpreted based on the properties of continued fractions. Numerical results confirm a typical regular multipeak structure of the total spectra as well as of the spectra corresponding to a given net number of photons absorbed. For extremely strong fields the peaks become closer and the lines overlap.

Two-photon ionization of helium is investigated by using a discretization technique. Perturbative cross sections are calculated with a L² basis set (B-splines), the efficiency of such an approach is discussed in the context of above threshold ionization (ATI). We propose a parametrization of the cross sections in the region of resonances. The role of the correlations is discussed in a P-Q Feshbach projection treatment.

The influence of solid-state effects on the K beta /K alpha intensity ratio of Ti and V has been found in proton induced X-ray emission (PIXE) and gamma-ray fluorescence ( gamma RF) studies of the compounds TiB₂, VB₂ and VN. The change in the K beta /K alpha ratio observed can be explained as being due to changes in the screening of 3p electrons by a varying 3d electron delocalization. The amounts of 3d electron delocalization obtained for the diborides differs considerably from the results of previous cohesive energy calculations and in the case of VN it is of the same order as given by the previous band structure calculations.

We investigate the dynamics of the ground state and the first three singlet ¹ Sigma ⁺ excited states of BH using the split-operator method. Ab initio calculations show that these states have strong non-adiabatic couplings, resulting in a complex topology of avoided crossing regions and double well potentials. We attempt to find a method to populate the second minimum of B ¹ Sigma ⁺, enabling experimental observation of its vibrational states.

Molecular nitrogen has been exposed to synchrotron radiation in the range 16-40 eV and the resulting fluorescence has been studied in various regions from 1100-8500 AA. Line structures are observed which are interpreted in terms of autoionization or dissociation of well known Rydberg states in N₂ converging to the B, C and D states in N₂⁺. In addition a new member of the Rydberg series converging to the D state is found.

For the wavepacket treatment of molecular collisions, the quantum flux spectral analysis method and the optical potential method improve the performances of one another. This association of these two methods changes the usual criteria for an optimum use of the optical potential and allows us to derive precise results even in cases where large spurious reflections are present. The study is based on four common examples of optical potentials.

The spectral properties of Hamilton operators perturbed by a complex absorbing potential (CAP) are studied. For a wide class of CAPS proper eigenvalues of the perturbed Hamilton operator converge to Siegert resonance eigenvalues of the unperturbed Hamiltonian with decreasing CAP strength. The errors in the calculation of complex resonance energies caused by the additional CAP and by finite basis set representation are examined. In order to minimize these errors a scheme of approximations is provided. The application of this method allows for the use of real L² basis sets. The feasibility and accuracy of the proposed method is demonstrated by calculations of resonance energies of a model potential and of the ² Pi _g shape resonance of N₂.

Single-centred expansion coupled states calculations are performed with basis sets large enough to achieve convergence. The particular collision system studied is p+H(1s) at energies of 15 keV and above. The results obtained establish the validity of this method for calculating excitation and electron removal cross sections even in situations where charge transfer is large. Cross sections for 1s to 2l', 3l' and 4l' excitation and for electron removal are presented and compared to experiment and to other theoretical calculations.

Large-basis single-centred expansion calculations have been previously shown to yield accurate cross sections for collisions between protons and ground state hydrogen atoms. The method is extended to p + H(n = 2) collisions. Cross sections for 2s and 2p to 3l' and 4l' excitation and for electron removal from 2s and 2p are presented and compared to other theoretical calculations.

Single ionization of lithium, beryllium and neon by ion impact is considered using the continuum-distorted-wave eikonal-initial-state model. Total cross sections, for ionization of lithium by proton, antiproton and alpha particle impact, and for the ionization of beryllium and neon by proton impact are presented. We also present doubly differential cross sections for 1 MeV proton and antiproton impact on lithium.

Relative emission cross sections were measured for all Ne(2p⁵3p) levels (2p₁-2p₁₀ in the Paschen notation) excited by He⁺ ions with incident energy E=0.2-20 keV. The polarization degree was also determined for each emission line. For most of the levels, the energy dependence of both the emission cross sections and the polarization degrees show oscillatory structures in the energy range observed. These were interpreted as Rosenthal oscillations. From the phase correlation between the oscillations of these emission cross sections and those of He I 3¹P, we conclude that the He I 3¹P level interferes with 2p₂-2p₈. From the polarization degree we determine the partial emission cross sections of the magnetic sublevels for J=1 (J is the total angular momentum) levels (2p₂, 2p₅ and 2p₇). For 2p₂ and 2p₅, the magnetic sublevels with M_J=+or-1 (M_J is the magnetic quantum number) were dominantly excited over the sublevel with M_J=0 and showed oscillations with larger amplitude than that for the M_j=0 level.

There is a marked disagreement between the universal cross sections extracted from experiment by Paul (1989) for K-vacancy production produced by fast protons incident on light atoms, and theoretically calculated cross sections for the same processes. The authors address this discrepancy by examining the corrections to the theory due to the neglect of dynamic correlation. They find that these corrections are important, though they can be significantly reduced by a better choice for the single-particle potential to be used in an independent particle model. The inclusion of dynamic correlation in the theory reduces the Paul discrepancy but some disagreement still remains.

Angular correlation parameters, lambda , R, the circular polarization parameter I and the differential cross section are measured for the electron impact excitation of hydrogen atoms to the 3²P_j states at an incident electron energy of 54.42 eV and for electron scattering angles up to 35 degrees . Comparison of the measurements with the convergent close-coupling calculations of Bray and Stelbovics (1992) shows excellent agreement with experiment.

Exchange collisions of electrons with Hg and Na atoms and O₂ and NO molecules have been studied by means of polarized electrons for energies between 2.5 and 15 eV and scattering angles up to 110 degrees . While significant exchange effects have been observed for collisions of electrons with Hg and Na atoms, differential spin exchange cross sections for elastic collisions from the open shell molecules O₂ and NO are much smaller. This is in agreement with theoretical calculations that predict significant exchange effects only for oriented molecules. A small but significant spin exchange structure for e-O₂ collisions between 8 and 15 eV at theta =100 degrees is attributed to a shallow ⁴ Sigma _u^- resonance.

Positron spin relaxation has been investigated in some organic compounds (biphenol, octadecane and PPO) by using static magnetic fields. Depolarization effects have been found in all the materials under study. A possible explanation in terms of an interaction between the magnetic moments of the positron and of the molecular triplet excited states has been proposed.

The significance of permanent electric dipoles in second harmonic generation is considered with regard to their role in both the non-linear susceptibility and also in the orientation of polar molecules. Under conditions of near-resonance with the first electronic excited state, the susceptibility is dominated by a two-level term governed by the difference between the ground and excited-state dipoles; this term also drives the harmonic conversion in electric field-induced second harmonic generation. If the resonant level is not the lowest electronic excited state, there is a secondary resonance feature which can contribute just as strongly as the two-level term, and which can dominate the harmonic conversion if the ground- and excited-state dipoles are either equal, or both zero.

An integral method for solving the problem of imprisonment of resonance radiation based on propagator functions is further developed. Earlier work was restricted to plane parallel and spherical geometries, to a Lorentz lineshape, and to the approximation of complete frequency redistribution. This work extends the method to cylindrical geometry, to a Voigt lineshape, and to include the effects of partial frequency redistribution. The method is ideal for calculating both the time-dependent and the steady-state densities of resonance atoms which result from an arbitrary production rate per unit volume. An emission spectrum is also generated in calculations involving partial frequency redistribution. The propagator function method is at least 50 times faster than the Monte Carlo method. The greater speed of the propagator function method makes it well suited to fully self-consistent kinetic simulations of glow discharge plasmas.