Table of contents

We report a calculation of satellite intensity factors F₂(s-f) of helium-like neon and argon ions for dielectronic transitions 1s²+ epsilon l to 1s2l'nl" to 1s²ml"' with n=5, 6, 7, and 8. The calculation has been performed in the intermediate coupling scheme using a multiconfiguration Hartree-Fock atomic model. The 1/n³ scaling, which has often been used to extrapolate satellite intensity factors for n>4, is tested against the calculated satellite intensity factors. We find that the estimates from the 1/n³ scaling can differ by as much as 50% from the calculated values.

We have studied multiphoton ionization and autoionization in atomic silicon using a configuration interaction Hartree-Fock-Dirac method (CIHFD). We have obtained new results for part of the autoionization spectrum as well as photoionization cross sections at a laser frequency of 0.121 au into three open continua. This information provides a starting point for further theoretical development as well as a basis for some initial experimental comparison in such complex atomic systems with presently available lasers.

It is demonstrated that a stochastic algorithm may conveniently be employed to mimic quantum effects in CTMC calculations. Using the collision system proton-hydrogen as an example, perspectives and limitations of extended CTMC models are discussed.

The results of a large scale (74 states) coupled channels calculation is presented for the cross section of Balmer alpha emission arising in collisions of protons with hydrogen atoms. The range of proton energies is from 15 to 100 keV. A two-centre symmetric basis set was employed which included some united atom orbitals. The results confirm earlier calculations and a disagreement with recent experimental data remains.

The 19-state and the 29-state R-matrix calculations have been employed to obtain the differential cross section and electron-photon coincidence parameters lambda and mod chi mod for the 1¹S-3³P excitation of helium at energies below the 5¹P threshold. The results of the calculation reported represent the first definitive calculation on these cross sections and lambda and |chi| values expressed as functions of electron energies. The convergence between the R-matrix calculations is good. There is good agreement with recent experimental observations of Allan (1992) on the 3³P excitation functions. However, the agreement with the experiments of Humphrey et al. (1987) on the values of lambda and mod chi mod at 23.5 eV is far from satisfactory.

Superelastic scattering of polarized electrons from laser-excited polarized chromium atoms (4⁷P₄^o) to 4⁷S₃ transition, energy gain 2.92 eV) has been measured for total collision energies of 6.8 and 13.6 eV (0.5 and 1.0 Ryd) and scattering angles ranging from 10 degrees to 140 degrees . The orientation parameter L_{perpendicular to} is determined both averaged over spin states, and separately for the sextet and octet total spin channels. The octet-to-sextet cross section ratio r is also determined. The chromium atom has six unpaired electrons with the configuration 3d⁵(⁶S_5/2)4l (l=0, 1) for the states under investigation. The scattering data show strong orbital orientation and spin effects, which the authors attribute to the interaction between the continuum electron and the outermost 4l target electron.

A resonance theory of electron-impact vibrational excitation of diatomic molecules is extended to the case where the final vibrational level of the molecule lies in the continuum. The extended theory is applied to resonant dissociation of molecular hydrogen by low-energy electron impact. Theoretical cross sections for dissociation of ground-state H₂ via the X ² Sigma _u⁺ and B ² Sigma _g⁺ resonances are presented and compared with theoretical cross sections obtained by other authors using non-resonant methods. An important aim of the present work is to study the effect of initial vibrational excitation on the dissociation cross section. It is found that the B ² Sigma _g⁺ resonance contributes significantly to the total dissociation cross section for all values of v_i, for incident energies between 12 eV and 18 eV, while the effect of the X ² Sigma _u⁺ resonance becomes appreciable only for larger values of v_i.

The first measurements of electron impact vibrational excitation of ozone are presented as a function of both incident electron energy, 3 eV<or=E_i<or=7 eV, and scattering angle, 40 degrees <or= theta <or=120 degrees . Evidence is given for the formation of a low-lying shape resonance at E_i approximately=4 eV. Preliminary vibrational excitation cross sections are also presented.

The wavefunctions and energy levels of the low-lying spectrum of the neutral calcium atom are computed using the configuration interaction method. The Hamiltonian for the two valence electrons is modified by the addition of a semi-empirical polarization potential. The inclusion of the polarization potential results in theoretical energy levels in much closer agreement with experiment. The largest discrepancy between the computed and experimental energies (relative to the Ca²⁺ limit) is smaller than 0.1 eV. Oscillator strengths are computed for all possible dipole transitions and found to be in better accord with experiment that previous calculations. Lifetimes are also computed and compared with experiment for those levels where lifetime data exists.

We calculate ¹S^e intra-shell wavefunctions of highly doubly excited two-electron atoms by diagonalizing the corresponding Hamiltonian in a Sturmian-type basis set representing symmetrically excited electrons. We take into account couplings between different intra-shell manifolds. The Z-dependence of the energies of all intra-shell states up to principal quantum numbers N=n=12 is examined. We investigate the nodal structure of the wavefunctions in the manifold N=8 for nuclear charges Z=2 and Z=10 to examine the validity of the Herrick classification scheme. It turns out that this scheme appropriately describes the structure of the lower states in the multiplets but it breaks down for the higher lying states. A comparison with wavefunctions from large-scale calculations reveals that the intra-shell diagonalization overestimates the radial independent-electron character of the wavefunctions. A semiclassical interpretation of the wavefunctions is given in terms of fundamental modes of the electron-pair motion.

The authors consider the preparation process and the time evolution of an angular wavepacket of Rydberg atomic states in the external magnetic field in the l-mixing regime. Two kinds of time evolution are observed. The wavepacket of states concentrated in the upper part of the diamagnetic Zeeman multiplet is periodically destroyed and revived. In the lower part the time dynamics is chaotic-like.

The polarization properties and the temporal shapes of the super-radiance pulses formed on degenerate levels are studied theoretically in a semiclassical approximation. The dependence of the super-radiance pulse polarization on the values of the total angular momenta of resonant levels and on the polarization and area of the pumping pulse is examined in detail. Non-linear equations describing the temporal shapes of the super-radiance pulses with different polarizations are obtained. Some examples with low values of angular momenta of the levels are discussed. It is shown that, in general, the temporal shape of the super-radiance pulse differs from hyperbolic secant.

Systematic calculations of energies and decay rates for the 2p⁵3snl core-excited states (with n<or=7) in Na have been performed using a configuration-interaction method, including spin-orbit interaction and core-polarization correlation through a semi-empirical model core potential. Data obtained extend and clarify considerably the classification of lines in the existing photoabsorption and ejected-electron spectra of Na. Lifetime determination included radiative decay to the bound 2p⁶nl levels and to the lower core-excited states as well as autoionization. Both correlational and relativistic effects play an important role, as expected, but in addition it was found that the non-orthogonality of bound and continuum orbitals, being a result of core excitation, is also very important in evaluating of autoionization rates. The predicted lifetimes are analysed with respect to their 'quasimetastability', which has recently attracted great attention in atomic spectroscopy and in the construction of XUV lasers.

The authors have studied double ionization of Ba 6snl Rydberg states through resonant two-photon excitation of 8snl double-Rydberg states with n=13 and 14 and different l. The laser intensity dependence of Ba²⁺ yield measured for l>or=8, when the electrons suffer long-range dipole interaction, shows a decrease from four-photon for l=8 to less than three for l=11. The four-photon dependence is due to autoionization decay populating the Ba⁺(7p) levels which gives an indirect signature to the Ba²⁺ production. By increasing l this decay vanishes and the data show evidence of a direct double-ionization process via planetary-electron states for l>or=10. Additional intensity measurements performed for low l=6 states strongly mixed with 5fn'l' ones by short-range multipole interaction, reveal indirect Ba²⁺ production and an individual case of partial stabilization due to destructive interference leading to direct double ionization.

A theoretical analysis of one-dimensional cooling of atoms is presented. The method of cooling is based on velocity-selective coherent population trapping. Both internal and translational degrees of freedom of the atoms are treated quantum mechanically. The analytic expression for atomic density matrix is obtained in the limit of large interaction time and small kinetic momentum of atom. It is demonstrated that: (i) there is a limit on the lowest temperature that can be achieved by such a method and it is determined by the small but finite rate of decay of the coherence between two low-lying states in the three-level Lambda -configuration; (ii) the fraction of atoms involved in the cooling process decreases to zero as the atom-field interaction time increases, because of the presence of such a relaxation which always takes place in real experimental systems.

Impact parameter dependence of the ionization probability in fast ion-atom collisions is theoretically studied. A role of projectile ion electrons in the ionizing collisions (or, equivalently, of target electrons in the electron loss process) is discussed. Within the framework of the first-order semiclassical approximation a method is suggested allowing inclusion of double electron excitations in the calculation. Using this method the screening and antiscreening effects of various values of impact parameter are quantitatively studied. The effective charge of the ion as a function of the impact parameter is introduced. It is shown that the effective charges for the ionization probability and for the mean energy transfer are different and have different dependence on the impact parameter.

We present a first application of the recently proposed doorway approximation of the time dependent optical potential to ion-atom collisions. The new formalism is tested for the one particle systems H⁺-H(1s) and H⁺-He⁺(1s). Results for elastic scattering, excitation, electron loss and ionization are compared with those of other theoretical approaches and with experimental data in the range of impact energies of 10-1000 keV for H⁺-He⁺(1s) and 2-1000 keV for H⁺-H(1s).

A symmetric eikonal model is developed to study electron excitation of hydrogenic atoms by impact of heavy ions (composed of a nucleus and N electrons) at intermediate and high collision velocities. Differential cross sections are calculated for impact of He⁺ ions on H targets and compared with existing experimental data for formation of H(n=2) atoms. Total cross sections for collisions of singly (H⁺, He⁺, Li⁺) and doubly charged (He²⁺, Li²⁺) projectiles on H targets are also computed. A good representation of measurements is obtained for He⁺ ions. The role played by the spatial distribution of projectile electrons is analysed.

A numerical study is performed for the locking of an electronic angular momentum j=1 to the molecular axis during a collision of two atoms interacting via a potential proportional to an inverse power of the interatomic distance, R. Limitations to the notion of the locking radius and slipping probability are discussed in connection with the steepness of the interaction governed by the exponent n. Numerical calculations confirm the authors' earlier analytical result: the optimal criterion for determination of the locking radius is a condition for the accumulated phase difference between two molecular states. Analytical expressions are suggested for the locking angle and the slipping probability. The implication of the locking approximation for calculation of the quasiclassical scattering matrix is discussed.

Results are presented of classical trajectory calculations for the problem of large-L generation due to post-collisional interaction after conversion of the double excited states of multicharged ions formed in double charge exchange. It is shown that the large-L states are produced by this interaction but that the simple model of this phenomenon, proposed by the author previously, appears to be too crude. It is shown that the influence of the inhomogeneity of the electric field of the residual ion on the distribution of final orbital angular momentum of the Rydberg electron is important. The results imply that the uniform distribution function of the parameter x=L_fin/n_fin provides a reasonable approximation.

Translational energy spectroscopy in an apparatus previously developed in this laboratory has been used to study one-electron capture by 4-28 keV N⁴⁺ ions in collisions with helium. The main excited N³⁺ product channels have been clearly identified and the relative cross sections determined. The two main channels are found to involve core rearrangement. While the N³⁺ 2p^{2 1} S product channel is dominant, it is shown that other channels involving large energy defects contribute significantly at the higher energies thereby resulting in an essentially energy invariant total one-electron capture cross section: MCLZ calculations, which have also been carried out, satisfactorily predict the main 2p^{2 1} S product channel but fail to account for the other channels arising from curve crossings at small internuclear separations.

The process of charge exchange with ion excitation is investigated theoretically. A multielectron and multichannel theoretical model of the process is proposed. The important point of the model is the determination of matrix elements of exchange coupling. The formulae for calculations of two-electron exchange coupling are obtained and discussed. The model is applied to collisions of helium ions with mercury atoms at thermal energies. The matrix elements of exchange coupling, the adiabatic potential energies, the partial cross section and the partial rate constant for the process under consideration are calculated. The matrix elements of exchange coupling are mainly determined by electron-core interactions. The effective potentials are used for these interactions. The calculated partial rate constant for excitation of Hg⁺(7p ²P_3/2) is 1.16*10^-10 cm³ s^-1 and agrees with the experimental value.

The work investigates theoretically triply and doubly differential cross sections for electron- and positron-impact ionization of atomic hydrogen. On the basis of the framework developed by Brauner et al. (1989-91), structures in the triply differential cross section are identified and their contributions to the doubly differential cross section are discussed. A novel structure in the triply differential cross section in the case of positron impact has been found. Results for the energy and angular distributions of secondary electrons are presented and compared with available experimental data. In general, the results are in good agreement with the experiment. Discrepancies between experiment and the theory exist for electrons ejected into the forward direction. It turns out that at low impact energy the angular and energy spectrum of the secondary electrons is strongly influenced by all three two-body interactions involved in the process. In the case of electron impact also exchange effects contribute to the shape and magnitude of the cross section. The authors conclude that triply and doubly differential ionization cross sections depend sensitively on the description of the three-body system.

Differential cross sections are calculated for the electron impact excitation of 1¹S-2³S transitions in helium at incident energies of 100 and 200 eV. A distorted wave approximation is used with the effect of target polarization taken into account. The results are compared with the experimental data of Sakai et al. (1991) and with the R-matrix method calculation of Nakazaki et al. (1991). The present cross section is in general agreement with the R-matrix calculation. However, the present calculation cannot reproduce the very large cross section obtained by Sakai et al. in the forward direction at 200 eV.

The complete valence shell electron separation energy spectra and momentum distributions are measured for benzene by electron momentum spectroscopy at a total energy of 1200 eV. The measured momentum distributions are compared with those calculated in the plane wave impulse approximation (PWIA) formalism using a (9s5p/4s1p) to (4s2p/2s1p) Gaussian basis set. The level of agreement between the measured momentum distributions and the PWIA-SCF orbital momentum distributions is, in general, surprisingly good, particularly given the restricted nature of the current SCF basis set. The inner valence 2e_2g, 2e_1u and 2a_1g orbitals are severely split by final state correlation effects. The agreement between the measured and Green function many-body method calculated spectroscopic factors and separation energies is good, although the measured separation energy spectra contain significant strength up to 51 eV, this strength being mainly of 2e_1u and 2a_1g origin.

The authors adopted mass-weighted hyperspherical coordinates to study the properties of Coulombic three-body systems where all three particles are different. Using an adiabatic approximation, they applied the finite-element method to the two-dimensional eigenvalue problems at fixed hyperradius. The authors have calculated the adiabatic hyperspherical potential curves, and examined the wavefunctions (in terms of density plots) and the non-adiabatic coupling terms for a number of three-body systems. By fixing the masses of two of the particles, they examined how these properties vary with the mass of the third particle. The existence of stable bound states versus the masses of the systems is also investigated.

The total cross section for positronium formation in helium for positrons of energies E from threshold (17.8 eV) to 250 eV has been measured using a gas cell transmission method. The results indicate that the cross section above E=60 eV is satisfactorily described by 0.5e^-0.02E pi a₀², tending to zero at energies close to 200 eV. The approximate description of the cross section at intermediate energies, falling as E^-2.5, is consistent with a number of theoretical predictions but is in conflict with earlier experiments, which show an E^-1 behaviour. The implications of these measurements are discussed, with particular reference to the observed merging of the total cross sections for positron- and electron-helium scattering at intermediate energies.

The effect of dissipation of plasmas on the spontaneous radiation of an ionized atom is investigated by Langevin equations and the modified Weisskopf-Wigner approximation to consider the quenching effect of the Einstein A coefficient. The dissipation of field in plasmas results from inverse bremsstrahlung of free electrons and absorption of the ions of the same kind as the radiator. Under certain conditions the Einstein A coefficient may be reduced significantly. The lifetime of the excited ion will be changed correspondingly.

The conversion efficiencies and the linewidth of the Stokes components which result from stimulated Raman scattering (SRS) in hydrogen gas pumped with a high power alexandrite laser have been studied. The pump laser was operated at 742 nm giving first and second Stokes components at 1074 nm and 1935 nm respectively. The pump laser could be operated either in the normal Q-switched configuration with a linewidth of the order of 0.2-0.3 nm or with injection seeding which narrowed the linewidth to 1-2 pm. Measurements of the Stokes components under these varying conditions reveal that there is no effect on the conversion efficiency by the narrowing of the pump linewidth and that the linewidth of the first Stokes component is broader than the expected linewidth of the injection seeded pump. Modelling of the conversion from the pump to the Stokes components shows a strong dependence of this conversion on a specific, resonant four wave mixing process. The study gives a complete set of information about the radiation resulting from SRS in molecular hydrogen pumped with a high power, narrow linewidth, tunable solid-state laser.