Table of contents

Brillouin-Wigner-like quasidegenerate perturbation theory for intermediate effective operators is discussed. Due to the self-consistent approach to the calculation of projectors onto perturbed model states, an a priori partitioning of the model space into a main model space and an intermediate space is avoided. Small energy denominators do not appear in any order. As for the intermediate states, the accuracy of results and the norm of effective interactions can be controlled; one can achieve an almost exact description of the lowest intermediate states and gradual increase of errors when going up in energy. Numerical examples are presented and discussed.

The absolute optical oscillator strengths for the individual He 1¹S to n¹P (n=2-7) transitions have been measured using dipole (e, e) spectroscopy at high energy resolution. The results, which include the first measurements above n=3, are in excellent agreement with earlier published quantum mechanical calculations performed using very accurate correlated wavefunctions for helium.

The photoionization of Ne was studied by photon-induced fluorescence spectroscopy (PIFS). The threshold range of the 2s electron was investigated in detail revealing new structures of the photoionization cross section which result from doubly excited atomic states. In parallel, the cross sections for the (³P)3s ²P and ⁴P satellites were measured in their threshold ranges. The energy of the exciting photons was varied in wider steps up to 120 eV, in order to compare the data obtained by PIFS with published data from PES. Good agreement is observed.

Ionization of calcium atoms through resonant laser pumping of the 4s4p ³P₁ metastable state is investigated. The ionization yield is measured as a function of laser power, oven temperature and vapour density. The small bias field between the collector electrodes is observed to have a definite role in initial 'seeding' and 'priming' of the medium prior to the arrival of the laser pulse in that it accelerates some of the thermionically emitted electrons through the vapour.

It is rigorously proved that, if adiabatic switching off of the interaction is properly implemented, as demanded by formal time-dependent S-matrix theory, even the Keldysh-type theories of multiphoton ionization require energy conservation without allowing for the ponderomotive energy, which only contributes an unobservable phase. The importance of ponderomotive effects in above-threshold ionization therefore needs to be re-examined.

The energy spectrum and angular distribution of electrons ejected during collisions of multicharged ions with hydrogen targets are studied, in the intermediate-energy range, with the CTMC method and the CDW-EIS approximation. Single narrow bands are obtained for the corresponding cross sections scaled by the projectile charge as a function of the electronic energy or emission angle provided that the impact energy per nucleon is also scaled. Such a result at the singly differential cross sections level leads to the well known scaling law for total ionization cross sections. Ionization probabilities as a function of the impact parameter are also examined.

The authors have studied in a beam experiment the energy pooling reaction between two laser-excited Na* (3p) atoms, where in the final state one Na atom is in the 5s or 4d state and the other in the ground state. They have found that this reaction is strongly dependent on the polarization of the laser light used to prepare the Na*(3p) atoms. An analysis of theoretical potential curves suggests that the reaction mainly proceeds via the input channel where one of the atoms is in the M=0 and the other in the M=1 substate. Their experimental results are in qualitative agreement with that suggestion.

A two-dimensional R-matrix propagation procedure is presented which provides an accurate numerical solution of the two-electron Schrodinger equation. Its application to the s-wave electron-hydrogen model problem establishes its numerical stability and illustrates its efficiency for electron-impact excitation of high Rydberg states, as well as for ionization in the threshold and intermediate energy regions, which have long defied accurate treatments.

The author has calculated dielectronic recombination rate coefficients for Al¹⁰⁺ using the AUTOSTRUCTURE code. The high-temperature zero-density results are in good agreement (30%) with those obtained from the general formula of Burgess (1965, 1969) and are a factor of 3.6 larger than the R-matrix results of Terao and Burke (1990). There are two reasons for this discrepancy. Firstly, the author obtains a nonnegligible contribution from high angular momentum (l>6) states, 40% of the total, unlike Terao and Burke. Secondly, the author's allowance for outer electron stabilization, which was neglected by Terao and Burke, increases the author's results by a factor of 1.65. The discrepancy increases to a factor of 10, or more, when dielectronic recombination is dominated by low-lying states as found in a high-density or low-temperature plasma.

The shifted large-N method is applied to the eigenvalue problem of Varshni's third potential, which provides a good representation of the potential energy curves of many covalent diatomic molecules. Analytical results for the vibrational and rovibrational energy levels of I₂(X), CO(X) and H₂(X) molecules compare fairly well with the exact ones obtained numerically. The accuracy of the eigenvalues is found to depend significantly on the width of the potential.

Hydrogen atom oscillator strengths for the transition between the low-lying levels and the levels of quadratic Zeeman multiplets with large principle quantum numbers are investigated. It is shown that in the l-mixing regime elementary analytical relations exist between the profiles of the oscillator strength distributions and the plot of energy spacings in the Zeeman multiplet. An explanation is given for the steep rise of intensity at the high-frequency edge observed if the upper states are even with respect to z to -z transformation. Other types of perturbations including the Stark-Zeeman effect in parallel fields are also considered.

Ag I autoionizing levels were populated by a combination of discharge and laser excitation. Twenty-six of the observed lines are assigned to dipole transitions to the Ag I 4d⁹5snl (nl=5p, 6s, 5d, 7s, 6d) configurations. An additional 37 unclassified lines were also observed. Asymmetry parameters q and lifetimes were determined for a number of lines. A combination of ab initio Hartree-Fock calculations, diagonalization of energy matrices and a least-squares fit to the experimental level energies yielded lifetimes in quite good agreement with the experimental values for the above mentioned configurations.

The dynamics of the ground-state population is investigated for photodetachment of an ionic model including a ground state and a continuum which is Fano pre-diagonalized due to a coupling with an autoionizing-like state. In the near-threshold region exact decay curves are obtained which exhibit an oscillatory behaviour and asymptotically tend to a nonzero value (population trapping).

Spin-change cross sections (SCCS) for the Li₂, Na₂, K₂, Rb₂ and Cs₂ systems were calculated by full quantum mechanics and semiclassical theory. Calculations are reported for temperatures ranging from 10 K to 1000 K. A comparison between quantum, semiclassical and experimental results is presented.

M-shell vacancy production cross sections, identified with Auger-electron production measurements in cadmium by H⁺ and He²⁺ ions with velocities near the M-shell electron velocities v_2M and taken as data for electron capture from krypton by protons with velocities of approximately 4v_2M, are compared with the first-order and second-order ionisation theories. The MeV projectile ions of Z₁=1, 2 in these experiments are of intermediate and high velocities, relative to v_2M in the targets of medium atomic number Z₂. For these data, Z₁/Z₂ is sufficiently large to discriminate between various ionisation theories. The ECPSSR theory, that accounts for the energy loss and Coulomb deflection of the projectile as well as for the perturbed stationary state and relativistic nature of the target M shell, gives better agreement with the experiments than first-order ionisation theories and the continuum-distorted-wave treatment of electron capture.

The authors have calculated single and double ionization cross sections in the first Born approximation for two-electron ions of varying nuclear charge, Z_N. Electron correlation is essential to this process, and the target nuclear charge scaling is used to study this correlation. As Z_N is increased the single ionization cross section is found to scale as Z_N^-4 and the double ionization as Z_N^-6, in agreement with simple analytical predictions based on a perturbative treatment of the electron-electron interaction.

The rate coefficients and product ions have been determined for the reactions of F⁺ ions with some fifteen molecular gases at 300 K using a SIFT apparatus. The reactant gases were N₂, CO, CO₂, H₂, D₂, NO, O₂, C₆H₆, C₂H₄, C₂H₂, CH₄, NH₃, H₂O, H₂S and OCS. The mechanisms of the reactions are interpreted in terms of the accessible electronic states of the parent molecular ions formed in the (predominantly) charge transfer processes that occur. For two reactions, the electronic states of the product ions have been identified experimentally; for the other reactions, the states of the product ions have been inferred in the light of the available data obtained from photoelectron spectroscopy studies.

The angular distribution and spin polarization of Auger electrons are discussed. A general expression for the anisotropy coefficients is derived for single-configuration closed-shell atomic systems. As special cases the usual parameters alpha ₂ and beta ₂ are calculated numerically for some noble-gas transitions and also for the first time alpha ₄ and beta ₄. The results for alpha ₂ and beta ₂ are compared with experimental data and other theoretical results.

The rate coefficient (v sigma ) for radiative recombination has been measured in the energy range from 0 to 1 eV for H-like ions of C, O and F and for Li-like O ions. The experimental data are compared with a calculation based on the theory of Bethe and Salpeter (1957). The theory was corrected for the low-n states according to Stobbe's calculation. The theories, originally developed for fully stripped ions, were modified to treat radiative recombination for nonbare ions by taking into account the Pauli exclusive principle and the use of an effective charge. The authors find that when the asymptotic ion charge is used in the calculation, good agreement is found with the experimental data.

The buildup of external bremsstrahlung (EB) intensity as a function of target thickness in metallic targets of Al, Cu, Sn and Pb by beta particles of ²⁰⁴Tl and ³²P beta emitters has been studied. Generation of EB intensity in successive layers of a thick target is described in terms of beta particle absorption while self-absorption of EB is evaluated by considering layer-to-layer transmission. The conventional beta stopper method is employed to measure the integral EB intensity in different target thicknesses. Comparison of calculated and experimentally measured EB growth curves shows a good agreement for various targets for both beta emitters.