Table of contents

A simple two-parameter wavefunction for the ground state of two-electron atoms has been obtained by introducing variational parameters in the Hamiltonian rather than in the trial wavefunction. The energy eigenvalues have been calculated for atoms with nuclear charge Z in the range I<or=Z<or=12. The binding energies so obtained are observed to be better than the corresponding one-parameter (only screening) values, and are in reasonable agreement with very accurate multiparameter results reported by earlier workers. The method proposed is only easily extendable to those configurations in which both the electrons occupy the same orbital.

By using synchrotron radiation to excite calcium atoms in the 3p-3d resonance region, excited calcium ions are generated. In the first measurement of this kind, we have measured the polarization of the fluorescent photons which result from the decay of these ions, in coincidence with the corresponding angle-resolved photoejected electrons. By combining this with the angular distribution parameter for these same electrons, measured in a separate experiment, we are able to calculate for this specific case, within the assumption of LS coupling, the ratio of the dipole amplitudes for the outgoing waves and their phase difference.

The radiative lifetime of the 1s2p¹P state of helium is measured in a two-colour ionization experiment with a 5% accuracy. The state is excited by the 13th harmonic of a tunable 80 ps laser and ionized by a synchronous ultraviolet laser, with a variable time delay. This experiment demonstrates that the high harmonics generated in a jet of rare gas exposed to an intense laser field provide a tunable short-pulse XUV source ideally suited for pump/probe type of studies and, in particular, short lifetime measurements.

L-subshell ionization of Au and Bi induced by boron impact has been investigated for impact energies ranging from 0.48 to 0.88 MeV/u. The energy dependence of the measured ionization cross section shows, for the first time, a plateau structure for all three subshells. The plateau structure revealed by previous data for proton and helium impact was for the L₁ subshell only and this had been attributed to the bimodal nature of the 2s electron density. The observed plateau structure for all the three subshells and its occurrence at a somewhat lower energy signifies a considerable amount of Stark mixing of target 2s and 2p atomic wavefunctions. Fresh calculations incorporating the Stark mixing effect in target atomic wavefunctions are necessary to improve agreement with the present data. The existing theories, however, are found to be inadequate.

Various theoretical small-angle electron differential cross sections (DCSs) for excitation of H 2p at 54.4 eV are contrasted through the generalized oscillator strength (GOS) and compared with the GOS from the recent universal function and the only measurement. Near zero scattering angle spread among the results is significant. A curve is established by continuing the measurement through 0 degrees scattering to the optical oscillator strength, which can be used to assess the reliability of both theoretical and experimental small-angle data.

A number of unexpected features of small molecules subjected to intense laser fields, with wavelengths ranging from infrared to ultraviolet, have been observed or predicted in the past few years: above-threshold dissociation, molecular bond softening, vibrational population trapping. We review these processes for the case of the molecular ion H₂⁺ and discuss the experimental and theoretical tools that are used to study this system. Both electron and proton energy distributions are used to interpret the experimental results. Theoretically, the fragmentation dynamics can be described equivalently as a laser-assisted half-collision process, using solutions of the time-independent Floquet theory, or as the evolution of a wavepacket subjected to a classical radiation held with a given pulse shape, using solutions of the time-dependent Schrodinger equation. A broad range of laser intensity and pulsewidth has been explored, with the short-pulse results (analysed in terms of 'dressed' potential curves) offering the best interface between theory and experiment. We finally report on a promising new avenue for coherent control of fragmentation dynamics, through the use of two-colour phase-locked radiation.

A closed formula for and interpretation of exact quantum mechanical off-diagonal matrix elements of powers of r is given in terms of orthogonal polynomials. The compact formulae for r^-k-2 and r^k-1 are in exact correspondence to the semiclassical (diagonal) expectation values and are most suitable for the practical evaluation of the matrix elements. The expressions for < nl | r^-k-2 | l'n > and < nl | r^k-1 | l'n > consist only of the minimum of n-l or k+1 terms. An off-diagonal generalization of the well known diagonal inversion formula is derived and the matrix elements can thus be evaluated for arbitrary (complex) powers k. Also, an analytic continuation to an arbitrary effective n* is possible. It is thus established that, complementary to the O(2,1) approach by Armstrong (1970, 1971), orthogonality properties for purely radial quantities can be obtained without the introduction of a fictitious angular variable. Using the analytical results obtained, certain deficiencies of the semiclassical Coulomb (or WKB) approximation are discussed. The normalization error involved in the WKB approximation is identified analytically and an improved version of the semiclassical limit, a correspondence limit, is derived. Furthermore, it is established that, like the exact results, the WKB matrix elements obey a three-term recursion relation and an inversion formula. For low k some explicit formulae for exact matrix elements are tabulated.

X-ray emission transition energies and probabilities have been calculated for the core-excited states of the oxygen atom in the N₂O molecule at the ab initio Delta SCF level. The oxygen core electron was excited to the 3 pi and 8 sigma unoccupied molecular orbitals and comparisons of the X-ray emission pathways were made with the core-ionized excited state. To measure the effects of geometry relaxation in the initial states, pointwise geometry optimizations were also performed on the core-excited states. It was determined that the inclusion of multicentre contributions for the initial and final states is essential for interpreting the calculated spectra.

The theoretical study of 5f^N, ns5f^N, np⁵5f^N, nd⁹ 5f^N configurations by application of Hartree-Fock-Pauli approximation has led to the determination of the electronic structure of these configurations, values of electrostatic parameters and energy of K and L X-ray lines for actinides of '+1', '+2', '+3' valencies. The energy of X-ray lines decreases with the increase of the valency of the ion. The determination of N- and Z-dependence of various electrostatic parameters improves the reliability for prediction of spectra and electronic states of actinides in solid state, plasma and galaxy, and can be used for prediction of spectroscopic properties of 5f^N7s¹ configurations.

The charge spectra of multiple ions resulting from vacancy cascades in neon, argon, krypton and xenon are calculated by straightforward construction of de-excitation trees originating from various initial inner-shell vacancies. Included in the calculation are all possible radiative and non-radiative decay channels for each vacancy, and shake-off processes caused by diagram Auger, Coster-Kronig, and radiative transitions. The single-configurational Hartree-Fock approximation is employed to calculate the decay channel branching ratios and the probabilities of shake processes. The energies of levels and transition energies are calculated in the Hartree-Fock-Pauli (HFP) approximation. The dependences of level positions, level splittings, branching ratios, and shake probabilities on a configuration of a decaying ion are accounted for. Good agreement with experiment and some of the earlier calculations is obtained. The role of double electron ejection processes, other than monopole, is discussed.

Various problems in atomic physics can be formulated in terms of two bands of potential curves which cross each other. Each band consists of parallel (non-interacting) diabatic potential curves equally spaced on the energy axis. The bands with infinite number of states are considered under the assumption of 'translational' symmetry along the energy axis. The adiabatic potential curves are constructed explicitly. The system of avoided crossings appears not only in the weak coupling case (which is obvious), but also in the strong coupling limit. This feature also holds in the generalized model where the bands are finite and non-equidistant. In the case of an infinite number of states in each band the dynamic description (i.e. evolution in time) is reduced to the two-state problem which contains an additional continuous parameter (analogue of the quasimomentum). Some peculiarities of the time propagation are discussed. The present model generalizes the famous Landau-Zener two-state case and Demkov-Osherov model (one level interacting with a band of levels).

Electron angular distributions due to autoionization of Ne, doubly excited to the (2p⁴3s²)¹D state in collisions with Li⁺ in the energy range 1.2-2.2 keV, are measured in coincidence with Li⁺ scattered into a well defined direction ( Phi =0 degrees , Theta _cm=10.8 degrees ). The experimental findings are analysed with the help of a collision model proposed earlier. In this model the initial excitation occurs by radial diabatic coupling to a molecular Sigma -state at small distances, followed by rotational coupling to Pi - and Delta -states at intermediate distances in the second half of the collision. The energy splitting between the Sigma -, Pi - and Delta -states is described by a model function. By adapting two parameters of this model function, the experimental findings can be reproduced within the experimental error in numerical calculations involving the relevant set of coupled differential equations.

The electronic stopping power for bare particles and antiparticles (Z_p=+or-1, +or-2, +or-3) incident on atomic hydrogen has been calculated by using a full classical and quantum-mechanical description of the energy-loss processes. For this purpose we have applied the classical-trajectory Monte-Carlo (CTMC) and the coupled-channel atomic orbital (AO) method. A comparison indicates that the classical results converge to the non-perturbative quantum-mechanical ones in the limit of strong projectile-induced perturbations, especially if the capture channels are of minor importance.

The transverse and longitudinal momentum distributions of He⁺ ions produced by 0.25-1 MeV u^-1 H⁺ and He²⁺ impact have been measured using a precooled supersonic gas jet target (cold-target recoil ion-momentum spectroscopy-COLTRIMS). The transverse momentum of the recoil ions is mainly determined by the impact parameter. In the longitudinal direction the recoil ions are slightly backwardly directed. The width of the longitudinal momentum distribution is close to the electron momentum distribution in the initial state. This distribution gets much broader for larger transverse momentum transfer (i.e. smaller impact parameters). The experimental results are in good agreement with nCTMC calculations.

Charge-exchange processes in 400-2400 eV Ar⁸⁺-He collisions are investigated: high energy resolution ion beam spectroscopy is performed with a recoil ion source (RIS) built in GANIL, total and partial cross sections for single-electron capture are measured using retarding field and energy gain techniques. Comparison with previous experimental results for total charge-exchange cross sections shows a rather good agreement and confirms the independence of these cross sections on the collision energy in the studied range. In contrast partial charge-exchange cross sections are shown to depend strongly on the collision energy. The experimental data are compared with theoretical results in the semi-classical impact parameter frame and with predictions made by other authors using the Landau-Zener model.

An average-atom model is employed to calculate differential cross sections for the elastic scattering of electrons from argon and iron plasmas. The results are in good agreement with other theoretical calculations and available experimental data in the low temperature and density limits. Numerical simulations show that the effect from temperature is more complicated than that from density, and the differential cross sections change greatly with the condition of plasmas.

Elastic differential and momentum transfer cross sections are reported for electron scattering by H₂O at impact energies ranging from 4 to 50 eV. The iterative Schwinger variational method in the fixed-nuclei, static-exchange approximation is used to calculate the low partial wave scattering amplitudes and the higher partial wave contributions are included via closure using the Born approximation for a point-dipole. Comparison of our calculated cross sections with recent experimental and other theoretical results is encouraging.

Total cross sections for electron scattering on SO₂, NO₂, OCS have been measured in the energy range from 90 to 4000 eV. Our data extrapolated to low energies match other experiments on OCS and NO₂; for SO₂ discrepancies amount to as much as 20%. The experimental values for these gases and for CO₂ (the latter measured previously in our laboratory) have hen fitted with a Born-like formula. Two parameters are sufficient to reproduce the SO₂, NO₂ and CO₂ cross sections from 20 to 4000 eV within the experimental errors. A double Yukawa potential with four parameters is needed to reproduce the OCS cross sections over the same energy interval.

The relative flow technique, utilizing helium as a cross section standard, has been applied to measure absolute elastic differential cross sections for electron scattering from nitric oxide (NO). The present elastic measurements were conducted at nine energies in the range 1.5-40 eV, which represents an important extension to the only previous study of this system by Kubo and co-workers (1981). Furthermore, the uncertainties in the present data are significantly reduced in comparison to those quoted in the earlier work. The current elastic differential cross sections were measured at the Australian National University over an angular range 10 degrees -130 degrees . We also report the first measurements of absolute differential cross sections for rovibrational (0 to 1, 2, 3, 4) excitation of the NO ground electronic state. These were conducted at six energies in the range 7.5-40 eV and were measured at Flinders University over the angular range 10 degrees -90 degrees . The differential cross sections for both the elastic and inelastic processes are compared with the results of the recent Born-closure Schwinger variational method calculation of Mu-Tao Lee and colleagues (1992).

Deuterium Rydberg states which were populated by radiative recombination in the electron cooler of CRYRING have been detected using state-selective field ionization in an external electric field. Absolute radiative recombination rate coefficients for Rydberg states between 12<or=n<or=18 are extracted and agree well with theoretical calculations using the Kramers cross section corrected by the Gaunt factor. The influence of the Stark effect on the Rydberg states has to be taken into account to explain the data. It was found that the relative population distribution of Rydberg states as well as the rate coefficient are very sensitive to the transverse electron temperature in the electron cooler.

Doubly and singly excited-state energies, autoionization and radiative rates, and branching ratios for dielectronic recombination (DR) have been calculated for Mo³⁴⁺ in the intermediate coupling and single-configuration approximation. Wavefunctions and energies are calculated using the Hartree-Fock method with relativistic corrections. The results of these calculations are then compared to predictions obtained using scaling relations developed previously from detailed calculations of DR for O-like Ar¹⁰⁺, Ti¹⁴⁺, Fe¹⁸⁺ and Se²⁶⁺. These relations are strictly valid for predicting DR data between Ar¹⁰⁺ and Se²⁶⁺, but they can also be extrapolated to predict the data for Mo³⁴⁺. We find that, except in a few cases where the data show strong Z dependences, they are accurate to within or better than a few per cent of the detailed, calculated Mo³⁴⁺ data. Comparisons are also made with results of other published work for Mo³⁴⁺ as well as for other O-like ions. As expected, from earlier calculations, the results of this calculation are in much better agreement with those obtained using a fully relativistic MCDF calculations than those obtained using a non-relativistic LS coupling calculation without any relativistic corrections.