Table of contents

The Stokes parameters for the radiation from the Ne state excited by transversally polarized electrons near threshold reveal a strong resonance. A simple model predicts the contribution to polarization from the resonance.

We apply a convergent close-coupling study of 54.4 eV electron-impact excitation of the n = 3 states of atomic hydrogen to circular polarization arising from the 3D state. There have been recent measurements followed by several theory calculations, some of which provide better agreement with the data than others. The most recent DWBA models suggest fair agreement with the data. In order to resolve the disagreements we have carried out a substantial set of convergent close-coupling calculations in order to estimate the effects of approximations present in the earlier theory models. Our conclusion is that the substantial discrepancy with experiment, first identified by Kumar et al, remains.

We have measured electron - ion recombination rates for bare ions of , , , and in a storage ring. For the multi-charged ions an unexpected energy dependence was found, showing a strong increase of the measured rates over the calculated radiative recombination rate for electron beam detuning energies below the electron beam transverse temperature. The measured enhanced rates increase approximately as with the charge state Z. A comparison of these rates with theoretical predictions for collisional - radiative recombination in the cold magnetized electron plasma, in particular three-body recombination including radiative de-excitation of electrons in Rydberg levels, is made.

The energy distributions of positrons scattered at following an ionizing collision with argon gas have been measured at impact energies of 60 and 100 eV. In the latter case, the experimental data are compared with previous measurements and values from a classical-trajectory Monte Carlo calculation. Whilst good agreement is found amongst the experimental values, theory is found to exceed the measurements by up to a factor of five at the highest scattered projectile energies. The shape of the experimental energy spectra is discussed and is tentatively linked to the contribution of close positron - atom collisions to the cross section. In addition, our scattered positron spectra showed no significant structure which could be attributed to the process of electron capture to the continuum.

Closed expressions for the adiabatic and non-adiabatic matrix elements for coupling between the Rydberg states of diatomic molecules are derived by a combination of single-channel quantum defect theory and semiclassical mechanics. Validity conditions on the derived results and limiting behaviours as are discussed. Applications of the theory to low-lying and states of the singlet p-complex of hydrogen are in good agreement with ab initio results.

We investigate the effect of a strong dressing field on the polarizability of atomic helium at harmonic frequencies. This study is carried out in a time-dependent non-perturbative calculation by looking at the harmonic spectrum resulting from the interaction with the combination of two fields: a strong pump field at the fundamental frequency and a weak probe field at some odd harmonic frequency. We present results for harmonic frequencies extending over the characteristic spectrum of high-order harmonic generation.

In this paper we present the Rydberg autoionization spectra of lead from to the ionization limit at excited from the 6p7f and intermediate levels. Term energies and FWHM of 40 lower n levels (n = 10 - 15) of the - 3 Rydberg series are reported for the first time.

The global theory of spectral lineshapes recently applied to Paschen-, Paschen- and Brackett- lines of hydrogen, is used to supplement these previous works by substantial new calculations which extend our published calculations to different temperatures and densities. A detailed exposition of the formalism is given. The effects of partial waves excluded by the exact resonance approximation applied in our previous applications of the global theory are considered for broadening by electrons and ions. The Griem table method is improved such as to be used to calculate the lineshapes of , , and upper lines. The tables of line profiles of Brackett- at temperatures varying from K to K and densities scaling from to are available upon request.

This work reports a collection of 200 Ar II transition probabilities of lines in the spectral region 360 - 750 nm for 16 lifetime values, all of them measured in an emission experiment. Relative intensity measurements have been made on a pulsed discharge lamp and the absolute values have been obtained by using data taken from the literature. The electron density, which ranges from 0.2 to in the plasma, has been determined simultaneously by interferometry and from -Stark broadening. The Ar II excitation temperature (15 000 - 27 000 K) has been calculated by a new method which also provides transition probabilities and plasma composition. The high number of measurements has allowed us to properly discuss the plasma equilibrium model. Comparisons have been made with the most recent experimental and theoretical data collections.

Absolute photoionization cross sections for Ba, measured using a Mach - Zender interferometer, are presented. The present value of the cross section at the ionization threshold, , is in good agreement with previous values. R-matrix calculations combined with MQDT techniques have been carried out to analyse the spectra obtained. The lineshape parameters obtained from the experimental data on the resonance lines are used to establish the identity of perturbing structure: the perturber is identified as the level via the present theoretical calculations.

Electron emission diffraction and x-ray absorption in the presence of an unscreened excited atom core hole - responsible for a long-range Coulomb force distorting the outgoing electron waves - are described within the muffin-tin potential model by means of the Green's operator multiple-scattering formalism: a case especially relevant for molecules and polyatomic clusters. The approach has the merit of allowing analytical and closed-form expressions to be obtained for all amplitudes and of yielding the possibility of expanding the scattering amplitudes in multiple-scattering terms. The treatment is mathematically consistent avoiding the appearance, in the expanded scattering amplitudes, of terms which are either indefinite or divergent.

We investigate semiclassically the photodissociation of water in the first absorption band. Experimental absorption spectra show weak undulatory structures, which are related to the vibrational symmetric stretch motion on the excited -state potential-energy surface (PES). Using the cycle expansion resummation we demonstrate that there are only small corrections due to other fundamental periodic orbits and negligible contributions from longer orbits. The error in the positions of the semiclassical resonances is of the same order or less as the variations when using different Born - Oppenheimer potential surfaces. For this light molecule the background term is strongly influenced by quantum effects. We discuss estimates for the importance of quantum corrections and a way to incorporate them.

Ionization of atomic helium by a fully stripped heavy-particle projectile nucleus is considered. The importance of electron correlation in the final-state bound - continuum wavefunction of this two-electron system is examined. We obtain an approximate solution of the two-electron Schrödinger equation for this particular configuration. The method used is a variational-perturbative technique, which is extensively used in approaches to the alternative distinct transitions (bound - bound and continuum - continuum) of this two-electron system. We then introduce an effective charge which we show greatly improves this description. We apply the wavefunctions derived to evaluate total and singly differential cross sections for single ionization of helium by various differently charged projectiles. Finally, we compare results with experiment.

A method for evaluating the excitation by particles of charge of autoionizing states of helium is presented which includes interference with degenerate single ionization continuum states both before and after excitation. The decay into the final continuum state may be influenced by Coulomb interaction with the projectile in the final state. These effects influence the terms which may be observed experimentally. Calculations using this method are presented and compared with other calculations and experimental data.

Our previous differential cross section measurements on argon were extended to impact energies of 120 eV and krypton was used as an alternative target gas. Some distinctive features were observed in the cross sections of the transfer and transfer-ionization reaction channel which await a theoretical explanation. The narrow feature at , observed for the transfer-ionization reaction at 75 eV, does not persist at 90 eV and could not be observed with krypton. Therefore, a connection to Thomas scattering ( is the Thomas angle for Ps formation) is not supported.

Triply differential cross sections for electron - atom bremsstrahlung have been calculated in the relativistic independent particle approximation. We confirm the substantial discrepancies between the results of this theory and experimental data in the hard photon regime which have been reported recently. First results for intermediate energy photon emission show, however, good agreement with the experimental data available. The importance of the projectile - target electron exchange interaction and of detailed structure effects due to the description of the target atom are studied.

Results are presented for the triple differential (e, 2e) cross sections in the region encompassing the helium n = 2 autoionization resonances at the respective incident energies 94.6, 96.6 and 99.6 eV. The scattered-electron angle in each case is and the range of ejected-electron angles are between and . The measured coincidence ejected-electron spectra are analysed in terms of the Shore - Balashov parametrization to obtain the direct (e, 2e) cross section and the resonance parameters and for the and resonances as a function of ejected-electron momentum. These derived parameters are compared against the results of a calculation where configuration-interaction expansions for the resonances and helium ground state, which employed hydrogenic and multiconfiguration Hartree - Fock orbitals, respectively, were used. Here the distorted-wave Born approximation was employed for the (e, 2e) cross section calculation. The calculated parameters agree quite well with the experimental results indicating that the theoretical description of the reaction mechanism is physically realistic. Further, both experiment and theory show strong correlations between the resonance amplitudes and the direct ionization amplitudes. Finally, we report and discuss our results of an experimental investigation into the post-collision interaction effects for the present kinematical conditions.

The angular distribution of metastable fragments produced by electron impact dissociative excitation of has been measured at two impact energies (15 and 40 eV) with time-of-flight (TOF) spectroscopy. A simple TOF system which can be utilized for such studies is introduced. The parent molecular symmetry before dissociation was determined from the angular distribution, using this system. The angular distribution indicates that the process is direct dissociation, predominantly from a weakly bound -state.

New data on positron collisions with atomic hydrogen leading to ionization were taken by using the high-intensity positron beam at Brookhaven National Laboratory (BNL). The measured cross section values for positron-impact ionization, which supersede earlier ones, are in good agreement with other experimental and most theoretical results, while the new data for positronium formation, formerly normalized to impact-ionization data, now disagree with recent theoretical predictions.

We analyse a pump - probe set-up for detecting the optical Stark effect on two-level atoms with a quantized pump field in terms of a density matrix approach (DMA). The approximation used reveals a method to calculate the probe absorption spectra without high computational effort. It is even applicable for large numbers of atoms where the exact calculation is not practicable. The probe spectra in the DMA reflect the photon statistics of the pump field and deviate only slightly from exact results for relevant experimental parameters.