Table of contents

Lifetimes are reported for the , and levels in Hg-like Tl II, measured using beam-foil excitation. The values obtained were , , , and . Isoelectronic trends for line strengths of the resonance and intercombination transitions are studied through comparisons with earlier measurements for Hg I - Bi IV.

We solve the time-dependent Schrödinger equation numerically in the case of a linear chain of atoms irradiated by a short (40 fs) Ti - Sa laser pulse whose intensity ranges from to . The electrostatic potential is chosen so as to reproduce, after diagonalization, the corresponding one-dimensional band structure of a `free-electron' metal. The case of aluminium is taken as an example. Ionization rates and harmonic spectra are calculated and discussed.

Electrons have been scattered on in the energy range from 0 - 35 eV. A resonance structure in the detachment channel was observed at an energy of about 10 eV. To explain the structure, a potential curve was constructed from the isoelectronic ground-state potential curve and the repulsive Coulomb potential, and it is argued that the structure may be related to the population of a state. The state is predicted to decay primarily by electron emission since a large potential barrier prevents dissociation into .

The spin asymmetry associated with the ionization of the L shell of gold by 300 keV electrons polarized perpendicular to the scattering plane has been measured in an (e, 2e) coincidence experiment carried out in coplanar asymmetric geometry. The agreement with theoretical results obtained in the relativistic distorted wave Born approximation is favourable.

Dissociative attachment of electrons to electronically excited produced by optical pumping is studied. An entirely new application of the dissociative attachment process has been demonstrated to unravel couplings between various states of a molecule which has not been possible by optical spectroscopy. Also, identification of a negative-ion resonance state based on initial state selection of the neutral molecule is achieved for the first time without making any angular distribution measurements.

The energy dependencies of the differential cross sections for elastic scattering, positronium formation and direct single ionization in positron - argon collisions at are reported in the energy range 40 - 150 eV. Of particular interest is the region from 55 - 60 eV, where earlier measurements by Dou and co-workers found a drop in the elastic scattering cross section by a factor of two. This structure was tentatively attributed to a cross-channel coupling effect with an open inelastic scattering channel. No structure has been found in the three differential cross sections presented herein.

In this work we study two functions, f(r) and g(r), related to spatial generalizations of Kato's cusp condition for the one-electron density. In particular, we evaluate these functions using accurate correlated densities for low-lying S-states of two-electron atoms. These functions reveal the effects of the electron repulsion and are very sensitive to Coulomb correlations, showing interesting features in the comparison between singlet and triplet states. In addition to this, some relationships between and radial expectation values, previously found from the HF properties of these functions, are extended to more accurate descriptions of these states.

The centrifugal distortion effects for a molecule containing two symmetric internal rotors are formulated. The explicit expressions that relate the distortion constants to fundamental molecular parameters are given, which are applicable for analysing molecular structure from the parameters determined from a fit-to-observed spectrum or for calculating the centrifugal distortion constants from well known fundamental molecular parameters. The reduced torsion - rotation Hamiltonian is given for a molecule with two internal rotors with a symmetry plane which contains both internal rotation axes.

The definition of a norm for scattering states is reviewed particularly with respect to the method of complex scaling used to determine resonant states. The meaning of complex expectation values which result from this procedure is illustrated with a simple two-channel model. In a further example autoionizing states of helium are analysed in terms of complex expectation values which are shown to provide a sensitive tool to probe details of complicated dynamics. We also demonstrate how Lewis structures for resonances may be defined through complex expectation values.

Vacuum polarization induced by the external field of a nucleus in the ground state interacts with internal nuclear degrees of freedom. This coupling leads to a combined vacuum polarization - nuclear polarization contribution to the Lamb shift of bound electrons, which is qualitatively distinct from ordinary finite-size effects. It may instead be envisaged as a measurable radiative correction to the nuclear form factor due to intrinsic nuclear dynamics. The corresponding energy shift arising from collective nuclear monopole excitations is derived. Numerical results for the contribution to the Lamb shift of K- and L-shell electrons are presented for . Compared with the effect of ordinary nuclear polarization it turns out to be a considerable correction with opposite sign and thus reduces the total nuclear polarization contribution for this particular nucleus.

Conventional electron energy-loss spectra and two-dimensional electron scattering spectra are presented with unusual features. These features are accounted for by considering the trapping of the wave-packet due to a quantum mechanical analogue of friction. A detailed analysis of the spectra is given in terms of the vibrational eigenfunctions and eigen-energies of the available decay channels. This analysis supports the suggestion of a frictional mechanism. Such a mechanism could be quite general for polyatomic molecules, giving a new way to study non-Markovian aspects of the Schrödinger equation.

We report new high resolution observations of the 5p inner shell and of the doubly excited states of caesium covering the spectral region between 600 and 800 Å. The spectra were recorded in the first order of a 3 m spectrograph equipped with a 5000 line/mm holographic grating. Synchrotron radiation emitted by the 500 MeV electron accelerator served as the background source of continuum. More than 300 new levels of caesium are reported which are interpreted as inner-shell transitions

and doubly excited transitions

All the observed levels lie well above the first ionization threshold and are arranged into Rydberg series converging onto eight conspicuous limits. The interactions of the continuum with the discrete levels have been parametrized using the multichannel quantum defect theory.

A comparison is made of experimental far-wing profiles for the system with quantum calculations of thermally averaged free - free continua. Using input adiabatic potentials and transition moment functions from both ab initio and semi-empirical approaches, the comparison shows: (i) new ab initio potentials for reproduce the spectral positions of the observed rainbow satellites well; the height of the barrier predicted agrees with experiment to within , whereas its position is too large by ; (ii) potentials obtained with various semi-empirical methods reproduce the satellite structure qualitatively, but are generally too repulsive in the state at intermediate and large internuclear separations; (iii) transition moment functions for the asymptotically forbidden transitions reproduce, with different degrees of accuracy, the intensity of the red-wing satellite related to the potential extremum around . The vibrational energies in the and states calculated with the ab initio potentials reproduce to within a few those obtained from rotationally resolved band spectra reported in the literature.

The full width at half maximum (FWHM) of the 696.5 nm Ar I spectral line profiles was measured in the plasma temperature range from 13 500 K to 24 000 K and free electron density from 1.2 to . Results were obtained for this line emitted from the near-cathode region of an atmospheric pressure, 200 A arc burning in pure argon. Our results are compared with data existing up to now. In the electron density range of our experiment, the 696.543 nm Ar I line Stark FWHM is linearly proportional to the electron density. The observed dependence of the FWHM on the plasma temperature and electron density is in qualitative agreement with Griem's theory; however, the ratio of calculated and measured FWHM was equal to 1.55, for the dependence on electron density and temperature. The dependence of FWHM on electron density, obtained by weighted least squares fitting of all our data with the constraint that the curve passes through the origin, is well represented by the following equation: , with in nm and in , for the temperature .

Various total cross sections for electron scattering on the heteronuclear molecules NO, CO, and are calculated at energies from 50 eV onwards. The approximate additivity rule used earlier has been modified by separating - molecule interactions into short-range and long-range parts. The resulting modified additivity rule for - molecule cross sections is simple but more effective and it yields better accord with experiments, especially above 100 eV incident energy.

Elastic cross sections, integral and differential, are calculated for using a new set of codes which employ the separable representation of exchange interaction and the density functional (DFT) description of correlation forces. The entire problem is treated within a close-coupling (CC) formulation of the quantum scattering equations given in a single-centre-expansion (SCE) representation. Comparison with experimental data, over a rather broad range of collision energies from 0.1 eV up to 100 eV, provides one of the best overall agreements with experimental findings found thus far by calculations. The effect of correlation forces is discussed in comparison with the exact static-exchange (ESE) results provided by the present approach and by earlier calculations.

The same quality target wavefunction employed to carry out close-coupling (CC) scattering calculations in a previous paper by Gianturco and Stoecklin is now used to generate effective adiabatic potentials within a model treatment of the full interaction between the electron and the target molecule. A set of approximate diabatic potentials is then obtained by a series of sector diagonalizations.

The present study clearly shows the presence of three shape resonance states for three of the contributing irreducible representations (IR), , and , which the present model calculations locate rather close to the energies given by the earlier, exact CC calculations by Gianturco and Stoecklin.

The molecular nature of the resonant scattering states is further analysed in terms of their spatial structures and of the non-adiabatic couplings between potential curves induced by the anisotropy of the electron - molecule interactions. The interplay between the specific symmetry of the fixed-nuclei target geometry and the strength of the anisotropic bound - continuum interaction is shown to control, within each IR, the important features of the resonances.

We report results in the energy range 0.5 to 64 eV for positron scattering by ground state rubidium and caesium in and coupled-state approximations, respectively. The pattern of results is similar to analogous calculations made on potassium by McAlinden et al. Except at the lowest energies, positronium formation is found to be almost entirely into excited states, with there being a dramatic collapse of the Ps(1s) cross section in the case of caesium. Ps(n=2), Ps(n=3) and cross sections are predicted to be of comparable size. formation is estimated from the calculated Ps(n=3) results using the scaling rule. Good agreement in shape and magnitude is obtained with the recent lower bound measurements of total positronium formation in rubidium by Surdutovich et al, although there are differences in detail. Good agreement is also achieved with the total cross section measurements of Parikh et al on rubidium after these have been corrected for discrimination against forward elastic scattering and renormalized upwards by 5%. These total cross section measurements display a pronounced peak near 6 eV which is well reproduced by the present theory but is at variance with the earlier Rb(5s,5p,6s,6p,4d) calculations of McEachran et al which neglect positronium formation. This peak, which also appears in the calculations on potassium and caesium, is primarily associated with the maximum in the total positronium formation cross section. At low energies elastic scattering is dominant, this dominance being directly taken over by the resonance excitation of the atom with increasing impact energy. The next most important cross section is positronium formation, followed by excitation of the lowest atomic d-state. The and transitions in rubidium and the and transitions in caesium are of relatively minor importance.

Integrated Stokes parameters for the transitions in helium (388.9 nm) and neon (640.2 nm) and (626.7 nm) have been measured over the electron energy range from threshold to 300 eV after excitation by polarized electrons. The measured zero values of for both the He 388.9 and Ne 640.2 nm photons are consistent with the LS coupling of the upper states for each transition. Similarly, the measured non-zero values of for the 626.7 nm photons provide clear evidence of the breakdown of LS coupling for the Ne state due to the spin - orbit interaction of the atomic electrons.

Non-zero values of the Stokes parameter indicate strong electron exchange effects, particularly near threshold. Negative ion resonances show considerable effects on the polarizations of the subsequent radiations. The effect of different core configurations on the alignment of excited states is discussed.

The K-shell ionization cross sections of Ni and Cr by electron impact have been measured. In the measurements thin targets with a thick substrate were used. The influence of the reflected electrons from the substrate on the measurements has been corrected by means of a detailed calculation of electron transport. By comparison it is shown that the corrected measurement data of K-shell ionization cross section by electron impact are in good agreement with existing experimental data.

We report here on extensive calculations of radiative and autoionization transition rates, x-ray wavelengths and fluorescence yields of hollow argon atoms with configurations with m = 0 - 2 and n = 1 - 6. The calculations have been performed in the intermediate coupling scheme using the multiconfiguration Hartree - Fock atomic model.

Total cross sections for electron scattering by molecules in the energy range 300 - 5000 eV have been measured with experimental errors of about 3%. The method was based on a transmission-beam technique and a detailed error-source analysis is included. Present results have been compared with available experimental and theoretical data. The dependence on electron energy of the total cross sections obtained shows an asymptotic behaviour, in agreement with the Born - Bethe approximation. In addition, an analytical formula is provided to extrapolate total cross sections to higher energies.

Selection rules in nonlinear optics which are associated with time reversal (T) symmetry are reviewed, corrected and extended. For example, we find that both in fluids and in the nonresonant limit all nonlinear optical transitions between T-conjugate states are forbidden in Kramers systems. We show that further restrictions exist in the nonresonant limit with regard to the group symmetries of the associated effective transition operator. These and other rules generalize results found in Rayleigh and Raman scattering to nonlinear optical processes of arbitrary order. As in Judd - Ofelt theory, a careful treatment is required to obtain gauge-invariant conclusions since the relevant electronic operators in velocity and position gauges have opposite HT signatures.

We present a theory of third harmonic generation with XUV radiation through a double resonance, and examine the possibilities of enhancing the conversion efficiency by an additional laser field. We apply the theory to He where a photon of 21.22 eV leads to a one-photon resonance with the bound state , and a three-photon resonance with the autoionizing state .