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A sample of stable isotopes containing the masses 40, 42, 43 and 44 was stored under buffer-gas conditions in a Paul ion trap. After population of the metastable 3D levels by spontaneous decay of the laser-excited 4P state we observed laser-induced fluorescence when we scanned a laser across the different 3D - 4P fine-structure lines at 850, 854 and 866 nm. From the Doppler-limited lines of these transitions we deduced the isotope shifts, which are of the order of several GHz due to the large specific mass shift of the metastable 3D levels.

The ion-counting rate was investigated in a photoionization experiment using polarized and aligned oxygen atoms and linearly polarized light. The cross section of the valence photoionization was found to depend on the angle between the polarization axis of the ionizing synchrotron radiation and the axis of atomic alignment. A significant asymmetry in the ion rates corresponding to and was observed close to the resonance. This asymmetry is caused by j-dependent resonant behaviour of the and continuum channels.

The one- and two-electron photodetachment of is studied by using accurate Hylleraas initial-state and correlated 3C final-state wavefunctions in the high-energy range of 1 - 10 keV. The ratio between double- and single-electron ejection displays an approximately high-energy behaviour, . The influence of both the initial- and final-state correlations is demonstrated.

By means of a crossed-beams technique we have measured absolute cross sections for the charge-transfer reaction by coincident detection of the product ions and for CM energies between 4 and 200 keV. Estimates of angular differential cross sections are made from measured scattering distributions of reaction products at CM energies of 5.15 and 15.86 keV. The total cross sections for charge transfer are calculated by close coupling of a two-centre atomic basis.

The potential of applying (e, 2e) spectroscopy to metallic clusters is discussed within the jellium model. It is shown that such experiments would allow the investigation of the analogy between cluster and nuclear structure. Distinct diffractive patterns in the state-selected triply differential ionization cross section as well as in the elastic channel reflect the effective cluster radius. Differences between the triply differential cross section for atomic systems and those for jellium spheres are expected even if the valence electron states are not energetically resolved.

We report on the generation of harmonic radiation (in the 70 - 90 nm range) from clusters of Xe atoms formed in a gas jet. We find that the harmonic yield from the clusters exhibits an anomalous cubic scaling with backing pressure to the gas jet. This scaling is consistent with a cluster dipole moment resulting from collective oscillations of electrons around the central ions of the cluster. Using a nanosecond ultraviolet prepulse to dissociate the clusters, we have also attempted to compare harmonic yields from clusters with those produced from monatomic Xe, under otherwise identical conditions. Our results suggest that yields from clusters might exceed those from monomers by up to a factor of five.

It is shown that, when the n-electron continuum state is treated in LS-coupling, both the amplitude and the cross section for photo- and electron-impact ionization to such a state can be expressed as a linear combination of n-polar harmonics in the angles of emission of the n electrons. The analysis utilizes re-coupling techniques, largely avoiding redundant sums over products of Clebsch - Gordan coefficients. The Clebsch - Gordan series is generalized to n particles. Two selection rules are proved and the operation of such selection rules is illustrated by calculations of the differential cross section for (i) the complete photo-fragmentation of the lithium atom and (ii) the complete fragmentation of the helium atom under electron impact.

An algorithm for the recursive evaluation of expectation values for a given state of the relativistic one-electron atom with a point-like nucleus of charge number Z is proposed. In contrast to previous approaches, the present algorithm is based on only a single recurrence relation, which is a recurrence relation for a special type of generalized hypergeometric series. Two sequences of values of such series are generated. Finally, the members of these two sequences are assembled to yield the expectation values. The algorithm can be considered as a relativistic analogue of the Kramers - Pasternack recursion for expectation values for states of the non-relativistic hydrogen-like atom. As an application closed-form expressions for , were derived. In addition, numerical values for are given for some representative states of one-electron atoms with Z = 1, 80 and 137.

The wavelengths of the (n = 4 - 10) transitions in He-like O VII have been measured in laser-produced gas puff plasmas. The accuracy of the present measurements (1.5 - 3.0 mÅ) is a large improvement over previous results. In addition, the resonance transitions from the levels of He-like O VII were observed for the first time. Finally, the Rydberg series is used to determine the ground-state ionization energy of O VII. This method gives . These experimental results are compared with theoretical data calculated by different methods.

Photon-induced fluorescence spectroscopy was applied for the first time to investigate the anisotropy of VUV radiation emitted from Kr II satellite states with total angular momentum after the photoionization of the Kr atom by linearly polarized synchrotron radiation near the 4s threshold. The measurements showed that the sign of the alignment parameter is practically independent of the exciting-photon energy for the ionic state with , whereas for the states with the alignment parameter changes sign with varying energy. Simple formulae connecting the angular distribution parameter and the alignment parameter with the kinematics of our experiment and with quantities describing the dynamics of the photoionization were derived in closed form. Partial and total cross sections for the photoionization of 4p, 4s and several satellite levels were calculated taking into account many-electron correlations and doubly-excited states. For the satellite states with the dependence of the angular distribution parameter on the exciting-photon energy was calculated. Good overall agreement between theory and experiment was obtained.

Absolute photoionization cross section profiles of Zn have been calculated at TDLDA and LDA level, employing a very accurate B-spline basis set and the modified Sternheimer approach. The van Leeuwen - Baerends exchange correlation potential has been used, since its correct asymptotic behaviour is able to support virtual states and describe core-excited resonances. A comparison with available theoretical and experimental data has been performed when possible. The present method has been proven to be robust to analyse wide photon energy regions (from threshold up to 200 eV) and discuss the various shapes of one-electron resonances.

Relative photoionization cross sections for and produced by the Auger decay of a 1s hole in atomic oxygen were measured by using synchrotron radiation between 525 and 553 eV. Energies and quantum defects of the members of the two Rydberg series converging to and ionization thresholds were determined. In addition, the and ionization thresholds were calculated from the two Rydberg series. The 182 meV resolution of the monochromator allowed a detailed study over both thresholds revealing evidence for post-collision interaction and allowing a comparison of the ionization continuum above both and thresholds with that of the ionization continuum above the Ar edges. This comparison indicates that the lifetimes of the Ar(2p) and O(1s) hole states are approximately the same.

The first detailed observations of the multielectron-dissociative-ionization of sulphur hexafluoride have been made using 60 fs laser pulses at 750 nm and . The fragmentation pattern displays features similar to those found in experiments with diatomic and triatomic molecules, such as a tendency towards symmetrically dissociative channels and fragment kinetic energies which for all channels are consistent with a Coulomb explosion at a single, critical, internuclear separation.

For six quartet states of the ion the potential energy functions, the electronic transition moments and the spectroscopic constants have been calculated using an internally contracted multireference configuration-interaction approach. The harmonic wavenumbers are expected to be accurate to within , the equilibrium distances to within 0.01 Å, and the electronic excitation energies to within . For several electronic transitions between the quartet states, the variations of the transition moments with the internuclear distance are found to be very large, excluding the use of the Franck - Condon approximation. In numerous studies the predissociation mechanism of the state of has been discussed. The present work provides a new explanation of this process via a state.

Application of the position-sensitive photoelectron - photoion - photoion (PSD-PEPIPICO) method to dissociative double-ionization processes in and at photon energies of 40.8 and 48.4 eV provides measurements of total kinetic energy releases and complete descriptions of the kinematics for three-body dissociations of the type . Comparisons of experimental observations with predictions from sequential and concerted bond breaking mechanisms suggest a unified model in which both bonds break on a time scale of , and electron ejection may be either faster (direct ionization) or competitive (indirect) on this time scale.

Quantum calculations are carried out for the scattering of electrons from sulphur dioxide molecules in the energy range from about 1.0 eV up to 30 eV. Integral and differential cross sections are obtained for the elastic process and for the excitation processes of the target asymmetric rotor into various final rotational levels. Comparison with experimental data, both of the momentum transfer cross sections and the angular distributions, indicates rather good agreement between measured and computed values over the whole range of the examined energies, while the integral elastic cross sections (rotationally summed) show disagreement at low energy with the experimental estimates. Born corrections to the calculations are also shown to provide the correct behaviour of the forward scattering angular distributions and a reliable guide to experimental extrapolation procedures.

Double translational energy spectroscopy (DTES) has been used to study state-selective one-electron capture by pure ground-state ions in collisions with He, Ne and Ar at energies within the range 1.75 - 8.0 keV. The main collision channels have been identified and their relative importance has been assessed quantitatively. These measurements avoid the ambiguities inherent in the interpretation of previous TES measurements carried out using primary ion beams containing unknown fractions of metastable ions. In the absence of any detailed theoretical studies, we have calculated Landau - Zener reaction windows for these processes and shown that they accommodate the observed collision channels with limited success.

The electronic redistribution of an ion or atom induced by a sudden recoil of the nucleus occurring during the emission or capture of a neutral particle is theoretically investigated. For one-electron systems, analytical expressions are derived for the electronic transition probabilities to bound and continuum states. The quality of a B-spline basis set approach is evaluated from a detailed comparison with the analytical results. This numerical approach is then used to study the dynamics of two-electron systems (neutral He and ) using correlated wavefunctions for both the target and daughter ions. The total transition probabilities to discrete states, autoionizing states and direct single- and double-ionization probabilities are calculated from the pseudospectra. Sum rules for transition probabilities involving an initial bound state and a complete final series are discussed.

The static-exchange model has been employed to investigate Ps - He atom scattering at low and intermediate energies using Hylleraas and Hartree - Fock wavefunctions for the He atom. The total elastic cross sections and s-, p- and d-wave phase shifts are reported and our present phase shifts are compared with those existing predictions. The effect of electron exchange is found to be significant at low and medium energies and the scattering parameters are found to be sensitive to the choice of wavefunction.

The R-matrix method is employed to calculate the total cross section for electron-impact excitation of the and transitions in neutral oxygen. Nineteen spectroscopic states together with 12 pseudo-states are included in the R-matrix wavefunction expansion. The pseudo-states are chosen to ensure that the static dipole polarizability is fully included for all of the three transition states. Although the present calculation is the largest ever performed for neutral oxygen, discrepancies are found for all three transitions between the present theoretical results and experimental data. It is essential that further experiments be performed.

We performed a 31-state semi-relativistic Breit - Pauli R-matrix calculation for electron-impact excitation of neon. The calculated energy levels were found to be very accurate. Results are presented for angle-integrated cross sections in the energy range from threshold to 30 eV for the and states, together with some selected differential cross sections. These predictions are compared with experimental and other theoretical data. A strong dependence of the near-threshold resonance structure and the cross section results on the details of the structure calculation is found.

The Breit interaction between the incident and atomic K-shell electrons is introduced in the calculation of the triple differential cross section of the K-shell ionization of heavy atoms by very energetic electrons . As the total spin is not conserved, all possible spin states of the three-electron system in the initial and final states are taken into account. The results obtained by a Dirac plane wave description of the continuum electrons and a relativistic helium-like bound state wavefunction, show a net improvement with respect to our recent calculations employing the same description without the Breit interaction.

We have investigated the low-energy (0 - 10 eV) electron attachment on and , using an improved energy resolution close to zero incident energy (0.030 - 0.060 eV). We have found evidence of a zero-energy attachment process (within 0.030 eV), for both and , indicating that a s-wave capture process is present at low energy for these two molecules. For , although in contrast with symmetry arguments, and with previous free-electron attachment experiments, this result is in full agreement with the conclusions presented by several groups, using the Rydberg atom energy transfer technique to investigate the low-energy attachment in these systems. Energy positions of several resonant states, appearing between 0.5 and 5 eV in the ion yield versus electron energy spectrum, are clearly established in and , and assignments are discussed. Lifetime measurements are reported for the fullerene anions. In the incident energy range 7 - 11 eV the lifetime of was found to vary from 500 to , respectively, in agreement with previous measurements. The thermionic model for detachment does not seem to describe the values obtained, and their observed variation in this energy range. The lifetime of is longer and could not be measured precisely with our time-of-flight conditions. An estimation of has been made at . Below the lifetime of was larger than and could not be measured precisely.

Positronium scattering off a hydrogen target has been studied employing a three-state positronium model close-coupling approximation (CCA) with and without electron exchange. Elastic, excitation and quenching cross sections are reported at low and medium energies. The effect of electron exchange is found to be significant at low energies. The ratio of quenching to the total cross section (the conversion ratio) approaches the value of 0.25 with increase of energy, as expected.

Information about the energy distribution of hot electrons generated by the interaction of short-pulse (0.7 ps pulse duration at laser wavelength) and high-intensity laser beams with solid targets is derived from measurements of the spectral and angular distribution of hard x-rays with energies above 10 keV. The measurements were carried out with thermoluminescence dosemeters and a nine-channel spectrometer. A comparison of experimental data and model calculations shows that under the conditions of our experiment the hot-electron distribution deviates significantly from a Maxwell - Boltzmann shape with a single-electron temperature parameter.