Table of contents

The autoionizing decay routes of doubly excited, , Rydberg states in helium have been investigated using two-dimensional photoelectron spectroscopy. Constant ionic state spectra for the (N = 4,5 and 6) states have been extracted from two-dimensional photoelectron spectra obtained in the photon energy region 75.5 - 79 eV. A preference was observed for autoionizing decays in which the change in the principal quantum number, N, of the electron which remains bound was minimized. This is in accordance with propensity rules determined from theoretical studies. The energies of the lower members of the most prominent Rydberg series converging on each of the (N = 5) and (N = 6) thresholds have been determined.

Mass-resolved two-photon ionization spectra of ArXe and KrXe have been recorded using both linearly and circularly polarized fundamental laser beams. From the resultant intensity changes, the published excited-state electronic symmetry assignments for the band systems of ArXe in the region of the resonance line are confirmed, while those for KrXe are shown to be incorrect. The spectroscopic constants of the newly assigned KrXe transition in this region can be understood if the adiabatic excited state involved is an admixture of diabatic states that dissociate to and .

We present high-resolution measurements of the angular distributions of , and fragments from in intense, polarized, picosecond laser fields. Distinctly anisotropic angular distributions are measured in all cases. Some evidence is obtained of lobe-like structure in these distributions; these are indicative of pendular motion of molecules aligned along the polarization axis of the laser field. Results of classical calculations of the dissociation dynamics on an excited state potential energy surface are presented which qualitatively reproduce the global features of the measured angular distributions.

Rovibronic cross sections for the excitation of various rotational and vibrational levels of by electron impact for scattering energies in the range of 1 - 4 eV have been calculated in the framework of the R-matrix approach. The new non-adiabatic phase matrix method (NADP) is employed for the nuclear vibrations. The results are compared with experiment and with other theoretical data and are found to be in excellent agreement with measurements in the energy region of the shape resonance.

The spatial image of the spectrum of Xe(M) and Xe(L) emissions produced by the development of stable channelled propagation of intense femtosecond ( fs) 248 nm pulses propagating through a gas-phase Xe cluster target has been obtained. Analysis (i) shows that the observed channel length exceeds 20 Rayleigh ranges, (ii) reveals enhanced emission from the channel by very highly excited species such as Xe ions having double 2p-vacancies Å), (iii) gives spectral evidence for a substantial departure of the excitation from conditions of thermal equilibrium in the channel, (iv) confirms earlier studies which indicated the development of intensities of in the channel, and (v) provides additional evidence for the role of an anomolously strong multiphoton coupling in the generation of the observed X-ray emission.

We report new calculations of the energy differences between the and levels of the , and ions; here n = 4, 5 and 6 for , and , respectively. The electron affinities of Ca, Ba and Sr have also been calculated. All estimates were obtained by performing a series of multiconfiguration Dirac - Fock calculations for the neutral atoms and their anions. Correlations among valence electrons in both the ground and excited states were included in these calculations. Our estimate of the fine structure of the ground state of is in good agreement with recent measurements. Our estimates of the electron affinities of Ca, Sr and Ba are in reasonable agreement with experiment.

A simple local ensemble exchange potential is proposed in the fractional occupation approach. It is emphasized that the excitation energies cannot generally be calculated as a difference of the one-electron energies. The extra term is determined.

A quasiclassical theory is presented for scattering of atoms in a spinless P state by a spherical target. The scattering amplitudes can be expressed analytically as a function of a few parameters which are not only the phaseshifts for elastic scattering along the and molecular potentials, but also the locking angles and the slipping probabilities. Interference effects, for one scattering angle, occur between contributions of classical trajectories corresponding to different potentials. The locking angle appears explicitly in the amplitude of the high-frequency oscillations. It turns out that this type of interference survives the statistical averaging over initial substates, resulting in an oscillating pattern of the differential cross section for scattering of unpolarized atoms. It is shown that the existence of the locking phenomenon is a necessary condition to the observation of such an effect.

The quasiclassical analysis of the differential cross section for the scattering of an excited non-polarized sodium atom on neon is presented. The oscillatory behaviour of the cross section observed experimentally and calculated numerically within the close coupling approach is interpreted as being due to the superposition of two interference patterns, one of them for waves scattered by the repulsive and attractive parts of the potential, the other one for waves scattered by the attractive part of the potential and repulsive potential. The latter appears only when the long-range Coriolis coupling is taken into account. It is argued that previous interpretations of the high-frequency oscillations misunderstood the underlying mechanism, which is a clear manifestation of the locking phenomenon.

The Stark widths of 13 Ar III spectral lines have been measured at an electron density of and electron temperature of 38 000 K in a linear pulsed arc plasma discharge containing an argon - helium mixture. The measured values were compared to the existing calculated values based on semiclassical and semiempirical approximations.

We have used a light-induced drift (LID) experiment to determine ratios of cross sections for diffusion of potassium in the excited , and the ground levels in five rare gases. The measured ratios are combined with the ground-state cross sections and statistically averaged excited cross sections, available from a previously reported light induced diffusive pulling experiment, to obtain the absolute cross sections for individual fine structure levels. We also report calculated cross sections based on available potential curves and a coupled-channel theory. Rough qualitative agreement is generally found between the absolute cross sections inferred from experiment and the theoretical values. The light-induced drift experiments, however, measure ratios of cross section differences that are highly sensitive to the potential curves. It is found that the available potential curves are not adequate for predicting these measured ratios.

A model photodetachment is considered in a non-perturbative approach which takes into account an infinite number of virtual photon and electron states. Closed analytical formulae for the photoelectron spectrum are obtained which result from some properties of particular infinite band matrices. The formulae give account of the most characteristic features of the above-threshold detachment. A numerical example is given.

The effect of monopole rearrangement of electron shells, correlational rarefication of electron shells density, vacuum correlations, and virtual processes of one-photon multiple excitation/ionization on the differential cross section of anomalous elastic scattering of non-polarized X-ray radiation by the Ar atom near its 1s-shell ionization threshold are studied. The results of the study are predictions.

We have completed both experimental and theoretical studies of the photoionization from the multiplet in barium, in the range 350 - 402 nm, by considering the , and states, after the state which we studied in Carré et al. We have measured differential cross sections from below to above the threshold, where a number of ; J and ; J resonances are excited, for different cases of laser polarizations. Angle-resolved partial cross sections critically depend on both modulus and phase of the dipole moments. The experimental results are compared with eigenchannel R-matrix calculations. The successful comparison performed over a large energy range strengthens the surmise that a good representation of states and final continua has been worked out in the frame of R-matrix formalism.

Angle-resolved photoelectron spectra encompassing the and the states of sulphur hexafluoride have been recorded in the 20 - 22 eV photon energy range using synchrotron radiation. The envelope of the photoelectron band displays a complicated structure which varies significantly both as a function of excitation energy and as a function of electron ejection angle. The data for the state have been analysed by dividing the band into eight regions, specified by their binding energies. The results show that, as the photon energy is varied, the branching ratio for each region exhibits an enhancement at an electron kinetic energy of about 2.5 eV. The photoelectron angular distributions also vary significantly across the band. Various interpretations for this resonant behaviour are discussed.

A Doppler free kinetic energy release spectrum of has been recorded at a line width (FWHM) of 40 - 60 meV using a coincidence time-of-flight technique. Vibrational structure has been resolved in the W , B , 1 , 1 , B and 1 electronic states. Complete active space self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) calculations have been performed to predict vibrational levels, Franck - Condon factors and dissociation channels. For the B and 1 states, spin - orbit interaction with the 1 state leads to predissociation into the ground-state ionic pair . Spin - orbit interaction with the repulsive 1 and C states accounts for the dissociation of the B , 1 and 1 electronic states into . The W and 1 states probably dissociate by tunnelling into the ion pair through potential barriers which separate these states from the dissociation continuum.

Single and multiple electron capture processes are studied in slow (8 - 16 keV) collisions between and molecular nitrogen. The contributions of dissociative and non-dissociative channels are determined as well as the kinetic energies of the fragments of the (q = 1 - 6) ions. From the kinetic energies of the fragments dissociation energies of the molecular ion are deduced.

Under the assumption of isotropic dissociation the following results are obtained: at most 7% of single-electron capture leads to dissociation of into and at least 90% of the double-electron capture results in fragmentation into .

If more than two electrons are captured, all -ions dissociate into . For even values of q we observe r = s, otherwise ; i.e. the maximum difference between r and s is 1. A charge distribution closest to the most symmetric one is preferred.

The measured dissociation energies are smaller than those expected from the model of Coulomb explosion. Therefore, potential curves for the and molecular ions have been calculated, and dissociation energies deduced from these potential curves have been compared to the measured ones. In both cases dissociation from the lowest molecular state of the multicharged molecule seems most probable.

The partial electron impact ionization cross section for metastable neon, , has been measured in the energy range from threshold to 200 eV. The crossed-beam experiment used a fast neutral target (both ground state and ground state plus metastable) formed by charge transfer between an 800 eV neon ion beam and neon gas or sodium vapour. The ionization cross section for the and metastable components, which are assumed to be equal, was placed on an absolute scale by utilizing the known ground-state cross section and assuming that the beam fractions of the mixed state were statistically distributed. The results are compared with previous experimental and theoretical work.

Dissociative electron attachment in effusive beams of has been studied in the incident electron energy range 0 - 11 eV. Two processes are observed at 2.4 and around 4.55 eV. From the ion kinetic energy and angular distributions, the symmetries of the dissociative resonant states have been determined as and . In the sulphur vapour formation occurs at zero energy and around 4 and 7.5 eV, in addition to the processes.

Coupled-state calculations in a K(4s,4p,5s,5p,3d) + Ps(1s,2s,2p,3s,3p,3d) approximation are reported for positron scattering by ground-state potassium in the energy range 0.5 - 60.0 eV. Comparison is made with the earlier work of Hewitt and co-workers in a K(4s,4p,5s,5p) + Ps(1s,2s,2p) approximation. For the first time cross sections for positronium formation in n = 3 states are obtained. We support the conclusion of Hewitt et al that above about 4 eV positronium formation is mainly into excited states, although we disagree significantly on the distribution of the excited state population. Fairly good agreement is obtained with the measurements of Zhou and co-workers for total positronium formation when scaling is used to estimate the amount of positronium in states with . Our cross sections for elastic scattering, 4s - 4p, 4s - 5s and 4s - 5p excitation of potassium are in relatively good agreement with those of Hewitt and co-workers but our calculations also emphasize the importance of the 4s - 3d excitation which is omitted in their approximation. Atom excitation to the n = 5 level is found to be a relatively minor process. Good agreement is obtained with the total cross section measurements of Kwan and co-workers and Parikh and co-workers when these are normalized upwards by 10% and corrections made for near forward elastic scattering. This upward normalization lies within the quoted overall 21% error on the measurements. Below 4 eV the calculated total cross section is dominated by elastic scattering while at high energies 4s - 4p excitation is the main contributor. In the vicinity of 6 eV, and consistent with the measurements of Parikh and co-workers, the total cross section displays a pronounced peak which derives largely, although not totally, from positronium formation. Positronium formation is not important above about 30 eV.

We have calculated the excitation spectra of a single electron in the excited state of a three-level atom interacting with two off-resonant laser fields at high intensities. We have also considered the process where one laser photon is absorbed by the ground state, as well as a Raman-type process where one photon of one of the laser fields is emitted while another photon of the other laser field is absorbed simultaneously by the second excited state of the atom. The net excitation spectra of the three processes in question consist of two peaks having always positive intensities indicating induced absorption (attenuation) and a third peak which has positive, zero and negative intensities for values of the Rabi frequencies , and , respectively. When , the Raman-type process prevails over the other two, and the resulting peak has a negative intensity indicating that significant amplification is likely to occur at the corresponding frequency.

The quantum theory is applied for the accurate computation of high-frequency (up to ) spectral intensities in Ar - Ar collision-induced light scattering (CILS) processes at room temperature. Numerically, this becomes possible by means of the two-point Fox - Goodwin integrator, propagating on a grid the ratio of the wavefunction at adjacent points. Wavefunctions are normalized in a handy way which is based on the notion of the local wavenumber. Various potentials and anisotropy models are tested and compared. For frequencies exceeding our results show significant deviations as compared to the theoretical predictions of the classical theory. When the self-consistent field (SCF) anisotropy is applied, a clear tendency of the quantum calculation to approach recent experimental data is observed.