Table of contents

The relativistic Breit Hamiltonian between electrons is transformed into an effective vector potential for the ith electron, having the structure of a recoil-corrected hyperfine operator. Apart from a small three-body operator, the Dirac - Breit equation is now easier to apply to relativistic magnetic properties of complex systems.

Helium double photo-ionization is studied by a novel coincidence technique which employs time-of-flight spectrometers. Using this technique it is possible to collect simultaneously all the electron pairs, with different energy sharing, emitted by the absorption of a single energetic incident photon. The measurements, in a configuration where the two electrons emerge at relative angle, provide the more complete information on the contribution of the ungerade amplitude to the triple differential cross section and allow the establishment of a relative scale for the full coincident angular distribution measured by other experiments at the same photon energies, but only for a few selected energy-sharing conditions.

The photoabsorption spectra of neon and argon in the energy region of the and doubly excited states have been studied. The resolution and flux of the new VUV undulator beam line at ELETTRA allowed the resolution of new features for Ne and the first observation for Ar. A comparison of the measured photoabsorption spectra and of the threshold photoelectron spectra in the energy region of the and satellite states provides information on the decay routes of the inner-shell doubly excited states.

Using the atomic beam technique and monochromatized synchrotron radiation, the photoion yield spectra of and in the region of 5p excitation between 45 eV and 85 eV were observed. The results are compared with a previous study using the dual laser plasma technique and with the theoretical photoionization cross section calculated in the framework of Fano resonances interacting with many continua.

The average nuclear geometry of molecules during the laser-induced multielectronic ionization and multifragmentation is probed by Coulomb explosion into multicharged atomic fragments. An evolution from the initial bent structure to a linear configuration is observed as the number of removed electrons increases from 3 to 11 in the laser intensity range. This observation shows that the multiple ionization does not follow a simple sequential pattern from the less bound to the more tightly bound electrons of the molecule.

Single, double and triple ionization cross sections of 10 MeV projectiles colliding with He atoms have been measured. This was achieved thanks to a new experimental set-up allowing for a complete collection and unambiguous identification (in particular with respect to their charge) of all the fragments emitted by the dissociative species (Q = 2, 3, 4). As a first attempt to understand the mechanisms of ionization in the cluster we use these results, together with those we obtained in the identical velocity 2 MeV system, for testing the predictions of an {independent atoms} - atom collision calculation within the impact parameter formalism and using the independent-electron approximation. The model is found to be surprisingly good in predicting the cluster - atom results when appropriate (i.e. able to reproduce the atom - atom case) impact parameter dependent ionization probabilities are used.

Longitudinal electron momentum distributions in single ionization of helium by proton and antiproton impact are calculated using the CDW - EIS model. The distributions are studied at intermediate and high energies. The present results are in good qualitative agreement with CTMC simulations at 100 keV impact energy. Large differences appear in comparison to the predictions from the saddle-point emission theory.

Results from an R-matrix with pseudo-states (RMPS) calculation are presented for electron impact excitation of the transitions in beryllium for incident energies from threshold up to 5 Ryd. At energies above the ionization threshold (0.685 Ryd), the representation of target continuum states is found to be crucial for accurate results to be obtained. A significant reduction, of up to a factor of two compared to the results of Fon et al, is found for the cross sections in this energy range. This result has important implications regarding the use of currently available e - Be cross section data for plasma diagnostics.

High-order optical nonlinearities of -toluene solutions were observed employing the transient degenerate four-wave mixing technique using 0.5 ps laser pulses at 497 nm. The intensity and fullerene concentration dependence of fifth- and seventh-order signals were investigated. The effect of the nonlinear absorption on the nonlinear optical response is examined. The ratios and are determined to be of the order of .

The variational finite-basis-set approach in a minimax formulation to the Dirac equation is considered. Application to the one-electron Dirac - Coulomb problem, and a test of the minimax solutions are presented. It is demonstrated that although the minimax technique does not guarantee the occurrence of an upper bound on the relativistic Rayleigh quotient, a stationarity principle alone eliminates the appearance of spurious roots in the variational spectrum for a general value of the Dirac symmetry number. Some computational issues and extension to the two-electron Dirac - Coulomb problem are outlined.

A procedure for analysing resonances in atomic and molecular collision theory is introduced, which exploits the analytic properties of R-matrix theory to obtain the energy derivative of the reactance matrix, without assuming a pure Coulomb potential at large distances. The QB method defines matrices and in terms of asymptotic solutions, the R-matrix and energy derivatives, such that , from which eigenphase gradients of the -matrix can be obtained. Resonance positions are defined at the points of maximum gradient; resonance widths are related to the inverse of the eigenphase gradients. Resonance properties such as identifications are discussed.

The QB method is illustrated for some overlapping resonances in between the and ionization thresholds. It is then tested for accuracy against recent experimental measurements of positions and widths for doubly excited states of He between the n = 2 and n = 3 ionization thresholds, and for positions, widths and profiles of resonances in Li above the first inner-shell threshold.

Ab initio adiabatic and diabatic symmetric stretch/bend potentials of Rydberg states of have been employed for calculations of corresponding quantum-defect functions. These are in turn employed for the generation of the potential-energy surfaces of higher-lying Rydberg states, for the calculation of vibrational levels of the linear state and for the calculation of autoionization line widths in the series. The results are generally in good agreement with existing experimental information.

Photoelectron Eu 4d-shell spectra registered in coincidence with photoions , and produced by vacancy cascades are calculated using the yields of ions obtained by straightforward construction of de-excitation trees. The significant difference of the coincidence spectra from the normal-mode-detection spectrum is explained by the fact that for the high-spin state the Auger 4d - 4f4f decays are forbidden. A similar situation is expected to be seen for the atoms with ground state configurations , n<7.

Quantum close-coupled calculations of the collisional self-broadening of neon lines involving transitions whose lower level is either a metastable or intercombination level, and , and upper level is , or are presented for temperatures of 77 and 273 K. These calculations are based on the quantum-mechanical impact theory of Baranger in which the interatomic interaction is represented by adiabatic molecular potentials calculated using model potentials for the electron - atom and atomic core - core interactions. Semiclassical calculations for lines involving transitions between these lower levels and either or at temperatures of 77 and 273 K are given and compared with the recent quantum close-coupled calculations of these lines by Leo et al (1995 28 4449 - 58). Also reported are parametric fits to the temperature dependence of the line widths and shifts in the quantal model for transitions from the and 3p levels to the metastable and intercombination levels for the temperature range 70 - 370 K.

Stark lineshapes of lasing radiation in the X - UV range in aluminium and sulphur plasmas are calculated with the help of a quick code. Intensities are then obtained from the radiative-transfer equation in the nonsaturated regime, and, in the same way as in the experiments, a gain coefficient is deduced from the variation with length of the intensity integrated over wavelength. In the cases investigated, many lasing transitions contribute to the x-ray beam, and the spectral gain as well as the emissivity have to be suitably defined. For 10.57 and 15.47 nm radiation in and 20.65 nm radiation in , the gain coefficients thus calculated are much smaller than those obtained with symmetrical Doppler profile functions. When the amplification length is increased each lineshape becomes more and more homogeneous and tends towards the shape of the most intense transition. In other words, as the beam is amplified the individual transitions, except the most intense one, contribute less and less to the output intensity. The effect of the x-ray beam on the populations of the lasing levels is investigated and a rate is calculated which is found to be much smaller than the collisional and spontaneous-emission rates, but increases rapidly with the gain - length product.

We consider the problem of a positronium atom interacting with one or two laser fields with the goal of extending its lifetime. Ps annihilates essentially only from S states, so the lasers are used to populate the excited P and D states. The coherent trapping phenomenon yields a state which can be arranged to have no S components and is therefore stable against annihilation. Additional phenomena such as resonance, fluorescence and ionization repopulate the S state and destroy the absolute stability. We find that the net result is an optimal extention of the lifetime by about a factor of 2 for the three-state triplet case and 20 for the three-state singlet case.

We calculate the quasienergies and wavefunctions of the hydrogen molecular ion perturbed by a strong external electric field. Results are obtained for static-field ionization rates of the lowest dressed states. The nonlinear Stark shifts are also calculated for weak and strong fields for a range of internuclear separations. Charge and current densities are obtained and used to explain the ionization resonance structure. The ionization currents are found to be very sensitive to field strength and internuclear distance. There is evidence of critical distances that indicate the validity of Coulomb explosion models and electron localization. However, for this system, the results do not agree well with simple over-barrier models of electron release. Multiphoton ionization rates of low-frequency intense laser fields are estimated by cycle averaging.

Photoelectron spectra and partial cross sections of ground-state and laser-excited Cr atoms in the range of the excitations are presented. The experimental spectra are compared with results obtained by calculations within the spin-polarized random-phase approximation with exchange (SP RPAE), in which the dynamic relaxation upon core hole creation is taken into account.

An anisotropic magneto-optical trap is studied as the potential source of a continuous beam of laser-cooled atoms. Capture efficiency and cloud temperature are measured experimentally and found to be lower than in an isotropic situation with the same cooling parameters. Intensity imbalance and static magnetic field are investigated as possible extraction mechanisms. While only small drift velocities are induced by means of intensity imbalance, velocities of tens of are observed with a static magnetic field; the shape of the observed two-peaked velocity distributions is related to the average depth of local light-shift-induced potential wells.

A two-centre coupled-Sturmian-pseudostate approach is employed to calculate single electron transfer, target excitation and total ionization cross sections for the (3s) collision system at 2 - impact energies. The Sturmian basis set, together with an analytic Hartree - Fock potential for the Na core, are chosen carefully so that all three channels (transfer, excitation and ionization) are included in the coupled scheme. The present state-selective and total charge transfer cross sections compare fairly well with experimental data, as well as with previous theoretical results, particularly below . The target excitation cross sections for the sodium 3p and 4d states are in very good agreement with the available measurements, while significant discrepancy exists between theory and experiment for the excitation of the Na (4p) state.

The boundary conditions and the conservation conditions are discussed for - H ionization within the framework of the s-wave monopole model and a scheme is described for calculating the energy-differential cross section. Illustrative calculations, using a boundary collocation method, are presented to show the apparent behaviour of the energy differential and total cross sections for s-wave scattering. Comparisons are made with the results of other calculations within the framework of the model.

We have measured the alignment of after simultaneous ionization and excitation by electron impact via the non-isotropic angular distribution of photons of the decay . The range of incident electron energy was between and . Within experimental errors we obtained a constant value of except in the range - where the alignment has values between -1.2 and -1.3. These larger negative values can be explained by the decay of two-electron excitations into . The alignment value of -0.92(14) indicates that in the investigated range of energy not only two-electron partial waves with L = 0 but also with L>0 contribute to the ionization-excitation cross section. Our value of -0.92(14) agrees reasonably well with a nearly constant theoretical value of obtained by a combined DWBA + R-matrix calculation.

The basic integral equation of the non-local theory of inelastic resonant electron - molecule collisions is recast in the form of a system of differential equations for a set of the electron resonance pseudostates. These pseudostates are related to the quadrature approximation for the spectral integral over the interval of the electron continuum with some set of N complex-valued resonance energies and residues . The system of differential equations is solved with the split propagation method for treatment of initial wavepacket time evolution. This approach is not restricted (in principle) by the dimensionality of the configurational space and by the form or representation of the basic characterisitics of the system. The method is applied for calculation of the inelastic cross sections for - HCl and DCl collisions. The results obtained are in excellent agreement with the previous calculations. In contrast with previous investigations of the problem, the present approach allows one to consider the process as Markovian. This feature is very important for the assessment of non-local theory applicability for the investigation of inelastic electron - polyatomic-molecule collisions.

A comprehensive study of the elastic and vibrationally inelastic scattering of low-energy electrons on ozone, as well as of the dissociative electron attachment has been carried out. Absolute differential cross sections (DCSs) for the elastic and vibrationally inelastic scattering are presented as a function of electron energy and of scattering angle. The elastic data are compared to existing experimental and theoretical work. Two shape resonances are observed in the vibrational DCSs, peaking at 4.2 and 6.6 eV. We used the Koopmans theorem to assign them as and , with temporary capture of the incident electron into orbitals which are very strongly antibonding with respect to the O - O bond length. Both resonances excite not only the totally symmetric vibrations and , but also the antisymmetric stretch vibration , and we assign the excitation of the latter to vibronic coupling between the resonances. Dissociative electron attachment below 5 eV yields and with considerable intensity. formation through three-body attachment or ion - molecule reactions is also observed. Four resonant peaks are observed in the dissociative attachment spectra, at 0.4 eV, 1.3 eV, 3.2 eV, and 7.5 eV. The ions are formed with moderate kinetic energies for the first, nearly zero kinetic energy for the second, and large kinetic energies for the third resonance. The resonances in the dissociative attachment spectra are assigned as the `high energy tail' of the ground state of , a core excited resonance, the shape resonance, and a Feshbach resonance, respectively. The shape resonance appears at a lower energy in the dissociative attachment channel than in the vibrational excitation channel because of kinetic shift. Angular distribution and approximate absolute values are given for the formation at 3.2 eV.

Dissociative electron attachment (DEA) in ozone has been explored for incident electron energies between 0 and 10 eV. Three resonant regions are observed centred around 1.4, 3.5 and 7.5 eV, respectively. Only the first of these had been reported prior to this work. For the 3.5 eV resonance, a single anionic state appears to be involved; decay is rapid, along the asymmetric stretch coordinate to and , each in its ground electronic state. Within the 7.5 eV resonance region, both two-body and three-body dissociations are active.

The frequency of optically excited plasmon oscillations in a metal cluster is studied as a function of cluster size. The model is based on the fact that a sphere filled with free-electron gas forms a cavity with size-dependent eigenfrequencies of the optically excitable TM plasmons. Excitation of plasmon resonances by light in clusters with appropriate radius, and the size-dependent radiative damping of the excited oscillations are studied. Dependence of the frequency of plasmon oscillations and of the dumping on the relaxation rate of electron motion in the metal is discussed. Size-dependent total scattering cross section resulting from the Mie theory is analysed as a function of the cluster radius. The contributions to the scattering of the secondary fields due to excitation of collective electron oscillations and eddy currents are considered. The study is illustrated by the example of sodium clusters interacting with the light field with wavelength of 488 nm. It is shown that the collective motion of free electrons can be excited when clusters approach the radii of 55 nm (l=1), 118 nm (l=2),... respectively, which are the cluster radii available in the experiment on light-induced clusters.

We present a systematic study of the angular distributions of high-order harmonics generated with a femtosecond laser. We investigate the influence of different parameters, namely laser intensity, nonlinear order, nature of the gas and position of the laser focus relative to the generating medium. We show that when the laser is focused before the atomic medium, harmonics with regular spatial profiles can be generated with reasonable conversion efficiency. Their divergence does not depend directly on the nonlinear order, the intensity or even the nature of the generating gas, but rather on the region of the spectrum the considered harmonic belongs to, which is determined by the combination of the three preceding elements. When the focus is drawn closer to the medium, the distributions get increasingly distorted, becoming annular with a significant divergence for a focus right into - or after - the jet. We perform numerical simulations of the angular distributions. The simulated profiles reproduce remarkably well the experimental trends and are thus used to interpret them.

We applied the state-specific expansion approach (SSEA) to the solution of the time-dependent Schrödinger equation describing the interaction of H with laser pulses of and peak intensities , , and , and computed ionization rates and high-order harmonic spectra. For the first three cases, our results are compared with those of Krause and co-workers (Krause J L, Schafer K J and Kulander K C 1992 A 45 4998) who employed the grid method. Whereas for convergence difficulties were reported for the grid method, no such difficulties were found for the SSEA. Given this fact, we tackled the problem of computing the harmonic spectrum for , the most difficult case treated thus far. This spectrum shows a cut-off at the 65th harmonic and an extended plateau containing low-intensity harmonics up to the 181st. The ionization rate is while at the end of the pulse 19.5% of the system is in the 1s ground state and 2.6% in the bound states up to n = 20, l = 19.

A set of tables of the Stark width of the first three lines of Lyman and Balmer series of hydrogen, useful for plasma diagnosis purposes, is presented in this work. These tables have been obtained by means of a fully recognized computer simulation technique. Computer experiments have been carried out for density values between and . Simulation temperatures range from 2245 K to 224 856 K. Ion dynamics effects are taken into account in a natural way and the most significative gas mixtures have been covered in the simulations, carried out at the reduced mass of the emitter - perturber pair , 0.6, 0.8, 0.9, 1.0, 1.25, 1.5, 1.75 and 2.0. A detailed discussion of the influence of the relevant plasma parameters on the characteristics of the profiles is also included. The results show an excellent agreement with up-to-date experimental results and confirm these kind of profile calculation techniques as now the most accurate.

A comparison is presented of the temporally resolved resonance-line emission from the Ne-like Ge XUV laser (pumped with nanosecond pulses) with the predictions for the same emission from the hydro-atomic code EHYBRID. The specific lines chosen were the two 3s - 2p Ne-like lines at 10.01 and 9.762 Å, and the 3s - 2p F-like group of lines in the 9.4 - 9.6 Å region. Modification of the code to include 112 excited levels of the F-like ion facilitated a direct comparison between experiment and model of (i) the temporal variation of the emissions and (ii) the variation of the peak intensity ratios of the F-like to Ne-like emissions with irradiance on target.

Agreement between experiment and model was generally good, with the trends of the F/Ne ratio variation with irradiance being clearly reproduced. The results show that the model is realistically simulating the hydrodynamics and atomic kinetics of the plasma. This provides further support that the model can simultaneously calculate the and the (sub-keV) transition intensities through population dynamics which are acceptably accurate.