Table of contents

Using a B-spline expansion in momentum space, we have solved the time-dependent Dirac equation numerically for a model, one-dimensional atom which is subjected to an ultra-intense, high-frequency laser field. We find that for a peak electric field strength of 175 atomic units (au) and for angular frequencies of 1 and 2 au, relativistic effects start to become apparent. Even under these extreme conditions the wavefunction remains localized in a superposition of field-free bound states and very-low-energy continuum states. Comparing our results with the numerical solution of the time-dependent Schrödinger equation, we find that the Dirac wavefunction is slightly more stable against ionization. We also find that the energy distribution of the ionized electrons is strongly concentrated near threshold and that a cut-off in the high-energy spectrum occurs at the energy corresponding to the maximum momentum of a classical electron in the laser field.

For neon 3p states excited by transversely polarized electrons, the integrated Stokes parameters, and , have been measured from threshold to 300 eV for the decay photons of 692.9, 667.8 and 717.4 nm wavelengths from the and states, respectively.

The and parameters for the 692.9 and 667.8 nm transitions indicate similar excitation mechanisms consistent with their similar `L S component states'. The non-zero indicates the breakdown of L S-coupling and the large values near threshold suggest a significant exchange process. For the 717.4 nm transition, the three Stokes parameters are much smaller than those of the other two lines. The low is surprising since the large (75.4%) -coupled component state was expected to cause large exchange effects.

Considering the current interest in the BBK theory for ionization, a BBK-type theoretical model for direct excitation of hydrogen is discussed. To examine its applicability, the theory is applied to the excitation of hydrogen by electrons at 54.4 eV, and its predictions of the differential cross sections and angular correlation and R parameters are compared with the results of other current theories. A most striking result is that in contrast to existing theories, the BBK-type model, for the first time, gives an excellent agreement with the experimental R parameter for larger scattering angles.

Cross sections have been measured for the electron-impact ionization of argon and neon in a coplanar symmetric geometry. Results are presented for impact energies of 115.8, 85.8 and 50 eV for argon and 50 eV for neon. A comparison is made with distorted-wave Born approximation calculations which include initial channel polarization and final channel post-collisional interactions. The lowest-energy argon data appear to indicate that polarization effects are very significant, and, in order to explore this, additional measurements have been performed in the coplanar constant geometry.

The analytical formula, derived by Iglesias-Groth et al for the interaction energy between shells of spherical carbon onions using the surface continuum approximation, is shown to be in very good agreement with the numerical values obtained by Lu and Yang from the discrete summation over the van der Waals Lennard-Jones CC potential as well as the density functional results of Pis Diez and Iñiguez. The empirical law, , with , giving the interaction energy between the jth and (j + 1)th shell derived from the discrete summation results, is recovered with an analytical formula for K in terms of the parameters.

High-resolution recoil-ion momentum spectroscopy (RIMS) is a novel technique to determine the charge state and the complete final momentum vector of a recoiling target ion emerging from an ionizing collision of an atom with any kind of radiation. It offers a unique combination of superior momentum resolution in all three spatial directions of with a large detection solid angle of . Recently, low-energy electron analysers based on rigorously new concepts and reaching similar specifications were successfully integrated into RIM spectrometers yielding so-called `reaction microscopes'.

Exploiting these techniques, a large variety of atomic reactions for ion, electron, photon and antiproton impact have been explored in unprecedented detail and completeness. Among them kinematically complete experiments on electron capture, single and double ionization in ion - atom collisions at projectile energies between 5 keV and 1.4 GeV have been carried out. Double photoionization of He has been investigated at energies close to the threshold up to . At the contributions to double ionization after photoabsorption and Compton scattering were separated kinematically for the first time. These and many other results will be reviewed in this paper. In addition, the experimental technique is described in some detail and emphasis is given to envisaging the rich future potential of the method in various fields of atomic collision physics with atoms, molecules and clusters.

An experimental method is described to obtain electron spectra with good resolution and high intensity following L-shell excitation or ionization of laser-excited Na atoms. We report on experimental and theoretical electron spectra in the energy range of 25 - 33 eV for laser excitation to , , and and for 1.5 keV electron impact. Due to the good agreement in line positions and intensities essentially all lines can be assigned. Furthermore, experimental relative cross sections for states excited from , and are determined and compared with theoretical values.

We investigate the influence of the off-shell effects on the scattering processes in the presence of a laser field in view of the recent experiments by Wallbank and Holmes. In these experiments a large discrepancy between the observed data for small-angle scattering and the on-shell Kroll - Watson formula was reported. We show in the present work that the off-shell effects are not responsible for this discrepancy by presenting results for the differential cross sections obtained, using the off-shell low-frequency approximation and the [1,1] Padé approximation. We model the scattering from helium and argon atoms by the scattering on a Yukawa-type potential. We show, using some examples, that higher laser field frequencies and intensities are necessary for obtaining observable off-shell effects. We also comment on possibilities of explaining the Wallbank and Holmes results. For one of these possibilities - double scattering - we compute, using our simple model, the relative differential cross sections and obtain a qualitative agreement with the experiments of Wallbank and Holmes, both for helium and argon.

Cross sections and rate coefficients have been computed for rovibrational transitions between and within the four lowest vibrational states of , induced in collisions with H atoms. Our quantal close-coupling calculations are compared with previous classical trajectory Monte Carlo results. At low temperatures, there are large discrepancies, whereas, as the temperature increases, the present and previously computed rate coefficients are found to be in satisfactory agreement.

It is shown that the transitions along the - and -superseries of hidden crossings in a heavy ion - atom collision system (, - 30) form a dominant mechanism for the vacancy production at low reduced relative collision velocities. The -vacancy creation probability as a function of the impact parameter, calculated within this model for several collision systems and collision energies, agrees well with the experimental observations. A scaling relationship is derived for the -vacancy creation cross section which adequately describes the experimental data for charge-symmetric heavy ion - atom systems.

The rotationally inelastic cross sections, integral and differential, are computed as quantum processes using a published interaction potential and are compared with recently available experimental data at low collision energies. Different coupling approximations are analysed and the mainly repulsive interaction potential is confronted with the one exhibited by the system. It is found that a full coupling of angular momenta is needed for accurate comparisons and that the long-range anisotropy of the employed intraction is still not adequate to quantitatively reproduce the experimental data.

We have measured the mass distribution of fragment ions of produced by collisions with 15.6 MeV carbon ions in different charge states. Close collisions were selectively measured using the coincidence method with the change of the projectile charge state. For the electron capture and loss by projectile ions, which are the K-electron processes, the multifragmentation was observed evidently. In L-electron loss channels of the projectile, the peaks of the multiply ionized ions and the multiply ionized fullerene-like fragment ions are more intense or as intense as the small fragment ions. We suppose that we can roughly classify the intermediate states of through the measurement of fragmentation in coincidence with the change of charge state for various projectiles.

This paper reports experimental and theoretical results for the influence of optical target preparation on electron transfer for the collision system at a proton impact energy of 2 keV. The spatial pattern of the forwardly scattered neutrals is found to depend sensitively on initial optical target preparation as well as on the energy defect of the process. The complete set of density matrix parameters has been determined for the channels corresponding to electron transfer into and . The results reveal opposite state propensities for initial target preparation for the quasi-resonant and the endoergic exit channels. The overall agreement is good between experimental observations and theoretical predictions based on the coupled-channel impact parameter method using a 36-state atomic basis.