Table of contents

Electron energy spectra from multiphoton ionization of krypton with 617 nm, 70 fs pulses have been recorded over a large intensity range. Near the appearance intensity, the spectra display Stark-induced resonances with s and d states. Up to about 8*10¹³ W cm^-2, the spectra broaden and the structures are progressively washed out. Above this intensity, a strong resonance emerges, corresponding to the 4f state for which the static detuning is 2.94 eV, a value larger than the photon energy. The influence of a large static detuning and populations of excited states with shifts different from the threshold shift are discussed.

A theoretical analysis of the collisional broadening and shift of Rydberg levels with angular momenta J>or=1 taking into account the anisotropy of the Rydberg-perturber potential is presented. The symmetry of the problem is used for the calculation of the collision S matrix in a semiclassical approximation with rectilinear trajectories. The obtained expressions for the broadening and shift cross sections allow an explanation of the observed differences between the broadening and shift of the anisotropic np, np and nP_3/2 levels in comparison with those for the spherical ns and nP_1/2 states.

The influence of the electronic structure on the molecular response to an intense picosecond laser at lambda =616 nm and I=2*10¹⁵ W cm^-2 is investigated for N₂, C₂H₂ and C₂H₄. The fragmentation pattern into charge-symmetric N^Z+N^Z'+ and C^Z++C^Z'+ Coulomb fragmentation channels is found to be similar for N₂ and C₂H₂ which have close ground state electronic configurations. The laser molecule coupling for the production of C^Z++C^Z'+ dissociation pathways is more efficient for C₂H₂ than for C₂H₄ due to the respectively different pi ⁴ and pi ² electron configuration of the C-C bond. The overall experimental data suggest that the laser energy is absorbed by molecular orbital type electrons, even in the case of a nuclear structure deformation, while the molecule decays into Coulomb fragmentation channels with kinetic energy releases independent of the laser excitation conditions as observed in previous experiments.

Multiply-charged secondary ions produced from condensed gas targets have been observed under energetic, heavy-ion impact. It is found that, though strongly suppressed over gaseous target collisions, multiply-charged ions are more efficiently extracted from condensed surfaces if molecules such as N₂, O₂, C₂H₂ are used as targets, compared with condensed rare-gas atom and other molecule (for example, CH₄) gas targets. This feature could be understood qualitatively as being due to their large initial kinetic energy provided through Coulomb explosion of multiply-charged molecular ions originally produced in heavy-ion collisions.

A propensity rule has been found for the magnetic substate distribution of excited states populated in collisions between multiply charged ions with atoms. It is observed that the m=-l substates are predominantly populated if the quantization axis is chosen to be perpendicular to the scattering plane. The result implies that it is possible to carry out reduced close-coupling calculations where only the m=-l substates for each l are included in the basis expansion. Such reduced close-coupling calculations are desirable for treating electron capture to high-n excited states. The range of applicability of the propensity rule is examined for C⁶⁺-H collisions.

The population of levels with principal quantum numbers n largely in excess of the n=3 (most populated level) in the single electron capture by O⁶⁺ from He (at 60 keV) is reported. The n=4 population involves an 'electron promotion mechanism' at small internuclear distances (R approximately=2.5 a₀). The authors' observations show population of levels up to n=7 (Rydberg levels) leaving open the theoretical interpretation.

The ² Pi _g shape resonance of the electron-C₂H₂ system in the vicinity of 2 eV is investigated under several local models for exchange interaction depending upon free-electron-gas and semiclassical theories. It is found that the modified version of the semiclassical exchange model, recently proposed by Gianturco and Scialla (1987), is a very successful model in reproducing the ² Pi _g shape resonance features in agreement with experimental studies on the electron-C₂H₂ collision. The polarization force is included parameter-free in the correlation-polarization approximation. The scattering problem is solved in the fixed-nuclei approximation under the close-coupling formalism.

It is shown that the (e,2e) simulation of the photoionization process can be used to perform the complete quantum-mechanical experiment provided the target atoms are polarized. The experimental technique developed earlier for simulation of the photoelectron angular distribution measurements can be used to obtain three additional parameters in the case of polarized atoms.

Distorted-wave Born approximation (DWBA) calculations are reported for singly differential and total cross sections for the electron impact ionization for atomic hydrogen at 25, 40, 60, 100, 150 and 250 eV. The theory is compared with available experiments. It predicts a less rapid decrease of the singly differential cross section with increasing secondary-electron energy than the absolute measurements of Shyn. Two different DWBA calculations are performed. The first approximation omits the post-collision interaction and the second one approximates it. The total ionization cross section in the second approximation agrees better in general with the shape of the experimentally measured cross sections.

The authors report distorted-wave Born approximation calculations of the triple differential cross section and spin asymmetry parameter for electron impact ionization of ground state lithium at 54.4 and 100 eV. Comparison is made with some preliminary experimental data.

Rotational rainbow structures in atom/electron-molecule scattering are analysed by means of simple dynamical approximations as hard-shell molecules, the infinite order sudden (IOS) approximation and the N-centre spectator model. Typical collision systems described are atom-molecule scattering for collision energies of 10^-1 eV and electron-molecule scattering at 10² eV. The IOS and spectator rotational rainbow patterns are compared and special features related to initially excited molecules, m-changing collisions, multiple rainbows, and inversion techniques are discussed. The rotational rainbow patterns for polyatomic molecules (symmetric tops) are discussed as well as vibrational effects in the rotational rainbow structures of diatomics.

A complete systematic study of the broadening and shifting of the Sr resonance line (5¹P₁-5¹S₀) by collisions with all five rare gases has been conducted with a high-resolution laser-induced fluorescence method. The method enabled the authors to measure, for the first time, asymmetries in the near wings of the Sr emission profiles. The measurements were performed at 720 K and pressures below 600 Torr. The values of the line asymmetry are compared with ones predicted from lineshape models. Quantitative comparisons of the measured and theoretical line asymmetries suggest that atomic interactions in the Sr-rare-gas systems may not be entirely due to van der Waals type interactions, and that correlations between thermal motions and collisional interactions could be the major source of the spectra line asymmetry.

The authors consider the probability for double-Auger decay of multiply-excited states in Nⁿ⁺ with two 1s vacancies. They have calculated the branching ratio R³ between the double-Auger decay filling both holes in the 1s shell and the normal Auger decay for initial 2p^N, 2s2p^N-1 and 2s²2p^N-2 configurations with N=3, 4, 5 and 6. They show that R³ depends more strongly on the number of 2s electrons than on the total number of electrons in the n=2 shells. They find that the 'shake-down' of a 2s electron due to the non-orthogonality between the initial 2s and the final 1s electron is the most important effect. Comparison with the result of recent studies of neutralization of slow highly-excited ions at surfaces suggests the presence of one or two 2s electrons in the ion at the time of the double-Auger decay.

The decay probabilities of the 4d_j hole states in xenon into the differently charged ion states, i.e. fluorescence decay, normal Auger together with discrete double Auger decay and continuous double Auger decay, are measured directly by means of coincidences between 4d_j photoelectrons and xenon ions. Special emphasis is given to the strength of continuous double Auger transitions which is a direct manifestation of strong electron correlations during the non-radiative decay. In order to obtain reliable values, the problem for the correct extraction of true coincidences from the observed ones is also discussed in detail.

Single photon double ionization of carbon disulphide at 40.8 eV has been studied by photoelectron-photoelectron and photoelectron-photion coincidence spectroscopy. Interpretation of these coincidence spectra shows that indirect double photoionization, via intermediate singly charged ion states, contributes significantly to the double ionization cross section at this photon energy. Population of the metastable ³ Sigma ^-_g ground state of CS²⁺₂ is shown to favour the emission of one high-energy and one low-energy electron.

The present work provides a theoretical treatment of transitions between the Zeeman magnetic sublevels (m) of atomic hydrogen induced by ion collisions in the presence of a uniform magnetic field. This process is important in discussing the polarization properties of the optical emission in plasmas accompanied by an external magnetic field. Owing to the long-range dipole interaction, n² sublevels with the same principal quantum number n are strongly coupled. However, by using a peculiar symmetry property of atomic hydrogen, the number of coupled channels is reduced from n² to n. Furthermore, useful scaling law is found.

Absolute cross sections for single and double ionization of helium have been measured for highly charged (24<or=q<or=92) and fast (3.6 MeV u^-1<or=Ep<or=1 GeV u^-1) heavy-ion impact. The ratios of double-to-single ionization are found to deviate drastically from the prediction of first-order theories: in the strong perturbation regime they fall below the scaling of Knudsen et al. (1984) even for 120 MeV u^-1 U⁹¹⁺ impact, whereas a strong increase by a factor of 3 relative to this semi-empirical prediction is observed in the regime of low perturbations for 0.5 and 1 GeV u^-1 Kr³⁶⁺ impact.

L X-ray line production cross sections for Eu, Tb and Er elements have been measured as a result of ⁷₃Li ion bombardment at energies from 4.2 to 19.6 MeV. The corresponding L₁-, L₂- and L₃-subshell ionization cross sections have been calculated and have been compared to the predictions of the ECPSSR and SCA theories. Possible reasons for observed disagreements between experiment and theory are discussed.

Relative double differential cross sections for electron emission from collisions of 0.6 MeV amu^-1 I⁷⁺ and I²³⁺ projectiles with Ar and Au¹¹⁺ projectiles with He were measured for electron energies from 100 eV to 2000 eV and for angles from 0' to 50'. Experimentally observed sudden shifts of the position of the binary encounter peak were interpreted as resulting from quantum interference in the scattering of target electrons from the partially stripped projectile ion. To this end calculations for the elastic scattering of free electrons initially at rest in the laboratory reference frame from the screened projectile field as well as model calculations of binary encounter electron production taking into account the target Compton profile were performed and compared with the experimental results. Good agreement was obtained for the angular locations of these shifts as well as for the angular distribution of the binary encounter electron yield.

A crossed beam technique incorporating time-of-flight spectroscopy and coincidence counting of fast ion-slow ion and electron-slow ion collision products has been used to obtain individual cross sections for electron capture, transfer ionization and pure ionization in collisions involving Mg atoms. Processes involving Mgⁿ⁺ formation for n between 1 and 4 have been considered for both 90-2000 keV amu^-1 H⁺ impact and 43-500 keV amu^-1 He²⁺ impact. The results extend available data to energies where, in addition to the outer 3s electron, the inner 2s and 2p electrons in Mg are expected to become important. Discrepancies with previous low energy data are believed to reflect, in part, the use of different normalization procedures. While Mg⁺ production is dominated by pure ionization in the present energy range, the important role of transfer ionization in the formation of Mg²⁺, Mg³⁺ and Mg⁴⁺ ions is clearly demonstrated.

Calculations have been carried out in the relativistic distorted-wave approximation for the excitation of calcium, strontium and barium from their ground ¹S₀ states to the lowest lying ^1,3P₁ states of these atoms for incident electron energies in the range of 20 to 100 eV. The authors present results for differential cross sections, generalized spin-polarization parameters and electron-photon coherence parameters and compare them to experimental and previous theoretical results where available.