Table of contents

The dissociation dynamics of bent molecules in an intense polarized light field produced by 35 ps wide laser pulses at 532 nm has been investigated by measuring the angular distributions of and ions emanating from dissociative ionization of . These angular distributions show a remarkable directional specificity: ions are preferentially formed in a direction which is parallel to the polarization vector of the applied laser field whereas, in contrast, ions are formed in a perpendicular direction.

For the first time theoretical predictions for (e, 2e) on a hydrogen minus, , target are presented. This study shows that the size and shape of the TDCS for this process is extremely sensitive to the form of the approximation used for the wavefunction. In particular, the TDCS is enormously model sensitive in the Bethe ridge region (when the recoil ion momentum is zero).

The semiclassical Deutsch - Märk (DM) formalism for the calculation of absolute cross sections for the single ionization of atoms has been extended to the calculation of cross sections for the multiple ionization of ground state atoms (degree of ionization m > 3). Comparisons are presented with the predictions of two recently published semi-empirical methods and with available experimental data for some selected targets. The DM formalism is conceptually straightforward to apply; the semi-empirically determined parameters can easily be related to physical quantities and the formalism has already been successfully applied to the single ionization of atoms, ions, molecules, radicals and clusters, as well as to the double and triple ionization of atoms. In those cases where predictions from all three methods and experimental data are available, we find that, with very few exceptions, our method yields a level of agreement with the experimental data which is comparable to or better than the predictions from the other two methods.

This letter reports the first application of the new R-matrix program package RMATRX II to electron impact excitation of a near neutral open d-shell ion. In this calculation for , all states corresponding to the configurations and have been included in the expansion of the total wavefunction. Thermally averaged collision strengths for forbidden transitions involving the even parity states are presented in tabular form for temperatures between 5000 K and 20000 K. The importance of including accurate CI expansions for both the target and the (N+1)-electron terms is demonstrated.

Levels of Ca ions with one, two or four electrons in the n = 3 shell have been studied for their decay properties and lifetimes. For the levels of Ca X and the level of Ca IX, arbitrarily normalized decay curve (ANDC) and cascade model analyses yield results with precisions of 10% and less and with low systematic uncertainties. The measured lifetime of the Ca IX level confirms the need to include relativistic effects in the calculation. The lifetime results are compared to other, experimental and theoretical, data available for the isoelectronic sequences.

Multielectron ionization of atoms in collisions with fast highly charged ions is considered for the following range of parameters , ; (Z and v are the projectile charge and velocity, respectively, are the orbital velocities of atomic electrons, n is the number of removed electrons). Simple scaling relations for multielectron ionization cross sections and an estimation of the cross sections have been obtained.

The results of many-body calculations of the 2s, 2p photodetachment cross section are presented. Many-electron correlations, in particular the polarization interaction between a photoelectron and atomic core electrons in both initial and final states, are taken into account within the Dyson equation method. The calculated binding energies for 2s and 2p electrons are found to be equal to 1.075 and 0.080 eV, respectively. The 2p photodetachment cross section agrees quite well with available experimental data and previous calculations. The 2s photodetachment cross section reveals the well known strong shape resonance just above the threshold, which is associated with a quasi-bound state, although the calculated peak value is less than the experimental one.

The energy and width of the autoionizing state of the Be atom were calculated. A series of multireference configuration-interaction (MRCI) calculations with an extensive Slater-type function (STF) basis set was carried out and the full CI energy was estimated for each of the neutral and ionized states. The estimated energy of is with respect to the ionized state. The photoionization cross section from the state was calculated by using the R-matrix method. The resultant energy and width are and , respectively, in reasonable agreement with the observed values of and . The energies and widths of the series up to n = 11 were also calculated.

Auger electron spectra of the methane molecule have been measured at various photon energies below and above the C 1s threshold. At energies corresponding to the higher Rydberg resonances the resonant Auger spectrum approaches the normal Auger spectrum both in form and peak position. Deviations from this trend are observed, however, when the core-to-Rydberg transition is accompanied by the excitation of vibrational modes. Auger spectra measured at the continuum resonance around 303 eV excitation energy display new decay features, which allows this resonance to be assigned to core-hole double excitations. Furthermore, above the C 1s threshold the satellite Auger features arising from the lowest shake-up initial states are identified.

Special reduction formulae for bipolar harmonics with higher ranks of internal spherical functions are derived, which will be useful in problems involving multiple expansions in spherical functions. Together with irreducible tensor operator techniques these results provide a new and effective approach, which enables one to extract the geometrical and dynamical factors from the cross sections of atomic processes with polarized particles with an accurate account of all the polarization effects. The angular distribution of polarized electrons and the circular dichroism in photoionization of polarized atoms with an arbitrary angular momentum are presented in an invariant vector form. A specific circular dichroism, which is caused by the correlation of electron and atom orientations, is discussed. The angular distribution of escaping electrons in double photoionization of unpolarized atom is presented in a simple form. A convenient parametrization is proposed for describing the dependence of the photoprocess cross sections on the polarization state of the photon beam.

The interplay of excited-state preparation and the action of environmental degrees of freedom is studied theoretically. As an example, the O - H stretching vibration in water molecules is chosen, which can be excited by the absorption of infrared photons. Carrying out a Floquet analysis the selective excitation of vibrational states in a gas-phase situation can be explained as an interference of two Floquet states which are related to the initial and final states of the preparation process. This well known result changes if the stretching vibration is coupled to a thermal reservoir (environment). To include the related dissipative effects and, thus, vibrational relaxation and dephasing, the density-matrix method is utilized. Mapping the density matrix in a representation of so-called instantaneous Floquet states the decreasing efficiency of the state preparation with an increasing coupling to the environment can be accounted for by the occupation of more than two Floquet states.

Theory of resonant x-ray elastic and inelastic scattering involving electronic continuum resonances is presented. As for excitation to discrete states the resonant x-ray spectral shape is found to depend strongly on the frequency of incoming photons and on the scattering angle. Contrary to excitation to discrete states, the energy scale of this dependence is defined by the finite delay time of the core-excited electron. It is shown that also the partial absorption cross sections for a continuum resonance can be obtained when the x-ray absorption spectrum is measured in the resonant inelastic x-ray scattering mode. The connection between frequency and angular dependences and the symmetries of the continuum and discrete states involved in the x-ray scattering is derived. The limiting factors for the spectral function under different circumstances are given.

A novel method for measuring the vibrational state distribution of molecular ions is presented, based on the wavelength dependence of the photodissociation yield. By using a strongly repulsive upper state potential, the vibrational wavefunction is directly mapped onto the photodissociation spectrum. This allows an accurate analysis of the population of the separate vibrational states under the assumption that the photodissociation cross sections are known. Test measurements on created by associative ionization in the range v = 0 up to 7 yield a strongly peaked distribution with a fraction of 33% in v = 3. The accuracy is typically 3%. Rotational excitation has only a minor influence on the results. The method is especially suited for molecular ions with a small vibrational constant.

Positron scattering from atomic hydrogen is studied at energies ranging from 4 to 100 Ryd using the close-coupling approach and cross sections from both the - H and Ps - p entrance channels are reported in a basis comprising 1s, 2s and 2p states of the hydrogen and positronium atoms. Comparative studies with widely used approximation methods such as the first Born, unitarized Born and second Born approximations are presented. With the exception of elastic scattering for the Ps(1s) - p entrance channel, it was found that on-shell approximations such as the first Born and the unitarized Born approximations are reasonably accurate for collisions not involving particle transfer provided the energy is sufficiently high. Rearrangement collisions on the other hand are dominated by off-the-energy-shell scattering and the accurate approximation at high energies is the second Born approximation. Further, the effects of positronium channels on the direct reactions are investigated and we conclude that the positronium formation channels can be omitted for energies greater than 20 Ryd. Elastic scattering in the Ps(1s) - p entrance channel is seen to be unique, the high-energy behaviour of the phase shifts is different from any other quantum system.

The close-coupling method based on semiclassical impact parameter theory has been applied to collisions in the range of proton impact energies from 30 keV through to 10 MeV. The cross sections for hydrogen production, target excitation and ionization have been computed using two-centre AO expansions with up to 55 atomic states and pseudostates. The results can be used as reference data for further close-coupling studies of the system as well as for assessing the validity range and accuracy of various perturbational models at both intermediate and high energies. Applications to current plasma research at JET where accurate data are required for diagnostic purposes, are discussed.

Absolute cross sections for 5) emission at 465.7 nm after collisions of ions with ground state Na(3s) and laser excited aligned Na(3p) atoms are measured over the collision energy range of . For Na(3s) polarizations are observed by measuring the linear polarization of the emitted light. In these collisions the polarization is found to be 0.33 and almost independent of the collision energy. Cross sections for collisions with the Na(3p) target are up to five times larger than for collisions with Na(3s). Slightly larger cross sections are measured when the laser excited Na(3p) target is -aligned as compared to a -alignment. Classical trajectory Monte Carlo (CTMC) calculations agree excellently with the experimental findings and yield predictions of strong positive polarizations for electron capture also from aligned Na(3p), indicating preferential capture to states.

It is shown that two parallel ion beams react at a rate that is independent of their density profiles when made to oscillate against each other in a two-dimensional scanning motion. An experimental set-up that makes use of this principle is described. Absolute cross sections obtained in this way are in good agreement with those obtained with the beams merging in the usual (static) mode. Cross sections for single-charge transfer between and in the energy range 5 - 4000 eV are presented and compared to other existing data.

A crossed beam technique incorporating time-of-flight analysis and coincidence counting of the collision products has been used to study and formation in both one- and two-electron capture by 5.3 - 66.7 keV ions in collisions with ground-state oxygen atoms. In the case of one-electron capture, cross sections for simple charge transfer, which are known to be dominated by the product channel at low energies, are found to attain a large peak value of about at about 20 keV . The low-energy trend of these cross sections is in satisfactory accord with the single value measured previously at 2 keV in this laboratory using a different technique. There have been no other experimental or theoretical studies of the - O collision system. Production of ions, mainly by transfer ionization involving one-electron capture, is surprisingly effective with the production ratio increasing from 0.23 to 0.64 over the energy range considered.

The influence of final-state correlations on the triple differential cross section for K shell ionization of many electron atoms in coplanar symmetric geometry is studied using a semirelativistic Coulomb-projected Born model including a correlation factor. For high nuclear charge Z, correlation effects are found to be mostly small, but significant changes in the calculated cross sections are predicted at small relative emission angles. For low and intermediate values of Z, there are indications for substantial post collision interaction effects even at impact energies of several tens of keV.

A new approach to computer experiments in electron - atomic collision studies is introduced. It is based on a path-integral representation for the physical values to be studied. The approach has been used in computer simulations of electron-impact excitation of the helium atom: .

We have applied the R-matrix with pseudo-states method (RMPS) to study electron - helium scattering at intermediate energies. Differential cross sections for elastic and inelastic collisions and electron-impact coherence parameters for excitation of the transition are presented for incident energies of 30, 50 and 80 eV. Excellent agreement with experiment and with the predictions of the convergent close-coupling calculation by Fursa and Bray is obtained. Since correlation effects and target continuum states are both accurately represented in a completely general way by this method, the associated program package can now be used to obtain reliable results at intermediate energies for arbitrary complex atomic and ionic targets of importance in applications.

In this work, we calculated the ionization cross sections of electron impact for the Li-like isoelectronic sequence. The excitation autoionization is also included in the calculation. The results of the calculations are fitted by empirical formulae to meet the requirements of the applications. The fitted results are in good agreement with the calculated results.

Using the operator representation of the Dirac Coulomb Green function the analytical method in perturbation theory is employed in obtaining solutions of the Dirac equation for a hydrogen-like atom in a time-dependent electric field. The relativistic dynamical polarizability of hydrogen-like atoms is calculated and analysed.

Interference stabilization of atoms is investigated numerically. Raman-type transitions to Rydberg states with higher values of the electron orbital momentum are taken into account. These transitions are shown to change qualitatively theoretical predictions concerning the dependence of the time of ionization on the light-field-strength amplitude : if the weak-field time of ionization, as usual, is a falling function of , in a strong-field region the function is predicted to be more or less constant and of the order of the classical Kepler period in a rather large interval of , in contrast with the earlier prediction according to which, in a strong field, was assumed to be a rather fast growing function. The results derived are shown to be in a rather good agreement with the existing experiments. Alternative theoretical models are discussed.

We study the classical and quantal ionization dynamics of a one-dimensional system by strong, low-frequency electric fields with slowly varying envelopes. We show that classical ionization can be described using the principle of adiabatic invariance, if the driving frequency is sufficiently small, and that after ionization the atomic potential does not significantly alter the motion. This dynamical picture suggests that the quantal system ionizes via a sequence of tunnelling events which produce a wavepacket oscillating back and forth across the nucleus to produce high harmonics in the spectrum of radiation emitted by the accelerating electron. We further show that it is the interference between components of this wavepacket which produces both the harmonics of the field and a sharp cut-off in the spectrum, neither of which are present in the classical spectrum.