Table of contents

The degree of localization of inner-shell excitations is studied using the generalized multistructural (GMS) wavefunction. The results of the calculations indicate that, for the molecules considered, the inner-shell hole states exhibit a great deal of localization (78% for N₂, 60% for C₂H₂ and 86% for CO₂, 1s(0) hole) and that the degrees of localization are consistent with the difference in nuclear charges.

A numerical study of the conditions for complete population return in a two-state system is presented. For the first time, a pulse with an infinitely smooth asymmetric envelope and a constant detuning is found which in principle can restore the system to its initial state. Several approximate population return values of the pulse parameters are given.

Calculations of elastic positron-atom scattering phaseshifts can sometimes yield rather inaccurate results when the target wavefunction is not known exactly. This difficulty can be partially avoided by using the method of models, but the method cannot be used consistently in a rearrangement process such as positronium formation. As a preliminary to a detailed study of positronium formation in positron-helium scattering, we have used the Kohn variational method with very elaborate trial wavefunctions and several helium wavefunctions to investigate the conditions under which well converged and accurate elastic positron-helium scattering phaseshifts can be achieved without using the method of models. We find that increasing the flexibility of the total trial function, by adding more short-range correlation terms, only yields improved and better converged results provided the target wavefunction remains of a superior quality to that of the trial function. Further increases in the flexibility of the trial function lead to increasingly inaccurate results unless the quality of the target function is improved also.

We discuss how results from scattered-electron-polarized-photon coincidence experiments on the impact excitation of unpolarized heavy atoms with unpolarized electron beams can be used to extract information about the depolarization of spin-polarized electrons through exchange collisions. The method is demonstrated with recent experimental and theoretical data for Hg (6s²)¹S₀^e to (6s6p)³p₁⁰ excitation.

We report the results of fully relativistic distorted-wave calculations for (e,2e) collisions at high energies on gold and uranium targets in a coplanar symmetric geometry. The results for gold are in excellent agreement with available absolute experimental data. We also observe a peak in the triple differential cross section at large angles which may be interpreted in terms of a multiple scattering mechanism. In the present case of large impact energies, the ratio of the large angle to the binary peak is several orders of magnitude larger than that previously observed at non-relativistic energies. Our calculations indicate that this peak should be accessible to experiment. We propose a new geometrical arrangement for experiments which we believe offers distinct advantages in the study of distortion effects in relativistic (e,2e) processes.

In this article we review our recent studies on molecular spectroscopy and photodissociation dynamics of small jet-cooled molecules and van der Waals complexes in the vacuum ultraviolet (VUV) wavelength region ( lambda =200-115 nm) performed using tunable, intense, and coherent VUV light, a so-called tunable VUV laser, generated by a two-photon resonance four-wave sum or difference frequency mixing (2PR-4WSM or 2PR-4WDM) scheme in non-linear media of Sr, Mg, Hg, Xe and Kr. We introduce our progress in high-resolution spectroscopy of electronically excited CO, Cl₂, HgRg(Rg=Ne, Ar, Kr), XeRg, Xe₂ and photodissociation dynamics of NO₂ and OCS. We also describe the generation principle and the characteristic features of the tunable VUV laser and the practical procedure for generating the VUV laser for spectroscopic applications. Emphasis is placed on how the combination of the VUV laser and the free jet expansion techniques is promising for the derivation of precise and substantial information regarding the level structure and the dynamical processes characteristic of highly excited molecular species excited in the VUV wavelength region.

A convenient basis of wavefunctions is constructed for describing the motion of the charged two-body system (hydrogen-like ion) in a strong magnetic field. The basis functions correspond to two integrals of motion of the system, and they are the superpositions of the Landau states of non-interacting particles moving in a magnetic field. The matrix elements of the Coulomb potential are numerically analysed using the new basis.

Photoabsorption by atomic Au in the 5p and 4f excitation region has been studied using the dual laser plasma (DLP) technique. The observed features are dominated by two prominent Fano-type resonance lines which can be attributed to 5p to 5d and 4f to 5d transitions of valence excited 5d⁹6s²(²D₅2/) Au followed by autoionization. The experimental results are compared to spectra calculated for the excitation of 5d¹⁰6s¹(²S₁2/) ground and 5d⁹6s²(²D₅2/, ²D₃2/) valence-excited states within the (R)Hartree-Fock theory and the relativistic time dependent local density approximation.

In the context of laser spectroscopy we consider the effects of interference between the spontaneous emission to a common lower level from neighbouring, closely spaced, excited-state levels. In particular the effects on the fluorescence spectrum and on the measured positions of the line centres are described. A formalism, based upon the atomic density matrix, is developed quantitatively to describe these effects within complex hyperfine systems such as the sodium D-lines. Where possible, we have tried to simplify the resultant equations, defining the magnitude of the various fluorescent components and establish a general result for the phase of each component. Finally, the formalism is applied to the particular case of the sodium D1-line, where very different results are obtained depending on whether optical pumping into unobserved states is included, or omitted, from the calculation.

The quenching by electron collisions of the He(4³P) sublevel has been studied by a laser resonance fluorescence method. The sublevel is optically pumped by a dye laser tuned to the transition 2³S-4³P, and the fluorescence from 4³P itself and from the 4³D state is observed and analysed. The measurement is performed at various times in the post discharge of a pulsed RF discharge, and it is correlated to the decay of the electron random flux. The latter is measured simultaneously by a time resolved Langmuir probe technique. Combination of the two results allows the measurement of the electron collision rate constants k_el(4³P)=(1.23+or-0.34)*10^-4 cm³ s^-1, for the total quenching of 4³P, and k_el(4³P to 4⁴D)=(1+or-0.24)*10^-4 cm³ s^-1. Such rate constants turn out to be constant, within the experimental accuracy, in the range 0.2-0.6 eV of electron average energy. A good agreement is obtained with theoretical calculations. The importance of electron collisions is also demonstrated by a qualitative analysis of the fluorescence from 5³D, 4¹D states.

The total photoion yield spectrum of atomic chromium has been determined in the range of the 2p thresholds. The experimental data are compared to Hartree-Fock calculations, which include the interaction between three configurations, and to a previously published electron energy loss spectrum of solid chromium.

It is demonstrated that the angular distribution of Auger electrons produced in photoionization of non-polarized atoms by circularly polarized light is different for left and right circular polarization, provided the Auger electron is detected in coincidence with the photoelectron ejected in a certain direction. A general formulation of the theory of circular dichroism in angular-resolved double photoionization is presented based on the statistical tensor formalism. Extreme cases of direct and two-step double photoionization are considered and the inherent similarities and differences between the two cases as applied to circular dichroism are discussed. As an example, photoionization in the 3p₃2/ subshell of Ca and the following M₃-N₁N₁ Auger transition is considered. The connection of the dichroism in the angular distribution of Auger electrons to the photoelectron spin polarization is discussed as well as their dependence on the dynamical properties of photoionization amplitudes.

We have carried out a generalization of the continuum-distorted-wave-eikonal-initial-state (CDW-EIS) approximation for ion-impact single ionization where the interaction of the active electron with the target is represented by a Hartree-Fock potential. We apply this model to the ionization of He, Ne and Ar by proton and multiply-charged bare-ion impact. Doubly differential and total cross sections are calculated from each subshell. These cross sections summed over all subshells show a much better agreement with experimental data than those obtained from the previous formulations of the CDW-EIS approximation which use hydrogenic wavefunctions with effective charges.

The Coulomb-Born approximation is applied to K-shell and L-subshell ionization of Ag and heavier target atoms by electrons above 100 keV. The existing theory is extended to describe the spin asymmetry of the triply differential cross section in the case of polarized electron impact. Particular attention is paid to the dip in the binary peak for L₃-subshell ionization at Bethe ridge conditions which is very sensitive to wavefunction effects. Comparison with experimental (e, 2e) data in coplanar geometry verifies the applicability of the Coulomb-Born theory for weakly relativistic target states.

Scattering of electrons and positrons on a hydrogen atom below the H(n=2) threshold is studied by means of directly solving the modified Faddeev equations in the total angular-momentum representation. Elastic phaseshifts of the e^-+H scattering and those of the e⁺+H scattering below the Ps(n=1) threshold are obtained. K-matrix elements for the e⁺+H and p+Ps scattering between the Ps(n=1) and H(n=2) thresholds are calculated. The results are in a good agreement with previous variational calculations.

Calculations of low-energy positron-hydrogen and positronium-proton scattering using the close coupling method are reported at energies below the ionization threshold. The channel space included six hydrogen (1s, 2s, 2p, 3s, 3p and 3d) and six positronium (1s, 2s, 2p, 3s, 4s and 3p) states. Comparisons with more accurate calculations demonstrate that this model should give integrated cross sections accurate to 10%. Cross sections are computed with a fine energy mesh and a total of 11 resonances associated with the H(n=2) and Ps(n=2) thresholds have been identified and the resonance positions and widths determined. Calculations of the total cross section and the positronium formation cross section for positron-hydrogen scattering are consistent with the available experimental data. Cross sections for elastic scattering in the Ps(1s)-p entrance channel are dominated by a strong polarization potential and vary from 1000 pi a₀² at threshold to 15 pi a₀² at 1.0 Ryd. Cross sections for the Ps(1s)+p to e⁺+H(nl) rearrangement collision are also reported since these provide baseline data relevant to experiments aimed at making antihydrogen atoms. Differential cross sections for elastic Ps(1s)-p and e⁺-H(1s) scattering and for electron transfer to the hydrogen atom ground state are also reported at selected energies.