H Klar

Triply-differential cross sections for ionisation of hydrogen atoms by electrons and positrons

A derivation is given of the exact form of the three-body Coulomb wavefunction in the asymptotic region where the separation of all particles tends to infinity. Using a modification of the method of Pluvinage (1951), an approximate three-body scattering wavefunction is derived that satisfies this boundary condition. Triply-differential cross sections (TDCS) for electron impact ionisation of atomic hydrogen calculated with this scattering wavefunction, which contains no free parameters, show excellent agreement with measurements at impact energies greater than 150 eV. The corresponding TDCS for positron impact ionisation are also presented.

Theory of collision induced rotational energy transfer in the Π-state of diatomic molecules

It is suggested that an interaction not being invariant about the internuclear molecular axis is useful for the description of rotational excitation of diatomic molecules in the electronic Pi -state. In the framework of the helicity-basis of angular momentum a complete scattering theory qualified for numerical use is presented. Collision induced excitation by rare gases, Sigma - and Pi -state molecules, are treated separately. Special attention is paid to polarization phenomena. The connection with the usual angular momentum coupling scheme is given explicitly.

Angular distribution of photoelectrons from polarised atoms exposed to polarised radiation

The structure of the angular distribution of photoelectrons from polarised targets exposed to polarised radiation has been analysed in the dipole approximation. It is shown in particular that a measurement of this angular distribution without a spin polarisation measurement of the photoelectron constitutes a complete experiment for polarisable one-electron atoms and many other atoms. The completeness of the type of experiment proposed permits one to extract both magnitude and phase of all participating dipole transition amplitudes from an experimentally observed angular distribution, and represents therefore the basis for a sensitive comparison between experimental and theoretical work.

Threshold multiple ionization of atoms. Energy dependence for double and triple escape

Many slow electrons in the field of a residual ion are shown to have Wannier-like stationary configurations. Solutions of the Schrodinger equation in this limited region of space are obtained and applied to the investigation of the cross section for multiple ionization near threshold. Numerical results are presented for double escape in several spin and angular momentum states and for triple escape in the 2S and 4S states.

An accurate treatment of two-electron systems using hyperspherical coordinates

The three-particle Coulomb problem has been investigated in the framework of hyperspherical coordinates. Energy levels of doubly excited states below n=2 of He and H- have been computed. Results agree reasonably well with experimental as well as other theoretical work.

Spin polarisation of Auger electrons

Auger electrons may be spin polarised either due to a transfer of orientation from the initial inner-shell ionisation process to the decay fragments, or due to relativistic electron-ion final-state interactions. In particular it is shown that polarised Auger electrons can be emitted even if both the ionising projectile and the initial atom have been unpolarised. The analysis of the state of polarisation of electrons from Auger transitions constitutes a new sensitive probe for the investigation of atomic interactions.

Threshold ionisation of atoms by positrons

The threshold behaviour of the ionisation of atoms by positron impact has been analysed. The total cross section is shown to rise as the 1.650th power of the excess energy; for ions this exponent is larger. The escaping electron and positron are emitted predominantly into the same direction with a uniform electron-positron energy distribution. These effects emerge explicitly from electron-positron correlations.

Triply differential cross sections for ionization of hydrogen atoms by electrons: the intermediate and threshold energy regions

Triply differential cross sections in coplanar geometry for the ionization of atomic hydrogen by electron impact have been measured and calculated at incident electron energies of 54.4, 27.2, 16.5 and 15.6 eV. The theory is the same as that used successfully at higher energies and is shown to reproduce nearly all features of the cross sections in this low-energy regime as well. Moreover, the physical origin of peaks and dips in the triply differential cross sections has been explained. The authors conclude that kinematical effects, exchange effects, correlation and the coherence between electron-electron and electron-proton scattering are all important at these energies.

Structures in triply and doubly differential ionization cross sections of atomic hydrogen

The work investigates theoretically triply and doubly differential cross sections for electron- and positron-impact ionization of atomic hydrogen. On the basis of the framework developed by Brauner et al. (1989-91), structures in the triply differential cross section are identified and their contributions to the doubly differential cross section are discussed. A novel structure in the triply differential cross section in the case of positron impact has been found. Results for the energy and angular distributions of secondary electrons are presented and compared with available experimental data. In general, the results are in good agreement with the experiment. Discrepancies between experiment and the theory exist for electrons ejected into the forward direction. It turns out that at low impact energy the angular and energy spectrum of the secondary electrons is strongly influenced by all three two-body interactions involved in the process. In the case of electron impact also exchange effects contribute to the shape and magnitude of the cross section. The authors conclude that triply and doubly differential ionization cross sections depend sensitively on the description of the three-body system.

Screening effects on Wannier's theory of atomic threshold ionisation

The Wannier-Peterkop-Rau theory (1953) for the ionisation of atoms by by electron impact near threshold has been extended taking screening into account. Spherically symmetric atomic potentials are shown quite generally to leave Wanniers threshold law-originally derived from pure Coulomb interactions-unchanged. In particular it is shown that screening may lead to an unexpectedly large energy range over which the threshold theory holds. Results are not inconsistent with experimental work.