T. Åberg

Behaviour of Raman resonance scattering across the K X-ray absorption edge

A generalisation of the Kramers-Heisenberg formula is used to demonstrate the evolution of inelastic X-ray scattering into fluorescence across the K absorption edge. As an illustration the results of a recent synchrotron radiation study of the inelastic scattering in the vicinity of the Mn K absorption edge in KMnO4 by Briand et al. (1981) are analysed and found to be in qualitative agreement with the theory.

The resonant Auger effect

A brief survey of the resonant Raman Auger effect is presented. An outline of experimental considerations and equipment is given. The theory of the resonant Auger effect is outlined within the context of a resonant photoionization process, which we hope will provide new researchers in the field a simplified feel for the physics involved. Our focus is on the atomic problem, but we include the connection to the molecular case with some accompanying discussion. Within this context we then discuss some examples which address a number of issues we feel are central to the subject: coherence between intermediate states, the treatment and meaning of correlation effects, the relation to the normal Auger effect, and the experimental complications due to incident-photon bandpass effects.

Interpretation of rare-earth ion yields using an effective potential approach

We use a simple potential well model to study the giant resonances in Ce, Sm, Eu and Gd. We show that photoionization of a d electron in a spherical potential well into a short-lived quasibound f state produces the observed asymmetric ion yield curves. In our approach a suitable normalization condition is applied to the final state wavefunction inside the well and it is assumed that the f electron escapes within a very short time from the well. We further show that our three-parameter model allows the resonance profiles to be parametrized according to the Fano profile of the photoionization cross section.

Polarization of nonresonant inelastic x-ray scattering

The general cross section is expressed in terms of Stokes parameters of incoming and detected radiation. The dependence on the experimental arrangement is made explicit. Circular dichroism is considered for both randomly oriented and polarized target systems. A simple example illustrates how incident left- and right-handed circularly polarized x-rays can be used to obtain orientational information. We also examine the validity of the impulse (Compton) and optical (Raman) approximation for scattering by 1s electrons of hydrogen and hydrogen-like ions using an exact scaling law.

Stochastic theory of equilibrium charge-state distributions of heavy ions in gases

A stochastic theory of equilibrium charge-state distributions is presented and its explicit relation to the underlying single ion-atom collision is shown. In this approach each individual collisions is regarded as a stochastic process in which either loss or capture of one or several electrons occurs. The result is a universal representation of charge-state distributions which reproduces the observed asymmetries in terms of a single energy-independent parameter. A statistical analysis shows that this parameter, which is related to a correlation function in single collisions, assumes essentially two values only, leading to the log normal and gamma distributions, respectively. Furthermore the theory predicts a scaling law for single collisions which was fulfilled by the recent cross section measurements of Astner and co-workers (1984).

Resonance fluorescence induced autoionization

We use multichannel resonant scattering theory for an extension of the conventional theory of autoionization resonances in photoabsorption. It incorporates autoionization induced by resonance fluorescence. As a result we obtain a modified Fano profile formula. The result shows that the new mechanism has in certain cases a strong influence on the shape of the resonances.

Collisions and half collisons in strong electromagnetic fields

The theory of laser-assisted (half)collisions is discussed with emphasis on the relationship between multiphoton ionization and electron capture processes.

Hydrogenic resonances in photoabsorption of positive ions

A preliminary mapping of photoabsorption spectra of positive heavy ions in which nl to n(l+1) discrete transitions are possible has been carried out. Relativistic effects are accounted for by using the Dirac-Fock method. The authors show that along isoelectronic series the oscillator strength is heavily concentrated into the nl to n(l+1) transition in a domain which is primarily determined by the orbital angular momentum quantum number l. The origin of this behaviour is traced back to a phaseshift of pi between the nl and n(l+1) wavefunctions in the Coulomb field. Their results may be useful for the diagnostics of positive ions at, for example, synchrotron radiation sources.