Interacting autoionising resonances

We now build up the K-matrix theory for autoionising series, which will lead to a detailed account of (i)-(viii) above. 

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Fig. 8.2. Diagram of a typical thermionic diode arrangement to observe interacting autoionising resonances (after W.G. Kaenders et al. [389]).

We have had occasion to note the existence of fluctuations in the widths of interacting autoionising resonances, with the occasional instance of a near-zero width (cf section 8.25). 

In the present chapter, we have described many aspects of the simplest problem which can arise when an isolated resonance is formed in a single continuum we have shown that autoionisation is an interference phenomenon and compared it with the behaviour of a discrete three-level system. Two different derivations of the Fano formula have been given, and its connection with MQDT has been described. A third approach will be provided in chapter 8. Beutler-Fano autoionising resonances occur in all many-electron atoms, and a number of examples will be provided in the next two chapters. In chapter 8, the interactions between autoionising resonances will be considered, and two further questions will be discussed, namely the influence of coherent light fields on autoionising lines, and the use of lasers to embed autoionising structure in an otherwise featureless continuum. 

Fig. 7.2. A typical inner-shell excitation spectrum the 3p spectrum of Ca. Note the wide doublet splitting between the two series limits due to the large spin-orbit interaction of the nearly-closed core, the prominent Rydberg series and the broad, asymmetric autoionising resonances (after J.-P. Connerade et aL [302]).

A fundamental issue in the description of even the simplest, isolated autoionising resonance in the parametric approach followed by Fano [391] - and further pursued in K-matrix theory - is that the atom cannot be deperturbed, that is one cannot access the so-called prediagonalised states which are imagined to exist prior to autoionisation being included as a perturbative interaction, since the effect is anyway internal to the atom and cannot truly be turned off. This has the disadvantage that the parameters, once they have been obtained, must still be calculated from an ab initio model of the atom for a full comparison with theory. It might seem that the parametric theory cannot really be checked independently of ab initio calculations whose accuracy is hard to ascertain. 

However, we pick out one specific aspect here, because its appreciation does not require a detailed preliminary discussion of the underlying high field interactions the use of a laser to create or embed autoionising structure in an existing continuum is of great significance to the study of how the symmetries of autoionising resonances can be reversed (the so-called q-reversal effect, first discovered in the spectrum of an unperturbed neutral atom [382]). 

Induced ion desorption by resonant photon-stimulated desorption has been also observed. The yield of FT desorbed ions as a function of the incident photon energy from CeOa has been studied by the group at Sandia Laboratory. [101] They observed a sharp resonance in PSD of using photon energies near the Ce 4d edge at 110 eV. The resonance is due to autoionisation from excited states from the interaction of the Ce 4d and 4f levels. 

Thus, laser spectroscopy, and in particular ionisation spectroscopy involving several photons, allow one to study the mechanism of autoionisation itself, as opposed to testing the quality of atomic wavefunctions used to obtain pre-diagonalised states. Thereby, interest in the subject of interacting resonances and their parametric representation is enhanced (see in particular section 8.20). 

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