MBPT and giant resonances

This interpretation has been confirmed experimentally [203] by an elegant experiment in which the different angular distributions of the escaping photoelectrons were used to separate the partial waves and determine directly how much /-wave character is present in the photoionisation spectrum 

26 Many-body perturbation theories and giant resonances 

A simplification we have made throughout this chapter is to discuss the excitation of giant resonances using the language of effective radial potentials. While this provides a useful zero-order picture, the remarks made above about strong term dependence show that it has severe limitations. A potential is only useful if it can be at least approximately correct for a group of states. If the potential needs to be recalculated for initial and final states, there are already problems in using it to describe the excitation process. Moreover, if each excited state requires a different potential to represent it, then the theoretical model is unsuitable. 

An alternative approach is to work in a frozen basis and to calculate excitations by systematic perturbation expansions, which are most elegantly written out using Feynman graphs. An excellent introduction to Feynman graphs in this context is given by Kelly [241]. 

Stategy (i) is the one followed by the school of Kelly [242] and his coworkers. It is known as many-body perturbation theory (MBPT) and has the advantage of being very flexible, as it can be applied to all atoms, including those with open shells, but the disadvantage that the calculi tions can only be taken to finite (and in practice rather low) orders of 

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