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Diexcitation électronique de He par un photon au voisinage du seuil de double ionisation

Abstract : This work presents a study of the 1p0 excited states of He that can be reached by absorption of a single photon carrying an energy close to the double ionization threshold (DIT) (79eV). Above the DIT, these states are the double continuum states, below, they are the double excited states. these two types of states are tightly coupled to the single continuum states with or without excitation of the residual ion He+, owing to their degeneracy in energy. in a one-photon process ,theses states can only be formed owing to the elecronic correlations in the system which must be well described to obtain quantitatively good results. our study is part of a line of work which at a unified description of all these doubly excited, ionized-excited, and double continuum states.
We use the Hyperspherical R-Matrix with Semiclassical Outgoing Waves (HRM-SOW) method, initially dedicated to double photoionization studies. We extend it ot extract information on the single continuum. This extension allows us to compute cross sections of single photoionization with or without excitaion up to n=50 for an excess energy of 100meV just above the double ionization threshold. A deep insight into his process is given by a partial wave analysis. The results obtained shed ligh on the key role of angular and radial correlations. The numerous data we obtain on double and single ionization allow us to establish relation between these two processes. We show that single ionization with an infinite excitation of the residual ion merges into double photoionization when the excess energy is redistributed between the two electrons. it appears that this relation is valid not only for low but also for high photon energy. Since the HRM-SOW method can produce the integrated cross section for double photoionization with a hig accuracy in the low energy domain, we check the Wannier threshold law. The parameters extracted support strongly this law and are in good agreement with experimental results.
We complete this work with the study of doubly excited autoionizing states. The latter manifest themselves as Fano-like resonances in the ionization cross section below the double ionization threshold. We compute partial integrated as well as differential cross sections which allow us to identify a large number of resonances up to the fifh single ionization threshold. Some of them are observed for the first time.
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Celsus Bouri. Diexcitation électronique de He par un photon au voisinage du seuil de double ionisation. Physique Atomique [physics.atom-ph]. Université Paris Sud - Paris XI, 2007. Français. ⟨tel-00265629⟩

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