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Etude théorique de l'interaction entre une impulsion laser intense et un agrégat de gaz rare

Abstract : The irradiation of nanometer-scale rare gas clusters by a short (a few hundreds of femtoseconds) and intense (I>10^{15} W/cm2) laser pulse yields multi-keV short X-ray bursts. We employ an hydrodynamic model, the so-called "nanoplasma model", to understand the mechanisms that tailor the interaction. In this model, the cluster is treated as a dielectric sphere embedded in the quasistatic laser field leading to the formation of a plasma of nanometric size. We have shown that this model cannot reproduce the experimental results such as the high ionization states and associated X-ray spectra. We have thus included in the model two additional mechanisms that significantly improve the ionization dynamics :

- We have introduced high order ionization processes involving intermediate excited states X^{q+} + e- -> X^{q+*} +e- -> ... -> X^{(q+1)+} +2 e-. We have used a model potential approach to describe the electronic structure of the cluster's ions (and atoms), and we have computed the total excitation and ionization cross-sections in the distorted-wave Born approximation.

- We have studied the influence of screening phenomena induced by the electronic density on the interaction dynamics. By using a sophisticated potential, we have shown that screening effects enhance ionization and lower excitation cross sections with respect to the unscreened data.

The improved nanoplasma model allows us to reproduce the populations of highly charged states experimentally observed, and the variation of argon He_alpha emission with respect to the various experimental parameters (cluster's size, laser pulse duration, intensity and wavelength). We have further computed time- and energy-resolved X-ray spectra which emphasize ultra-short emission duration (less than 100 fs), and therefore indicate that cluster-based X-ray sources are adequate to ultrafast X-ray science applications.
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Contributor : Samuel Micheau <>
Submitted on : Sunday, November 11, 2007 - 3:08:13 AM
Last modification on : Thursday, July 26, 2018 - 12:07:30 PM
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  • HAL Id : tel-00186659, version 1




Samuel Micheau. Etude théorique de l'interaction entre une impulsion laser intense et un agrégat de gaz rare. Physique Atomique [physics.atom-ph]. Université Sciences et Technologies - Bordeaux I, 2007. Français. ⟨tel-00186659⟩



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