Transport of Fast Ions through Solid and Clusters under Intense Laser Pulses: Interaction dynamics probed by X‐ray spectroscopy - Archive ouverte HAL
Hdr Année : 2012

Transport of Fast Ions through Solid and Clusters under Intense Laser Pulses: Interaction dynamics probed by X‐ray spectroscopy

Résumé

When studying the interaction between a fast heavy ion projectile and a solid target, we can be interested either by the target itself (for instance, the formation of tracks in insulating materials) or by what happens to the ion. These different studies are obviously strongly connected. In our group, we mainly concentrate on the transport of electrons in ion excited states. Two extreme representations may be used to evaluate, especially, the electronic ion stopping in matter. These two aspects are linked to the response of the target electrons: In the first picture, the solid is seen as an assembly of atoms and the HCIs undergo a series of binary ion‐ atom collisions with the target electrons. This pure collisional approach has been used in the Bethe theory to calculate the ion stopping power. The ensemble of ion‐atom cross sections forms the database of any theoretical treatment. In the other picture, closer in spirit to the dielectric theory first proposed by Bohr, the target electrons are considered to respond collectively to the passage of the projectile. The HCIs induce a polarization of the medium described as a wake of electronic density fluctuation trailing the ion, the so‐called wake field. The gradient of the wake potential leads to the establishment of an electric field, and its local value at the projectile can be used to calculate the stopping power. Its value can be as high as that of the electric field experienced by an electron on the first Bohr orbit, i.e., 5.4 109 V/cm. Both types of calculations appear to achieve good agreement with stopping power measurements, despite a rather different physical picture for the behavior of the target electrons. It is clear that the response of the target electrons to the passage of the ion has direct consequences on the projectile ion: it is slowed down, but not only, the populations in its excited states are also altered by the presence of the environment (the solid). Notably, the presence of the wake field may mix the ion excited states by Stark effect in a coherent manner while a pure collisional response of target electrons destroys any coherence. Hence, the group tackled the study of the production and transport of projectile excited states. The analysis of the de‐excitation of these excited states allow us to probe the solid response. High values of electric field are also reachable with intense laser pulses inducing also a strong perturbation in the matter. Laser excitation/ionization of nanometer‐sized atomic clusters offers opportunity to explore ultrafast many particle dynamics. Indeed, submitted to strong optical fields, the response of free clusters is often very different from that of single atoms/molecules. Large clusters, similarly to solids, couple very efficiently to intense sub‐picosecond laser pulses. Near 100 % of the laser radiation can be absorbed giving rise to the observation of highly charged ions with energies reaching MeV and warm electrons with a fraction having keV energies. Another fascinating feature of this interaction is its efficiency for converting photons in the eV range to X‐rays with keV energies. This emission is due to the de‐excitation of HCIs having inner shell vacancies. The lifetime of these excited states being very short (down to some fs), their observation gives access to the dynamical evolution of the irradiated cluster on a time scale comparable to that of the laser pulse duration. Our group has performed several experiments to measure the evolution of absolute photon yields and charge state distributions of the emitting ions with different physical parameters governing the interaction; namely intensity, polarization, pulse duration and wavelength of the laser as well as the size of the clusters. These various studies have allowed us to determine the ionization mechanisms in inner shells and their time competition. In this manuscript, I only present the influence of the laser intensity.
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Dates et versions

tel-01103310 , version 1 (14-01-2015)

Identifiants

  • HAL Id : tel-01103310 , version 1

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Emily Lamour. Transport of Fast Ions through Solid and Clusters under Intense Laser Pulses: Interaction dynamics probed by X‐ray spectroscopy. Atomic Physics [physics.atom-ph]. Université Pierre et Marie Curie, 2012. ⟨tel-01103310⟩
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