X-ray emission: a tool and a probe for laser - cluster interaction
Résumé
Interaction of high intensity ultrashort laser pulses with matter has received considerable attention in the last few years. With clusters in intense optical fields (F > 109 V/cm), the energy coupling between radiation and matter is extremely efficient, providing specific features that are not observed when atoms, small molecules or even solid are used as targets : multicharged ions reaching the MeV, hot electrons up to few keV and keV x-ray photons are produced in the laser heated cluster plasma. Here, we focus on the x-ray generation related to the production of highly stripped and excited ions. We study deexcitation of species from Ar12+ to Ar16+ with K vacancy for Ar clusters and from Xe24+ to Xe34+ with L vacancies for Xe clusters. The goal is to understand how electrons initially produced by Optical Field Ionisation (OFI) can reach enough energy to generate a strong production of ions with inner shell vacancies. We have performed quantitative studies(*) on the evolution of absolute photon emission yields and complete charge state distributions with different physical parameters governing the interaction ; namely intensity, polarization, pulse duration and wavelength of the laser, size and atomic number of the clusters. KeV x-rays are studied from rare-gas clusters irradiated with intense (I ~ 1014–1018 W cm-2) infrared and blue laser pulses of 50 – 2000 fs duration. Our measurements give rise to fundamental results like an optimum heating time in the x-ray production and an intensity threshold unexpectedly low (~ 1015 W/cm2 at 50 fs and ~ 2 1014 W/cm2 at 575 fs). The set of data and the behavior of x-ray yields with the physical parameters explored will be presented and discussed. (*) Experiments are performed on the LUCA (Laser Ultra Court Accordable) in Saclay, France.