Observation of H-like ions within argon clusters irradiated by 35-fs laser via high-resolution x-ray spectroscopy

Abstract

Plasmas have been created with bulk temperatures $>~400$ eV and densities greater than $1\ifmmode\times\else\texttimes\fi{}{10}^{21}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ in some regions by irradiating atomic clusters of argon with a 0.4 TW ultrashort (35 fs) laser pulse. An improved nozzle has been designed so that larger clusters can be formed, allowing the clusters to maintain high-density cores after the preplasma is formed due to the laser prepulse. Ar is the element with the highest atomic number, Z, for which the Lyman-\ensuremath{\alpha} resonance line $({\mathrm{Ly}}_{\ensuremath{\alpha}})$ has been observed in ultrashort $(<100$ fs) pulse laser experiments for any target. The unambiguous spectroscopic evidence, including high-resolution measurements of the ${\mathrm{Ar}}^{+17}$ ${\mathrm{Ly}}_{\ensuremath{\alpha}}$ spectrum, is consistent with a theoretical two-temperature collisional-radiative model of irradiated atomic clusters incorporating the effects of highly energetic electrons $(\ensuremath{\sim}5$ keV).