Evidence of supercritical density in 45-fs-laser-irradiated Ar-cluster plasmas

Abstract

Plasmas have been created by irradiating atomic clusters of argon with high-contrast, ultrashort $(<100\mathrm{fs})$ laser pulses and recording the corresponding K-shell emission spectra. The high-resolution Ar K-shell spectra were analyzed using a theoretical two-temperature collisional-radiative model of irradiated atomic clusters incorporating the effects of highly energetic electrons $(\ensuremath{\sim}5\mathrm{keV}).$ Comparison of the measured spectrum with the calculations indicates that a supercritical density near ${N}_{e}=2\ifmmode\times\else\texttimes\fi{}{10}^{22}{\mathrm{cm}}^{\ensuremath{-}3}$ was attained in the shortest-pulse and highest-contrast shots. Results from additional experiments using pulses on the order of 300--400 fs are also consistent with a hydrodynamic model of laser-cluster interactions.