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

Plasmas have been created by irradiating atomic clusters of argon with high-contrast, ultrashort (<100fs) 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 (∼5keV). Comparison of the measured spectrum with the calculations indicates that a supercritical density near N_e=2×10²²cm^-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.