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

Plasmas have been created with bulk temperatures $>~400$ eV and densities greater than 1×10²¹ cm^-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-{α} resonance line (Ly_α) has been observed in ultrashort $(<100$ fs) pulse laser experiments for any target. The unambiguous spectroscopic evidence, including high-resolution measurements of the Ar⁺¹⁷ Ly_α spectrum, is consistent with a theoretical two-temperature collisional-radiative model of irradiated atomic clusters incorporating the effects of highly energetic electrons (~5 keV).