Absolute keV photon yields from ultrashort laser-field-induced hot nanoplasmas

We study the x-ray L-shell production from large krypton clusters submitted to ultrashort and intense laser pulses. The x-ray photon emission pattern appears to be isotropic and the absolute x-ray photon yields per laser pulse are measured as a function of the laser intensity and of the estimated mean cluster size in the supersonic expansion. In particular, up to 4×10⁶ x-ray photons per laser shot are detected at intensities approaching 5×10¹⁷ Wcm^-2. This allows us to determine precisely a maximum conversion efficiency of 1.7×10^-8 between the incoming IR photon and the generated x-ray photon energies. Finally, the x-ray photon emission is understood as the result of highly stripped ion production with L-shell electron-impact ionization and excitation in laser-heated cluster-sized nanoplasmas.