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 df the estimated mean cluster size In the supersonic expansion, In particular, up to 4x10(6) x-ray photons per laser shot are detected at intensities approaching 5x10(17) W cm(-2). This allows us to determine precisely a maximum conversion efficiency of 1.7x10(-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.