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 $4\times {10}^6$ x-ray photons per laser shot are detected at intensities approaching $5\times {10}^{17}$ ${\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}.$ This allows us to determine precisely a maximum conversion efficiency of $1.7\times {10}^{-8}$ 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.