Low intensity threshold values for keV X-ray production in laser-cluster   interaction
  Absolute Ar XK (3 keV) and Xe XL (4 keV) photon yields are measured as a function of laser peak intensity, for large (104 - 105 atoms) argon and xenon clusters submitted to intense laser pulses. A clear threshold behavior is observed. The intensity threshold is found as low as 2 1014 Wcm-2, a value matching the production of singly-charged argon or doubly-charged xenon by optical field ionization. Possible mechanisms involved in the inner-shell ionization are discussed. The outcome on the optimization of keV X-ray production is underlined. 
C. Prigent, L. Adoui, E. Lamour, J.-P. Rozet, D. Vernhet, O. Gobert,   P. Meynadier, D. Normand and M. Perdrix