  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.
