Two-photon double ionization of rare gases by a superposition of harmonics

Two XUV-photon double ionization of Kr and Ar by an intense superposition of higher-order harmonics of a Ti:sapphire laser is demonstrated. The logarithm of the Kr2+ and Ar2+ signals measured as a function of the logarithm of the XUV intensity depicts a linear dependence with a slope of 1.8 +/- 0.2. Under the XUV intensities and photon energies employed, this slope value provides evidence that to the observed double ionization the direct process has a significant contribution. Applications of the above process to attosecond pulse metrology are also discussed.