Two-photon ionization and three-photon above-threshold ionization of argon

Studies of nonlinear laser-matter interaction have been so far limited to wavelengths from the near Ultraviolet to Infrared, because of the low brightness of currently available sources outside this range. However nonlinear processes in the VUV/Soft Xray domain would initiate multiphoton innershell spectroscopy, XUV nonlinear optics and applications of such processes to metrology (for instance, autocorrelation measurements of ultrashort XUV pulses).