XUV induced non-linear transitions

We observe two photon transitions induced by high order harmonics of a Titanium Sapphire laser with harmonic orders larger than 11 (i. e. h < 73 nm). We generate high order harmonics by focussing an 800 nm, 50 fs pulse into a I cm gas cell (filled with Xe, Kr or Ar) and use this harmonics to observe two XUV photon processes. After spectral filtering of the harmonic beam (by transmission through a thin Aluminium filter that only transmits harmonics with orders higher than 1 I), the XUV beam is focussed onto a second rare gas jet with a 5 cm focal length, platinium coated mirror. This gaz jet is located inside the sensitive zone of a time of flight mass spectrometer that allows us to detect singly and doubly charged ions. When the XUV beam is focussed into the sensitive zone of the spectrometer, one could observe doubly charged ions (we could observe A?, K? and Xe” by using Ar, Kr or Xe). This signal disapears when the XUV beam crosses the sensitive zone without being focussed which shows that the observed processes are intensity dependent. Among the several pathes that could lead to observation of such doubly charged ions, only two photons transitions involving harmonic order larger than 1 I are consistent with our observations. These transitions can be sequential or direct but both can help measuring the XUV pulse duration via autocorrelation like techniques. 0-7803-7734-6/03/$20.0