Few-photon multiple ionization of ne and ar by strong free-electron-laser pulses.

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of Ne{sup 2+} production reveals the dominance of nonsequential two-photon double ionization at intensities of I<6x10{sup 12} W/cm{sup 2} and significant contributions of three-photon ionization as I increases. Ne{sup 2+} recoil-ion-momentum distributions suggest that two electrons absorbing ''instantaneously'' two photons are ejected most likely into opposite hemispheres with similar energies.