Separation of Recollision Mechanisms in Nonsequential Strong Field Double Ionization of Ar: The Role of Excitation Tunneling

Vector momentum distributions of two electrons created in double ionization of Ar by 25fs, 0.25 PW/cm2 laser pulses at 795nm have been measured using a ''reaction microscope.'' At this intensity, where nonsequential ionization dominates, distinct correlation patterns are observed in the two-electron momentum distributions. A kinematical analysis of these spectra within the classical ''recollision model'' revealed an (e,2e) -like process and excitation with subsequent tunneling of the second electron as two different ionization mechanisms. This allows a qualitative separation of the two mechanisms demonstrating that excitation-tunneling is the dominant contribution to the total double ionization yield