Separation of Recollision Mechanisms in Non-Sequential Strong Field Double Ionization of Ar: The Role of Excitation-Tunneling

Vector momentum distributions of two electrons created in double ionization of Ar by 25 fs, 0.25 PW/cm² laser pulses at 795 nm 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