Strong-field non-sequential ionization: The vector momentum distribution of multiply charged Ne ions

COLTRIMS (COLd Target Recoil-Ion Momentum Spectroscopy) was used to measure the vector momentum distribution of Nen+ (n=1,2,3) ions formed in ultrashort (30 fsec) high-intensity (≅1015 W/cm2) laser pulses with center wavelength at 795 nm. To a high degree of accuracy the length of the Nen+ ion momentum vector is equal to the length of the total momentum vector of the n photoelectrons released, with both vectors pointing into opposite directions. At a light intensity where non-sequential ionization of the atom dominates the Ne2+ and Ne3+ momentum distributions show distinct maxima at 4.0 a.u. and 7.5 a.u. along the polarization axis of the linearly polarized light beam. First, this is a clear signature of non-sequential multiple ionization. Second, it indicates that instantaneous emission of two (or more) electrons at electric field strength maxima of the light wave can be ruled out as main mechanism of non-sequential strong-field multiple ionization. In contrast, this experimental result is in accordance with the kinematical constraints of the 'rescattering model'