Nonsequential double ionization of Ar by 45 fs laser pulses (800 nm) at $(4–7)\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ was explored in fully differential measurements. Well below the field-modified recollision threshold we enter the multiphoton regime. Strongly correlated back-to-back emission of the electrons along the polarization direction is observed to dominate in striking contrast to all previous data. No effect of Coulomb repulsion can be found, the predicted cutoff in the sum-energy spectra of two emitted electrons is confirmed, and the potential importance of multiple recollisions is discussed.

• Received 22 January 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.053001