Observation of nonsequential double ionization of helium with optical tunneling

We have measured the ion yields for helium ionized by 120 fs, 614 nm laser pulses for intensities near 10¹⁶ W/cm². We have found that for these ultrashort pulses the He⁺² data exhibit a feature which saturates in parallel with the He⁺ signal indicating that the ionization may proceed nonsequentially. We propose a new mechanism, which can exist only in the tunneling regime, for such nonsequential ionization.