Wavelength-dependent nonsequential double ionization of Ar at laser intensities below the recollision threshold

Using the semiclassical ensemble model, we investigated the correlated electron dynamics in the nonsequential double ionization (NSDI) of Ar driven by a few-cycle laser pulse at a fixed ponderomotive energy U-p = 0.2 a.u. and over a wide range of wavelength. When the laser wavelength increases from near-infrared to mid-infrared, the kinetic energy of the retuning electrons and the energy transfer efficiency of recollision both decrease and more NSDI events occur in the second half optical cycle after recollision, which leads to the switching of the electron momentum correlation along the laser polarization direction from side-by-side to back-to-back dominant emission. Our results also show that a significant part of NSDI events undergo a "hard recollision" through which the struck electron acquires more energy and ionizes earlier than the returning electron. This type of NSDI event is responsible for the high momentum part, especially the part with momentum higher than 2 root U-p in the correlated electron momentum spectra along the laser polarization direction. (C) 2019 Optical Society of America