Strong-field double ionization at the transition to below the recollision threshold

We investigate the strong-field double ionization using a semiclassical model, in which the recollision-induced excitation-tunneling (RIET) effect has been taken into account with the Wentzel-Kramers-Brillouin (WKB) approach. When the laser intensity is below the recollision threshold, we find that both RIET and multiple recollisions become significant and can produce the anticorrelated (back-to-back) electron pair. Distinct footprints left by these two mechanisms on the correlated momentum spectra have been identified. As another signature of the transition to below the recollision threshold regime, we find that the V-shaped (or fingerlike) structure in the correlated momentum spectra fades away. Our model calculations have been compared with a recent experiment on argon atoms.