Intensity-dependent recollision dynamics in strong-field nonsequential double ionization

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Using the three-dimensional classical ensemble model, we systematically investigate the strong-field nonsequential double ionization (NSDI) of He atom by intense linearly polarized laser pulses at different intensities for 750 nm and 1500 nm in wavelength. In the intensity range of 0.4−0.8 PW/cm<sup>2</sup> considered in this work, for 750 nm wavelength the correlated electron pairs are always distributed mainly near the diagonal but for 1500 nm wavelength, with increasing laser intensity the population of electron pairs moves from the diagonal to the two axes, forming a near-axis V-shaped structure at 0.8 PW/cm<sup>2</sup>. The analysis indicates that for 750 nm with increasing laser intensity the contribution from the single-return events to NSDI decreases sharply and the contribution from the multiple-return events increases. For 1500 nm wavelength when the laser intensity increases, the contributions from one-, two- and three-return trajectories decrease and the contributions of other trajectories increase. It is because most of ionized electrons have a non-zero initial transverse momentum. After the excursion of the ionized electron, when it returns to the parent ion at the first time there is a distance in the transverse direction between the free electron and the parent ion, which hinders the recollision and NSDI from occurring. The transverse deviation can be significantly reduced by the Coulomb attraction from the parent ion to the free electron when it returns back to the parent ion in the longitudinal direction. Higher intensity results in larger returning velocity for the free electron. The free electron faster passes by the parent ion and the Coulomb attraction has less time to pull the free electron to the parent ion. For each return the compensation of the Coulomb attraction for the transverse deviation for high intensity is weaker than for low intensity. Thus for higher intensities more returns are required to compensate for the transverse deviation. Moreover, numerical results show the recollision distance in NSDI is smaller for the longer wavelength and higher intensity. It is attributed to the larger returning velocity of the free electron at the longer wavelength and higher intensity, which can more easily overcome the strong Coulomb repulsion between the two electrons and achieve a smaller recollision distance. Finally, electron correlation behaviors for those trajectories where recollision occurs with different return times are studied.