We investigate the ultrafast electron correlation effects during non-sequential double ionization (NSDI) of argon subjected to a combined femtosecond field composed of counter-rotating two-color circularly polarized (TCCP) pulse laser using a 3D classical ensemble model (CEM). Our simulation results reveal that manipulation of the carrier-envelope phase (CEP) of the external driving field modulates the dynamical behavior of the two electrons, resulting in a notable sensitivity of their momentum distribution to the relative phase of two components of the counter-rotating TCCP field. Through inversion analysis, we uncover the capability to direct electrons toward a single direction, thereby facilitating focused ion-electron collisions on the attosecond timescale. Furthermore, we observe that adjusting the CEP exerts a substantial influence on the NSDI ionization process, modifying the relative contributions of the recollision-induced ionization (RII) mechanism and recollision excitation with subsequent ionization (RESI) mechanism.

We have investigated the Rydberg state population of an argon atom by an intense laser pulse and its wavepacket dynamics monitored by another successive laser pulse in the tunneling regime. A wavepacket comprising a superposition of close high-lying Rydberg states is irradiated by a multicycle laser pulse, where the sub-wave components in the wavepacket have fixed relative phases. A time-delayed second laser pulse is employed to apply on the excited Rydberg atom. If the time is properly chosen, one of the sub-wave components will be guided towards the ionization area while the rest remains intact. By means of this pump–probe technique, we could control and monitor the Rydberg wavepacket dynamics and reveal some interesting phenomenon such as the survival rate of individual Rydberg states related to the classical orbital period of electron.

Utilizing a classical ensemble model (CEM), we have studied the rescattering behavior of quasi-free tunneling electron and correlation dynamics of electron pair during nonsequential double ionization (NSDI) of Xe atoms induced by a counter-rotating, two-color, elliptically polarized (TCEP) laser field. Our numerical results reveal that the momentum of the first tunneling electron is predominantly located in the third quadrant of momentum spectrum, whereas the momentum spectrum of the second electron presents a triangular structure along the direction of the negative vector potential of driving field. This phenomenon occurs because the first electron has greater emission kinetic energy than the second one, rendering the latter more sensitive to the influence exerted by the electric field vector potential. Moreover, we also observe that increasing ellipticity significantly alters ion momentum distributions by reducing weights at non-zero peaks, which indicates a weakened electron correlation. The large ellipticity field progressively decreases recollision energy, reflected in broadening delays between recollision and double ionization time, ultimately modifying ionization pathways and correlation features of electron pair.

We experimentally investigate the dissociative single ionization process, O ₂ → O ⁺ + O, of aligned O ₂ molecule in intense laser fields. The yield of a vibrational structure in the kinetic energy release spectra is measured as a function of alignment angle. By quantitative comparison of the measured angle-dependent dissociation probability with the simulation of a classical model that considers ionization and thus additional interaction within the laser pulse, we are able to distinguish the dissociation pathway that contribute to the vibrational structure. It is found that for a relative low laser intensity, the vibrational structure are produced from the dissociation pathway of a ⁴Π _u → f ⁴Π _g − 1ω. As the laser intensity increases, the increasing of the population probability of higher vibrational states and the absorbtion probability of more photons makes another two dissociation pathways open and become dominant.

We theoretically investigate the modulation structures of atomic Rydberg excitation in tunneling ionization and show its various features with diversity of population amplitude as a function of laser peak intensity. Simulating by the time-dependent Schrödinger equation, we find that the distribution of bound states with low-lying principal quantum number n tends to be dominant on total excitation probability with opposite phase compared with other higher n states. At the same time, a well-defined parity angular momentum state makes the main contribution under one certain laser intensity. This Rydberg residual population effect of a rare-gas atom surviving from the laser field is elaborated by the repopulation of Rydberg states via Λ -type Raman transitions joining with a low-energy free electron, which confirms that multiphoton resonance exists in the tunneling regime. It helps in understanding the peak structures of the excited state population quantitatively as well as the atomic stabilization established in the tunneling regime. A physical scenario is also established for the role of Rydberg atom radial size in the ionization.