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Based on the strong-field approximation (SFA), we investigate the interference between different returning orbits of the rescattering electron in nonsequential double ionization. We find that the effect of interference is negligible in the recollision-impact ionization process but is prominent in the recollision excitation with subsequent ionization process and induces fast oscillation with constant period in the laser intensity dependence of the asymmetry parameter. The dominant contribution to the fast oscillation comes from the mixed interference between different pairs of returning orbits in different excitation channels. However, the oscillation disappears after laser focus averaging is performed due to its rather small period.

The consensus on the mechanism of atomic excitation in an intense laser field has not been reached so far because the proposed mechanisms may dominate in different regimes. In this paper, we show a clear transition of the underlying physical mechanism of strong-field atomic excitation from multiphoton resonance to coherent recapture by investigating the intensity dependence of the yields of neutral excited atoms $({\mathrm{Ar}}^{*})$ and singly charged ions $({\mathrm{Ar}}^{+})$ of argon experimentally and theoretically. Our results indicate that, for 400 nm, the multiphoton resonance mechanism plays a significant role at low intensity and coherent recapture mechanism becomes important at high intensity. While for 800 nm, distinctive out-of-phase oscillations of ${\mathrm{Ar}}^{*}$ and ${\mathrm{Ar}}^{+}$ yields are identified, implying that coherent recapture mechanism dominates. Our work provides a more comprehensive understanding of the atomic excitation mechanism in an intense laser field.

A Wigner distributionlike function based on the improved strong-field approximation theory is proposed to calculate the rescattering time-energy distribution (RTED) of high-energy photoelectrons of atomic above-threshold ionization process in few-cycle laser fields with different frequencies. The RTED shows bell-like stripes and the outermost stripe is compared with semiclassical results given by the simple-man model with consideration of different positions of tunnel exit and different initial longitudinal momenta. Analysis indicates the existence of the tunnel exit. However, though it shifts farther away from the core with decreasing frequency, the position of the tunnel exit is significantly less than the prediction by adiabatic theory even for the low-frequency case which is well in the tunneling regime. Our results also imply that the effect of the tunnel exit is more important than that of the initial longitudinal momentum at the tunnel exit for the backward-scattering electrons. Moreover, the inner stripe structures in the RTED are attributed to interference between electrons with the same final energy emitted at different ionization times.

We investigate the ionization dynamics of one-electron model diatomic molecules with different orbital symmetries ($1{\ensuremath{\sigma}}_{g}$ and $1{\ensuremath{\sigma}}_{u}$) in a strong laser field via the full three-dimensional time-dependent Schr\"odinger equation. It is shown that the ionization probabilities of the molecules with different initial states ($1{\ensuremath{\sigma}}_{g}$ and $1{\ensuremath{\sigma}}_{u}$) oscillate out of phase vs internuclear distance, and the photoelectron spectra of the aforementioned two molecules also show out-of-phase oscillation structures. The above intriguing features are well reproduced by the simulations of the length-gauge strong-field approximation, which can be ascribed to the distinct interferences of electronic wave packets released from different atomic centers.

Quantum interference plays an important role in various intense-laser-driven atomic phenomena, e.g., above-threshold ionization and high-order-harmonic generation, and provides a useful tool in ultrafast imaging of atomic and molecular structure and dynamics. However, it has eluded observation in nonsequential double ionization (NSDI), which serves as an ideal prototype to study electron-electron correlation. Thus far, NSDI usually could be well understood from a semiclassical perspective, where all quantum aspects have been ignored after the first electron has tunneled. Here we perform coincidence measurements for NSDI of xenon subject to laser pulses at 2400 nm. It is found that the intensity dependence of the asymmetry parameter between the yields in the second and fourth quadrants and those in the first and third quadrants of the electron-momentum-correlation distributions exhibits a peculiar fast oscillatory structure, which is beyond the scope of the semiclassical picture. Our theoretical analysis indicates that this oscillation can be attributed to interference between the contributions of different excited states in the recollision-excitation-with-subsequent-ionization channel. Our work demonstrates the significant role of quantum interference in NSDI and may create an additional pathway towards manipulation and imaging of the ultrafast atomic and molecular dynamics in intense laser fields.

We experimentally investigate the recoil-ion momentum distribution along the laser polarization direction for nonsequential double ionization of Xe by 50 fs, 2400 nm laser pulses at intensities of $(22\ensuremath{-}68) \mathrm{TW}/{\mathrm{cm}}^{2}$. The observed doubly charged ion momentum distribution exhibits a distinct transition from a flat-top structure near zero longitudinal momentum at $22\phantom{\rule{0.16em}{0ex}}\mathrm{TW}/{\mathrm{cm}}^{2}$ to the one with two maxima at nonzero longitudinal momentum at $37\phantom{\rule{0.16em}{0ex}}\mathrm{TW}/{\mathrm{cm}}^{2},\phantom{\rule{4pt}{0ex}}52\phantom{\rule{0.16em}{0ex}}\mathrm{TW}/{\mathrm{cm}}^{2}$, and $68\phantom{\rule{0.16em}{0ex}}\mathrm{TW}/{\mathrm{cm}}^{2}$, which is remarkably different from the case of 800 nm. Simulation based on a semiclassical model is used to obtain the ratios of contributions from the recollision-impact ionization (RII) and the recollision-induced excitation with subsequent field ionization (RESI) in nonsequential double ionization. Our calculation reveals that the increasing contribution of the RII channel is responsible for the more prominent double-hump structure at longer wavelength or higher laser intensity. Moreover, a simple fitting based on the calculated ratios allows one to reproduce the experimental ion momentum distributions well and obtain contributions from these two channels.