Attosecond angular streaking (or “attoclock”) is an insightful technique for probing the ultrafast electron dynamics in strong laser fields. Up until recently, this technique relied solely on an accurate measurement of the photoelectron momentum distribution and has remained restricted to atomic targets. Here, we propose a novel attosecond angular streaking scheme applicable to molecules, for which the ionic fragments of dissociative ionization are detected in the polarization plane of a close-to-circular polarized laser light. Our ionic attoclock measurements are consistent with theoretical results from a numerical solution of the time-dependent Schrödinger equation and an upper bound of 10 as on the tunneling time from the attoclock readings in the ${\mathrm{H}}_2$ molecule has been given, which is significantly smaller than any definitions of tunneling time available in the literatures.

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 $\left({\mathrm{Ar}}^{*}\right)$ and singly charged ions $\left({\mathrm{Ar}}^{+}\right)$ 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.

Strong-field ionization and Rydberg-state excitation (RSE) near the continuum threshold exhibit two phenomena that have attracted a lot of recent attention: the low-energy structure (LES) just above and frustrated tunneling ionization just below the threshold. The former becomes apparent for longer laser wavelengths, while the latter has been especially investigated in the near infrared; both have been treated as separate phenomena so far. Here we present a unified perspective based on electron trajectories, which emphasizes the very important role of the electron-ion Coulomb interaction as expected in this energy region. Namely, those trajectories that generate the LES can also be recaptured into a Rydberg state. The coherent superposition of the contributions of such trajectories with different travel times (each generating one of the various LES peaks) causes an oscillation in the intensity dependence of the RSE yield, which is especially noticeable for longer wavelengths. The theory is illustrated by RSE experiments at 1800 nm, which agree very well with the theory with respect to position and period of the oscillation. The wavelength scaling of the RSE oscillation is also discussed. Our work establishes a solid relationship between processes below and above the threshold and sheds new light on atomic dynamics driven by intense laser fields in this critical energy region.

We present theoretically obtained photoelectron momentum distributions (PMDs) for the strong field ionization of argon in an elliptically polarized laser field at a central wavelength of 400 nm. Three different theoretical approaches, namely, a numerical solution of the time-dependent Schrödinger equation (TDSE), a nonadiabatic model, and a classical-trajectory Monte Carlo (CTMC) model are adopted in our calculations. From the TDSE calculations, it is found that the attoclock offset angle (most probable electron emission angles with respect to the minor axis of the laser’s polarization ellipse) in the PMD increases with rising ATI order. While this result cannot be reproduced by the CTMC model, the nonadiabatic model achieves good agreement with the TDSE result. Analysis shows that the nonadiabatic corrections of the photoelectron initial momentum distribution (in both longitudinal and transverse directions with respect to the tunneling direction) and nonadiabatic correction of the tunneling exit are responsible for the ATI order-dependent angular shift.

Experimentally and theoretically, we explore the photoelectron momentum distributions of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ and their companion atoms Ar and Xe, which possess nearly equal electron binding energies to the corresponding molecules, subject to close-to-circularly polarized laser fields. Compared to Xe, significant deviations of the results for ${\mathrm{O}}_2$ have been observed experimentally. In the meantime, the measured photoelectron momentum distributions of ${\mathrm{N}}_2$ match those of Ar. With the theoretical analysis based on strong field approximation, the results that ${\mathrm{N}}_2$ behaves just like its companion atom Ar and ${\mathrm{O}}_2$ behaves significantly different from Xe have been attributed to the two-center interference effect of diatomic molecules, which is disparate for ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ due to the parity difference of their molecular ground-state wave functions.

In this work, we have experimentally and theoretically investigated the high-lying Rydberg state excitation for noble gas atoms subject to an intense near-infrared laser field. To obtain the signal of these high-lying Rydberg atoms which are further ionized by a constant electric field $\left(F_c\right)$, coincident detection of the photoelectrons and photoions with well-chosen arrival times is performed. Based on a fitting to the time dependence of the coincidently measured photofragment yields with a semiempirical formula, we can extract a principal quantum number distribution (PQND) of the population of the excited states in a range closely related to the strength of $F_c$. The extracted valid PQND is in qualitative agreement with a semiclassical model calculation. Our work thus provides a method to extract the PQND and study the ultrafast high-lying Rydberg state excitation.