We report an experimental and theoretical study of very low-energy photoelectrons in tunneling ionization process from noble gas atoms interacting with ultrashort intense infrared laser pulses. A universal peak structure with electron energy well below 1~eV in the photoelectron spectrum, corresponding to the double-hump structure in the longitudinal momentum distribution, is identified experimentally for all atomic species. Our quantum and semiclassical analysis reveal the role of long-range Coulomb potential in the production of this very low-energy peak structure.

So far, nonsequential double ionization (NSDI) of atoms can be well understood within a semiclassical or even classical picture. No quantum effect appears to be required to explain the data observed. We theoretically study electron correlation resulting from NSDI of argon in a low-intensity laser field using a quantum-mechanical $S$-matrix theory. We show that quantum interference between the contributions of different intermediate excited states of the singly charged argon ion produces a transition from back-to-back to side-by-side emission with increasing laser intensity, which is in close agreement with the experimental data. For higher intensities, this transition is enhanced by the consequences of depletion of the excited states.

A semiclassical model is developed to investigate the atomic nonsequential double-ionization process in elliptically polarized intense laser field. First, the ellipticity dependence of the ion yield of ${\text{Ne}}^{2+}$ is calculated and a good agreement with the experiment observation is found. Second, the frequency dependence of the ratio of ${\text{Ne}}^{2+}:{\text{Ne}}^{+}$ is investigated for fields with different ellipticities. In the high-frequency regime, the ratio increases rapidly with increasing wavelength and is not dependent on the ellipticity. However, the ratio reaches maximum which decreases with increasing ellipticity and decreases with wavelength in the long-wavelength regime.

The study of the ionization process of molecules in an intense infrared laser field is of paramount interest in strong-field physics and constitutes the foundation of imaging of molecular valence orbitals and attosecond science. We show measurement of alignment-dependent ionization probabilities of the lower-lying orbitals of the molecules by experimentally detecting the alignment dependence of fluorescence emission from tunnel ionized carbon dioxide molecules. The experimental measurements are compared with the theoretical calculations of the strong field approximation and molecular Ammosov-Delone-Krainov models. Our results demonstrate the feasibility of an all-optical approach for probing the ionization dynamics of lower-lying orbitals of molecules, which is until now still difficult to achieve by other techniques. Moreover, the deviation between the experimental and theoretical results indicates the incompleteness of current theoretical models for describing strong field ionization of molecules.

Rresonancelike enhancement of groups of adjacent peaks in the photoelectron spectrum of high-order above-threshold ionization has been well documented for noble-gas atoms subjected to intense infrared laser pulses. However, its physical origin is still under debate. In this Rapid Communication, we investigate experimentally and theoretically high-order above-threshold ionization of diatomic nitrogen and oxygen molecules in order to shed more light on the underlying mechanism. The resonancelike enhancement is experimentally observed for N${}_{2}$ but is absent for O${}_{2}$ molecules. A simulation on the basis of $S$-matrix theory and the strong-field approximation reproduces the experimental observations. This implies that the resonancelike enhancement can be attributed to the channel-closing effect. The specific molecular structure plays a decisive role for the presence or absence of this enhancement in molecular systems.

Using a semiclassical model, we evaluate the correlated electron momentum distribution from nonsequential double ionization (NSDI) of neon in an elliptically polarized laser field. The momentum distribution pattern exhibits a nontrivial change with the increase of the laser ellipticity, which can be understood as a combined effect of the extra laser electric field and the ionic Coulomb potential on the tunnel-ionized electron trajectory. Especially for large ellipticities ($$\epsilon${}>0.2$), multiple return collision trajectory contributes dominantly to the NSDI yield and determines the correlated electron momentum distribution, revealing an indispensable role of ionic Coulomb potential in NSDI dynamics under elliptical polarization. Moreover, the nontrivial evolution of the correlated electron momentum distribution with the ellipticity revealed in our model should be testable by a realistic experiment.