We present high resolution photoelectron energy spectra of noble gas atoms from high intensity above-threshold ionization (ATI) at midinfrared wavelengths. An unexpected structure at the very low-energy portion of the spectra, in striking contrast to the prediction of the simple-man theory, has been revealed. A semiclassical model calculation is able to reproduce the experimental feature and suggests the prominent role of the Coulomb interaction of the outgoing electron with the parent ion in producing the peculiar structure in long wavelength ATI spectra.

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 calculation of the second-order (rescattering) term in the $S$-matrix expansion of above-threshold ionization is presented for the case when the binding potential is the unscreened Coulomb potential. Technical problems related to the divergence of the Coulomb scattering amplitude are avoided in the theory by considering the depletion of the atomic ground state due to the applied laser field, which is well defined and does not require the introduction of a screening constant. We focus on the low-energy structure, which was observed in recent experiments with a midinfrared wavelength laser field. Both the spectra and, in particular, the observed scaling versus the Keldysh parameter and the ponderomotive energy are reproduced. The theory provides evidence that the origin of the structure lies in the long-range Coulomb interaction.

We present energy-resolved angular distributions of photoelectrons generated in above-threshold ionization (ATI) of nonaligned diatomic molecules ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ in high-intensity short laser pulses, with emphasis on the most energetic part of the spectra. The angular distribution for photoelectrons with energy of $10U_p$ ($U_p$: ponderomotive energy), i.e., the plateau cutoff in ATI spectra, is found to be broader in ${\mathrm{O}}_2$ than in ${\mathrm{N}}_2$. Resorting to the analyses from both an $S$-matrix theory and an intuitive semiclassical model, we attribute the observation to the effect of the ground state molecular orbital structure on high-energy electron emission in strong-field molecular ionization.

We experimentally investigate the single and double ionization of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ molecules in bicircular two-color femtosecond laser pulses, and compare with their companion atoms of Ar and Xe with comparable ionization thresholds. Electron recollision assisted enhanced ionization is observed in ${\mathrm{N}}_2$ and Ar by controlling the helicity and field ratio between the two colors, whereas the enhanced ionization via the recollision is almost absent in ${\mathrm{O}}_2$ and Xe. Our $S$-matrix simulations clearly reveal the crucial role of the detailed electronic structures of ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ on the two-dimensional recollision of the electrons driven by the bicircular two-color laser fields. As compared to Ar, the resonant multiphoton excitation dominates the double ionization of Xe.