We consider high-order above-threshold ionization of polyatomic molecules by a strong laser field. An improved molecular strong-field approximation which takes into account the electron rescattering off the molecular centers is developed. The presented theory is applied to calculate the photoelectron energy and angular distributions for the ozone molecule. The obtained spectra exhibit pronounced minima, and this is explained as a three-point destructive interference of the rescattered electron wave packets.

Additional support for the concept of ”quantum orbits” is presented that emphasizes in particular the importance of ”long orbits” where the time between ionization and rescattering may be many periods of the laser field. Two examples are discussed, above-threshold ionization by an elliptically polarized laser field and intensity-dependent enhancements of certain spectral regions within the backscattering plateau, where we compare experimental data and the theoretical quantum-orbit simulations. In both cases, good agreement is obtained.

Abstract Plasmonic field-enhanced high-order harmonic generation (HHG) is investigated theoretically using the solutions of the three-dimensional time-dependent Schrödinger equation and a (semi)classical three-step model. Plasmonic field is modeled by a spatially inhomogeneous field with a time-dependent cosine squared pulse envelope. The dependence of the HHG yield on the carrier-envelope phase (CEP) is investigated. It is shown that the position of the cutoff of the HHG spectra is very sensitive to the value of CEP and that for the inhomogeneous field the dependence on the CEP is modulo. This enables both the CEP control of the plasmonic field-enhanced HHG and the determination of the CEP modulo by the measurement of the HHG cutoff position. Contrary to the homogeneous field case, for inhomogeneous field the difference between the maximum and minimum HHG cutoff (for the CEP in the interval from 0 to) remains substantial even for pulses longer than 10 optical cycles.

Electron-ion radiative recombination assisted by a few-cycle laser pulse is analyzed in the frame of the $S$-matrix theory. The result obtained in the first Born approximation corresponds to the direct recombination of electrons with ionic targets, while the result in the second Born approximation corresponds to the recombination preceded by electron-ion scattering. Driven by the laser pulse, the once scattered electron may return to the same ion and recombine with it. The x-ray photon emitted in such a process may have high energies. The dependence of the energy spectrum on various parameters, such as the carrier-envelope phase, peak intensity, and duration of the laser pulse, and the incident electron energy and angle, is investigated. The abrupt cutoffs of the plateau structures in the energy spectra of the process are explained by classical analysis.

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.

Atoms and molecules submitted to a strong laser field can emit electrons of high energies in the above-threshold ionization (ATI) process. This process finds a highly intuitive and also quantitative explanation in terms of Feynman's path integral and the concept of quantum orbits [P. Salieres et al., Science 292, 902 (2001)]10.1126/science.108836. However, the connection with the Feynman path-integral formalism is explained only by intuition and analogy and within the so-called strong-field approximation (SFA). Using the phase space path-integral formalism we have obtained an exact result for the momentum-space matrix element of the total time-evolution operator. Applying this result to the ATI we show that the SFA and the so-called improved SFA are, respectively, the zeroth- and the first-order terms of the expansion in powers of the laser-free effective interaction of the electron with the rest of the atom (molecule). We have also presented the second-order term of this expansion which is responsible fo...

$S$-matrix theory of electron-atom scattering assisted by a few-cycle laser pulse is introduced. The result obtained in the first Born approximation corresponds to the direct or single scattering, while the result in the second Born approximation corresponds to the rescattering or double scattering contribution to the laser-assisted scattering process. The rescattered electrons may acquire high energies while moving in the laser pulse. The dependence of the energy spectrum on the value of the carrier-envelope phase and the duration of the laser pulse is investigated. The abrupt cutoffs of the plateau structures in the energy spectra of the process are explained by the classical analysis. It is shown that the height of both the single and double scattering plateaus can be increased by orders of magnitude by choosing the heavier atomic targets.

Plasmonic field‐enhanced high‐order harmonic generation is investigated theoretically. The plasmonic field is modeled as a spatially inhomogeneous field with a time‐dependent cosine squared pulse envelope. A method for solution of the three‐dimensional time‐dependent Schrödinger equation for a hydrogen atom in such a field is presented. It was found that the harmonic cutoff can be significantly increased. For larger values of the inhomogeneity parameter and longer pulses additional plateaus with higher cutoffs appear in the harmonic spectra. A semiclassical method, which takes into account nonzero initial coordinate and momentum of the ionized electron, is also developed. Using this method the observed multiplateau structure and cutoff positions are explained.

We consider above-threshold ionization of polyatomic molecules by a strong laser field within the molecular strong-field approximation (MSFA). A polyatomic molecule is modeled by an $(N+1)$-particle system, which consists of $N$ heavy atomic (ionic) centers and an electron. After the separation of the center-of-mass coordinate, the dynamics of this system is reduced to the relative electronic and nuclear coordinates. Two forms of the MSFA, one with the field-free and the other with the field-dressed initial molecular bound state, are derived. For neutral polyatomic molecules the ionization amplitude takes a simple form which allows interpretation as a multiple-slit-type interference. This is illustrated by a numerical example for the ozone molecule.