No abstract available

No abstract available

[en] The multiple-slit type destructive interference in molecular high-order harmonic generation (HHG) leads to the minima in the harmonic yield for particular molecular orientation angle θ_L and the harmonic order n. For the case of homonuclear diatomic molecules, we have found a condition for this destructive interference in the form of a nonlinear equation over n and θ_L. This condition strongly depends on the molecular symmetry, on the atomic orbitals of which a particular highest occupied molecular orbital (HOMO) consists and on the internuclear distance R. Therefore, one can learn about the molecular structure and dynamics analysing the HHG spectra of aligned molecules. Using the example of an Ar₂ molecule, having 5σ_u HOMO, for various internuclear distances, we analyse this interference condition and show that, in the (n, θ_L) plane, it manifests in the form of one or several continuous curves. Furthermore, in the (Rcos θ_L, λ_min) plane, where λ_min is an effective de Broglie wavelength of the recombining electron, this condition leads to a set of lines which are positioned between the limiting cases of molecular orbitals consisting of only one type (even or odd) of atomic orbitals (s and p in the Ar₂ case). (fast track communication)

[en] In our recent paper (2010 Phys. Rev. A 82 023412) we introduced a theory of high-order harmonic generation by diatomic molecules exposed to an elliptically polarized laser field and have shown that the nth harmonic emission rate has contributions of the components of the T-matrix element in the direction of the laser-field polarization and in the direction perpendicular to it. Using both components of the T-matrix element we now develop a theoretical approach for calculating ellipticity and the offset angle of high harmonics. We show that the emitted harmonics generated by aligned molecules are elliptically polarized even if the applied field is linearly polarized. Using examples of N₂, O₂ and Ar₂ molecules we show the existence of extrema and sudden changes of the harmonic ellipticity and the offset angle for particular molecular alignment and explain them by the destructive two-centre interference. Taking into account that the aligned molecules are an anisotropic medium for high harmonic generation, we introduce elliptic dichroism as a measure of this anisotropy, for both components of the T-matrix element. We propose that the measurement of the elliptic dichroism may reveal further information about the molecular structure.

[en] We present a theory of ionization of diatomic molecules by a strong laser field. A diatomic molecule is considered as a three-particle system, which consists of two 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. The exact S-matrix element for ionization is presented in a form in which the laser-molecule interaction is emphasized. This form is useful for application of the molecular strong-field approximation (SFA). We introduced two forms of the molecular SFA, one with the field-free and the other with the field-dressed initial molecular bound state. We relate these two forms of our modified molecular SFA to the standard molecular SFAs, introduced previously using the length gauge and the velocity gauge. Numerical examples of the ionization rates of N₂ and O₂ molecules are shown and compared for all four versions of the molecular SFA and we suggest that our modified molecular SFA should be used instead of the standard molecular SFA

[en] In order to explain the recently observed strong intensity enhancement of a single high-order harmonic we analyze high-order harmonic generation (HHG) from low-ionized laser plasma ablation in the presence of a strong radiative transition in plasma ions. If an integer multiple of the laser photon energy is resonant with this transition, a coherent superposition of ground and excited states having different parity is formed by pumping the excited state by HHG radiation. Resonant HHG from this superposition of states produces strong radiation at the plasma ions' transition frequency. The population of the excited state is increased and the stimulated emission becomes significant. The described physical picture of this process is illustrated by numerical examples related to the recent experiments

[en] Experimentally observed strong enhancement of a single high-order harmonic in harmonic generation from low-ionized laser plasma ablation is explained as resonant harmonic generation. The resonant harmonic intensity increases regularly with the increase of the laser intensity, while the phase of the resonant harmonic is almost independent of the laser intensity. This is in sharp contrast with the usual plateau and cutoff harmonics, the intensity of which exhibits wild oscillations while its phase changes rapidly with the laser intensity. The temporal profile of a group of harmonics, which includes the resonant harmonic, has the form of a broad peak in each laser-field half cycle. These characteristics of resonant harmonics can have an important application in attoscience. We illustrate our results using examples of Sn and Sb plasmas.

[en] The role of incoherent scattering in above-threshold ionization (ATI) of atoms and above-threshold detachment of negative ions is investigated. We assume that the ionized (detached) electron may scatter on a target other than its parent ion (atom). We call such a process incoherent in order to distinguish it from the coherent rescattering of an electron on its parent ion (atom). The rescattering process is known to be responsible for high-order ATI (HATI). We show that the contribution to the ionization rate of ATI with a subsequent incoherent scattering can be higher than that of the rescattering-induced HATI if the density of atomic targets is high enough. The spectra of ATI with incoherent scattering have a cutoff-like behaviour, similar to that of ATI and HATI. Our numerical results show that the cutoff energy of the ATI with incoherent scattering is higher than that of the rescattering-induced HATI. These results are supported by classical analysis

[en] We investigate how various versions of the molecular strong-field approximation (MSFA) agree with the experiment by Grasbon et al. [Phys. Rev. A 63, 041402(R) (2001)], in which the suppression of the ionization yield in the low-energy spectrum of the O₂ molecule, compared to the spectrum of its companion atom Xe, was observed. In this experiment, it was also found that the spectrum of the N₂ molecule is comparable to the corresponding spectrum of its companion atom Ar. We show that the length-gauge version of the MSFA with the initial state dressed by the laser field gives the best agreement with the experimental data for both O₂ and N₂ molecules.

[en] Using the example of electron-atom scattering in a strong laser field, it is shown that the oscillatory structure of the scattered electron spectrum can be explained as a consequence of the interference of the real electron trajectories in terms of Feynman's path integral. While in previous work on quantum-orbit theory the complex solutions of the saddle-point equations were considered, we show here that for the electron-atom scattering with much simpler real solutions a satisfactory agreement with the strong-field-approximation results can be achieved. Real solutions are applicable both for the direct (low-energy) and the rescattering (high-energy) plateau in the scattered electron spectrum. In between the plateaus and beyond the rescattering cutoff good results can be obtained using the complex (quantum) solutions and the uniform approximation. The interference of real solutions is related to the recent attosecond double-slit experiment in time