We theoretically investigate high-order above-threshold ionization (HATI) of heteronuclear diatomic molecules applying the molecular strong-field approximation which includes dressing of the molecular bound state. We consider HATI of nitrogen monoxide molecules, which are characterized by the π symmetry of their highest occupied molecular orbital. We show that the HATI spectra of NO exhibit characteristic interference structures. We analyze the differences and similarities of the HATI spectra of NO molecules and the spectra of CO (σ symmetry) and O(2) (π(g) symmetry) molecules. The symmetry properties of the molecular HATI spectra governed by linearly and elliptically polarized fields are considered in detail. The yields of high-energy electrons, contributing to the plateau region of the photoelectron spectra, strongly depend on the employed ellipticity.

A theory of high-order harmonic generation by heteronuclear diatomic molecules exposed to an elliptically polarized laser field is presented. This theory is based on the molecular strong-field approximation, which takes into account the dressing of the initial and final molecular bound states caused by the laser field. The laser-field-caused Stark shift of the effective ionization potential of the highest occupied molecular orbital changes the structure of the harmonic spectra. Both the even and odd harmonics appear in the harmonic spectra due to the broken inversion symmetry of the heteronuclear diatomic molecule. An interference minima condition that is valid for arbitrary heteronuclear diatomic molecules is derived. Our results are illustrated using examples of CO and NO molecules.

In this paper, we present a quantum-mechanical theory in order to describe laser-induced and laser-assisted atomic and molecular processes with an emphasis on an elliptically polarized strong laser field. The features of the high-energy regions of spectra and their cutoffs are analyzed in detail for different values of ellipticity, laser intensities and various atomic and molecular species. Our theoretical study is focused on the ellipticity dependence of the plateau structures that appear in the energy spectra of atomic and molecular processes in strong laser fields.

We present a quantum-mechanical laser-assisted scattering theory and show how the rescattering effects can be described within the second Born approximation. Also, we use two versions of the atomic (molecular) strong-field approximation in order to describe high-order above-threshold ionization. In the improved strong-field approximation, the rescattering of the ionized electron off the parent ion is described within the first-order Born approximation. A better approximation is the so-called low-frequency approximation in which the elastic rescattering amplitude is calculated exactly.

Laser-induced electron detachment or ionization of atoms and negative ions is considered. In the context of the saddle-point evaluation of the strong-field approximation (SFA), the velocity maps of the direct electrons (those that do not undergo rescattering) exhibit a characteristic structure due to the constructive and destructive interference of electrons liberated from their parent atoms/ions within certain windows of time. This structure is defined by the above-threshold ionization rings at fixed electron energy and by two sets of curves in momentum space on which destructive interference occurs. The spectra obtained with the SFA are compared with those obtained by numerical solution of the time-dependent Schrödinger equation. For detachment, the agreement is excellent. For ionization, the effect of the Coulomb field is most pronounced for electrons emitted in a direction close to laser polarization, while for near-perpendicular emission the qualitative appearance of the spectrum is unaffected.

The molecular strong-field approximation (MSFA), which includes dressing of the molecular bound state, is introduced and applied to above-threshold ionization of heteronuclear diatomic molecules. Expressions for the laser-induced molecular dipole and polarizability as functions of the laser parameters (intensity and frequency) and molecular parameters [molecular orientation, dipole, and parallel and perpendicular polarizabilities of the highest occupied molecular orbital (HOMO)] are presented. Our previous MSFA theory, which incorporates the rescattering effects, is generalized from homonuclear to heteronuclear diatomic molecules. Angle- and energy-resolved high-order above-threshold ionization spectra of oriented heteronuclear diatomic molecules, exemplified by the carbon monoxide (CO) molecule, exhibit pronounced minima, which can be related to the shape of their HOMO-electron-density distribution. For the CO molecule we have found an analytical condition for the positions of these minima. We have also shown that the effect of the dressing of the HOMO is twofold: (i) the laser-induced Stark shift decreases the ionization yield and (ii) the laser-induced time-dependent dipole and polarizability change the oscillatory structure of the spectra.

Above-threshold ionization of diatomic molecules by infrared carrier-envelope phase (CEP) stable few-cycle laser pulses is analyzed both experimentally and theoretically. The theoretical approach is based on the recently developed molecular improved strong-field approximation (ISFA), generalized to few-cycle pulses. Instead of using the first Born approximation, the rescattering matrix element in the ISFA is now calculated exactly. This modification leads to the appearance of characteristic minima in the differential cross section as a function of the scattering angle. Experimental angle-resolved photoelectron spectra of N{sub 2} and O{sub 2} molecules are obtained using the velocity map imaging technique. A relatively good agreement of experimental and simulated angle-resolved spectra, CEP-dependent asymmetry maps, and extracted electron-molecular ion elastic scattering differential cross sections is obtained.

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.

The so-called low-frequency approximation (LFA) is an improved version of the strong-field approximation. The LFA describes the rescattering step of high-order above-threshold ionisation by utilising the exact field-free scattering amplitude, which has to be calculated separately. We apply the LFA to high-order above-threshold detachment of the fluorine negative ion. By comparing the so-obtained angle- and energy-resolved spectra with the exact results obtained as solutions of the time-dependent Schrödinger equation we show that the LFA is a more adequate approximation than the so-called improved strong-field approximation used previously. The LFA is also superior to the recently introduced quantitative rescattering (QRS) model. We show this by applying the LFA to the analysis of the intensity-dependent enhancements in above-threshold ionisation spectra of argon atoms. These enhancements can be explained as channel-closing-induced effects. In particular, we have observed and explained the behaviour of these channel-closing enhancements for higher values of the electron emission angle.

We present a theory of high-order harmonic generation by diatomic molecules exposed to an elliptically polarized laser field. This theory is based on the molecular strong-field approximation with the laser-field-dressed initial bound state and the undressed final state. The interference minima, observed for linear polarization, are blurred with the increase of the laser-field ellipticity. The $n$th 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. We analyze the destructive interference condition for this perpendicular component. 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 and discuss possibilities of its use for determining the molecular structure.

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 ${\mathrm{O}}_{2}$ molecule, compared to the spectrum of its companion atom Xe, was observed. In this experiment, it was also found that the spectrum of the ${\mathrm{N}}_{2}$ 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 ${\mathrm{O}}_{2}$ and ${\mathrm{N}}_{2}$ molecules.

We present experimental and theoretical results on photodetachment of Br(-) and F(-) in a strong infrared laser field. The observed photoelectron spectra of Br(-) exhibit a high-energy plateau along the laser polarization direction, which is identified as being due to the rescattering effect. The shape and the extension of the plateau is found to be influenced by the depletion of negative ions during the interaction with the laser pulse. Our findings represent the first observation of electron rescattering in above-threshold photodetachment of an atomic system with a short-range potential.

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.