Strong-field photoionization of argon dimers by a few-cycle laser pulse is investigated using electron–ion coincidence momentum spectroscopy. The momentum distribution of the photoelectrons exhibits interference due to the emission from the two atomic argon centres, in analogy with a Young's double-slit experiment. However, a simulation of the dimer photoelectron momentum spectrum based on the atomic spectrum supplemented with a theoretically derived interference term leads to distinct deviations from the experimental result. The deviations may have their origin in a complex electron dynamics during strong-field ionization of the Ar2 dimer.

The strong-field approximation for ionization of diatomic molecules by a strong laser field [D. B. Milo\ifmmode \check{s}\else \v{s}\fi{}evi\ifmmode \acute{c}\else \'{c}\fi{}, Phys. Rev. A 74, 063404 (2006)] is generalized to include rescattering of the ionized electron wave packet off the molecular centers (the electron's parent ion or the second atom). There are four rescattering contributions to the ionization rate, which are responsible for the high-energy plateau in the electron spectra and which interfere in a complicated manner. The spectra are even more complicated due to the different symmetry properties of the atomic orbitals of which a particular molecular orbital consists. Nevertheless, a comparatively simple condition emerges for the destructive interference of all these contributions, which yields a curve in the $({E}_{{\mathbf{p}}_{f}},\ensuremath{\theta})$ plane. Here $\ensuremath{\theta}$ is the electron emission angle and ${E}_{{\mathbf{p}}_{f}}$ is the electron kinetic energy. The resulting suppression of the rescattering plateau can be strong and affect a large area of the $({E}_{{\mathbf{p}}_{f}},\ensuremath{\theta})$ plane, depending on the orientation of the molecule. We illustrate this using the examples of the $3{\ensuremath{\sigma}}_{g}$ molecular orbital of ${\mathrm{N}}_{2}$ and the $1{\ensuremath{\pi}}_{g}$ molecular orbital of ${\mathrm{O}}_{2}$ for various orientations of these molecules with respect to the laser polarization axis. For ${\mathrm{N}}_{2}$, for perpendicular orientation and the equilibrium internuclear distance ${R}_{0}$, we find that the minima of the ionization rate form the curve ${E}_{{\mathbf{p}}_{f}}\phantom{\rule{0.2em}{0ex}}{\mathrm{cos}}^{2}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}={\ensuremath{\pi}}^{2}∕(2{R}_{0}^{2})$ in the $({E}_{{\mathbf{p}}_{f}},\ensuremath{\theta})$ plane. For ${\mathrm{O}}_{2}$ the rescattering plateau is absent for perpendicular orientation.

We investigate the laser-wavelength dependence of high-order above-threshold ionization and high-order harmonic generation rates. For both processes we have found pronounced enhancements in the spectra for those wavelengths for which the channel-closing condition is satisfied. These enhancements are explained in two complementary ways. One is in terms of the constructive interference of a large number of long quantum orbits, while the second interpretation is based on threshold anomalies. There are two types of these enhancements, which leave very different footprints in the spectra. When the ground-state parity of the atom changes, the two types are interchanged.

We analyzed high-order harmonic generation from the plasma plumes prepared on the surfaces of complex targets. The studies of In-Ag targets showed that the characteristics of the high-order harmonics from the double-target plume were the same as those from the single-target plasmas. For the chromium-tellurium plasma, the enhancements of the 29th and 27th harmonics were obtained, thus indicating the appearance of the enhancement properties from both components of the double-target plasma. These comparative studies also showed higher enhancement of a single harmonic in the case of atomic plasma (Sb) with regard to the molecular one (InSb). The additional component can only decrease the enhancement factor of the medium, due to the change of the oscillator strength and spectral distribution of the transitions involved in the resonance enhancement of the specific harmonic order. The theoretical calculations have shown the enhancement of specific harmonics for the Sb, Te, and Cr plasmas in the double-target configurations.

The strong-field approximation for ionization of diatomic molecules by an intense laser field is generalized to include rescattering of the ionized electron off the various centers of its molecular parent ion. The resulting spectrum and its interference structure strongly depend on the symmetry of the ground state molecular orbital. For N2, if the laser polarization is perpendicular to the molecular axis, we observe a distinct minimum in the emission spectrum, which survives focal averaging and allows determination of, e.g., the internuclear separation. In contrast, for O2, rescattering is absent in the same situation.

Sub-femtosecond control of the electron emission in above-threshold ionization of the rare gases Ar, Xe and Kr in intense few-cycle laser fields is reported with full angular resolution. Experimental data that were obtained with the velocity-map imaging technique are compared to simulations using the strong-field approximation (SFA) and full time-dependent Schrödinger equation (TDSE) calculations. We find a pronounced asymmetry in both the energy and angular distributions of the electron emission that critically depends on the carrier-envelope phase (CEP) of the laser field. The potential use of imaging techniques as a tool for single-shot detection of the CEP is discussed.

The very pronounced intensity-dependent enhancements of groups of peaks of high-order above-threshold-ionization spectra of rare-gas atoms are investigated using an improved version of the strong-field approximation, which realistically models the respective atom. Two types of enhancements are found and explained in terms of constructive interference of the contributions of a large number of long quantum orbits. The first type is observed for intensities slightly below channel closings. Its intensity dependence is comparatively smooth and it is generated by comparatively few (of the order of 20) orbits. The second type occurs precisely at channel closings and exhibits an extremely sharp intensity dependence. It requires constructive interference of a very large number of long orbits (several hundreds) and generates cusps in the electron spectrum at integer multiples of the laser-photon energy. An interpretation of these enhancements as a threshold phenomenon is also given. An interplay of different types of the threshold anomalies is observed. The position of both types of enhancements, in the photoelectron-energy--laser-intensity plane, shifts to the next channel closing intensity with the change of the ground-state parity. The enhancements gradually disappear with decreasing laser pulse duration. This confirms the interpretation of enhancements as a consequence of the interference of longmore » strong-laser-field-induced quantum orbits.« less

In the customary formulation of the strong-field approximation (SFA) for laser-induced ionization, the initial bound state is taken as field-free. In the formulation of a length-gauge SFA for ionization of a molecule described by a two-center binding potential with sufficiently large internuclear separation, we argue that the initial state has to be dressed in order to account for the different scalar potentials at the various centers. We propose a ``dressed-state'' SFA to this end.

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.

Above-threshold detachment of electrons from negative ions by a bicircular laser field is analyzed within the strong-field approximation. A bicircular field consists of two counter-rotating circularly polarized fields with angular frequencies rω and sω with integer r and s. The energy and angle resolved spectra of the detached electron are invariant with respect to a rotation by the angle 360°/(r+s). The results obtained are explained in terms of the interference of contributions to the T matrix from different complex solutions of the saddle-point equation.