Introduction

In the past decade, the phenomena occurring when matter interacts with short laser fields, whose intensity is of the order of 1013 W/cm2 or higher, have been looked at as powerful attosecond (10−18 s) imaging tools[1, 2, 3, 4]. Recent examples include the reconstruction of the highest occupied molecular orbital (HOMO) of N2[5] and the imaging of attosecond electron−hole dynamics[6] employing high-order harmonic generation, or the velocity-map imaging of atomic orbitals employing above-threshold ionization[7, 8]. This is a direct consequence of the fact that these phenomena result from the laser-induced rescattering or recombination of an electron with its parent ion, or molecule[9]. Recombination of the active electron with a bound-state leads to high-order harmonic generation (HHG), while rescattering may cause above-threshold ionization (ATI) or laser-induced non-sequential double ionization (NSDI). These processes occur within a fraction of a driving field cycle, which for typical parameters used in experiments is roughly = 2.7 fs. Hence, their duration comprises hundreds of attoseconds. Furthermore, upon recollision, the active electron will probe the structure of the target, and this will leave imprints in the high-harmonic or photoelectron spectra.

The above-mentioned imprints may be static or dynamic. Widely investigated structural effects include interference fringes due to recollision at spatially separated centers, and the influence of the geometry of the molecular orbitals involved, such as nodal planes or orbital orientation, on the spectra. These effects have been mostly studied by considering the core to be static, and assuming the HOMO to be the sole active orbital. Recent studies, however, have shown that the dynamics of the core, in particular electron−electron correlation, core excitation and relaxation are expected to play an important role[6]. Indeed, when the active electron is removed, a hole will be created in the core, which, potentially, will migrate. Apart from that, the electric field may polarize the core, or induce excitation and relaxation processes, especially if one electron has been removed[10, 11].

This chapter is a brief review of our recent study on HHG and NSDI, both in atoms and molecules, together with novel results or aspects which have not been brought up before. Throughout, particular emphasis is placed on attosecond imaging.