We formulate a transparent measure that quantifies chirality in single electron ionization triggered in atoms, which are achiral systems. We do so in the context of Ar driven by a new type of optical fields that consists of two non-collinear laser beams giving rise to chirality that varies in space across the focus of the beams. Our computations account for realistic experimental conditions. To define this measure of chirality, we first find the sign of the electron final momentum scalar triple product p k • (pi × pj) and multiply it with the probability for an electron to ionize with certain values for both p k and pipj. Then, we integrate this product over all values of p k and pipj. We show this to be a robust measure of chirality in electron ionization triggered by globally chiral electric fields.

At intensities below-the-recollision threshold, we show that re-collision-induced excitation with one electron escaping fast after re-collision and the other electron escaping with a time delay via a Coulomb slingshot motion is one of the most important mechanisms of non-sequential double ionization, for strongly-driven He at 400 nm. Slingshot-NSDI is a general mechanism present for a wide range of low intensities and pulse durations. Anti-correlated two-electron escape is its striking hallmark. This mechanism offers an alternative explanation of anti-correlated two-electron escape obtained in previous studies.

We formulate a three-dimensional semiclassical model to address triple and double ionization in three-electron atoms driven by intense infrared laser pulses. During time propagation, our model fully accounts for the Coulomb singularities, the magnetic field of the laser pulse, and the motion of the nucleus at the same time as for the motion of the three electrons. The framework we develop is general and can account for multielectron ionization in strongly driven atoms with more than three electrons. To avoid unphysical autoionization arising in classical models of three or more electrons, we replace the Coulomb potential between pairs of bound electrons with effective Coulomb potentials. The Coulomb forces between electrons that are not both bound are fully accounted for. We develop a set of criteria to determine when electrons become bound during time propagation. We compare ionization spectra obtained with the model developed here and with the Heisenberg model that includes a potential term restricting an electron from closely approaching the core. Such spectra include the sum of the electron momenta along the direction of the laser field as well as the correlated electron momenta. We also compare these results with experimental ones.

For two-electron diatomic molecules, we investigate magnetic field effects in nonsequential double ionization where recollisions prevail. We do so by formulating a three-dimensional semiclassical model that fully accounts for the Coulomb singularities and for magnetic field effects during time propagation. Using this model, we identify a prominent signature of nondipole effects. Namely, we demonstrate that the recolliding electron backscatters along the direction of light propagation. Hence, this electron escapes opposite to the direction of change in momentum due to the magnetic field. This is in striking contrast to strongly driven atoms where the recolliding electron forward scatters along the direction of light propagation. We attribute these distinct signatures to the different gate that the magnetic field creates jointly with a soft recollision in molecules compared to a hard recollision in atoms. These two different gates give rise, shortly before recollision, to different momenta and positions of the recolliding electron along the direction of light propagation. As a result, we show that the Coulomb forces from the nuclei act to backscatter the recolliding electron in molecules and forward scatter it in atoms along the direction of light propagation.

We demonstrate significant enhancement of frustrated double ionization (FDI) in the two-electron triatomic molecule D + 3 when driven by counter-rotating two-color circular (CRTC) laser fields. We employ a three-dimensional semiclassical model that fully accounts for electron and nuclear motion in strong fields. For different pairs of wavelengths, we compute the probabilities of the FDI pathways as a function of the ratio of the two field-strengths. We identify a pathway of FDI that is not present in strongly-driven molecules with linear fields. In this pathway the first ionization step is "frustrated" and electronic correlation is essentially absent. This pathway is responsible for enhancing FDI with CRTC fields. We also employ a simple model that predicts many of the main features of the probabilities of the FDI pathways as a function of the ratio of the two field-strengths. PACS numbers: 33.80.Rv, 34.80.Gs, 42.50.Hz Formation of highly excited Rydberg states, during the interaction of atoms and molecules with laser fields, is a fundamental problem with a wide range of applications. Rydberg states underlie, for instance, acceleration of neutral particles [1], spectral features of photoelectrons [2], formation of molecules via long-range interactions [3], and inversion of N 2 in free-space air lasing [4]. Recently, the formation of Rydberg states in weakly-driven H 2 was accounted for by electron-nuclear correlated multiphoton resonant excitation [5]. For H 2 driven by intense infrared laser fields (strongly-driven), this latter process was shown to merge with frustrated double ionization (FDI) [5]. FDI accounts for the formation of Rydberg fragments in strongly-driven two-electron molecules. In frustrated ionization an electron first tunnel ionizes in the driving laser field. Then, due to the electric field, this electron is recaptured by the parent ion in a Rydberg state [6]. In FDI an electron escapes while another one occupies a Rydberg state at the end of the laser pulse.For linear laser fields, FDI is a major process during the breakup of strongly-driven molecules, accounting for roughly 10% of all ionization events. Hence, FDI has been the focus of intense experimental studies in the context of H 2 [7], D 2 [8] and of the two-electron triatomic molecules D + 3 and H + 3 [9-11]. For strongly-driven twoelectron diatomic and triatomic molecules, FDI proceeds via two pathways [12][13][14]. One electron tunnel ionizes early on (first step), while the remaining bound electron does so later in time (second step). If the second (first) ionization step is "frustrated", we label the FDI pathway as FSIS (FFIS), previously referred to as pathway A (B) [12]. Electron-electron correlation, underlying pathway FFIS [12,15], can be controlled with orthogonally polarised two-color linear (OTC) laser fields [14].Here, we show that counter-rotating two-color circular (CRTC) laser fields are a powerful tool for controlling FDI in strongly-driven molecules. CRTC fields have attracted a lot of interest due to their applicabi...