Agapi Emmanouilidou

Construction of a natural partition of incomplete horseshoes

We present a method for constructing a partition of an incomplete horseshoe in a Poincare map. The partition is based only on the unstable manifolds of the outermost fixed points and eventually their limits. Consequently, this partition becomes natural from the point of view of asymptotic scattering observations. The symbolic dynamics derived from this partition coincides with the one derived from the hierarchical structure of the singularities of the scattering functions.

Intensity dependence of strong-field double-ionization mechanisms: From field-assisted recollision ionization to recollision-assisted field ionization

Using a three-dimensional quasiclassical technique we explore molecular double ionization by a linearly polarized, infrared (800 nm) and ultrashort (6 fs) laser pulse. We first focus on intensities corresponding to the tunneling regime and identify the main ionization mechanisms in this regime. We devise a selection of observables, such as, the correlated momenta and the sum of the momenta parallel to the laser field as a function of the inter-electronic angle of escape where all the main mechanisms have distinct traces. Secondly, we address intensities above but close to the over-the-barrier intensity regime. We find a surprising anti-correlation of electron momenta similar to the experimental observations reported in Phys. Rev. Lett. \textbf{101}, 053001 (2008). There, however, the anti-correlation was observed in very low intensities corresponding to the multiphoton regime. We discuss the mechanism responsible for the anti-parallel two-electron escape.

Direct versus delayed pathways in strong-field non-sequential double ionization

We report full-dimensionality quantum and classical calculations of the double ionization (DI) of laser-driven helium at 390 nm. Good agreement between the quantum and classical results is observed. We identify the relative importance of the two main non-sequential DI pathways: the direct—with an almost simultaneous ejection of both electrons—and the delayed. We find that the delayed pathway prevails at small intensities independently of total electron energy, but at high intensities the direct pathway predominates up to a certain upper limit in total energy, which increases with intensity. An explanation of this increase with intensity is provided.

Stark and field-born resonances of an open square well in a static external electric field

The resonance positions, widths (inverse lifetimes), and wave functions of a square-potential well in the presence of a static electric field are calculated by using the outgoing boundary conditions. Our study concentrates on the field-born states that, unlike the well-known Stark resonances, are not associated with the field-free bound states. The effect of a lower cutoff of the static field on the field-born resonance phenomena is studied. The feasibility of experiments, where the isolated long-lived and overlapping short-lived field-born resonances can be explored, is discussed.

Initial State Dependence in Multielectron Threshold Ionization of Atoms

It is shown that the geometry of multielectron threshold ionization in atoms depends on the initial configuration of bound electrons. The reason for this behavior is found in the stability properties of the classical fixed point of the equations of motion for multiple threshold fragmentation. Specifically for three-electron breakup, apart from the symmetric triangular configuration also a breakup of lower symmetry in the form of a T shape can occur, as we demonstrate by calculating triple photoionization for the lithium ground and first excited states. We predict the electron breakup geometry for threshold fragmentation experiments.

The effect of electron-electron correlation on the attoclock experiment

We investigate multi-electron effects in strong-field ionization of Helium using a semi-classical model that, unlike other commonly used theoretical approaches, takes into account electron-electron correlation. Our approach has an additional advantage of allowing to selectively switch off different contributions from the parent ion (such as the remaining electron or the nuclear charge) and thereby investigate in detail how the final electron angle in the attoclock experiment is influenced by these contributions. We find that the bound electron exerts a significant effect on the final electron momenta distribution that can, however, be accounted for by an appropriately selected mean field. Our results show excellent agreement with other widely used theoretical models done within a single active electron approximation.

Multiple electron trapping in the fragmentation of strongly driven molecules

We present a theoretical quasi-classical study of the formation, during Coulomb explosion, of two highly excited neutral H atoms (double H*) of strongly driven H2. In this process, after the laser field is turned off each electron occupies a Rydberg state of an H atom. We identify the route for forming two H* atoms and show that two-electron effects are important. We also find that both ionization steps are ‘frustrated’ in double H* formation, whereas only one ionization step is ‘frustrated’ for both the routes leading to single H* formation, as was shown by Emmanouilidou et al (2012 Phys. Rev. A 85 011402). Moreover, we compute the screened nuclear charge that drives the explosion of the nuclei during double H* formation.

Prevalence of different double ionization pathways and traces of three-body interactions in strongly driven helium

We present a unified treatment of the prevalence of different nonsequential double ionization pathways in strongly driven helium as a function of the sum of the final electron energies and of the laser frequency and intensity. At high total energy the pathway that dominates is the one where one electron is ionized with a delay following recollision. At low total energy, a double ionization pathway dominates where recollision the two electrons and the nucleus form a bound compound for at least a quarter of a laser cycle. We also identify asymptotic observables that trace the prevalence of these pathways.

Quasiclassical double photoionization from the 2 S-1,S-3 excited states of helium including shake-off

We account for the different symmetries of the 2 1,3S helium excited states in a quasiclassical description of the knockout mechanism augmented by a quantum shake-off contribution. We are thus able to formulate the separate contributions of the knockout and shake-off mechanisms for double photoionization for any excess energy from the 2 1,3S states. Photoionization ratios and single-differential cross sections calculated for the 2 1,3S excited states of helium are found to be in very good agreement with recent theoretical results.

The two-electron attosecond streak camera for time-resolving intra-atomic collisions

We generalize the one-electron attosecond streak camera to time-resolve the correlated two-electron escape dynamics during a collision process involving a deep core electron. The collision process is triggered by an extreme ultraviolet (XUV) attosecond pulse (single-photon absorption) and probed by a weak infrared field. The principle of our two-electron streak camera is that by placing the maximum of the vector potential of the probing field at the time of collision, we get the maximum splitting of the inter-electronic angle of escape. We thereby determine the time of collision.