arXiv subjects

A. Emmanouilidou

Publications and source records attributed to A. Emmanouilidou.

At least 37 records · Page 2Linked to original sources

arXiv2015

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.

physics.atom-ph↗

arXiv2015

Traces on ion yields and electron spectra of Ar inner-shell hollow states with Free-Electron Lasers

We explore the formation by Free-Electron-Laser radiation of Ar hollow states with two or three inner-shell holes. We find that even charged Ar ion states can be more populated than odd charged Ar ion states. This depends on the pulse intensity and the number of energetically accessible inner- shell holes. Fully accounting for fine structure, we demonstrate that one electron spectra bare the imprints of Ar hollow states with two inner-shell holes. Moreover, we show how the Auger spectra of these hollow states can be extracted from two-electron coincidence spectra.

physics.atom-ph↗

arXiv2014

A toolkit for semiclassical computations for strongly-driven molecules: "frustrated" ionization of H$_{2}$ driven by elliptical laser fields

We study the formation of highly excited neutral atoms during the break-up of strongly-driven molecules. Past work on this significant phenomenon has shown that during the formation of highly excited neutral atoms ($\mathrm{H^{*}}$) during the break-up of H$_{2}$ in a linear laser field the electron that escapes does so either very quickly or after remaining bound for a few periods of the laser field. Here, we address the electron-nuclear dynamics in $\mathrm{H^{*}}$ formation in elliptical laser fields, through Coulomb explosion. We show that with increasing ellipticity two-electron effects are effectively "switched-off". We perform these studies using a toolkit we have developed for semiclassical computations for strongly-driven multi-center molecules. This toolkit includes the formulation of the probabilities of strong-field phenomena in a transparent way. This allows us to identify the shortcomings of currently used initial phase space distributions for the electronic degrees of freedom. In addition, it includes a 3-dimensional method for time-propagation that fully accounts for the Coulomb singularity. This technique has been previously developed in the context of celestial mechanics and we currently adopt it to strongly-driven systems. Moreover, we allow for tunneling during the time-propagation. We find that this is necessary in order to accurately describe the fragmentation of strongly-driven molecules.

physics.atom-ph↗

arXiv2014

Auger spectra following inner-shell ionization of Argon by a Free-Electron Laser

We explore the possibility of retrieving Auger spectra with FEL radiation. Using a laser pulse of 260 eV photon energy, we study the interplay of photo-ionization and Auger processes following the initial formation of a 2p inner-shell hole in Ar. Accounting for the fine structure of the ion states we demonstrate how to retrieve the Auger spectrum of $\mathrm{Ar^{+}\rightarrow Ar^{2+}}$. Moreover, considering two electrons in coincidence we also demonstrate how to retrieve the Auger spectrum of $\mathrm{Ar^{2+}\rightarrow Ar^{3+}}$.

physics.atom-ph↗

arXiv2013

Evidence for a {\bf T}-shape break-up pattern in the triple photoionization of Li

We examine the angular distributions of all three electrons ionized from Li by a single photon near the triple ionization threshold using a fully quantum-mechanical treatment. We find strong evidence for a {\bf T}-shape break-up pattern at 5 eV excess energy as previously predicted by quasi-classical simulations [A. Emmanouilidou and J. M. Rost, \jpb {\bf 39}, 4037 (2006); A. Emmanouilidou, P. Wang, and J. M. Rost, \prl {\bf 100}, 063002 (2008)]. This finding is in conflict with the expected Wannier break-up dynamics of three electrons moving at mutual angles of 120$^{\circ}$, which is expected to hold at energies a few eV above threshold. We use our quantum-mechanical approach to explore the physical mechanisms behind this unusual break-up configuration.

physics.atom-ph↗

arXiv2012

Attosecond electron-electron collision dynamics of the four-electron escape in Be close to threshold

We explore the escape geometry of four electrons a few eV above threshold following single-photon absorption from the ground state of Be. We find that the four electrons leave the atom on the vertices of a pyramid instead of a previously-predicted tetrahedron. To illustrate the physical mechanisms of quadruple ionization we use a momentum transferring attosecond collision scheme which we show to be in accord with the pyramid break-up pattern.

physics.atom-ph↗

arXiv2012

Frequency Resolved Optical Gating for Time-Resolving Intra-Atomic Knock-Out in Double Ionization with Attosecond Pulses

We develop the two-electron attosecond streak camera under realistic conditions using a quasi-classical model. We assume extreme ultra-violet (XUV) attosecond pulses with a full width at half maximum (FWHM) of 24 attoseconds, centered at 120 eV and a streaking infrared laser field of 1600 nm, and intensity of $1.8 \times 10^{12}$ W/cm$^2$. The proposed method is shown to be capable to time resolve intra-atomic collisions in double ionization.

physics.atom-ph↗

arXiv2012

Multiple electron trapping in the fragmentation of strongly driven molecules

We present a theoretical quasiclassical study of the formation, during Coulomb explosion, of two highly excited neutral H atoms (double H$^{*}$) of strongly driven H$_2$. In this process, after the laser field is turned off each electron occupies a Rydberg state of an H atom. We show that two-electron effects are important in order to correctly account for double H$^{*}$ formation. We find that the route to forming two H$^{*}$ atoms is similar to pathway B that was identified in Phys. Rev. A {\bf 85} 011402 (R) as one of the two routes leading to single H$^{*}$ formation. However, instead of one ionization step being "frustrated" as is the case for pathway B, both ionization steps are "frustrated" in double H$^{*}$ formation. Moreover, we compute the screened nuclear charge that drives the explosion of the nuclei during double H$^{*}$ formation.

physics.atom-ph↗

arXiv2011

Energy Sharing in the 2-Electron Attosecond Streak Camera

Using the recently developed concept of the 2-electron streak camera (see NJP 12, 103024 (2010)), we have studied the energy-sharing between the two ionizing electrons in single-photon double ionization of He(1s2s). We find that the most symmetric and asymmetric energy sharings correspond to different ionization dynamics with the ion's Coulomb potential significantly influencing the latter. This different dynamics for the two extreme energy sharings gives rise to different patterns in asymptotic observables and different time-delays between the emission of the two electrons. We show that the 2-electron streak camera resolves the time-delays between the emission of the two electrons for different energy sharings.

physics.atom-ph↗

arXiv2011

Two-electron effects pervading the formation of highly excited neutral atoms in the break-up of strongly driven H$_{2}$

We present the first theoretical treatment of the formation of highly excited neutral H atoms (H$^{*}$) for strongly driven H$_{2}$. This process, with one electron ionizing and one captured in a Rydberg state, was recently reported in an experimental study in Phys. Rev. Lett {\bf 102} 113002 (2009). We show that two mechanisms underlie this process: a non-sequential one resembling non-sequential double ionization, and a sequential one resembling double ionization through enhanced ionization. We also predict a new feature, asymmetric energy sharing between H$^{*}$ and H$^{+}$ with increasing intensity. This feature is a striking demonstration of the influence the electron has on the nuclear motion.

physics.atom-ph↗

arXiv2011

Strongly driven molecules: probing the tunneling phase in the over-the-barrier regime and prevalence of different double ionization pathways

Using a three-dimensional quasiclassical technique we explore molecular double ionization by a linearly polarized, infrared (800~nm) 27~fs laser pulse. For intensities ranging from the tunneling to the over-the-barrier regime, we identify the double ionization pathways in a unified way as a function of total electron energy. For the tunneling regime, we discuss the differences in the interplay of double ionization (DI) pathways between strongly driven He and strongly driven $N_{2}$. For intermediate intensities in the over-the-barrier regime, we find that both the correlated momenta and the double ionization probability distribution as a function of total energy probe the tunneling phase of the re-colliding electron. This allows for a direct verification of the re-colliding electron tunneling at a large phase of the laser field in the over-the-barrier regime in contrast to a small tunneling phase in the tunneling regime.

physics.atom-ph↗

arXiv2010

Direct versus Delayed pathways in Strong-Field Non-Sequential Double Ionization

We report full-dimensionality quantum and classical calculations for double ionization of laser-driven helium at 390 nm. Good qualitative agreement is observed. We show that the classical double ionization trajectories can be divided into two distinct pathways: direct and delayed. The direct pathway, with an almost simultaneous ejection of both electrons, emerges from small laser intensities. With increasing intensity its relative importance, compared to the delayed ionization pathway, increases until it becomes the predominant pathway for total electron escape energy below around 5.25 $U_{p}$. However the delayed pathway is the predominant one for double ionization above a certain cut-off energy at all laser intensities.

physics.atom-ph↗

arXiv2010

Time-resolving intra-atomic two-electron collision dynamics

We generalize the one electron attosecond streaking 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 XUV attosecond pulse 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 thus identify the time of collision.

physics.atom-ph↗

arXiv2009

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.

physics.atom-ph↗

arXiv2008

Recoil collisions as a portal to field assisted ionization at near-UV frequencies in the Strong Field Double Ionization of Helium

We explore the dependence of the double ionization of the He atom on the frequency of a strong laser field while keeping the ponderomotive energy constant. As we increase the frequency we find that the remarkable "finger-like" structure for high momenta recently found for $ω=0.055$ a.u. \cite{Staudte, Rudenko} persists for higher frequencies. At the same time, at $ω=0.187$ a.u. a new X-shape structure emerges for small momenta that prevails in the correlated momenta distribution. The role of this structure as a signature of the frequency dependence of non-sequential double ionization is discussed.

physics.atom-ph↗

arXiv2008

Electron stripping and re-attachment at atomic centers using attosecond half-cycle pulses

We investigate the response of two three-body Coulomb systems when driven by attosecond half-cycle pulses: The hydrogen molecular ion and the helium atom. Using very short half-cycle pulses (HCPs) which effectively deliver ``kicks'' to the electrons, we first study how a carefully chosen sequence of HCPs can be used to control to which of one of the two fixed atomic centers the electron gets re-attached. Moving from one electron in two atomic centers to two electrons in one atomic center we then study the double ionization from the ground state of He by a sequence of attosecond time-scale HCPs, with each electron receiving effectively a ``kick'' from each HCP. We investigate how the net electric field of the sequence of HCPs affects the total and differential ionization probabilities.

physics.atom-ph↗

arXiv2004

Above-well, Stark, and potential-barrier resonances of an open square well in a static external electric field

Besides the well known Stark resonances, which are localized in the potential well and tunnel through the potential barrier created by the dc-field, "strange" long and short-lived resonances are analytically obtained. These resonances are not localized inside the potential well. We show that the narrow ones are localized above the potential well. These narrow resonances give rise to a {\it peak structure} in a 1D scattering experiment. We also show that the broad overlapping resonances are associated with the static electric field potential barrier. These "strange" overlapping resonances do not give rise to a {\it peak structure} in a 1D scattering experiment. We propose a 2D experimental set-up where in principle these short-lived states should be observed as {\it peaks}. Broad overlapping resonances, associated only with the static electric field potential barrier, could also have observable effects in a $N>1$ array of quantum wells in the presence of a truncated static electric field. This last problem is associated with the resonance tunnelling phenomena which are used in the construction of resonance-tunnelling diodes and transistors.

physics.atom-ph↗

arXiv2003

Construction of a natural partition of incomplete horseshoes

A method is presented to construct a partition of an incomplete horseshoe in a Poincare map which is only based on unstable manifolds of outermost fixed points and eventually their limits. Thereby 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 scattering functions.

nlin.CD↗