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1.
Phys Rev E ; 102(3-1): 032218, 2020 Sep.
Article in English | MEDLINE | ID: mdl-33075872

ABSTRACT

We study the double ionization of atoms subjected to circularly polarized (CP) laser pulses. We analyze two fundamental ionization processes: the sequential (SDI) and nonsequential (NSDI) double ionization in the light of the rotating frame (RF) which naturally embeds nonadiabatic effects in CP pulses. We use and compare two adiabatic approximations: The adiabatic approximation in the laboratory frame (LF) and the adiabatic approximation in the RF. The adiabatic approximation in the RF encapsulates the energy variations of the electrons on subcycle timescales happening in the LF and this, by fully taking into account the ion-electron interaction. This allows us to identify two nonadiabatic effects including the lowering of the threshold intensity at which over-the-barrier ionization happens and the lowering of the ionization time of the electrons. As a consequence, these nonadiabatic effects facilitate over-the-barrier ionization and recollision-induced ionizations. We analyze the outcomes of these nonadiabatic effects on the recollision mechanism. We show that the laser envelope plays an instrumental role in a recollision channel in CP pulses at the heart of NSDI.

2.
Phys Rev Lett ; 124(25): 253203, 2020 Jun 26.
Article in English | MEDLINE | ID: mdl-32639777

ABSTRACT

Increasing ellipticity usually suppresses the recollision probability drastically. In contrast, we report on a recollision channel with large return energy and a substantial probability, regardless of the ellipticity. The laser envelope plays a dominant role in the energy gained by the electron, and in the conditions under which the electron comes back to the core. We show that this recollision channel efficiently triggers various nonlinear and nonperturbative phenomena-such as multiple ionization-with an elliptically polarized pulse.

3.
Phys Rev Lett ; 121(11): 113202, 2018 Sep 14.
Article in English | MEDLINE | ID: mdl-30265122

ABSTRACT

Electron motion in combined strong laser and Coulomb fields is central to laser-matter interactions. By mapping this problem onto the motion of a guiding center, we derive a reduced model which naturally embeds important Coulomb effects such as focusing and asymmetry, and clearly distinguishes direct versus rescattered electron ionization processes. We demonstrate the power of this tool by unraveling the bifurcation in photoelectron momentum distributions seen in experiments.

4.
Chaos ; 26(5): 053101, 2016 05.
Article in English | MEDLINE | ID: mdl-27249941

ABSTRACT

Hamiltonian dynamics are characterized by a function, called the Hamiltonian, and a Poisson bracket. The Hamiltonian is a conserved quantity due to the anti-symmetry of the Poisson bracket. The Poisson bracket satisfies the Jacobi identity which is usually more intricate and more complex to comprehend than the conservation of the Hamiltonian. Here, we investigate the importance of the Jacobi identity in the dynamics by considering three different types of conservative flows in ℝ(3): Hamiltonian, almost-Poisson, and metriplectic. The comparison of their dynamics reveals the importance of the Jacobi identity in structuring the resulting phase space.

5.
Phys Rev Lett ; 112(13): 133003, 2014 Apr 04.
Article in English | MEDLINE | ID: mdl-24745413

ABSTRACT

We present a purely classical recollision scenario, i.e., without tunneling, which, in contrast to the standard three-step model, takes into account the ionic core potential fully at all stages of the recollision process and is valid at all intensities. We find that a key periodic orbit drives the recollisions by guiding electrons away and back to the core. At sufficiently high intensity, we connect our scenario to the three-step model, and explain why the three-step model leads to good agreement with the cutoff in high harmonic generation despite neglecting the core potential after tunneling.

6.
Phys Rev Lett ; 110(25): 253002, 2013 Jun 21.
Article in English | MEDLINE | ID: mdl-23829734

ABSTRACT

We show that a family of key periodic orbits drives the recollision process in a strong circularly polarized laser field. These orbits, coined recolliding periodic orbits, exist for a wide range of parameters, and their relative influence changes as the laser and atomic parameters are varied. We find the necessary conditions for recollision-driven nonsequential double ionization to occur. The outlined mechanism is universal in that it applies equally well beyond atoms: The internal structure of the target species plays a minor role in the recollision process.

7.
Phys Rev E Stat Nonlin Soft Matter Phys ; 85(6 Pt 2): 066205, 2012 Jun.
Article in English | MEDLINE | ID: mdl-23005194

ABSTRACT

We analyze the dynamical processes behind delayed double ionization of atoms subjected to strong laser pulses. Using reduced models, we show that these processes are a signature of Hamiltonian chaos which results from the competition between the laser field and the Coulomb attraction to the nucleus. In particular, we exhibit the paramount role of the unstable manifold of selected periodic orbits which lead to a delay in these double ionizations. Among delayed double ionizations, we consider the case of recollision excitation with subsequent ionization (RESI) and, as a hallmark of this mechanism, we predict oscillations in the ratio of RESI to double ionization yields versus laser intensity. We discuss the significance of the dimensionality of the reduced models for the analysis of the dynamical processes behind delayed double ionization.


Subject(s)
Ions/chemistry , Ions/radiation effects , Lasers , Models, Chemical , Nonlinear Dynamics , Oscillometry/methods , Computer Simulation
8.
Phys Rev E Stat Nonlin Soft Matter Phys ; 85(1 Pt 2): 016204, 2012 Jan.
Article in English | MEDLINE | ID: mdl-22400640

ABSTRACT

We analyze the dynamics of a valence electron of the buckminsterfullerene molecule (C60) subjected to a circularly polarized laser field by modeling it with the motion of a classical particle in an annular billiard. We show that the phase space of the billiard model gives rise to three distinct trajectories: "whispering gallery orbits," which hit only the outer billiard wall; "daisy orbits," which hit both billiard walls (while rotating solely clockwise or counterclockwise for all time); and orbits that only visit the downfield part of the billiard, as measured relative to the laser term. These trajectories, in general, maintain their distinct features, even as the intensity is increased from 10(10) to 10(14) Wcm-2. We attribute this robust separation of phase space to the existence of twistless tori.

9.
Phys Rev Lett ; 108(6): 063001, 2012 Feb 10.
Article in English | MEDLINE | ID: mdl-22401062

ABSTRACT

When intense laser pulses release electrons nonsequentially, the time delay between the last recollision and the subsequent ionization may last longer than what is expected from a direct impact scenario [recollision excitation with subsequent ionization (RESI)]. We show that the resulting delayed ionization stems from the inner electron being promoted to a sticky region. We identify the mechanism that traps and releases the electron from this region. As a signature of this mechanism, we predict oscillations in the ratio of RESI to double ionization yields versus laser intensity.

10.
Phys Rev E Stat Nonlin Soft Matter Phys ; 83(3 Pt 2): 036211, 2011 Mar.
Article in English | MEDLINE | ID: mdl-21517579

ABSTRACT

We consider the classical dynamics of a two-electron system subjected to an intense bichromatic linearly polarized laser pulse. By varying the parameters of the field, such as the phase lag and the relative amplitude between the two colors of the field, we observe several trends from the statistical analysis of a large ensemble of trajectories initially in the ground-state energy of the helium atom: high sensitivity of the sequential double-ionization component, low sensitivity of the intensities where nonsequential double ionization occurs, while the corresponding yields can vary drastically. All these trends hold irrespective of which parameter is varied: the phase lag or the relative amplitude. We rationalize these observations by an analysis of the phase-space structures that drive the dynamics of this system and determine the extent of double ionization. These trends turn out to be mainly regulated by the dynamics of the inner electron.

11.
Phys Rev Lett ; 105(8): 083002, 2010 Aug 20.
Article in English | MEDLINE | ID: mdl-20868094

ABSTRACT

It is generally believed that the recollision mechanism of atomic nonsequential double ionization is suppressed in circularly polarized laser fields because the returning electron is unlikely to encounter the core. On the contrary, we find that recollision can and does significantly enhance double ionization, even to the extent of forming a "knee," the signature of the nonsequential process. Using a classical model, we explain two apparently contradictory experiments, the absence of a knee for helium and its presence for magnesium.

12.
Phys Rev Lett ; 104(4): 043005, 2010 Jan 29.
Article in English | MEDLINE | ID: mdl-20366707

ABSTRACT

We examine the nature and statistical properties of electron-electron collisions in the recollision process in a strong laser field. The separation of the double ionization yield into sequential and nonsequential components leads to a bell-shaped curve for the nonsequential probability and a monotonically rising one for the sequential process. We identify key features of the nonsequential process and connect our findings in a simplified model which reproduces the knee shape for the probability of double ionization with laser intensity and associated trends.

13.
Phys Rev Lett ; 102(17): 173002, 2009 May 01.
Article in English | MEDLINE | ID: mdl-19518779

ABSTRACT

We identify the phase-space structures that regulate atomic double ionization in strong ultrashort laser pulses. The emerging dynamical picture complements the recollision scenario by clarifying the distinct roles played by the recolliding and core electrons, and leads to verifiable predictions on the characteristic features of the "knee", a hallmark of the nonsequential process.

14.
J Chem Phys ; 130(16): 164105, 2009 Apr 28.
Article in English | MEDLINE | ID: mdl-19405559

ABSTRACT

Finding the causes for the nonstatistical vibrational energy relaxation in the planar carbonyl sulfide (OCS) molecule is a longstanding problem in chemical physics: Not only is the relaxation incomplete long past the predicted statistical relaxation time but it also consists of a sequence of abrupt transitions between long-lived regions of localized energy modes. We report on the phase space bottlenecks responsible for this slow and uneven vibrational energy flow in this Hamiltonian system with three degrees of freedom. They belong to a particular class of two-dimensional invariant tori which are organized around elliptic periodic orbits. We relate the trapping and transition mechanisms with the linear stability of these structures.

15.
Phys Rev E Stat Nonlin Soft Matter Phys ; 78(3 Pt 2): 036407, 2008 Sep.
Article in English | MEDLINE | ID: mdl-18851169

ABSTRACT

The Hamiltonian description of the self-consistent interaction between an electromagnetic plane wave and a copropagating beam of charged particles is considered. We show how the motion can be reduced to a one-dimensional Hamiltonian model (in a canonical setting) from the Vlasov-Maxwell Poisson brackets. The reduction to this paradigmatic Hamiltonian model is performed using a Lie algebraic formalism which allows us to preserve the Hamiltonian character at each step of the derivation.

16.
Phys Rev E Stat Nonlin Soft Matter Phys ; 77(3 Pt 2): 036314, 2008 Mar.
Article in English | MEDLINE | ID: mdl-18517518

ABSTRACT

Tailored mixing inside individual droplets could be useful to ensure that reactions within microscopic discrete fluid volumes, which are used as microreactors in "digital microfluidic" applications, take place in a controlled fashion. In this paper we consider a translating spherical liquid drop to which we impose a time periodic rigid-body rotation. Such a rotation not only induces mixing via chaotic advection, which operates through the stretching and folding of material lines, but also offers the possibility of tuning the mixing by controlling the location and size of the mixing region. Tuned mixing is achieved by judiciously adjusting the amplitude and frequency of the rotation, which are determined by using a resonance condition and following the evolution of adiabatic invariants. As the size of the mixing region is increased, complete mixing within the drop is obtained.

17.
J Chem Phys ; 128(17): 174105, 2008 May 07.
Article in English | MEDLINE | ID: mdl-18465908

ABSTRACT

We discuss the influence of periodic orbits on the dissociation of a model diatomic molecule driven by a strong bichromatic laser fields. Through the stability of periodic orbits, we analyze the dissociation probability when parameters, such as the two amplitudes and the phase lag between the laser fields, are varied. We find that qualitative features of dissociation can be reproduced by considering a small set of short periodic orbits. The good agreement with direct simulations demonstrates the importance of bifurcations of short periodic orbits in the dissociation dynamics of diatomic molecules.

18.
Phys Rev Lett ; 100(8): 083001, 2008 Feb 29.
Article in English | MEDLINE | ID: mdl-18352620

ABSTRACT

Vibrational energy flows unevenly in molecules, repeatedly going back and forth between trapping and roaming. We identify bottlenecks between diffusive and chaotic behavior, and describe generic mechanisms of these transitions, taking the carbonyl sulfide molecule OCS as a case study. The bottlenecks are found to be lower-dimensional tori; their bifurcations and unstable manifolds govern the transition mechanisms.

19.
Phys Rev Lett ; 101(26): 260603, 2008 Dec 31.
Article in English | MEDLINE | ID: mdl-19437631

ABSTRACT

We investigate the dynamics of many-body long-range interacting systems, taking the Hamiltonian mean-field model as a case study. We show that regular trajectories, associated with invariant tori of the single-particle dynamics, prevail. The presence of such tori provides a dynamical interpretation of the emergence of long-lasting out-of-equilibrium regimes observed generically in long-range systems. This is alternative to a previous statistical mechanics approach to such phenomena which was based on a maximum entropy principle. Previously detected out-of-equilibrium phase transitions are also reinterpreted within this framework.

20.
Phys Rev E Stat Nonlin Soft Matter Phys ; 76(4 Pt 2): 046217, 2007 Oct.
Article in English | MEDLINE | ID: mdl-17995092

ABSTRACT

Transport and mixing properties of passive particles advected by an array of vortices are investigated. Starting from the integrable case, it is shown that a special class of perturbations allows one to preserve separatrices which act as effective transport barriers, while triggering chaotic advection. In this setting, mixing within the two dynamical barriers is enhanced while long range transport is prevented. A numerical analysis of mixing properties depending on parameter values is performed; regions for which optimal mixing is achieved are proposed. Robustness of the targeted mixing properties regarding errors in the applied perturbation are considered, as well as slip/no-slip and/or boundary conditions for the flow.

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