66 research outputs found
Two-mode squeezed vacuum state coupled to the common thermal reservoir
Entangled states play a crucial role in quantum information protocols, thus
the dynamical behavior of entanglement is of a great importance. In this paper
we consider a two-mode squeezed vacuum state coupled to one thermal reservoir
as a model of an entangled state embedded in an environment. As a criterion for
entanglement we use a continuous-variable equivalent of the Peres-Horodecki
criterion, namely the Simon criterion. To quantify entanglement we use the
logarithmic negativity. We derive a condition, which assures that the state
remains entangled in spite of the interaction with the reservoir. Moreover for
the case of interaction with vacuum as an environment we show that a state of
interest after intinitely long interaction is not only entangled, but also
pure. For comparison we also consider a model in which each of both modes is
coupled to its own reservoir.Comment: replaced with version published in J. Phys.
Non-sequential double ionization of molecules in a strong laser field
We consider the final stage of double ionization of molecules by
short linearly polarized laser pulses. The saddles of the effective adiabatic
potential energy close to which simultaneous escape of electrons from a
molecule takes place are identified. The analysis of the saddles and numerical
simulations of the ionization indicate that to observe clear signatures of
simultaneous electron escape in double ionization of molecules
stronger and much shorter laser pulses than those used in the recent experiment
[E. Eremina, {\it et al}, Phys. Rev. Lett. {\bf 92}, 173001 (2004)] should be
applied.Comment: 5 pages, 3 figures, Conference proceedings from 13th International
Laser Physics Workshop, Triest, Italy, July 12-16, 200
Non-sequential double ionization of molecules
Double ionization of diatomic molecules by short linearly polarized laser
pulses is analyzed. We consider the final stage of the ionization process, that
is the decay of a highly excited two electron molecule, which is formed after
re-scattering. The saddles of the effective adiabatic potential energy close to
which simultaneous escape of electrons takes place are identified. Numerical
simulations of the ionization of molecules show that the process can be
dominated by either sequential or non-sequential events.
In order to increase the ratio of non-sequential to sequential ionizations
very short laser pulses should be applied.Comment: 7 pages, 7 figures, submitted to PR
Strong-field triple ionisation of atoms with valence shell
The interaction of strong pulsed femtosecond laser field with atoms having
three equivalent electrons in the outer shell ( configuration, e.g.
nitrogen) is studied via numerical integration of a time-dependent
Schr\"{o}dinger equation on a grid approach. Single, double and triple
ionization yields originating from a completely antisymmetric wave function are
calculated and extracted using a restricted-geometry model with the soft-core
potential and three active electrons. The direct triple ionization channel is
found to produce a larger yield than the channel connected with single and then
direct double ionization. Compared against earlier results investigating the configuration, we propose that the differences found here might in
fact be accessible through electron's momentum distribution.Comment: 5 pages, 3 figure
Strong-field ionization of atoms with valence shell: Two versus three active electrons
For a model atom with the valence shell we construct consistent three-
and two-active electrons models enabling their direct comparison. Within these
models, we study the influence of the third active electron on the double
ionization yield in strong femtosecond laser fields. We reveal proportionality
between double ionization signals obtained with both models in the field
intensity region where non-sequential ionization dominates. We derive
analytically a correspondence rule connecting the double ionization yields
obtained within the three- and two-active electrons models.Comment: version accepted for Phys. Rev.
Nonsequential double ionization of atoms in strong laser pulses
It is now possible to produce laser pulses with reproducible pulse shape
and controlled carrier envelope phase. It is discussed how that can be explored
in double ionisation studies. To this end we solve numerically the
Schr¨odinger equation for a limited dimensionality model which nevertheless
treats electron repulsion qualitatively correctly and allows to study correlation
effects due to the Coulomb repulsion
Double ionization of a three-electron atom: Spin correlation effects
We study the effects of spin degrees of freedom and wave function symmetries
on double ionization in three-electron systems. Each electron is assigned one
spatial degree of freedom. The resulting three-dimensional Schr\"odinger
equation is integrated numerically using grid-based Fourier transforms. We
reveal three-electron effects on the double ionization yield by comparing
signals for different ionization channels. We explain our findings by the
existence of fundamental differences between three-electronic and truly
two-electronic spin-resolved ionization schemes. We find, for instance, that
double ionization from a three-electron system is dominated by electrons that
have the opposite spin
Strong-Field Double Ionization in a Three-Electron System: Momentum Distribution Analysis
We study strong-field double ionization in a three-electron system by
applying a simplified, reduced-dimensionality model with three active
electrons. The influence of the spin-induced symmetry of the spatial part of
the wavefunction on the final two-photoectron momentum distribution is
discussed. We identify partial momentum distributions originating from
different sets of spins of outgoing electrons providing in this way a quantum
support connection between V-structure and direct ionization typically
explained classically. Changes in the momentum distribution with increasing
field amplitude obtained in our simplified model are shown to be
well-correlated with experimental data known from the literature. The possible
relation between the observed dependencies and different ionization mechanisms
is discussed.Comment: 9 pages, 4 figure
Quantum model for double ionization of atoms in strong laser fields
We discuss double ionization of atoms in strong laser pulses using a reduced
dimensionality model. Following the insights obtained from an analysis of the
classical mechanics of the process, we confine each electron to move along the
lines that point towards the two-particle Stark saddle in the presence of a
field. The resulting effective two dimensional model is similar to the aligned
electron model, but it enables correlated escape of electrons with equal
momenta, as observed experimentally. The time-dependent solution of the
Schr\"odinger equation allows us to discuss in detail the time dynamics of the
ionization process, the formation of electronic wave packets and the
development of the momentum distribution of the outgoing electrons. In
particular, we are able to identify the rescattering process, simultaneous
direct double ionization during the same field cycle, as well as other double
ionization processes. We also use the model to study the phase dependence of
the ionization process.Comment: 14 pages, 16 figures, version accepted for publication in Phys. Rev.
Restricted space ab initio models for double ionization by strong laser pulses
Double electron ionisation process occurs when an intense laser pulse
interacts with atoms or molecules. Exact {\it ab initio} numerical simulation
of such a situation is extremely computer resources demanding, thus often one
is forced to apply reduced dimensionality models to get insight into the
physics of the process. The performance of several algorithms for simulating
double electron ionization by strong femtosecond laser pulses are studied. The
obtained ionization yields and the momentum distributions of the released
electrons are compared, and the effects of the model dimensionality on the
ionization dynamics discussed
- …