1,179 research outputs found
Two-color stabilization of atomic hydrogen in circularly polarized laser fields
Dynamic stabilization of atomic hydrogen against ionization in high-frequency
single- and two-color, circularly polarized laser pulses is observed by
numerically solving the three-dimensional, time-dependent Schr\"odinger
equation. The single-color case is revisited and numerically determined
ionization rates are compared with both, exact and approximate high-frequency
Floquet rates. The position of the peaks in the photoelectron spectra can be
explained with the help of dressed initial states. In two-color laser fields of
opposite circular polarization the stabilized probability density may be shaped
in various ways. For laser frequencies and ,
and sufficiently large excursion amplitudes distinct
probability density peaks are observed. This may be viewed as the
generalization of the well-known ``dichotomy'' in linearly polarized laser
fields, i.e, as ``trichotomy,'' ``quatrochotomy,'' ``pentachotomy'' etc. All
those observed structures and their ``hula-hoop''-like dynamics can be
understood with the help of high-frequency Floquet theory and the two-color
Kramers-Henneberger transformation. The shaping of the probability density in
the stabilization regime can be realized without additional loss in the
survival probability, as compared to the corresponding single-color results.Comment: 10 pages, REVTeX4, 11 eps-figures, see also
http://www.physik.tu-darmstadt.de/tqe/dieter/publist.html for a manuscript
with higher-quality figure
A numerical ab initio study of harmonic generation from a ring-shaped model molecule in laser fields
When a laser pulse impinges on a molecule which is invariant under certain
symmetry operations selection rules for harmonic generation (HG) arise. In
other words: symmetry controls which channels are open for the deposition and
emission of laser energy---with the possible application of filtering or
amplification. We review the derivation of HG selection rules and study
numerically the interaction of laser pulses with an effectively one-dimensional
ring-shaped model molecule. The harmonic yields obtained from that model and
their dependence on laser frequency and intensity are discussed. In a real
experiment obvious candidates for such molecules are benzene, other aromatic
compounds, or even nanotubes.Comment: 5 pages, 3 figure
Expansion of a finite size plasma in vacuum
The expansion dynamics of a finite size plasma is examined from an analytical
perspective. Results regarding the charge distribution as well as the
electrostatic potential are presented. The acceleration of the ions and the
associated cooling of the electrons that takes place during the plasma
expansion is described. An extensive analysis of the transition between the
semi infinite and the finite size plasma behaviour is carried out. Finally, a
test of the analytical results, performed through numerical simulations, is
presented.Comment: 4 pages with 5 figure
Harmonic generation by atoms in circularly polarized laser fields: far-off and near resonances regimes
The generation of harmonics by atoms interacting with two circularly
polarized and frequency related laser fields is addressed through ab initio
numerical simulations. A detailed charaterization of a few specific harmonics
is given. In particular, the two different cases where the total energy
absorbed through photons is far-off or close to the energy gap between
different atomic states are investigated. It is found that the conversion
efficiency in the harmonic generation is strongly dependent on the inner atomic
structure and in certain specific cases it can be significantly enhanced within
a small frequency range.Comment: Submitted to Appl. Phys. B, 4 page
Electric field dynamics and ion acceleration in the self-channeling of a superintense laser pulse
The dynamics of electric field generation and radial acceleration of ions by
a laser pulse of relativistic intensity propagating in an underdense plasma has
been investigated using an one-dimensional electrostatic, ponderomotive model
developed to interpret experimental measurements of electric fields [S. Kar et
al, New J. Phys. *9*, 402 (2007)]. Ions are spatially focused at the edge of
the charge-displacement channel, leading to hydrodynamical breaking, which in
turns causes the heating of electrons and an "echo" effect in the electric
field. The onset of complete electron depletion in the central region of the
channel leads to a smooth transition to a "Coulomb explosion" regime and a
saturation of ion acceleration.Comment: 9 pages, 7 figures, final revised version, to appear on Plasma Phys.
Contr. Fus., special issue on "Laser and Plasma Accelerators", scheduled for
February, 200
Electron correlation versus stabilization of atoms in intense laser pulses
We present a numerical study of the stabilization
process for a fully correlated two-electron model atom
in an intense laser pulse. A comparison with calculations
for a "real" 3D He atom is also done. We concentrate
on the very high frequency regime, where the photon energy
exceeds the ionization energy of both electrons, outer
and inner. Our results show that when correlation
effects are included the ionization probability (IP) is
enhanced. Nevertheless, we still observe a decreasing IP
within a certain intensity domain. The results from the
fully correlated simulations are compared with those from
simpler, approximate models. The full numerical treatment
for the He atom is not yet possible. We therefore present
results obtained with "single active electron"
approximation and time-dependent density functional theory.
Our numerical simulations can be useful for the future
understanding of the stabilization phenomenon for more-than-one
electron atoms
Representation theory of wreath products of finite groups.
This is an exposition on the representation theory of wreath products of finite groups, with many examples worked out
Harmonic generation by atoms in circularly polarized two-color laser fields with coplanar polarizations and commensurate frequencies
The generation of harmonics by atoms or ions in a two-color, coplanar field
configuration with commensurate frequencies is investigated through both, an
analytical calculation based on the Lewenstein model and the numerical ab
initio solution of the time-dependent Schroedinger equation of a
two-dimensional model ion. Through the analytical model, selection rules for
the harmonic orders in this field configuration, a generalized cut-off for the
harmonic spectra, and an integral expression for the harmonic dipole strength
is provided. The numerical results are employed to test the predictions of the
analytical model. The scaling of the cut-off as a function of both, one of the
laser intensities and frequency ratio , as well as entire spectra for
different and laser intensities are presented and analyzed. The
theoretical cut-off is found to be an upper limit for the numerical results.
Other discrepancies between analytical model and numerical results are
clarified by taking into account the probabilities of the absorption processes
involved.Comment: 8 figure
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