49,406 research outputs found

### Small rare gas clusters in XUV laser pulses

Semi-classical molecular dynamics simulations of small rare gas clusters in
short laser pulses of 100 nm wavelength were performed. For comparison, the
cluster response to 800 nm laser pulses was investigated as well. The inner
ionization dynamics of the multi-electron atoms inside the cluster was treated
explicitly. The simulation results underpin that at XUV wavelengths collisions
play an important role in the energy absorption. The generation of the
surprisingly high charge states of Xe atoms inside clusters, as they were
observed in the free-electron laser experiment at DESY, Hamburg, Germany
[Wabnitz et al., Nature 420, 482 (2002)], is due to the reduced ionization
potential of atoms inside charged clusters, the ionization ignition mechanism,
and collisions.Comment: 6 pages, 6 figures, uses Springer style svjou

### Maximal entropy random networks with given degree distribution

Using a maximum entropy principle to assign a statistical weight to any
graph, we introduce a model of random graphs with arbitrary degree distribution
in the framework of standard statistical mechanics. We compute the free energy
and the distribution of connected components. We determine the size of the
percolation cluster above the percolation threshold. The conditional degree
distribution on the percolation cluster is also given. We briefly present the
analogous discussion for oriented graphs, giving for example the percolation
criterion.Comment: 22 pages, LateX, no figur

### QPROP: A Schroedinger-solver for intense laser-atom interaction

The Qprop package is presented. Qprop has been developed to study laser-atom
interaction in the nonperturbative regime where nonlinear phenomena such as
above-threshold ionization, high order harmonic generation, and dynamic
stabilization are known to occur. In the nonrelativistic regime and within the
single active electron approximation, these phenomena can be studied with Qprop
in the most rigorous way by solving the time-dependent Schr\"odinger equation
in three spatial dimensions. Because Qprop is optimized for the study of
quantum systems that are spherically symmetric in their initial, unperturbed
configuration, all wavefunctions are expanded in spherical harmonics.
Time-propagation of the wavefunctions is performed using a split-operator
approach. Photoelectron spectra are calculated employing a window-operator
technique. Besides the solution of the time-dependent Schr\"odinger equation in
single active electron approximation, Qprop allows to study many-electron
systems via the solution of the time-dependent Kohn-Sham equations.Comment: 40 pages, LaTeX; to obtain the QPROP source code visit
http://www.qprop.de, accepted for publication in Computer Physics
Communication

### 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 $\omega_1$ and $\omega_2=n\omega_1$,
$n=2,3,...$ and sufficiently large excursion amplitudes $n+1$ 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

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