Low Field Laser Ionization of Argon Clusters: A new Remarkable Fragmentation Dynamics
Argon clusters (a weakly bonded atomic assembly) are shattered by short and intense femtosecond laser pulses. Electrons are generated immediately and recollide the structure when the laser field reverses; this may generate extra ions inducing Coulomb explosion and eventually X ray generation, a very complex situation. Here in difference, we have investigated the intermediate situation of an Argon cluster interacting with an ultrashort laser pulse of "moderate" energy, above the threshold for field ionisation. We have observed evidence for the fission of the structure and the surprising formation of well defined values (rings here) in the fragments speed distribution, appearing as a "quantification" in the energy relaxation dynamics.
In our interpretation a single excited ion is formed within the cluster, by electron recollision. In contrary to the unexcited argon atom, this ion can now build a chemical bond. While the atom attaches a neighboring atom, this latter one is suddenly ionized by the excess energy stored in the ion. Hence forming a single pair of positive ions is generated at a short, well defined separation distance. A strong repulsion follows inducing the fission of the cluster. Accordingly, the "quantification" corresponds to the recoil of well defined partners dissipating asymmetrically and directly their excess energy. LC11454
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Golden quantum dots on a thin magnesium oxide film over silver
.GIF)
Recent progress in the controlled growth of ultrathin metal-oxide films
has made it possible to fabricate well-defined templates for
spectroscopic studies of insulator-supported metal particles and
molecule-metal ¿systems. Nanosized metal clusters deposited or grown on
these films are model systems regarding nanoparticle growth,
low-dimensional electron physics, nanocatalysis, and molecular
electronics. Now, researchers at the Nanoscience Center in the
University of Jyväskylä in Finland predict how small (from a few atoms
to 20 atoms) gold nanoclusters adsorbed on three-layer thick magnesium
oxide films, grown on silver, would appear in the scanning tunneling
microscopy (STM) imaging. They investigated the electronic structure of
the clusters by using the density functional theory, and observed that
those electron states of the gold clusters that participate in the
tunneling from/to the STM tip, are delocalized over the metal cluster.
Furthermore, they display clear symmetries that can be understood by
considering the cluster as a ¿quantum dot¿ with a finite number of
electrons confined in the volume of the cluster. As a consequence, in
the STM experiments single atoms in the clusters cannot be ¿seen¿ and
determinations of the precise atomic count of the clusters become
indirect as it will require comparisons of the observed image symmetries
to theory.
Attached Figures:
Au13MgO.gif: A low-resolution image of the atomic structure of a 13-atom
gold cluster (yellow) adsorbed on a MgO surface-defect (oxygen vacancy).
The lowest atom in the cluster (blue) pins the cluster to the defect. O
atoms are red and Mg atoms grey.
Au13STM.gif: A low-resolution simulated scanning tunneling microscopy
image for a low negative bias voltage, displaying a highly symmetric
single electron state close to the Fermi energy. The diamonds depict the
positions of the gold atoms projected on a plane parallel to the MgO
surface.
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Proteins at rest - an investigation of a jamming transition using proteins as model nanoparticles
The use of proteins as model nanoparticles has allowed us to investigate the interplay between phase separation and dynamical arrest in colloidal suspensions. The investigation of non-equilibrium phenomena such as gel and glass formation in colloidal suspensions has emerged as one of the most important fields of soft matter research. Areas such as dynamical arrest or jamming in suspensions of (weakly) attractive colloids, the interplay between spinodal decomposition and glass formation or the formation of ordered versus amorphous photonic materials attract considerable attention from the experimental and theoretical soft matter community. In this paper we were now able to unambiguously demonstrate that temperature quenches into the spinodal region and below an arrest temperature lead to the formation of a bicontinuous network, where the dense phase undergoes dynamical arrest once it reaches the glass line at this temperature. Microscopically, this corresponds to a coexistence of a dilute fluid with a dense percolated glass phase. These measurements have allowed us for the first time to quantitatively locate the glass line in the unstable region below the spinodal, and thus provide a new test ground for computer simulations and theoretical calculations in the current attempt to understand and generalize dynamical arrest in soft matter. LZ10476
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Intense laser light, dark cavities and cool molecules
Laser cooling has been demonstrated to be a powerful technique
in experiments with atomic vapors.
It was instrumental in the achievement of Bose-Einstein
condensation, and is routinely applied in experiments dealing with
cold atoms for high-precision measurements and quantum information
processing. When applied to molecules, however,
direct implementations of laser cooling techniques
proved to be inefficient, because they in general heat the rotational and
vibrational motion of the molecules. Now, researchers in Barcelona, Munich
and Garching have devised a new method to cool molecules' external,
rotational and vibrational motion simultaneously by laser light in
combination with
high-quality optical cavities. The results are obtained with state of the art
quantum-chemical simulations for a prototype molecule, OH, and show
that the vibrations and rotations of OH molecules can be brought to
the ground state, while the motion is simultaneously cooled to
temperatures of the order of few microkelvin in a fraction of a
second. This proposal opens novel perspectives in the preparation
and control of ultracold matter of larger complexity. LC155455
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Molecular "compass needles" in dying stars point to some of the largest magnetic fields in the galaxy
The methine molecule has been employed as a sensitive "compass needle" in the atmosphere of a dying star to determine the strength and direction of its magnetic field. Using this new technique we find with great accuracy an enormous field strength, nearly 15 million times stronger than the natural field of the Earth. Eventually, our Sun will also turn into one of such stars called white dwarfs. These stellar remnants tell us a fascinating story on how the galaxy may have looked like a few billion years ago when the Sun and the Earth were born. LW10449
