PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 705 October 20, 2004
by Phillip F. Schewe, Ben Stein

CRYSTALLINE ORDER AT 40,000 K.  Physicists at the Christian-Albrechts
Universit„t in Kiel and Ernst-Moritz-Arndt Universit„t in Greifswald
(Germany) have been able to rig a ball of dust particles holding to a
crystalline structure even in the middle of a hot plasma.  Most
crystals---that is, solid materials in which atoms are arrayed in a regular
stacked-cannonball order---melt at temperatures of hundreds or thousands of
degrees.  The heartiest crystal, diamond, succumbs at 4000 K.  The heat is
just too much for the atomic bonds and the defining gridiron structure
weakens and melts.  Another sort of "crystal," at low
temperatures, is the optical crystal consisting of an artificial and
diffuse array of atoms held at the interstices of a 3-dimensional lattice
by the
electric fields of cross-cutting laser beams.   The plasma crystal, by
great contrast, consists of a herd of charged 3.5-micron-sized polymer
particles amidst a gas-discharge.  Juggling two mighty forces---the mutual
repulsion of the particles among themselves and the compressive force on
them by the surrounding plasma---
the particles manage to arrange themselves into neat concentric spheres, to
a total ball diameter of several mm (see figure at www.aip.org/png).  It is
ironic that J.J. Thomson, the discoverer of the electron, had suggested in
1904 that the layout of the Periodic Table of elements could be explained
if atoms had exactly this sort of onionlike architecture, with negative
charges held poised in a wider sea of positive charges.  This idea was
wrong for atoms but does describe the arrangement of the dust particles in
this plasma. To sum up: in a plasma where the electron temperature is
40,000 K (the positive-ion temperature is less than 1000 K), an orderly
Coulomb ball consisting of aligned, concentric shells of dust particles can
survive for long periods.  The two outstanding features of the ball (other
than its survival at such high
temperatures) are that it represents a true transparent crystal; with a
microscope and video camera individual particles in the middle of the
structure can be imaged by laser light. The other feature is the slowness
of the dynamics.  The particles move about with a characteristic  timescale
of milliseconds rather than the femtosecond scale of atoms in a
conventional crystal.  The study of laboratory plasma crystals, the
experimenters believe, gives fundamental insight into strongly coupled
matter and applies directly to the study of intergalactic nebulae, comet
tails, the rings of Saturn and, back here on Earth, in the improvement of
various microchip processing steps.   (Oliver Arp et al., PhysicalReview
Letters, upcoming article; contact  Dietmar Block block{at}physik.uni-kiel.de,
49-431-880-3862)

ATOMS CAN TRANSFER THEIR INTERNAL "STRESS" TO OTHER ATOMS, new
experiments have revealed.   Compared to atoms that are all by themselves,
atoms with a close neighbor have a very efficient and surprising way to get
rid of excess internal energy.  An excited atom can hand over its energy to
a neighbor, a research team led by the University of Frankfurt has
demonstrated experimentally in a measurement carried out at the Berlin
synchrotron facility BESSY II (R. Doerner, doerner{at}hsb.uni-frankfurt.de).
Predicted in 1997 by a group at Heidelberg University (Cederbaum et al.,
Phys Rev. Lett, 15 Dec 1997), this decay mechanism occurs when atoms or
molecules lump together. Once an excited particle is placed in an
environment of other particles such as in clusters or fluids, the novel
de-excitation mechanism, called "Interatomic Coulombic Decay,"
leads to the emission of very low-energy electrons from a particle that is
neighboring the initially excited one (see figure at www.aip.org/png). The
researchers demonstrated the effect in a pair of weakly bound neon atoms. 
The two neon atoms were separated by 3.4 Angstroms (about 6 times the
radius of the neon atom) and held together by a weak "van der
Waals" bond.  Removing a tightly bound electron from one of the neon
atoms allowed one of the less tightly bound atoms to jump down to the
tightly bound spot and in the process gained energy.  The extra energy was
not sufficient to liberate any of the remaining electrons in the same neon
atom, but it was sufficient to release an electron in the neighboring atom.
This newly verified effect may have a wide-ranging impact in chemistry and
biology since it is predicted to happen frequently in most hydrogen-bonded
systems, most prominently liquid water. Furthermore, it may be an
important, and so far unknown, source of low-energy electrons, which have
recently been shown to cause damage to DNA (see
http://www.aip.org/pnu/2003/split/636-1.html). (Jahnke et al., Physical
Review Letters, 15 October 2004; also see researchers' website at
http://hsb.uni-frankfurt.de/photoncluster/ICD.html)

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