Text 224, 75 rader
Skriven 2004-10-22 18:26:04 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 705
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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
Universität in Kiel and Ernst-Moritz-Arndt Universität 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@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@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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 * Origin: Big Bang (1:106/2000.7)