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03-29-2007, 08:05 PM
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 817 #March 29, 2007 by Phillip F. Schewe, Ben Stein
http://www.aip.org/pnu
SLOW SALT. #Through laser cooling it's relatively easy to cool atoms
to microkelvin temperatures. #This method is not useful for
molecules, which possess a variety of internal vibrational and
rotational motions. #By indirect methods, however, stationary
samples of molecular vapors have been chilled to mK temperatures by
cooling molecules in cold helium or by decelerating polar molecules,
or to microkelvin temperatures by welding together pairs of
already cooled atoms. Another cooling technique employs a spinning
beam source whose speed cancels the velocity of the molecules
emerging from the source. #Molecular speeds down to around 60 m/s
have been obtained. #Now, two physicists at the Universitat
Bielefeld (Germany) have produced a beam of potassium-bromine
molecules (essentially a kind of salt) with an average molecular
speed of 42 m/s; an estimated 7% of the beam travels even slower
than 14 m/s (below 1.4K). #At this speed, some of the molecules
could be loaded into a trap. #The cold KBr molecules are made by
sending a beam of K atoms into a counter-propagating beam of HBr
molecules where the velocity of both species have to be tuned
properly. #Within the intersection #zone the slow KBr molecules are
formed by chemical reaction. There the density of trappable
molecules is about #two million molecules per cubic centimeter, but
the researchers believe this can be increased by a thousandfold.
Besides KBr, beams of other heavy salt molecules can be produced
(such as CsI) as well as beams of radicals (reactive molecules with
unpaired electrons) such as CaBr and BaI. # According to Hansjuergen
Loesch (mailto:loesch@physik.uni-bielefeld.de), slow molecules are a
prerequisite for performing cold chemistry, which would simulate
conditions in cold planetary atmospheres or in cold interstellar
clouds. #If the chemistry is cold enough, new quantum effects might
emerge. (Liu and Loesch, Physical Review Letters, 9 March
2007) # #
# #
THE EVER-SHIFTING FACE OF PLUTONIUM. #A new theory explains some of
the unusual properties of plutonium, the radioactive metal best
known for its proclivity to undergo nuclear fission chain reactions,
making it a potent fuel for nuclear weapons and power plants.
Plutonium is one of the most unusual metals --it's not magnetic and
it does not conduct electricity well. #The material also changes its
size dramatically with even the slightest changes in its temperature
and pressure. #The atom's unusual set of properties distinguishes it
from even its closest neighbors on the periodic table, such as
americium. #What makes plutonium unique? #In the new theory,
developed by condensed-matter theorists at Rutgers University in New
Jersey, plutonium's eight outermost or "valence" electrons can
circulate among different orbitals, or regions around the atom. #In
plutonium's 5f orbital, the one with the greatest influence on its
atomic properties, the number of valence electrons it contains is
most often five (approximately 80% of the time), but can also be six
(about 20% of the time) or four (less than 1% of the time),
according to the theory. These electrons shuttle in and out of the
5f orbital very quickly -- on the order of femtoseconds, or
quadrillionths of a second, the researchers say. #Plutonium is an
example of a strongly correlated material, in which the valence
electrons interact with each other to a great degree, and cannot be
treated as independent agents. #Taking these interactions into
account, the researchers combined two theoretical approaches to
solid materials, called the local density approximation and
dynamical mean field theory, to come up with their sophisticated
analysis. #As their analysis shows, the 5f orbital dictates many of
plutonium's key properties, such as its lack of conductivity and net
magnetism. #With their theory, the researchers have also explained
the magnetic and electrical properties of americium and curium.
They hope their approach will also elucidate the properties of
rare-earth elements on the periodic table (Shim et al., Nature, 28
March 2007.)
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. #It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
AUTO-SUBSCRIPTION OR DELETION: By using the expression
"subscribe physnews" in your e-mail message, you
will have automatically added the address from which your
message was sent to the distribution list for Physics News Update.
If you use the "signoff physnews" expression in your e-mail message,
the address in your message header will be deleted from the
distribution list. #Please send your message to:
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(Leave the "Subject:" line blank.)
The American Institute of Physics Bulletin of Physics News
Number 817 #March 29, 2007 by Phillip F. Schewe, Ben Stein
http://www.aip.org/pnu
SLOW SALT. #Through laser cooling it's relatively easy to cool atoms
to microkelvin temperatures. #This method is not useful for
molecules, which possess a variety of internal vibrational and
rotational motions. #By indirect methods, however, stationary
samples of molecular vapors have been chilled to mK temperatures by
cooling molecules in cold helium or by decelerating polar molecules,
or to microkelvin temperatures by welding together pairs of
already cooled atoms. Another cooling technique employs a spinning
beam source whose speed cancels the velocity of the molecules
emerging from the source. #Molecular speeds down to around 60 m/s
have been obtained. #Now, two physicists at the Universitat
Bielefeld (Germany) have produced a beam of potassium-bromine
molecules (essentially a kind of salt) with an average molecular
speed of 42 m/s; an estimated 7% of the beam travels even slower
than 14 m/s (below 1.4K). #At this speed, some of the molecules
could be loaded into a trap. #The cold KBr molecules are made by
sending a beam of K atoms into a counter-propagating beam of HBr
molecules where the velocity of both species have to be tuned
properly. #Within the intersection #zone the slow KBr molecules are
formed by chemical reaction. There the density of trappable
molecules is about #two million molecules per cubic centimeter, but
the researchers believe this can be increased by a thousandfold.
Besides KBr, beams of other heavy salt molecules can be produced
(such as CsI) as well as beams of radicals (reactive molecules with
unpaired electrons) such as CaBr and BaI. # According to Hansjuergen
Loesch (mailto:loesch@physik.uni-bielefeld.de), slow molecules are a
prerequisite for performing cold chemistry, which would simulate
conditions in cold planetary atmospheres or in cold interstellar
clouds. #If the chemistry is cold enough, new quantum effects might
emerge. (Liu and Loesch, Physical Review Letters, 9 March
2007) # #
# #
THE EVER-SHIFTING FACE OF PLUTONIUM. #A new theory explains some of
the unusual properties of plutonium, the radioactive metal best
known for its proclivity to undergo nuclear fission chain reactions,
making it a potent fuel for nuclear weapons and power plants.
Plutonium is one of the most unusual metals --it's not magnetic and
it does not conduct electricity well. #The material also changes its
size dramatically with even the slightest changes in its temperature
and pressure. #The atom's unusual set of properties distinguishes it
from even its closest neighbors on the periodic table, such as
americium. #What makes plutonium unique? #In the new theory,
developed by condensed-matter theorists at Rutgers University in New
Jersey, plutonium's eight outermost or "valence" electrons can
circulate among different orbitals, or regions around the atom. #In
plutonium's 5f orbital, the one with the greatest influence on its
atomic properties, the number of valence electrons it contains is
most often five (approximately 80% of the time), but can also be six
(about 20% of the time) or four (less than 1% of the time),
according to the theory. These electrons shuttle in and out of the
5f orbital very quickly -- on the order of femtoseconds, or
quadrillionths of a second, the researchers say. #Plutonium is an
example of a strongly correlated material, in which the valence
electrons interact with each other to a great degree, and cannot be
treated as independent agents. #Taking these interactions into
account, the researchers combined two theoretical approaches to
solid materials, called the local density approximation and
dynamical mean field theory, to come up with their sophisticated
analysis. #As their analysis shows, the 5f orbital dictates many of
plutonium's key properties, such as its lack of conductivity and net
magnetism. #With their theory, the researchers have also explained
the magnetic and electrical properties of americium and curium.
They hope their approach will also elucidate the properties of
rare-earth elements on the periodic table (Shim et al., Nature, 28
March 2007.)
***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
magazines, and other news sources. #It is provided free of charge
as a way of broadly disseminating information about physics and
physicists. For that reason, you are free to post it, if you like,
where others can read it, providing only that you credit AIP.
Physics News Update appears approximately once a week.
AUTO-SUBSCRIPTION OR DELETION: By using the expression
"subscribe physnews" in your e-mail message, you
will have automatically added the address from which your
message was sent to the distribution list for Physics News Update.
If you use the "signoff physnews" expression in your e-mail message,
the address in your message header will be deleted from the
distribution list. #Please send your message to:
listserv@listserv.aip.org
(Leave the "Subject:" line blank.)