PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 629 March 19, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

THE SHARPEST EVER OPTICAL IMAGE OF MOLECULAR VIBRATIONS, revealing details
as small as 20 nanometers, has been produced by a Rochester-Harvard-Portland
State  collaboration (Lukas Novotny, 585-275-5767 ,
novotny{at}optics.rochester.edu). The image shows individual carbon
nanotubes with single-atom-thick walls (see figure at www.aip.org/mgr/png ).
 Looking beyond this result, the researchers are striving for even higher
sensitivity, which could supply very useful images of proteins, only 5-20
nanometers in size. Other, non-optical imaging techniques, such as scanning
tunneling microscopy, can show smaller details, but this is the highest
resolution image that uses light, a probe that can potentially extract lots
more information.  The researchers employed a sophisticated version of
"near-field optical microscopy," in which a small probe (in this case, a
gold wire with an extremely narrow tip) is placed very close to the surface.
With the wire only a few nanometers away from the surface, researchers
circumvented the usual roadblock to resolution, known as the "diffraction
limit," in which optical details are ordinarily limited to half the
wavelength of the light being used.  In their technique, called "near-field
Raman spectroscopy," the researchers shine laser light at the gold wire. The
light strikes the wire's electrons, which then generate electric fields.
These fields interact with vibrating atoms in the sample, which then release
light of specific colors (frequencies). The spectrum of frequencies provides
information on the chemical composition and molecular structure of the
sample.  From this information, an image can be created.  In designing their
probe, the researchers made use of the "surface-enhanced Raman scattering
effect," in which the interaction with atomic vibrations is greatly
increased by the use of nanometer-sized metal particles (in this case, the
tip itself).  In the future, researchers hope to use their technique to
determine presently unknown structural details of carbon nanotubes, such as
the different ways the nanotubes can interconnect with one another.   With
better resolution, the researchers hope to take detailed pictures of
proteins in cell membranes.  Such data can potentially shed new insights on
how proteins act in a cell membrane and offer clues for designing better
drugs.  (Hartschuh et al., Physical Review Letters, 7 March 2003)

HOW DOES THE SUN SHINE?  The SNO and Super-Kamiokande detectors have done a
handy job of accounting for the neutrinos coming from the decay of boron-8
nuclei in the sun.  But the flux from B-8 decays represents a mere 0.02% of
the predicted flux of solar neutrinos, and one
wants to study other types of nu production in order to get a better grip
on nuclear physics in the sun's core.  One would especially like to know
more about neutrinos from Be-7, N-13 and O-15 decays (catalyzed by
carbon-12), and from proton-proton reactions.  (The p-p neutrinos, probably
amounting to 90% of the sun's nu flux, have relatively low energies, below
0.5 MeV, whereas the nu's seen directly in terrestrial detectors typically
have been in excess of 5 MeV.)  In the 1930's, nuclear pioneer Hans Bethe
argued that energy produced in the nuclear reactions involving the heavier
elements (the CNO cycle) were a more important energy-producing mechanism
for the Sun than was the fusion of the lighter elements (the p-p cycle).
Nowadays solar scientists believe the CNO reactions are predominant for
stars a bit heavier than our sun but that in the sun itself the p-p cycle
will be more important.  A new paper by John Bahcall and Carlos Pena-Garay
(Institute for Advanced Study) and Concha Gonzales-Garcia (Stony Brook)
addresses this issue using recent data from solar neutrino and reactor
experiments.  Bahcall and his colleagues determine that the fraction of
energy produced in the sun via CNO reactions is less than 7.3%.  This is a
tenfold improvement over the best previous estimation for the CNO
contribution.  (Physical Review Letters, upcoming article; contact John
Bahcall, jnb{at}ias.edu, 609-734-8054; see neutrino website at
www.sns.ias.edu/~jnb )

BLOOD VESSEL NETWORKS.  A new mathematical model is leading to insights
about the formation of blood vessel networks. The model, proposed by
researchers from several Italian institutions (contact A. de Candia,
decandia{at}na.infn.it, 011+39-081676805), accurately mimics vascular
structures formed by cells randomly spread on a gel matrix. Chemical cues
entice cells on a growing medium to migrate and aggregate into groups. Below
a certain cell density, the model and related experiments show many
disconnected groups are formed. Above a critical density known as the
percolation limit, a spanning cluster of cells connected across large
distances is formed (images at www.aip.org/mgr/png/2003/182.htm ). Exactly
at the percolation threshold, such a cluster exhibits a fractal structure
with a fractal dimension of about 1.9. (The fractal dimension specifies how
much of the available space is filled. For a 2-dimensional gel plate, the
surface is entirely filled at a fractal dimension of 2.) In addition, both
experiment and the new model point out that the fractal dimension is
different when the cells are observed at different scales. At scales of
about 0.8 millimeters or less, the fractal dimension of the cell networks
drops to about 1.5. The researchers speculate that the change in dimension
may be indicative of the dynamics that led to the formation of the cellular
networks in the first place. The good agreement between the model and
in-vitro experiments on gel growing media suggests that we may soon gain a
better understanding of the formation of vascular networks in living
creatures, as well as the pathological vascular formation that accompanies
certain cancers and other ailments. (A. Gamba et al., Physical Review
Letters, 21 March 2003)

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PHYSICS NEWS UPDATE is a digest of physics news items arising
from physics meetings, physics journals, newspapers and
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