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Skriven 2006-02-21 19:06:47 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 766
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PHYSICS NEWS UPDATE                                                            

The American Institute of Physics Bulletin of Physics News
Number 766  February 21, 2006 by Phillip F. Schewe, Ben Stein, and
Davide Castelvecchi
                
THE AMERICAN INSTITUTE OF PHYSICS (AIP) will observe its 75th
anniversary this year. AIP was established in New York City in 1931
to help facilitate publishing and other services for five scientific
organizations: the American Physical Society (APS), the Optical
Society of America (OSA), the Acoustical Society of America (ASA),
the Society of Rheology (SOR), and the American Association of
Physics Teachers (AAPT).  Later five more Member Societies were
added: the American Crystallographic Association (ACA), the American
Astronomical Society (AAS), the American Association of Physicists
in Medicine (AAPM), AVS: Science & Technology of Materials,
Interfaces, and Processing, and the American Geophysical Union
(AGU).  Today, AIP is one of the largest physics journal publishers
in the world, and the non-overlapping membership of its 10 member
societies numbers more than 100,000 (general AIP website:
http://www.aip.org/index.html ).  Physics News Update, the weekly
summary of physics research you are reading at this moment, is
prepared the AIP Media and Government Relations (MGR) division,
operating out of AIP's headquarters in College Park, MD, just
outside Washington, DC (associated websites are www.aip.org/pnu and
www.aip.org/news/links.html ).  To mark AIP's 75th anniversary, we
plan to run a series of occasional comparisons between noteworthy
physics topics from 1931 and 2006.  Herewith the first of these:
                                
PHASE CONTRAST IMAGING WITH AN ELECTRON MICROSCOPE. Physicists in
Germany have taken a crucial step towards achieving sharper images
of biological samples and other "weak-contrast" objects.  Typically
microscope images of samples made of low-weight elements like
hydrogen, carbon, nitrogen, and oxygen, are characterized by poor
contrast.  In the new approach, contrast will be improved for a
transmission electron microscope (TEM) by imposing a large relative
phase shift to the electron waves scattered from samples.  The use
of a beam of electrons as an illumination source for microscopy was
pioneered in the early 1930s by Ernst Ruska, who won a Nobel Prize
for the effort half a century later.  Since then, electron
microscopes have been a workhorse for imaging small things, often
with a spatial resolution superior to that available with light
microscopes.  Nevertheless, even electron microscopes have
resolution problems.  In a TEM device most of the electrons pass
through the thin electron-transparent sample without scattering.
Scattering of electron waves, when it does happen, occurs not
because of absorption---the amplitude of the electron beam is
largely undiminished---but through the shifting of the electron
phase.  Scattered and unscattered waves are focused and recombine
downstream of the sample in a recording medium, typically a charged
coupled device (CCD).
Unfortunately, in weak phase objects the phase shifting is slight,
resulting in poor contrast.  What scientists at the University of
Karlsruhe and the Max-Planck Institute for Biophysics in Frankfurt
have done to remedy this situation is to interpose a special
free-suspended micro-scaled electrostatic lens beyond the sample;
this electrostatic lens has the effect of shifting the phase of the
unscattered waves by a further 90 degrees but leaving the scattered
waves unshifted (see figure at http://www.aip.org/png/2006/249.htm
).   This dramatically improves the contrast in the resultant
images. This electrostatic lens is called a Boersch phase plate in
honor of Hans Boersch, who proposed the technique in 1947.  It has
not been achieved until now because of its demanding size
specifications.  (Schultheiss et al., Review of Scientific
Instruments, March 2006; contact Fabian Perez-Willard,
perez@lem.uni-karlsruhe.de; website,
http://www.lem.uni-karlsruhe.de/ )

MOLECULES GET MORE CLASSICAL at high pressures.  That is, a new
study of molecules being squeezed in a diamond anvil cell shows that
as the pressure goes up, the force between atoms in a diatomic
molecule behaves more and more like the classic Hooke's law,
according to which the force between two objects connected by an
elastic spring is proportional to the contraction or extension of
the spring.  Two scientists at the Carnegie Institution of
Washington, and Lawrence Livermore National Laboratory, Alexander
Goncharov and Jonathan Crowhurst, have loaded several species of
molecule, such as H2, D2, and N2, into their cell and then observed
what happened at high temperature and high pressure.  By varying
these two parameters the molecular sample can often be transformed
from a fluid into a crystal or back again, or the molecules
themselves might even be broken apart.  The researchers (contact
Goncharov at goncharov@gl.ciw.edu) first heated the samples using a
near-infrared laser and then probed the various excited vibrational
quantum states using the technique of Raman spectroscopy. By
carefully noting the frequency and linewidths of these stretching
modes, they could deduce the energetics of the binding between the
atoms even as the molecule was being subject to the extreme
conditions.  The findings, such as the realization that the binding
becomes more like a classical harmonic oscillator at high pressure,
should aid in such pursuits as the quest to observe metallic
hydrogen.  (Physical Review Letters, 10 February 2006)

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