From: Eugene.Leitl@lrz.uni-muenchen.de
Date: Sat Jun 02 2001 - 11:28:36 MDT
______________________________________________________________
ICBMTO : N48 10'07'' E011 33'53'' http://www.lrz.de/~ui22204
57F9CFD3: ED90 0433 EB74 E4A9 537F CFF5 86E7 629B 57F9 CFD3
5. ASTROPHYSICS: A BROWN DWARF RADIO STAR
Brown dwarf stars are formed by the contraction of a lump of
gas with a mass too small (less than approximately 0.08 the mass
of the Sun) for nuclear reactions to begin in the core. Such a
star has a relatively short-lived luminosity (approximately 100
million years to several billion years) as the result of
conversion of gravitational energy to radiation. The surface
temperature of a brown dwarf is below 2500 kelvins. As recently
as 1994, brown dwarfs were "theoretical" stars, with no brown
dwarfs considered to be unambiguously identified. In the past few
years, however, a number of stars have been recognized as brown
dwarfs, and they are under intensive study.
In this context, the term "gas giant" (giant planet) refers
to a planet of much larger mass and diameter than the Earth, and
which consists mostly of gas. In our own Solar System, Jupiter,
Saturn, Uranus, and Neptune are gas-giant planets.
A synchrotron is a device for accelerating electrons or
protons in closed orbits in which the frequency of the
accelerating voltage and the strength of an applied magnetic
field are simultaneously varied to keep the orbit radius
constant, and laboratory synchrotron radiation is electromagnetic
radiation generated by the acceleration of charged relativistic
particles in a synchrotron (or in any magnetic field). In
astronomy, the term "synchrotron emission" refers to
electromagnetic radiation emitted by charged particles moving in
a magnetic field at a velocity close to that of light, and
emissions from various types of astronomical objects are
apparently synchrotron emissions. In general, relativistic
electrons gyrating in a magnetic field emit radio waves at high
harmonics of the electron gyrofrequency, so emission of radio
waves can be an indication of synchrotron emission by
astronomical objects.
The term "radio waves" refers to electromagnetic radiation
of wavelength longer than approximately 1 millimeter (30
gigahertz). The longest radio waves observable in astronomy have
a wavelength of approximately 30 meters. The shortest radio
wavelengths, from approximately 1 millimeter to 30 centimeters,
are known as "microwaves". In this context, the term "radio
emission" refers to radio wave emission from an astronomical
source. Astronomical radio sources produce either continuum
radiation or line radiation. Line radio radiation is emitted at
only one specific wavelength and is equivalent to an optical
spectral line. The most important of such lines is the 21-
centimeter line emitted by neutral hydrogen atoms. Of continuum
radio radiation there are two kinds: a) thermal radio radiation
is electromagnetic energy emitted by hot ionized interstellar
gases; b) non-thermal radio radiation is a result of a process of
synchrotron emission, the release of radiation by electrons
spiraling in magnetic fields at speeds near the speed of light.
In this context, the term "x-ray emission" refers to x-rays
emitted from various astronomical sources. Most stars emit only
an extremely small fraction of their energy as x-rays, with young
massive stars the most powerful x-ray emitters. In general, gases
heated to temperatures above 10 million kelvins will emit x-rays.
An "M star" is a star of spectral type "M", i.e., with a
very cool surface (below 3900 kelvins), appearing reddish in
color and emitting most of its radiation in the infrared. M-type
dwarf stars ("red dwarfs") have masses below 0.5 solar-masses and
potential lifetimes longer than the present age of the Universe.
In this context, the term "corona" refers to the hot
outer atmosphere of certain stars, where the temperature can be 2
million kelvins or more. The term "flare" refers to a sudden
release of corona energy.
... ... E. Berger et al (14 authors at 14 installations, US)
report the discovery of radio emission from a brown dwarf star,
the authors making the following points:
1) The authors point out that brown dwarf stars are not
massive enough to sustain thermonuclear fusion of hydrogen at
their centers, but they are distinguished from gas-giant planets
by their ability to burn deuterium. Brown dwarf stars older than
approximately 10 million years are expected to have short-lived
magnetic fields and to emit only weak radio waves and x-rays from
their coronas. An x-ray flare was recently detected on the brown
dwarf star LP944-20.
2) The authors report the discovery of both quiescent and
flaring radio emission from LP944-20, with luminosities several
orders of magnitude larger than that predicted by the empirical
relation between the x-ray and radio luminosities that has been
found for many types of stars. An analysis of the radio data
within the context of synchrotron emission indicates that the
brown dwarf star LP944-20 has an unusually weak magnetic field in
comparison to M-dwarf stars.
... ... In a commentary on the above work, Arnold O. Benz (ETH-
Zentrum Zurich, CH) makes the following points:
1) The author (Benz) points out that this is the first time
that radio emission has been recorded from a bona fide brown
dwarf star. Although brown dwarf stars are too small to burn
hydrogen, they can fuse deuterium in their cores, provided they
are at least 12 times the mass of Jupiter. But this fusion of
deuterium is possible only when the brown dwarf star is young, in
the first 10 million years or so. The brown dwarf star LP944-20,
however, is apparently 500 million years old and should have no
deuterium source left in its core. This suggests that the radio
emission of LP944-20 may be synchrotron emission associated with
a hot corona. However, the apparently weak magnetic field of this
star makes this explanation for the radio emission tenuous. The
author (Benz) concludes: "The whistling brown dwarf is a new
mystery for astronomers to puzzle over, and may indicate that
these dusky little objects have more surprises in store."
-----------
E. Berger et al: Discovery of radio emission from the brown dwarf
LP944-20.
(Nature 15 Mar 01 410:338)
QY: E. Berger: ejb@astro.caltech.edu
-----------
Arnold O. Benz: Brown dwarf is a radio star.
(Nature 15 Mar 01 410:310)
QY: Arnold O. Benz: benz@astro.phys.ethz.ch
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 1Jun01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON BROWN DWARF STARS
Brown dwarf stars are formed by the contraction of a lump of gas
with a mass too small for nuclear reactions to begin in the core
[*Note #1]... The surface temperature of a brown dwarf is
estimated to range from below 2500 kelvins to less than 1000
kelvins...
... ... C.G. Tinney (Anglo-Australian Observatory Epping, AU)
presents a review of recent observations of brown dwarf stars,
the author making the following points: 1) Most stars spend most
of their lives in a state of pressure balance maintained between
gravitational contraction and the energy generated by nuclear
reactions. In 1963, Kumar suggested there may exist a class of
star-like bodies with masses too low to create the central
temperature and densities required to ignite nuclear fusion
reactions. These "failed stars" became known to astronomers as
"brown dwarfs". 2) The lowest-mass ordinary stars can
theoretically maintain a quasi-equilibrium luminosity for almost
6000 billion years. Brown dwarf stars, in contrast, are expected
to fade throughout their lifetime, cooling to temperatures below
1000 kelvins and becoming undetectable by direct observation
after just a few billion years. This has engendered considerable
interest in brown dwarf stars as possible candidates for the
*dark matter which apparently composes more than 90 percent of
the mass our Galaxy. 3) The past 4 years have seen success
finally achieved in the hunt for brown dwarf stars. These
detections have confirmed predictions that both methane and dust
play an important role in determining the spectral behavior of
these objects. But the detection of brown dwarf stars in
significant numbers, when combined with results for the space
density of low-mass stars and *gravitational microlensing
results, allows us to conclude that brown dwarf stars do not make
a significant contribution to the dark matter of our Galaxy. The
author concludes: "No matter how nicely brown dwarfs would solve
the *baryonic dark matter problem, it appears we must look
elsewhere for a solution to this long-standing astronomical
quandary."
-----------
C.G. Tinney: Brown Dwarfs: The stars that failed.
(Nature 7 Jan 99 397:37)
QY: C.G. Tinney, Anglo-Australian Observatory, PO Box 296, Epping
NSW 1710, AU.
-----------
Text Notes:
... ... *Note #1: Present theoretical models predict a lower
mass-limit for fusion burning stars with the same element mix as
the Sun of 0.07 solar-mass, equivalent to 74 times the mass of
Jupiter.
... ... *dark matter: In general, in this context, the term "dark
matter" refers to material whose presence can be inferred from
its effects on the motions of stars and galaxies, but which
cannot be seen directly because it emits little or no radiation.
It is believed that at least 90 percent of the mass in the
Universe exists as some form or dark matter.
... ... *gravitational microlensing: Gravitational lensing is the
bending of light and other radiation by a massive gravitational
entity such as a star, a black hole, a galaxy, or a cluster of
galaxies. The effect is predicted by Einstein's theory of
relativity and was first detected during a total solar eclipse by
Eddington in 1919. Large-scale gravitational lensing causes
multiple images of an object, the type and arrangement of the
images determined by the specifics of the lensing entity.
Gravitational "microlensing" is a small-scale lensing effect, the
gravitational field of the lensing object not strong enough to
form distinct images of the background source, but instead
causing an apparent brightening of the source. Stars are expected
to vary in brightness in a characteristic manner if low-mass
stars pass in front of them.
... ... *baryonic dark matter: Ordinary matter too dim to be
observed. A baryon is a nuclear particle, e.g., a proton, built
from 3 quarks (fundamental particles that combine to make up
protons, neutrons, and mesons).
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 19Mar99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
THE DUSTY ATMOSPHERE OF A BROWN DWARF STAR
... C.A. Griffith et al (3 authors at 3 installations, US) now
report observations of the brown dwarf star Gliese 229B, which
exhibits certain unique characteristics. At 900 kelvins, the
atmosphere of this object is too warm to contain ice clouds
like those on Jupiter and too cool to contain silicate clouds
like those on low-mass stars. These unique conditions (high
gravity and the lack of high clouds) permit spectroscopic
visibility of the atmosphere down to higher pressures (i.e.,
closer to the surface) than possible in cool stars or planets.
The authors investigated the structure of the atmosphere of
Gliese 229B by analyzing its optical spectrum in the interval
0.85 to 1.0 micron, the spectrum obtained at the *Keck 1
telescope. The authors report that the spectrum of Gliese 229B
indicates deep-atmosphere particulate matter with the optical
properties of neither ice nor silicates. The authors suggest the
reddish color of the particles indicates an organic composition
characteristic of aerosols in planetary stratospheres, and that
the *mass fraction of the particles agrees with a photochemical
origin involving incident radiation from its companion primary
star (Gliese 229A).
-----------
C.A. Griffith et al: The dusty atmosphere of the brown dwarf
Gliese 229B.
(Science 11 Dec 98 282:2063)
QY: Caitlin A. Griffith, Northern Arizona University 520-523-5511
-----------
Text Notes:
... ... *Keck 1 telescope: The Keck telescopes are a pair of twin
telescopes at the W. M. Keck Observatory on Mauna Kea, HI US,
each with a 10 meter mirror, the pair constructed 1992-1996. The
installation is managed by the University of California (US) and
the California Institute of Technology (US).
... ... *mass fraction: The mass fraction of aerosols is related
to the *eddy diffusion coefficient k, the mass density of the
atmosphere d, the net mass flux f, and the scale height of the
atmosphere h according to F = fh/kd.
... ... *eddy diffusion coefficient: (turbulent diffusion
coefficient) The exchange coefficient for the diffusion of a
conserved property by eddies in a turbulent flow. In general, an
"eddy" is a vortex-like motion of fluid running contrary to the
main current.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12Feb99
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
6. CHEMICAL PHYSICS:
MEASUREMENT OF SHORT-RANGE CHEMICAL BONDING FORCES
In atomic and molecular systems, forces can be categorized
as either long-range or short-range, depending on the relative
distances over which the forces are effective. For example, the
Coulomb force, which falls off with the inverse square of the
distance, is long-range; van der Waals forces, which fall off
with the inverse 6th power of the distance, are short-range, as
are the interatomic forces responsible for the various types of
chemical bonds. In general, forces that fall off less rapidly
than the inverse 4th power are called "long-range".
In an increasing number of laboratories, the technique of
atomic force microscopy is being used to examine the interaction
between atoms and surfaces at angstrom-scale distances. In
general, in atomic force microscopy, a tip is fixed to a
cantilever whose position is monitored while the tip scans a
surface. The force between the tip and the surface determines the
position of the cantilever, and when recorded in atomic
resolution, one can obtain an image that represents a map of
atomic forces at the surface. But the technique has also been
used to investigate the behavior of single atoms and molecules at
surfaces, and it is beginning to provide important insights into
the physical chemistry of adsorption phenomena.
In this context, the term "adatom" refers to an atom
adsorbed on a surface.
... ... M.A. Lantz et al (8 authors at University of Basel, CH)
report quantitative measurements of short-range chemical bonding
forces, the authors making the following points:
1) The authors point out that although the atomic force
microscope was originally intended to be used as a tool capable
of measuring the forces acting between a single pair of atoms, it
has evolved into an instrument capable of producing atomically
resolved images of surfaces. Such atomic-scale images are
generally interpreted as resulting from the short-range chemical
interaction between an atomically sharp atomic-force-microscope
tip and the nearest atoms on the surface of the sample. In
principle, it should therefore be possible to map the chemical
bonding potential between the foremost atom on an atomic force
microscope tip and a specific atom on the sample.
2) The authors report quantitative measurements of the
short-range chemical bonding force between the apex of a silicon
atomic force microscope tip and specific atomic sites on a
silicon sample. The force is measured over a large range of tip-
sample distances using a technique developed by the authors for
resolving force-distance characteristics. The short-range and
long-range forces are accurately separated by compensating the
electrostatic force and measuring the van der Waals force above
nonreactive sites. The authors report that the magnitude and
range of the measured short-range force are in good agreement
with first=principles calculations designed to model the same
situation. In addition, the authors report they have demonstrated
that the measured short-range force results from the formation of
a single bond.
3) The authors suggest these measurements may provide
insight into covalent bond formation at surfaces. The authors
conclude: "The measurements presented here demonstrate the
possibility of directly measuring and quantifying local surface
reactivity with the atomic force microscope. The range of systems
that can be studied by such measurements could be extended by
depositing different materials or even single atoms on the tip.
The resolution of our measurements was sufficient to reveal
differences in the interaction potential between inequivalent
silicon adatoms; hence, it may eventually be possible to use
similar methods to distinguish between different atomic
species..."
-----------
M.A. Lantz et al: Quantitative measurement of short-range
chemical bonding forces.
(Science 30 Mar 01 291:2580)
QY: M.A. Lantz: mark.lantz@unibas.ch
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 1Jun01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
CHEMISTRY: PROBE CHARACTERIZATION OF SINGLE BOND FORMATION
In this context, "tunneling" is a quantum mechanical phenomenon
involving an effective penetration of an energy barrier by a
particle resulting from the width of the barrier being less than
the wavelength of the particle. First available in the early
1980s, the technique of scanning tunneling microscopy involves an
atomically sharp metal tip brought in atomic proximity (e.g., 0.5
to 1 nanometer) to a flat surface so that electrons can tunnel
between the two systems. Recording the atomic modulation of the
atomic structure while scanning the tip across the surface allows
one to image adsorbed species and surface morphologies.
... ... H.J. Lee and W. Ho (Cornell University, US) now report
the use of a scanning tunneling microscope to manipulate the
bonding of a carbon monoxide (CO) molecule and to analyze the
structure and vibrational properties of individual products.
Individual iron (Fe) atoms were evaporated and coadsorbed with CO
molecules on a silver surface [Ag(110)] at 13 kelvins in
ultra-high vacuum. A CO molecule was transferred from the surface
to the scanning tunneling microscope tip and bonded with an Fe
atom to form Fe(CO). A second CO molecule was similarly
transferred and bonded with Fe(CO) to form Fe(CO)(sub2). The
authors suggest this controlled bond formation and
characterization at the single-bond level is effectively probing
chemistry at the spatial limit. The authors conclude: "The
binding of a diatomic molecule to an atom constitutes one of the
simplest chemical transformations involving a molecule. By
combining the present manipulation approach with other mechanisms
such as 'sliding', 'pulling', and 'pushing', extension of
spatially controlled bond formation to other atoms and molecules
is envisioned. The ability to control step-by-step bond formation
of adsorbed chemical species at the single-molecule level
provides a real-space understanding and direct visualization of
the nature of the chemical bond."
-----------
H.J. Lee and W. Ho: Single-bond formation and characterization
with a scanning tunneling microscope.
(Science 26 Nov 99 286:1719)
QY: Wilson Ho: wilsonho@ccmr.cornell.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 18Feb00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
MEASUREMENT OF THE RUPTURE FORCE OF SINGLE COVALENT BONDS
>From an elementary standpoint, what we call a "chemical bond" is
something that ties two or more atoms together to form a
molecular entity with sufficient stability for us to measure or
describe some of the entity's properties. The idea of the
chemical bond, including the notion of valence and the way we
draw the connections between atoms in a molecule, is usually
ascribed to Friedrich Kekule (1829-1896), but in fact it was
Edward Frankland (1825-1899) who first suggested the idea of
valence, and Archibald Couper (1831-1892) who first suggested the
depiction of the bond between 2 atoms by a dash. The use of
Couper's dashes to depict bonds was made popular by a chemist
well-known at the time for various discoveries in synthetic
chemistry, Richard Erlenmeyer (1825-1909). So today we have the
irony that Erlenmeyer is known primarily for the name of a glass
flask that he designed, Cooper and Frankland are hardly known at
all, and Kekule is often described to students as having
concocted these beginnings of structural chemistry in a dream.
Kekule may have indeed dreamt of the resonating benzene ring, but
his dream had the intimate help of his generation of chemists.
Putting aside the personal history of the concept of the chemical
bond, during its relatively short 150-year-old existence, this
concept has without any doubt been one of the most important
ideas in modern science. During this century, the focus has been
to understand the chemical bond in terms of quantum physics, but
certain classical aspects remain of great practical interest,
particularly the question of the relative strengths of the bonds
between different types of atoms. There are various ways to
approach this question of "bond strength", including actual
determinations of the mechanical force necessary to rupture
bonds. The mechanical stabilities of covalent bonds, bonds that
involve sharing of electrons, have in the past been investigated
indirectly in ensemble measurements or by flow-induced chain
fracture in liquids. The recent development of nanoscale
manipulation techniques has made it possible to directly address
single atoms or molecules and probe their mechanical properties.
... ... M. Grandbois et al (5 authors at 3 installations, DE US)
now report a study of the rupture force of single covalent bonds
under an external load measured with an *atomic force microscope.
Single polysaccharide molecules were covalently anchored between
a surface and an atomic force microscope tip and then stretched
until the molecule became detached. The authors report that by
using different surface chemistries for the attachment, it was
found that the silicon-carbon bond ruptured at 2.0 +- 0.3
nanonewtons, whereas the sulfur-gold anchor ruptured at 1.4 +-
0.3 nanonewtons, at force-loading rates of 10 nanonewtons per
second. The authors report these results agree with bond rupture
probability calculations based on *density functional theory. The
authors conclude: "Although chemical compounds play a dominant
role in material sciences, the forces that chemical bonds can
withstand could previously not be directly measured in
experiments. The experiments reported here demonstrate that the
individual chemical bonds can be probed in mechanical
experiments. An important feature of such experiments is the
mechanical activation of chemical bonds (here in the simplest
form as bond rupture), which can now be studied on an individual
basis."
-----------
M. Grandbois: How strong is a chemical bond?
(Science 12 Mar 99 283:1727)
QY: Hermann E. Gaug, Lehrstuhl fur Angewandte Physik, Ludwig-
Maximillians-Universitat, Amalienstrasse 54, D-80799 Munich, DE.
-----------
Text Notes:
... ... *atomic force microscope: In general, in atomic force
microscopy, a tip is fixed to a cantilever whose position is
monitored while the tip scans a surface. The force between the
tip and the surface determines the position of the cantilever.
When recorded in atomic resolution, the image represents a map of
atomic forces at the surface. In the present study, the active
polymer molecule was first coupled to the substrate surface. The
tip was then slowly brought into contact with the surface,
allowing the polymer to bind to the tip (which occurred in
approximately 30 percent of the cases). The tip and the substrate
where then gradually separated while the force was recorded. The
polymer was repeatedly stretched and relaxed through one or more
conformational transitions. After analysis confirmed that a
single molecule was bound, the force was gradually increased
until the molecular bridge ruptured.
... ... *density functional theory: For atomic force calculations
on solids, the current method of choice is density functional
theory, due to Kohn, Hohenberg, and Sham. Its name comes from its
predicted connection between the total ground state electronic
energy of a system and the electronic charge density. The theory
was first proposed in 1964, and has since been useful as a
simplifying alternative to more rigorous but intractable many-
electron wavefunction calculations. In general, in density
functional theory, it is the electron density which is the
fundamental variable: the ground state of a system is defined by
that electron density distribution which minimizes the total
energy. In this approach, once the ground state electron density
is known, all other ground state properties (lattice constants,
cohesive energies, etc.) follow, at least in principle. In
mathematics, a "functional" is a function whose value depends on
the set of all values of another function. In density functional
theory, the ground state properties of a system are functionals
of the ground state electron density function.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 4Jun99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON SINGLE MOLECULE PHYSICS AND CHEMISTRY
Only a few decades ago, most scientists believed that individual
molecules would not come within the domain of experimental
observations within their lifetime, if ever, and that the
statistical ensemble properties of molecules were therefore the
only properties of relevance. That view has now undergone a
dramatic alteration as a consequence of technological advances,
and there is much excitement evident in many laboratories over
the prospects of single-molecule explorations in physics,
chemistry, and biology.
... ... C. Bai et al (4 authors at 3 installations, CN US)
present a short review of recent work in single-molecule physics
and chemistry, the authors making the following points:
1) The authors point out that when Richard Feynman (1918-
1988) was bothered while looking through one of the first
*scanning tunneling microscopes, he was upset to have been
interrupted because seeing the images of singe atoms was a
"religious experience". For many generations of scientists, the
molecule was both the concrete ultimate entity upon which our
understanding of the everyday world was based, and at the same
time an elusive intellectual construct whose very existence could
only be inferred circumstantially by experiments on macroscopic
samples. Thus, seeing an individual atom or molecule in motion
brings immediate emotional impact to this central concept of
modern thought.
2) The authors ask: "When is molecular individuality
important?" The new possibility of studying single molecules is
important because molecular individuality does finally come into
play when the molecule is a complex entity. This may occur
because the molecule itself may have an intricate internal
structure -- e.g., a biomolecule -- resulting in a complex energy
landscape. Alternatively, the molecule may be part of a complex
environment that substantially changes the behavior of the
molecule. Here, distinguishing different molecules at different
locales is crucial for understanding the system as a whole.
Biomolecules in living cells are examples of this. Even simple
inorganic molecules on structured surfaces or in disordered
systems such as viscous liquids or glasses provide situations in
which molecular individuality matters. In all of these cases, the
capability of studying an individual molecule over time can
provide new insights unavailable by straightforward experiments
on macroscopic populations of molecules.
3) With the aid of *scanning probe microscopy, direct
observations of entire arrays of atoms, molecules, and the fine
structures of molecular aggregates have become possible. The
ability to precisely control probes permits the study of long-
range structures made by molecules lying on surfaces. However,
although pretty pictures of such systems are easy to construct,
obtaining quantitative characteristics of surface-bound molecules
is not entirely straightforward, and the rigorous interpretation
of scanning probe microscopy images requires substantial
theoretical as well as experimental effort.
4) The authors conclude: "We are only at the beginning, but
it is clear there is much to be discovered of a fundamental
nature about complex molecules viewed as individuals. Perhaps
equally important will be the idea of single molecule control.
Now that experiments interact with molecules at an individual
level, we can try to control them as individuals, not as
populations. A molecule under active control by an adaptive
environment will be a new beast. Such tamed molecules may well
resemble much more the elegant engineered machinery of everyday
experience than the unruly, wild molecules we are used to
studying today."
-----------
C. Bai et al: Single molecule physics and chemistry.
(Proc. Natl. Acad. Sci. US 28 Sep 99 96:11075)
QY: Chunli Bai, Institute of Chemistry, The Chinese Academy of
Sciences, Beijing 100080 CN.
-----------
Text Notes:
... ... *scanning tunneling microscopes: The general approach in
scanning probe microscopy research is illustrated by
consideration of two major techniques, scanning *tunneling
microscopy (STM) and atomic force microscopy (AFM). In scanning
tunneling microscopy, an atomically sharp metal tip is brought in
atomic proximity (e.g., 0.5 to 1 nanometer) to a flat surface so
that electrons can *tunnel between the two systems. The probe is
slowly moved across the surface and raised and lowered so as to
keep the tunneling current constant. A computer-generated contour
map of the surface is thus produced. The technique can resolve
individual atoms, but requires electrically conducting materials.
In atomic force microscopy, a tip is fixed to a cantilever whose
position is monitored while the tip scans the surface. The force
between the tip and the surface determines the position of the
cantilever. When recorded in atomic resolution, the image
represents a map of atomic forces at the surface. The advantage
of atomic force microscopy is that the probed surface does not
need to be electrically conducting.
... ... *tunneling: "Tunneling" is a quantum mechanical
phenomenon involving an effective penetration of an energy
barrier by a particle resulting from the width of the barrier
being less than the wavelength of the particle.
... ... *scanning probe microscopy: A general term comprising all
atomic-level probe techniques.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 5Dec99
For more information: http://scienceweek.com/swfr.htm
This archive was generated by hypermail 2.1.5 : Sat Nov 02 2002 - 08:07:54 MST