Number 132, June 10, 1993 by Phillip F. Schewe and Ben Stein
THE STM CAN NOW DISTINGUISH AMONG SIMILAR MOLECULES adsorbed onto a
metal surface. Identifying molecules with scanning tunneling microscopy
(STM) is actually more challenging than identifying individual atoms because
the electrons that the STM "sees" when scanning molecules are
often arranged in delocalized orbitals that are not centered on specific
atoms. Researchers at IBM-Almaden, with the help of chemists at the Technical
Hochschule Darmstadt in Germany, imaged a series of isomers--molecules
with the same chemical composition but different structures--adsorbed onto
a platinum substrate. The IBM scientists are the first to demonstrate the
power of combining STM images with improved calculations to recognize such
similar molecules as three different isomers of the methylazulene molecule,
a double-ring aromatic molecule with a CH3 group attached at different
locations. This ability to distinguish such similar molecules with the
STM is a first step toward allowing researchers to identify directly the
individual reactants, products and, perhaps, intermediate configurations
of chemical reactions on surfaces. (V.M. Hallmark et al, Phys. Rev. Lett.,
14 June 1993.)
VERTICAL CAVITY SURFACE EMITTING LASERS (VCSEL's) , lasers that emit
light from the face of a semiconductor chip rather than from the cleft
edge of the chip, now operate at visible wavelengths (as short as 631 nm)
and can be powered with electricity instead of having to be pumped by another
laser. The new VCSEL's, built by Richard Schneider and James Lott at Sandia,
consist of numerous 10-nm layers of semiconductors which serve as quantum
wells for trapping electrons and holes and as mirrors for reflecting and
focusing light. Such surface-emitting, visible-light lasers on a chip are
much more compact that the helium-neon gas lasers used in grocery-checkout
scanners and generate light beams that are more focused (coming from through
a 10-micron aperture) than the light beams from edge-emitting lasers on
a chip. (Science News, 22 May 1993; Science, 28 May 1993.)
WHY DO LARGE THINGS RISE TO THE TOP? Shaking grains of different sizes
in a container creates large-scale flow patterns which are responsible
for separating the grains by size, new experiments have shown. An important
problem in industry has been to determine why, when one shakes a pile of
sand, mixed nuts, or other granular material in a container, the larger
particles end up on top and the smaller ones wind up on the bottom. New
experiments, performed by Sidney R. Nagel (312-702-7190) and his colleagues
at the University of Chicago, uncover a previously unsuspected mechanism:
the separation is caused by large-scale flow patterns, or "convection,"
in the grains. The researchers studied the rise of a single large glass
bead through a vibrating cylinder filled with smaller beads. They found
that the large bead, once it had reached the top of the pile, was unable
to follow the convection cycle through a very narrow region of downward
motion along the walls of the container. The researchers discovered that
container boundaries play an important role in the convective process that
leads to separation. (James B. Knight et al., Phys. Rev. Lett., 14 June
1993.)
|
