Number 110, January 15, 1993 by Phillip F. Schewe and Ben Stein
ATOMIC HYDROGEN IN A MAGNETIC TRAP has been studied directly using optical
spectroscopy for the first time. Hydrogen usually exists on Earth as a
diatomic molecule, but atomic hydrogen (H) can be created and maintained
at low temperatures by spin polarizing the atoms in a magnetic field; the
Pauli exclusion principle then keeps the atoms from pairing up. This effort
is worthwhile since atomic hydrogen is the one substance in nature that
remains a gas even at a temperature of absolute zero, making it an interesting
test ground for quantum mechanics. Previous studies of trapped H had observed
the atoms only after they had left the trap. Now, Jook Walraven and his
colleagues at the University of Amsterdam have studied the transmission
spectroscopy of trapped H in situ using pulsed (third-harmonic generated)
laser light at the so-called Lyman-alpha wavelength (121.6 nm, corresponding
to the 1S-2P transition). The spectrum can be used to monitor the temperature
and density of the hydrogen sample as a function of time, which will be
useful for later studies of H at even lower temperatures. (O.J. Luiten
et al., 1 Feb. issue of Physical Review Letters.)
STELLAR OBSERVATIONS WITH 2-MILLIARCSECOND RESOLUTION at optical wavelengths
have been made using interferometry techniques. Richard Simon of the Naval
Research Lab (NRL) reported at last week's meeting of the American Astronomical
Society the measurement of the diameters of 10 giant stars with the Mark
III Optical Interferometer at the Mt. Wilson Observatory. Mark III, which
determines diameters but cannot make images, uses the Young's-double-slit
technique: light from a source (including stars) passing through each of
two slits (or, in this case, telescopes separated by as much as 31 m) interferes
with itself, producing light and dark bands (fringes) at a detector. Changing
the telescope configuration changes the fringes in a way that is characteristic
of the source's size. One of Simon's observations showed that the star
Gamma Cassiopeia is slightly ellipsoidal. The NRL is currently building
the Big Optical Array at the Lowell Observatory. This six-element interferometer
should actually produce images, with resolutions as good as 200 microarcseconds
(100 times better than that of the Hubble Space Telescope). Shri Kulkarni
of Caltech predicted that at the AAS meeting ten years hence perhaps as
many as one third of all papers would draw on optical-interferometer measurements.
BUCKYBALLS EMIT LIGHT. When exposed to laser light, fullerenes usually
re-radiate light in the infrared. The result was different when David Leigh
and co-workers at the University of Manchester inserted carbon-60 atoms
into the spaces of a zeolite crystal known as VPI-5. Shining blue laser
light on this compound caused the buckyballs to re-radiate visible light.
Researchers speculate that electrons confined to the 12.5-angstrom zeolite
channels may undergo quantum changes similar to those proposed for electrons
in light-emitting silicon. Light-emitting fullerenes may have many practical
applications as an optical material if the re-emission efficiency (only
1%) could be improved and if the visible-light emission could be stimulated
by electricity rather than light. (Science, 18 December 1992.)
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