PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 625 February 19, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

FROM FEMTOCHEMISTRY TO ATTOPHYSICS.  Amid a fast game in a vast venue,
sports photography seeks to freeze motion and isolate small portions of
space for special consideration.  In the scientific world of the ultrafast
and ultrasmall, stroboscopic effects are achieved with greatly attenuated
laser pulses.  The advent of laser light served up in femtosecond (or 10^-15
second) bursts  has helped to elucidate the molecular world by freezing
their vibrational and rotational motions.  Scientists would of course like
to instigate and monitor even shorter times and distances.
A collaboration between scientists at the Technical University of Vienna
and the Max Planck Institute for Quantum Optics (MPQ) has now done precisely
this.  They have produced a series of 2.5-fsec pulses, each consisting of
only a few cycles of a carrier light signal modulated within an amplitude
envelope.  In the case of the Vienna-MPQ experiment, however, all the pulses
are identical (a feat not achieved previously) and the phase of the carrier
wave within the envelope is controlled with a time resolution of about 100
attoseconds.
When the intense (100 GW) few-cycle pulse strikes an atom, an electron can
be stripped away quickly, and reabsorbed just as quickly.  This violent
excursion results in the emission of a sharp x-ray spike with a duration
even shorter than the pulse that excited the reaction.  In fact the x-ray
pulses are about 500 attoseconds long.  Moreover, because all the waveforms
of the optical pulse are identical, and controlled, the subsequent electron
motions and x-ray emissions are also highly controlled and reproducible.  At
a talk at this week's meeting of the American Association for the
Advancement of Science (AAAS) in Denver, Vienna physicist Ferenc Krausz said
that this sub-femtosecond control of electron currents represented true
attophysics, a new technique for directing and watching atomic processes at
unprecedentedly short time intervals.  (See Baltuska et al., Nature, 6
February 2003.)

A NEW LIMIT ON PHOTON MASS, less than 10^-51 grams or 7 x 10^-19 electron
volts, has been established by an experiment in which light is aimed at a
sensitive torsion balance; if light had mass, the rotating balance would
suffer an additional tiny torque.  This represents a 20-fold improvement
over previous limits on photon mass.  Photon mass is expected to be zero by
most physicists, but this is an assumption which must be checked
experimentally.  A nonzero mass would make trouble for special relativity,
Maxwell's equations, and for Coulomb's inverse-square law for electrical
attraction.   The work was carried out by Jun Luo and his colleagues at
Huazhong University of Science and Technology in Wuhan, China
(junluo{at}mail.hust.edu.cn, 86-27-8755-6653).  They have also carried out a
measurement of the universal gravitational constant G (Physical Review D, 15
February 1999) and are currently measuring the force of gravity at the
sub-millimeter range (a departure from Newton's inverse-square law might
suggest the existence of extra spatial dimensions) and are studying the
Casimir force, a quantum effect in which nearby parallel plates are drawn
together.  (Luo et al., Physical Review Letters, upcoming article, probably
28 Feb)

A MOLECULAR SWITCH TOOK ONLY 47 ZEPTO-JOULES (47 x 10^-21 joules, or 0.3
eV) to operate in a recent experiment, some 10,000 times less than
transistor switches used in current high-speed computers.    The molecular
switch in question consists of rotating one of the four phenyl legs attached
to a complicated porphyrin molecule (abbreviated as Cu-TBPP) from one stable
position to another.  A group of scientists from the University of Basle,
IBM Zurich, and the CEMES-CNRS Lab in Toulouse used an atomic force
microscope (AFM) tip both to rotate the leg and to measure the force
expended and energy used.  The use of a single chemical bond as a switch
would greatly reduce the power dissipation in electronic circuits, but this
new development will take time to implement, along with other
molecular-electronic elements.  (Loppacher et al., Physical Review Letters,
14 February 2003; contact Christian Loppacher, loppacher{at}iapp.de,
49-351-4633-4903; http://www.iapp.de/iapp/index.php )

***********
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