Embargoed until 2 p.m. EST
NSF PR 02-17 - March 6, 2002
Hydrogen Reaction Experiment Reaps a Surprise
Scientists got a surprise recently when a team of physical
chemists at Stanford University studied a common hydrogen
reaction. The experiment and an associated new theory
revealed behaviors completely opposed to what had
previously been expected.
In the effort to learn more about fundamental chemical
reactions, scientists have intensively studied the
hydrogen exchange reaction, which occurs when a hydrogen
atom (H) collides with a hydrogen molecule (H2).
In the recent experiment, scientists supported by
the National Science Foundation (NSF) used a sophisticated
laser laboratory at Stanford to observe the collision
between a single hydrogen atom (H) and a heavy form
of hydrogen molecule (D2, or deuterium
molecule). In hydrogen, the nucleus of the atom is
a single proton, while deuterium's nucleus consists
of a proton and a neutron bonded together.
One of the products resulting from the collision, HD,
traveled in an unexpected direction. As predicted
by longstanding theories of scattering, the HD is
expected to recoil, or scatter, in the opposite direction
from that of the incoming H atom. In this experiment,
however, the resulting product moved forward, in the
same direction the single atom had been traveling.
The experiment also revealed a time delay before the
HD product began its forward motion. These results
are reported in the March 7 issue of Nature.
"Even in the simplest kinds of chemical reactions,
including hydrogen atoms colliding with hydrogen molecules,
we are still finding surprises," said Donald Burland,
acting director of NSF's chemistry division. "These
results demonstrate the importance of continued research
in fundamental chemistry."
"This news is quite exciting for the world of chemistry
because it suggests that the 'simple' hydrogen reaction
is a more complicated process than previously thought,
involving more than one reaction mechanism," said
Stanford chemist and team leader Richard Zare.
The results of Zare's experiment demonstrate that the
hydrogen reaction involves more than the direct atom-molecule
exchange mechanism commonly observed. The explanation,
the Stanford scientists believe, is that in addition
to the direct reaction mechanism that leads to backward
scattering, as predicted, another indirect mechanism
at the quantum mechanical level (the level of subatomic
particles) leads to forward scattering after a brief
delay.
Both the forward motion and the time delay are in keeping
with new theories recently developed by a team at
the University of Durham, England, led by Stuart Althorpe.
Zare served as chair of the National Science Board,
the policy body of NSF, in 1996-98.
For animated graphics, see: http://www.dur.ac.uk/chemistry/publications/sc_althorpe/nature.html
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