experiment has helped us to understand the specifics
of what happens when we excite molecules with energetic
ultraviolet light," said Brookhaven chemist Arthur
Suits, the experiment's lead scientist. The results
are described in the February 27, 2004 issue of Physical
also learn, more generally, how electrons rearrange
in molecules in response to the light, the nature
of chemical bonds, and the dynamics of bond-breaking
processes," he said.
gas used in this experiment is called methyl fluoride.
Each molecule can be thought of as the sum of two
parts: a negatively charged fluorine atom, or fluorine
ion, and a positively charged methyl ion containing
three hydrogen atoms bonded to one carbon atom.
When Suits and his group aimed the ultraviolet beam
from the DUV-FEL at a beam of methyl fluoride gas,
the gas molecules each absorbed a single energetic
photon, or tiny "packet," of light, which
caused them to separate, or dissociate, into their
positive and negative fragments, called ion pairs.
dissociation is a special case, Suits explained. When
a molecule absorbs a great deal of energy, normally
it will spit out an electron and become a positive
ion. But sometimes, molecules can temporarily exist
in "superexcited" states, absorbing enough
energy to ionize, but without doing so. Instead, they
can dissociate into fragments. By studying the parts,
scientists can gain information about the whole molecule
and the details of the bond-breaking process.
this experiment, Suits and his collaborators used
a technique called "ion pair imaging" to
learn about methyl fluoride's reaction to the light.
After breaking the molecules into ion pairs, they
tracked the motion of the fluorine ions by causing
them to strike a screen that records each ion's impact
location. The resulting pattern gives the scientists
information about the speed and trajectory of the
fluorine ions. By working backward, they can learn
about the properties of the entire methyl fluoride
molecule and its interaction with the light.
fluoride was a good test case to use in the first
FEL experiment, but we hope to study other dissociation
processes," said Suits. "We have yet to
take full advantage of the DUV-FEL's many unique features
and capabilities, which will help us learn more about
how electrons move in molecules as chemical bonds
are changed and broken."
DUV-FEL is located at the National Synchrotron Light
Source (NSLS), a facility at Brookhaven that produces
infrared, ultraviolet, and x-ray light by accelerating
electrons in a circular path at very high speeds,
but it does not work the same way. Instead, the electrons
are first accelerated in a straight line down a linear
accelerator (linac). They then pass through a "wiggler,"
a device that uses a series of permanent magnets to
force them to "wiggle" in a wavy path. This
wiggling motion causes the electrons to emit light.
this point, the electrons need a little help in order
to produce light with the qualities DUV-FEL scientists
need to perform their experiments -- a steady wavelength
and frequency, and very short pulses. Thus, as they
traverse the wiggler, the electrons are simultaneously
coupled with light from a "seed laser."
The seed laser light gives the electrons a boost by
regulating their motion so that they emit more concentrated,
laser-like light. Next, the electrons enter a "bunch
compressor" device, where they are grouped into
tiny clusters. Finally, the electrons are sent into
a second, longer wiggler, where they emit light pulses
that can be used for experiments.
DUV-FEL is a very intense source of high-energy light
Suits said. "But the FEL light has other unique
properties. It is very coherent, or steady, and each
pulse lasts less than one trillionth of a second.
These short, intense 'flashes' allow us to make very
rapid 'snapshots' of brief molecular processes, such
as chemical reactions, using the light. These are
the kinds of experiments we hope to perform in the
research was funded by the Office of Basic Energy
Sciences in the Department of Energy's Office of Science,
the National Science Foundation, and the Robert A.