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Good vibes: seeing a single molecule move.

The quest to see a single molecule has long taunted physicists. How does one hold a barely detectable speck, bombard it with electrons, photons, or X-rays, and observe how it moves without disturbing, destroying, or sweeping the sample away?

But researchers now tell of a new technique for watching a single molecule move. Gary M. McClelland, Fumiya Watanabe, and Harr Heinzelmann, all physicists at IBM's Almaden Research Center in San Jose, Calif., report directly observing phthalocyanine - perched on a tungsten tip. Their report appears in the Nov. 19 SCIENCE

In essence, the group has invented a new kind of instrument, called a femto-second field emission camera, which combines the features of a video camera and an oscilloscope. "It operates much like a streak camera. We sweep a tightly focused beam of electrons from a tip across a detection screen. The tip itself [on which the molecule sits] is a single crystal that comes to an apex of just three atoms," McClelland says. "We observe the molecule's motion by recording the changes in the field emission intensity."

This new system provides the first "direct and continuous, high-resolution, real-time look at the motions of individual atoms and small molecules," McClelland says.

"We've actually recorded the vibration of a molecule as it osciallates back and forth at a particular site," he says. "It's the first time a record has been made with a single molecule in real time."

The physicists chose to observe a copper phthalocyanine molecule because it is relatively large, rigid, and stable -- "something that would vibrate slowly, with a large amplitude," McClelland adds.

After sharpening a tungsten tip and colling it to 80 Kelvins, the researchers deposited a single copper phthalocyanine molecule on the tip's end. They then sujected the tip to roughly 20 sweeps of a pulsed electric field until "the molecule decompose or diffused off the end of the tip," says McCelland. The team used a video camera to make single-frame recordings of the electrons emitted from each sweep - producing a sort of electronic snapshot.

In the end, they analyzed 270 such sweeps, comparing them to 100 sweeps of a clean tungsten tip, from which they recorded no large "peaks," or signals.

The ability to watch events taking place in single atoms and molecules while they are going on opens up "entirely new possibilities in molecular dynamics," the physicists state, adding that the technique allows them to observe spontaneous processes that cannot otherwise be seen.

"We think this is about as close as possible to directly viewing molecular motion," concludes McClelland.
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Title Annotation:femto-second field emission camera allows scientists to observe vibrations in molecules
Author:Lipkin, Richard
Publication:Science News
Article Type:Brief Article
Date:Nov 20, 1993
Words:423
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