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Buckyballs combine to make giant fullerenes.

When magicians wave their wands, objects go poof! and disappear. But when those round, all-carbon molecules called fullerenes go poof! they sometimes just turn into bigger versions of themselves.

Scientists often use lasers to make fullerenes by evaporating carbon atoms from graphite rods. But zapping fullerene films with lasers causes the 60-carbon buckyballs and their 70-carbon big brothers to vaporize and combine like soap bubbles, says Chahan Yeretzian, a physical chemist at the University of California, Los Angeles. These united molecules sometimes billow to form stable fullerenes with 400 or more carbon atoms, he and his colleagues report in the Sept. 3 NATURE.

Most molecules would fall apart when subjected to such intense laser pulses, but fullerenes just open up their cages and then reseal themselves as bigger, smaller, or sometimes metal-filled molecules, says Mark M. Ross, a physical chemist at the Naval Research Laboratory in Washington, D.C. "I don't know of any other molecules that do this sort of thing," he says. In addition, the new results suggest that fullerenes form not only by the incremental addition of carbon atoms, but also by the linking of large precursors, says Ross.

"This could be a very efficient way to produce specific larger fullerenes that today are very hard to get," says Yeretzian.

Other scientists had suggested that fullerenes energized by lasers might temporarily open up their cages. Already, at the Naval Research Laboratory, physical chemist Stephen W. McElvany had used lasers to promote the entrapment of one or two yttrium atoms inside fullerenes. He reported on his work in the June 11 JOURNAL OF PHYSICAL CHEMISTRY.

For his experiments, Yeretzian flowed helium into the chamber where he zapped the fullerene film. The helium briefly confines tens of trillions of vaporized fullerenes into a space much smaller than the head of a pin. Thus the fullerenes behave like molecular bumper cars, Yeretzian explains. A mass spectrometer detected new fullerenes with 70 to 400 carbon atoms. By varying experimental conditions, the researchers can skew the sizes of molecules produced, he adds.

With pure [C.sub.60] as the starting material, the mass spectra peaks revealed that many molecules contain carbon atoms in near-multiples of 60; the rest increase or decrease in size by two-carbon increments (see figure). The researchers suggest that coalescence causes the molecules to eject two-carbon fragments, which other merging molecules then take on. - E. Pennisi
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Title Annotation:intense laser pulses produce specific fullerenes
Author:Pennisi, Elizabeth
Publication:Science News
Date:Sep 5, 1992
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