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Late atomic bursts from cracked crystals.

When brittle crystals break, they often stage a spectacular light-and-particle show, spewing copious quantities of photons, electrons, ions, single atoms and even clusters of atoms. Now, researchers have discovered that certain crystals also emit intense bursts of atoms and molecules after the material has cracked in two -- sometimes as much as 250 milliseconds later.

"This is a huge delay in terms of the normal kinds of mechanisms [proposed to account for emissions accompanying fracture]," says physicist J. Thomas Dickinson of Washington State University in Pullman. "It's a very peculiar effect."

Dickinson and his collaborators describe their findings in the April 22 PHYSICAL REVIEW LETTERS.

The researchers first noted this delayed emission when cracking reactangular, single-crystal wafers of sodium chloride and lithium fluoride. In each case, they observed rapid bursts of either sodium and chrlorine atoms or lithium and fluorine atoms, as well as neutral sodium chloride or lithium fluoride clusters. These appeared 0.5 to 250 milli-seconds after the crystal had finished breaking. Each burst typically lasted a few milliseconds.

More recent experiments show that cracked germanium crystals similarly produce delayed bursts of single germanium atoms and molecules consisting f pairs of germanium atoms. "The presence of the emission is very reproducible," Dickinson says. "But because every fracture is different, the time delay is fairly erratic."

To explain this delay, Dickinson and his co-workers propose that the tip of a crack streaking through a breaking crystal induces microscopic dislocations, or shifts, in the crystal's orderly array of atoms or ions. Driven by stresses in the material, these crack-injected "ripples" travel into the crystal, reverse direction, then come speeding back to the newly formed fracture surface. The simultaenous arrival of several mobile dislocations at the surface furnishes sufficient energy to eject an atom or a cluster of atoms.

"The dislocations basically dump their energy at the surface, relasing atomic and molecular species into the vacuum," Dickinson says. However, "the details of the transfer of this energy to the atoms and molecules are not clear."

Monitoring the atomic and molecular emissions that result from dislocation "popout" may nonetheless prove useful for studying such important processes as energy dissipation during fracture. At the very least, the researchers say, such signals could provide a relatively simple way of investigating dislocations generated by fractures.
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Title Annotation:crystals that emit bursts of atoms and molecules when cracked
Author:Peterson, Ivars
Publication:Science News
Date:May 4, 1991
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