Bunches of atoms madly morph

Minuscule clusters of atoms don’t hold their shapes as well as hold-in-your-hand solids do. Understanding such instability is a growing priority as circuits, machines, and other structures shrink to atomic scales (SN: 2/15/03, p. 110: Streams plus nanostrands equals electricity).

Now, using ultrafast lasers, physicists at the University of Virginia in Charlottesville have observed that clusters randomly morph between different arrangements, or isomers, many times within a nanosecond.

In the Feb. 14 Physical Review Letters, Andrew J. Dally and Louis A. Bloomfield report rapid cycling among three isomers–a cube, a ladder, and a ring–by seven-atom clusters of cesium iodide salt.

To focus on suspected shape-shifting by clusters, the Charlottesville team repeatedly condensed cesium iodide vapor in cooler helium gas to form a stream containing hundreds of clusters.

The researchers then zapped the cluster stream with a laser pulse strong enough to shatter only the cubic clusters and waited up to a nanosecond for subsequent shape changes to take place among the remaining isomers. Then the scientists measured the proportions of all three isomers in the stream to see whether some of the other isomers changed into cubic ones.

By overlaying thousands of such measurements, Dally and Bloomfield say they have demonstrated that shape changes indeed take place and that those transformations recur at extremely brief intervals of tens to hundreds of trillionths of a second.

Shiv N. Khanna of Virginia Commonwealth University in Richmond calls the new results “a very important contribution.” Besides verifying theoretical predictions, he says, the findings may also help researchers figure out how to select and stabilize isomers with desirable traits for use as nanometer-scale building blocks for novel structures.


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