Salt: The Movie

Solid, liquid, melting, and freezing are concepts that refer to bulk matter, and not to individual atoms. But what about a cluster of a dozen atoms?

Louis Bloomfield (University of Virginia) has assembled a nano-sized grain of salt, a seven-atom blob of consisting of 4 cesium atoms and 3 iodide atoms. Compare this to an ordinary salt grain, with a size of 0.2 mm and about 1.5 million atoms along each side of its cubical structure.

By spraying this cluster with picosecond pulses of light, Bloomfield has been able to make a "movie" of sorts showing how the cluster rearranges its geometry: sometimes a 2 x 2 x 2 cube, sometimes a flat 2 x 4 ladder, sometimes an octagonal ring, all by virtue of the cluster's own internal thermal energy; they don't image the cluster directly, but their locations can be inferred from a mixture of measurement and theory (for figures and cool movie, see http://rabi.phys.virginia.edu/research/ ). Separate laser pulses are used to heat or to view the clusters.

One outcome of the experiment: "melting" of the tiny crystal begins at a "temperature" of 225 C rather than 626 C, the melting temperature of the bulk material. Studies like this are pertinent to the production of nm-sized circuitry since one should know whether a wire or some other structure will retain its basic shape or shift into something else over time. (Dally and Bloomfield, Physical Review Letters, 14 February 2003, bloomfield@virginia.edu, 434-924-4576; see also How Things Work: The Physics of Everyday Life, chapter 15)