Salt: The movie

Solid, liquid, melting, and freezing are concepts that refer to bulk matter, not to individual atoms. But what about a small cluster of atoms or molecules? Louis Bloomfield and Andrew Dally (University of Virginia) looked at a pulsed beam of clusters of a salt; each cluster contained dozens of molecules that each had four cesium atoms and three iodine atoms. An ordinary salt grain has more than a million atoms along each side of its cubical structure. The Cs₄I₃ molecule can take on three different shapes or “isomers”: a cube, ladder, or ring. The researchers sent the salt clusters through a laser interaction region, where the cubic isomer was depleted. Using probing lasers downstream, the researchers watched at a cinematic 30 “frames” per second as the population of the cubic form was restored at the expense of ladders and rings. The interconversion, known as isomerization, happened more quickly with higher temperature. In fact, at about 500 K, the molecules spent only enough time in any one shape to convert into another, the signature of a phase transition from solid to liquid in a bulk system. Interestingly, the melting temperature of bulk cesium iodide is about 900 K. (A. J. Dally, L. A. Bloomfield, Phys. Rev. Lett. 90, 063401, 2003 http://dx.doi.org/10.1103/PhysRevLett.90.063401 .)