Watching Rapid Melting at the Atomic Scale

At last month's CLEO/QELS optics meeting in Baltimore, Dwayne Miller of the University of Toronto (dmiller@lphys.chem.utoronto.ca) described how he and his colleagues are capturing the first atomic-level view of the melting process, one of the simplest transformations of matter, on the timescale of femtoseconds, or quadrillionths of a second. Rapidly heating metals and watching how their atoms rearrange themselves can provide insights into extreme states of matter, e.g., of matter that approaches fusion temperatures or under the extreme conditions in the interiors of planets.

In the University of Toronto setup, an intense, ultrafast pulse of laser light melts the target material. This pulse is followed by a beam of electrons that diffracts off the atoms in the sample to provide information on the positions of the atoms at any given instant. The experiments are revising scientists' basic knowledge of what happens during rapid melting. Raising the temperature of solid aluminum to approximately 1000 degrees in less than 1 picosecond, the researchers found that the aluminum atoms, initially arranged in an face-centered cubic lattice (much like oranges in a grocery display), are vigorously shaken by the heating caused by the laser beam, with the atoms at the corners shaken off first, followed by those closer inside (see Siwick et al, Science, 21 November 2003).

Recently, the researchers have begun to investigate the melting and the equation of state of pure carbon, the element with the highest melting point; the results might help answer a question in planetary science, namely whether liquid carbon exists inside Neptune and Uranus. (Presentation CTuAA1, "Femtosecond Electron Diffraction: An Atomic-Level View of Condensed Phase Dynamics"; http://lphys.chem.utoronto.ca/)r