U of T team makes 'movie stars' of atoms

November 20, 2003

Chemists at the University of Toronto have captured atom-scale images of the melting process-revealing the first images of the transition of a solid into a liquid at the timescale of femtoseconds, or millionths of a billionth of a second.

The result is an unprecedented "movie" detailing the melting process as solid aluminum becomes a liquid. This new study, led by Professor R. J. Dwayne Miller of the Departments of Chemistry and Physics, received the prestigious cover position of the Nov. 21 issue of Science.

"Imagine being able to see atoms as they move in real time," says Miller, who holds the Canada Research Chair in Femtoscience. "Chemistry and biology are fundamentally governed by changes in atomic structure. We now have a tool that will let us observe the most fundamental processes at the atomic level of inspection with sufficient time resolution to allow us to see chemical and biological events as they happen."

Since no camera shutter can open and close at the femtosecond time scale, the team built a special system using a laser and an electron gun inside a vacuum chamber. The energy of the laser's blast superheated small sections of the aluminum to over 1,000 degrees Celsius, exceeding the metal's melting point of 660 degrees Celsius.

Releasing a 600-femtosecond electron pulse at virtually the same moment of the laser blast, they captured an image of the aluminum atoms. This revealed the melting process at 0.5-picoseconds (one thousandth of a billionth of a second) after the laser struck the aluminum. However, capturing the complete melting sequence required that they repeat the process several times, each time firing the electron pulse a few hundred femtoseconds later. This revealed the melting process at 1.5-, 2.5- and 3.5-picoseconds after the laser pulse.

The "movie" the group saw when they put the frames together revealed that the solid literally shook itself apart at the atomic level. Liquids are fundamentally different than solids in that the atomic positions are random in liquids but ordered in solids. The team was able to watch, step by step, as the initially well-ordered arrangement of aluminum atoms in the solid changed into the disordered state of the liquid. The aluminum melted in an astonishingly short time-within 3.5 picoseconds.

This work represents the first atomic level view of the melting process, one of the simplest structural changes of matter. The team stresses the scientific implications of being able to watch atoms rearrange themselves on the femtosecond timescale. "Chemists think of reactions in terms of atoms moving around as bonds are broken and formed," says Jason Dwyer, a graduate student in Miller's laboratory and a co-author of the paper. "It is one of the dreams of chemistry to be able to actually watch that as it happens, and we now have a technique that lets us do that."
The research is funded by the Canada Foundation for Innovation, the Ontario Innovation Trust, the Natural Sciences Engineering Research Council of Canada and the U of T Connaught Fund.


R.J. Dwayne Miller
Department of Chemistry
(currently in Germany)
011-49-30-897-48221 2-6pm EST
011-49-30-6392-1429 7am-12pm

Brad Siwick
Department of Physics

Jason Dwyer
Department of Chemistry

Robert Jordan
Department of Chemistry

Nicolle Wahl
U of T Public Affairs

University of Toronto

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