The Varsity

U of T researchers have created an unprecedented movie of what happens when a solid changes into a liquid on the atomic scale.

Dwayne Miller of the Departments of Chemistry and Physics and his coworkers fired short laser pulses at a thin film of aluminum, then tracked what happened as the material melted. Their findings graced the cover of the Nov. 21 issue of the journal Science.

The new technique has allowed them to witness some never-before-seen details of the melting process. Shortly after melting, for example, the aluminum seems to retain a faint memory of its former orderly arrangement before it gets completely washed away by the unruly motion of the atoms in the liquid state.

Scientists have been able to see atomic level detail in the past, but “they’re usually talking about molecules when they’re not doing anything,” says Brad Siwick, a PhD student who coauthored the paper. “We’re able to capture the material in almost an instant of time. And by doing the experiment over and over again and capturing it at different instants in time…you can basically produce a movie.”

The researchers aren’t able to see individual atoms with the technique, but it does allow them to deduce details of their arrangement. They can determine how far apart they are from each other on average, for example, and how they are moving with respect to each other.

The key to making the movies is the ability to create extremely short snapshots of the system. Chemical reactions happen very quickly, on the scale of a picosecond, which is one trillionth of a second. Even high-speed cameras that can capture a bullet in mid flight are about a billion times too slow.

Miller’s group has pioneered a technique that uses extremely short pulses of electrons to make his snapshots. The electrons are deflected by varying degrees as they pass through the material, building up an image on the screen behind it.

Each pulse lasts just 0.6 picoseconds. On this timescale, everything appears to move very, very slowly. Even light itself, traveling at the blistering speed of 300,000 km per second, moves only a hair’s width in one picosecond.

A series of still shots shows the material evolving from a solid to a liquid over a period of just 3.5 picoseconds. Shortly after the laser pulse hits, the material is still solid but the atoms are vibrating like crazy. Eventually these vibrations shake the material apart and the atoms begin to move about randomly in their new liquid state.

The technique should allow movies to be made of many other processes of interest, Siwick says. “There’s a whole host of things that you might be interested in studying with this sort of a technique.”

The advance is “very exciting indeed,” says Todd Stuckless, a chemistry professor at UBC. It is fast enough that “one could obtain something similar to ‘time lapse photography’ of a chemical bond being formed or broken.”