By William J. Cromie

Gazette Staff

A unique way to slow down, trap, and study atoms and molecules has been successfully tested by Harvard researchers.

Molecules flying around in a gas, such as air, normally travel at bullet speed, vibrate, and tumble. Slowing them to the speed of a gnat, then holding them suspended in a container was, until recently, thought to be impossible.

Last year, John Doyle, assistant professor of physics, used a technique he invented to slow down and trap atoms, from which molecules are built. Now he's on the brink of attempting to trap molecules, something that has never been done.

"By confining molecules or atoms for hours or days, we can do studies and make measurements never done before," Doyle says. "It's like a painting by Brueghel; the longer you look, the more new things you see and understand."

A molecule is the smallest unit of any substance that retains all the physical and chemical characteristics of that substance. When put in a container at room temperature, molecules or atoms zing around at high speed, bounce off the walls, vibrate, and rotate. Doyle uses a combination of very cold temperatures and helium gas to slow them down to the speed of a slowly flying bird, about 5 miles an hour.

In experiments done last year, Doyle and senior research fellow Bretislav Friedrich, trapped a thimbleful of atoms of Europium, a rare metallic element, in a supercold container in the basement of Jefferson Laboratory. These atoms collided with atoms of helium, losing so much energy they could be captured in the grip of a magnetic field.

The experimenters then pumped out the helium, leaving the metal atoms suspended in an ultra-high vacuum. It was the first time that such a helium buffer had been used, and the first time such big, unruly atoms had been trapped. Europium atoms are as large as some small molecules.

"We expect to use the same system to trap molecules in the coming months," Doyle says. He and Friedrich say they'll first try to catch metal oxides, chemically reactive compounds that can be easily held by magnetic fields. "Other types of molecules may follow suit," notes Friedrich. In the future, it may be possible to trap the organic molecules from which living creatures are made.

Building a Better Light Trap

Friedrich and Doyle are also looking at capturing molecules with light. A web of laser beams would attract and hold these molecules by interacting with their electrons.

"The molecules act like a moth drawn to a flame," notes Dudley Herschbach, Baird Professor of Science. "The moth has enough energy to fly by, but doesn't. Molecules would also have enough energy to escape but collisions with the helium gas should slow them enough to fall into the trap."

Herschbach won the 1986 Nobel Prize in Chemistry for research with beams of molecules that are collided with each other. By capturing and measuring the fragments, he and his colleagues obtained the first detailed picture of what happens when two molecules react chemically.

"Before such techniques, we could only deal with great unruly mobs of molecules speeding in every direction," Herschbach recalls. "Trapping is the next step in taming molecular mobs."

Recently, he and Friedrich developed a way to eliminate the random tumbling of molecules so that the molecules all can be oriented in the same direction. "That will enable us to obtain a slow-motion view of what goes on when two chemicals react with each other."

Doyle, Friedrich, and Herschbach believe they'll be able to build the first light traps in a year or two. Herschbach will describe their experiments at a meeting of the American Chemical Society in San Francisco next month.

Waves Matter

Air is pumped out of the traps, of course, to prevent unwanted collisions with air molecules. And they are cooled down to within a fraction of the lowest temperature possible, near minus 460 degrees Fahrenheit. Heat provides the energy of motion, so that in such cold, molecules move very slowly.

That's not the way it is in nature. However, "studying chemical reactions at low temperatures will teach us a lot about what goes on at normal temperatures," Doyle points out. "Such reactions will also enable us to learn things we could never learn otherwise, such as how atoms bind to each other to form molecules, and the nature of weak interaction between molecules when they are very far (relatively) from each other."

"If you slow down atoms and molecules enough, you begin to see a whole different side of matter," Herschbach comments. "Everything has a dual nature, an atom can behave like a solid particle, or like a cloud of waves. Take a dump truck; it has such a huge mass, you're not aware of its wave character. But an extremely lightweight particle, an atom or molecule, moving at very low speed, can behave like a wave. That means we could see a whole new type of chemistry, one we've never before explored.

"It should be an intriguing intellectual experience," Herschbach enthuses.

Physicists see other intriguing intellectual experiences at the very cold, very slow, very light end of the matter tunnel. For example, there's the puzzling problem of an asymmetric universe. According to theory, when time and the universe began with a Big Bang some 15 billion years ago, there should have been equal amounts of matter and antimatter. Today, however, we live in a matter-dominated universe. Where did the antimatter go?

"That's one of the fundamental questions about the universe," Doyle points out. "We hope to use ultra-cold molecules as probes of the physics that was important just after the birth of the universe."

Asked about possible practical applications of confining a fistful of molecules in a supercold container, Herschbach admitted: "None, at this moment." But, he added, "that's what Ben Franklin said when he experimented with electricity in the 1760s. Franklin actually apologized for working on something with no practical use."