Unprecedented Step : Scientists Able to See ‘Birth’ of New Molecules

Times Science Writer

Caltech scientists, using extraordinarily short bursts of laser light, have witnessed the creation of chemical molecules--an unprecedented step in the study of chemical reactions.

“For the first time, we are able to see the birth of new molecules which make us and form the world around us,” said chemist Ahmed H. Zewail. “We are able to see how bonds (between atoms) are broken and made and how molecules are formed in a millionth of a billionth of a second,” he said.

The achievement “has opened up a new branch of chemistry. . . . (The) work is a milestone,” said chemist Richard B. Bernstein of UCLA.

The techniques Zewail used may ultimately improve chemists’ ability to synthesize new chemicals such as medicines, plastics and industrial chemicals more cheaply, experts said.


Had Inferred Interaction

Until now, the movement and interaction of atoms in the course of a reaction had to be inferred from changes in energy states or in the development of a product after a chemical reaction. Now scientists can determine the instant a new molecule is born.

Zewail’s results were published last summer with little fanfare in the Journal of Chemical Physics, but were announced this week by Caltech. “Chemists are fundamentally interested in how one molecule is transformed into another, how A goes to B,” Zewail said Wednesday. “In the past, people have studied such reactions by looking at B and inferring what happened.”

Researchers would study the reaction products and see what kind of light they emit or how fast their bonds are vibrating, and from that information deduce the pathways by which the molecules were formed. In fact, chemists Dudley Herschbach, Yuan Lee, and John Polanyi won the 1986 Nobel Prize for chemistry for developing techniques to study reaction products immediately after they are formed.


But what chemists have been trying to observe “since modern chemistry developed,” said chemist Kenneth Eisenthal of Columbia University in New York City, is the short-lived transition state--the stage in which the molecules that are reacting are loosely joined together before breaking down into products.

Zewail has made “an extremely important contribution,” Eisenthal said, by finding a way, in effect, to take a series of “snapshots” of gaseous molecules while they are in the transition state.

To do this, chemicals to be studied are placed in a small metal vacuum chamber with a quartz window. A single very short laser burst is fired into the gas to start the chemical reaction. A few millionths of a billionth of a second later, a second short burst is fired.

The molecules, which are then in the transition state, absorb light from the second laser beam and re-emit it at a different wavelength. By analyzing the characteristics of this emitted light, Zewail was able to determine the structure of the transition state as it was when the laser was fired.


By firing the laser repeatedly at short intervals, Zewail said, he can make “a movie of snapshots” that shows the whole reaction.

In one study, for example, Zewail and his colleagues studied the reaction of a hydrogen atom with carbon dioxide--a reaction that could be used in the production of synthetic fuels from coal.

They began by placing carbon dioxide and hydrogen iodide gas into the reaction chamber. These two molecules bind together loosely.

When the first laser was fired, it instantaneously broke apart the hydrogen iodide, freeing the iodine atom and producing a hydrogen atom that could react with the carbon dioxide.


As the second laser was fired repeatedly, the researchers were able to watch the hydrogen form a bond to one of the oxygens of carbon dioxide, and then to see that oxygen break its bond to the carbon atom, forming the ultimate products, carbon monoxide and hydroxyl.

Zewail found that the transition state persisted about 10 times as long as had been predicted by theorists.

“But the most important thing is that he saw it while it was happening,” Bernstein said. “That has never been done before.”