'Snapshots' of bond breaking and making.

"Snapshots' of Bond Breaking and Making

It's hardly satisfying to see merely the beginning and the end of a movie. One misses most of the human drama and can only guess at the intricacies of the plot as it has unfolded.

The same is true for molecules and chemical reactions. Scientists have been able to study the reagents and products of a chemical reaction--its beginning and end. But because they have not had tools fast enough to catch the action in between, they've missed out on directly observing the movements and play of atoms during the making and breaking of bonds. Most studies using lasers to investigate reaction dynamics have used nanosecond-long (10(-9) sec) pulses, which are too long to allow a glimpse of the atomic drama taking place within picoseconds (10(-12) sec) during a reaction.

In the last few years, however, lasers with shorter pulses, in the femtosecond (10(-15) sec) range, have come of age. And using such lasers, Ahmed H. Zewail and his colleagues at the California Institute of Technology in Pasadena have for the first time directly probed the events involved in the birth and destruction of molecules. "It's like having an ultrafast camera now to view these processes,' says Zewail, who discussed his work this week in Lake Tahoe, Nev., at the International Conference on Lasers 87.

"We're very excited about it,' says Larry Davis of the Air Force Office of Scientific Research in Washington, D.C. "It's the first time a chemical reaction itself is being followed with such a narrow time slice that you can actually get a handle on what's happening as the [molecules] separate from one another.'

"It's brilliant work,' concurs Kenneth Eisenthal at Columbia University in New York City. "It will stimulate a lot of theoretical work and further experiments.'

So far, Zewail's group has examined two elementary reactions: the dissociation of cyanogen iodide (ICN) into iodine (I) and cyanide (CN); and the interaction between hydrogen (H) and carbon dioxide (CO2) to form carbon monoxide (CO) and hydroxyl (OH). In future studies, Zewail plans to examine more complex reactions.

In their technique, the researchers initiate a reaction by using one laser pulse to pump more than enough energy into a gas of reactants to begin breaking bonds. (In the carbon dioxide reaction, the "pump' pulse breaks hydrogen away from another molecule to which it had been bound and sends it reeling into carbon dioxide molecules.) Zewail's group then sends in a series of "probe' pulses at different delay times and of different energies. When the energy and timing are just right, these pulses are absorbed by molecules undergoing the various transition stages from reactants to products. The researchers detect this absorption by monitoring the light that is reemitted by the molecules, a process called laser-induced fluorescence.

The resulting real-time data, though not "photographs,' yield information that paints a graphic portrait of the reaction. From the energy of the pump pulse-- which reveals the velocity of the reactant molecules--and from the times of the absorbed probe pulses, Zewail's group can determine the distances between molecules as they separate or come together. And from the energies of the absorbed probes, they can also identify transition species and eventually glean information about their structure, rotations, vibrations, bond lengths and other aspects of their energy states as they evolve in time.

In the hydrogen-carbon dioxide reaction, Zewail says his group confirmed suggestions from earlier, indirect studies that "hydrogen dances around carbon dioxide for a while,' forming what is known as a collision complex, rather than stripping off an oxygen immediately. Moreover, the technique enabled the researchers to clock the lifetime of this transition state--5 picoseconds.

Knowing how molecules transform from one species into another is the essence of chemistry, says Eisenthal. Indeed, last year's Nobel Prize in chemistry was shared by three researchers working on such reaction dynamics. But these studies could offer only indirect hints about transition states. With his technique, Zewail has added an important new wrinkle, enabling chemists to start asking questions such as how transition states are altered by the presence of other molecules, temperature changes or isotopic substitutions. "It's an important first,' he says.