Atomic rebound makes breaking up hard.

When chemical reactions occur in solution, solvent molecules exert a strong influence on the making and breaking of bonds. A team of researchers has now obtained the first glimpse of what happens when a laser light pulse lasting only a few femtoseconds (quadrillionths of a second) excites and splits an iodine molecule surrounded by a layer of argon atoms.

"With femtosecond time resolution, we can obtain snapshots of chemical reactions in real time," says chemical physicist Ahmed H. Zewail of the California Institute of Technology in Pasadena. This capability permits researchers to monitor motions on an atomic scale and to investigate what role solvent molecules play in easing bond formation or bond breaking.

Zewail and his Caltech co-workers studied the behavior of iodine molecules enveloped in clusters of argon atoms traveling in a molecular beam. From previous studies of isolated iodine molecules in the gas phase, they already knew that extremely short light pulses at different wavelengths cause iodine molecules to break up in different ways.

At a wavelength of 614 nanometers, a light pulse lasting only a few femtoseconds causes such a rapid breakup of an iodine molecule into two iodine atoms that the two atoms shoot away from each other at high speed. At a wave-length of 510 nanometers, the dissociation of iodine molecules occurs much more slowly.

The presence of argon atoms changes the dynamics of this chemical reaction considerably. In the first case, the two iodine atoms speed apart until they hit the surrounding "wall" of argon atoms. The iodine atoms rebound along their original paths and recombine into a molecule. This "hot" iodine molecule then gradually cools down via repeated collisions with its shell of argon atoms.

"We see the recombination of the atoms, which you don't see in the gas phase," Zewail says.

In the second case, because the iodine molecule breaks up more slowly, the surrounding argon atoms have sufficient time to get between the two iodine atoms. "There is plenty.of time for the solvent to rearrange, and as a result of that, we lose this [fast] recombination of the atoms," Zewail says.

The results demonstrate that the dynamics of the dissociation of iodine molecules depend critically on how swiftly bond breaking occurs relative to how quickly solvent atoms rearrange themselves. The researchers can also pinpoint the rebound of iodine atoms within their argon trap as the mechanism responsible for a process by which a solvent may enhance bond formation in solution by trapping reactive atoms or molecules in "solvent cages."

"Now we want to try to generalize this result," Zewail says. "There are a lot of other solvents we would like to do." By studying what effect solvent composition has on the dynamics of this and other chemical reactions, researchers may obtain important insights into the atomic forces that lead to various types of chemical behavior.

Zewail, Qianli Liu, and Juen-Kai Wang describe their findings in the July 29 NATURE.