Chemistry Nobel spotlights fast reactions.

For more than 10 years, Ahmed H. Zewail of the California Institute of Technology in Pasadena has been using extremely short laser pulses to track intimate details of chemical reactions. This week, the Royal Swedish Academy of Sciences awarded the 1999 Nobel Prize in Chemistry to Zewail for his groundbreaking investigations of atom movements during the birth and destruction of molecules.

When molecules undergo chemical reactions, their atoms shift so quickly that scientists require laser pulses lasting only a few femtoseconds (quadrillionths of a second) to obtain snapshots of the making and breaking of chemical bonds. Zewail and his coworkers developed laser techniques for observing such interactions and used them to investigate a variety of fundamental chemical processes.

In femtosecond spectroscopy, researchers initiate a reaction with a laser pulse that pumps more than enough energy into a gas of reactants to begin breaking bonds. They then send in a series of weaker laser pulses of different energies to detect the altered molecules.

Initially, Zewail and his team studied such elementary reactions as the dissociation of iodocyanide (ICN) into iodine (1) and cyanide (CN) and the interaction between hydrogen (H) and carbon dioxide ([CO.sub.2]) to form carbon monoxide (CO) and hydroxyl (OH). In the hydrogen-carbon dioxide reaction, the chemists showed that a hydrogen atom temporarily joins with a carbon dioxide molecule to form a surprisingly long-lived, loose unit before it strips away an oxygen atom (SN: 12/12/87, p. 372).

Zewail's group later used femtosecond laser pulses to observe atomic movements in iodine and other two-atom molecules, capturing in detail individual rotations and vibrations of the molecules (SN: 3/3/90, p. 135). Another study highlighted how solvent molecules exert an influence on bond breaking (SN: 7/31/93, p. 71).

Recently, Zewail has focused on biologically important chemical processes of large molecules, such as chlorophyll and retinal. He and his coworkers have demonstrated experimentally that light can convert a molecule from one structure to another with high efficiency because energy is quickly channeled to the appropriate place in the excited molecule.

"Zewail pioneered ... femtosecond methods to investigate photochemical reactions," says Nicholas J. Turro of Columbia University. "His research involves brilliant technique development and experimental conception and execution."

Scientists are now using femtosecond spectroscopy to study how catalysts or molecular electronic components function and to probe what mechanisms underlie biological processes.