THE 1999 NOBEL PRIZE IN CHEMISTRY goes to Ahmed H. Zewail of Caltech, for developing a technique that enables scientists to watch the extremely rapid middle stages of a chemical reaction. Relying on ultra-fast laser pulses, "femtosecond spectroscopy" can provide snapshots far faster than any camera--it can capture the motions of atoms within molecules in the time scale of femtoseconds (10^-15 s).
An atom in a molecule typically performs a single vibration in just 10-100 femtoseconds, so this technique is fast enough to discern each and every step of any known chemical reaction. Shining pairs of femtosecond laser pulses on molecules (the first to initiate a reaction and the second to probe it) and studying what type of light they absorb yields information on the atoms’ positions within the molecules at every step of a chemical reaction. With this technique, Zewail and his colleagues first studied (in the late 1980s) a 200-femtosecond disintegration of iodocyanide (ICN-->I+CN), observing the precise moment at which a chemical bond between iodine and carbon was about to break.
Since then, femtochemistry has revealed a whole new class of intermediate chemical compounds that exist less than a trillionth of a second between the beginning and end of a reaction. It has also provided a way for controlling the courses of chemical reaction and developing desirable new materials for electronics. It has provided insights on the dissolving of liquids, corrosion and catalysis on surfaces (see Physics Today, October 1999, p. 19); and the molecular-level details of how chlorophyll molecules can efficiently convert sunlight into useable energy for plants during the process of photosynthesis. (Official announcement and further info at www.nobel.se/announcement-99/chemistry99.html; see also Scientific American, December 1990.)