Zewail, Ahmed Hassan (1946-) is an Egyptian-born American chemist who won the 1999 Nobel Prize in chemistry for his experiments showing how molecules change during chemical reactions. In such reactions, chemical bonds (attractions between atoms that make up molecules) form and break extremely rapidly. And in many cases, what seems to be a single reaction actually takes place in two or more steps. Zewail and his colleagues at the California Institute of Technology (Caltech) in Pasadena developed the first experimental techniques that are fast enough to detect such steps and measure their speed. In these techniques, brief pulses of laser light produce and measure the reactions.

Zewail was born on Feb. 26, 1946, in Damanhur, Egypt, east of Alexandria. He was the only son in a family of three daughters, and his parents wanted him to receive a top education. His father worked for the government and also had his own business. As a boy, Zewail became interested in mathematics, mechanics, and chemistry. In his bedroom, he built a small chemistry set using his mother's oil burner for making Arabic coffee and a few glass tubes.

In 1967, Zewail received his bachelor's degree from Alexandria University. He earned a master's degree there in 1969, while also working as a chemistry instructor and researcher. He went on to the United States to study and, in 1974, he received his doctorate in chemistry from the University of Pennsylvania in Philadelphia. He worked as a research fellow at the University of California in Berkeley until 1976, when he joined the Caltech faculty. Zewail became a full professor at Caltech in 1982. In the same year, he became a United States citizen.

Over two decades at Caltech, Zewail conducted his pioneering research in the field of femtochemistry, the branch of chemistry in which chemical reactions are detected as they occur and the reaction times are measured. Femtochemistry gets its name from the fact that these times are measured in femtoseconds. One femtosecond is one-millionth of one-billionth of a second. This number is written out as a decimal point followed by 14 zeros and a 1. Scientists believe that no chemical reaction can take place more rapidly than this.

There are many kinds of chemical reactions. For example, a molecule may break apart, or the atoms in the molecule may change their positions. In a reaction involving two molecules, one or more atoms may move from one molecule to the other, creating a new molecule. But all reactions involve changes in chemical bonds (attractions between the atoms that make up the molecules). In some reactions, bonds break, while in others, they form. In still other reactions—as when an atom jumps from one molecule to another—bonds both break and form. Furthermore, many reactions occur in stages. For instance, when a complex molecule breaks apart, bonds may break one at a time.

In a simple femtochemistry experiment, a laser shoots two extremely short pulses of laser light, one after the other, into a group of molecules. The pulses are precisely timed so that the second pulse reaches the molecules a certain number of femtoseconds after the first pulse.

The first pulse, known as the pump pulse, causes a chemical reaction—or a stage of a reaction—to occur in the molecules. The pulse acts by breaking bonds. Light is a form of energy, and if a molecule absorbs a certain amount of energy, a bond will break.

The second pulse, called the probe pulse, also puts energy into the molecules. The molecules then react by emitting (sending out) light energy. The intensity and color of the emission depend on the structure of the molecules when the probe pulse arrived. Light sensors measure the emissions, and a computer analyzes the results.

Zewail also won the Buck-Whitney Medal from the American Chemical Society in 1985, the 1993 Earl K. Plyler Prize from the American Physics Society, the 1993 Wolf Prize in Chemistry, and the 1993 Medal of the Royal Netherlands Academy of Arts and Sciences, and the Benjamin Franklin Medal of the Franklin Institute in 1998. Egypt honored Zewail in 1995, when Egyptian President Hosni Mubarak presented him with the Order of Merit, First Class. He also earned Egypt's Grand Collar of the Nile, the highest state honor, in 1999. The Egyptian government also issued two postage stamps with Zewail's portrait. He belonged to many academies and societies, including the National Academy of Sciences and the American Academy of Arts and Sciences.

Zewail and his family live in San Marino, California.