Ahmed Zewail

Who he was

Ahmed Zewail grew up in Alexandria, Egypt, and studied chemistry at Alexandria University. He moved to the United States for his PhD and joined Caltech in 1976. He remained proud of his roots throughout his life and worked to build science education in Egypt and across the Arab world.

Key contributions

Catching the transition state. Every lesson on reaction rates draws an energy profile with a peak, the transition state, where old bonds are half broken and new ones half formed. For a century this was a theoretical idea: it lasts far too short a time to see. Bonds vibrate on a timescale of about 10 to 100 femtoseconds, where

$$1\ \text{fs} = 10^{-15}\ \text{s}$$

A femtosecond is to one second roughly what one second is to 32 million years.

Femtochemistry. Zewail’s group used a pump–probe method. A first, ultrashort laser pulse (the pump) starts the reaction; a second pulse (the probe), arriving a precisely controlled number of femtoseconds later, records what the molecules look like at that instant. Repeating with different delays builds up a “film” of the reaction. In 1987 they watched the molecule ICN fly apart into I and CN in a few hundred femtoseconds.

Later he developed 4D ultrafast electron microscopy, which uses pulses of electrons instead of light to image structures changing in space and time.

Prizes and legacy

He received the 1999 Nobel Prize in Chemistry alone, “for his studies of the transition states of chemical reactions using femtosecond spectroscopy”. He was the first Egyptian to win a Nobel Prize in a science. Femtosecond lasers are now used in chemistry, biology, materials science and eye surgery.

Connections to your lessons

His work brings to life the lessons on activation energy and reaction rates (transition states and energy profiles), and his tools build on the light absorption you meet in the Beer–Lambert law.