London

Zewail: honoured in his native Egypt. Credit: EGYPTIAN POST OFFICE

The Nobel Prize for Chemistry has been awarded to Ahmed Zewail of the California Institute of Technology for his role in developing techniques of ultrafast spectroscopy for studying molecular change in real time.

The fundamental time-scale of chemical reactions is set by the speed at which the reactant molecules pass through the transition state. This involves atomic motions which typically occupy just a few tens of femtoseconds (10−15 s).

Short pulses of laser light are used to capture this change. A series of femtosecond pulses provides a strobed sequence of the molecules' transformation. Each pulse records a slice through the absorption spectrum, modified by the change in the molecule.

This process requires, however, that one knows a ‘zero’ time from when the change began. This involves synchronizing two pulses with femtosecond precision. The initial ‘pump’ pulse provides the stimulus for a photochemical process — for example, boosting a collection of molecules into an excited state. Subsequent pulses then interrogate the molecules as they pass through the transition state to the products.

Zewail has pioneered the implementation of this scheme. His early experiments focused on the simplest case: unimolecular photodissociation, whereby a molecule absorbs a photon and fragments into two. His later work encompasses more complex reactions, such as the dissociation of C2I2F4 into three fragments, bimolecular processes such as the reaction of carbon dioxide with a hydrogen radical, and the influence of solvent cages on reaction dynamics in solution.

John Simons of the University of Oxford says that Zewail has been “marvellously effective in demonstrating the potential of ultrafast laser techniques — of showing quantum behaviour happening before your eyes”. These methods are now being applied beyond elementary test cases, for example to elucidate some of the rapid electron transfer processes in photosynthesis.

It seems inevitable that ultrafast change in biological systems will receive increasing attention. Another goal is the use of pulsed laser techniques for ultrafast diffraction, so that precise changes in atomic coordinates can be tracked during chemical change.

Zewail has been a favourite for the award for several years.