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17 Jun 2002
This year's Nobel Prize in chemistry has been awarded to Ahmed Zewail of California Institute of Technology, US, for probing chemical reactions with femtosecond (10-15 second) laser pulses. The studies have helped understand how catalysts and biological processes function and how molecular electronics should be designed.
In the late 1980s, Zewail began imaging molecules, as they react, by bombarding them with laser pulses at intervals of tens of femtoseconds. The technique became known as femtosecond spectroscopy, or femtochemistry. Chemical reactions occur on a timescale of typically 10-100 femtoseconds so the imaging technique allows chemists to watch bonds within molecules break and reform.
The technique now allows chemists to observe the unstable, short-lived species formed as intermediates during reactions.
In femtosecond spectroscopy the original substances are mixed as beams of molecules in a vacuum chamber. An ultrafast laser then injects two pulses, a 'pump' pulse and then a 'probe' pulse. The first, high-energy pulse starts the reactions and the second, weaker pulse shows what is happening. Altering the time interval between the two pulses allows chemists to see how the molecular structure changes during the reaction. Each step in the reaction gives a characteristic spectrum that serves as a fingerprint. Comparing this with theoretical calculations gives the structure of the intermediate products.
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