A femtosecond is one millionth of one billionth of a second (Femto is a danish word that means "15"), or about the time it takes light to travel 1/100th of the width of a human hair. Using pulsed lasers,Dr.Zewail and his research group have devised techniques for catching atoms in the act of coming together and reacting to form molecules, and for catching molecules in the act of falling apart to form new molecules. These "molecular births" occur in times of less than a millionth of a millionth of a second. And before Dr.Zewail's work, scientists had been unable to study such ultrafast events directly in real time.And,the field has expanded over the last decade into many areas of chemistry, physics, and biology.
At the end of the 1980s, Zewail performed a series of experiments using a high-speed camera to image molecules in the actual course of chemical reactions and trying to capture pictures of them just in the transition state. The camera was based on new laser technology with light flashes of some tens of femtoseconds. The time it takes for the atoms in a molecule to perform one vibration is typically 10-100 fs. That chemical reactions should take place on the same time scale,as when atoms oscillate in the molecules,they may be compared to two trapeze artists "reacting" with each other on the same time scale as that on which their trapezes swing back and forth.
The first success was the discovery of substances formed along the way from the original one to the final product, substances termed intermediates. To begin with these were relatively stable molecules or molecule fragments. Each improvement of the time resolution led to new links in a reaction chain, in the form of increasingly short-lived intermediates, being fitted into the puzzle of understanding how the reaction mechanism worked.
Femtochemistry in femtosecond spectroscopy, the original substances are mixed as beams of molecules in a vacuum chamber. An ultrafast laser then injects two pulses: first a powerful pump pulse that strikes the molecule and excites it to a higher energy state, and then a weaker probe pulse at a wavelength chosen to detect the original molecule or an altered form of this. The pump pulse is the starting signal for the reaction while the probe pulse examines what is happening. By varying the time interval between the two pulses it is possible to see how quickly the original molecule is transformed. The new shapes the molecule takes when it is excited - perhaps going through one or more transition states - have spectra that may serve as fingerprints. The time interval between the pulses can be varied simply by causing the probe pulse to make a detour via mirrors. Not a long detour: the light covers the distance of 0.03 mm in 100 fs!