Chemical reactions can, as we all know, take place at very varying velocities - compare a rusting nail and exploding dynamite! Common to most reactions is that th2eir velocity increases as  temperature rises, i.e. when molecular motion becomes more violent. 

For this reason researchers long believed that a molecule first needs to be activated, 'kicked' over a barrier, if it is to react. When two molecules collide, nothing normally happens, they just bounce  apart. But when the temperature is high enough the collision is so violent that they react with one  another and new molecules form. Once a molecule has been given a sufficiently strong  'temperature kick' it reacts incredibly fast, whereupon chemical bonds break and new ones form.  This also applies to the reactions that appear to be slow (e.g. the rusting nail). The difference is 
only that the 'temperature kicks' occur more seldom in a slow reaction than in a fast one. 

The barrier is determined by the forces that hold atoms together in the molecule (the chemical bonds) roughly like the gravitational barrier that a moon rocket from Earth must surmount before it  is captured by the Moon's force field. But until very recently little was known about the molecule's  path up over the barrier and what the molecule really looks like when it is exactly at the top, its  'transition state'. 

Hundred years of research  Svante Arrhenius (Nobel laureate in Chemistry 1903), inspired by van't Hoff (the first Nobel  laureate in Chemistry, 1901) presented just over a hundred years ago a simple formula for  reaction speed as a function of temperature. But this referred to many molecules at once  (macroscopic systems) and relatively long times. It was not until the 1930s that H. Eyring and M. Polanyi formulated a theory based on reactions in microscopic systems of individual molecules. 

The theoretical assumption was that the transition state was crossed very rapidly, on the time scale  that applies to molecular vibrations. That it would ever be possible to perform experiments over such short times was something no-one dreamed of. 

But this is exactly what Zewail set out to do. At the end of the 1980s he performed a series of  experiments that were to lead to the birth of the research area called femtochemistry. This  involves 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 the atoms oscillate in the molecules 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. 

To better understand what happens, the fingerprint and the time elapsing are then compared with  theoretical simulations based on results of quantum chemical calculations (Nobel Prize in  Chemistry 1998) of spectra and energies for the molecules in their various states. 

The first experiments In his first experiments Zewail studied the disintegration of iodocyanide:   ICN -->I + CN. His team were able to observe a transition state exactly when the I-C bond was  about to break: the whole reaction takes place in 200 femtoseconds. 

In another important experiment Zewail studied the dissociation of sodium iodide (NaI): NaI -->  Na + I. The pump pulse excites the ion pair Na+ I - which has an equilibrium distance of 2.8 Å  between nuclei (Fig. 1) to an activated form [NaI]* which then assumes covalent bonding. 
However, its properties change when the molecules vibrate; when the nuclei are at their outer turning points, 10-15 Å apart, the electron structure is ionic, while at short distances it is covalent. 

At a certain point on the vibration cycle, just when the nuclei are 6.9 Å apart, there is a great  probability that the molecule will fall back to its ground state or decay into sodium and iodine atoms. 

Zewail also studied the reaction between hydrogen and carbon dioxide: 
H + CO2 --> CO + OH a reaction that takes place in the atmosphere and in combustion. He  showed that the reaction crosses a relatively long state of HOCO (1 000 fs). 

A question that has occupied many chemists is why certain chemical bonds are more reactive than  others and what happens if there are two equivalent bonds in one molecule: will they break  simultaneously or one at a time? To answer this kind of question Zewail and his co-workers  studied the disassociation of tetrafluordiiodethane (C2I2F4) into tetrafluorethylene (C2F4) and two  iodine atoms (I): 
 

They discovered that the two C-I bonds, despite their equivalence in the original molecule, break one at a time. 

Research is extra interesting when the results are unexpected. Zewail studied what may be thought  the simple reaction between benzene, a ring of six carbon atoms (C6H6) and iodine (I2), a  molecule consisting of two iodine atoms. When the two molecules become sufficiently close together they form a complex. The laser flash causes an electron to be shot from the benzene molecule into the iodine molecule. This then becomes negatively charged while the benzene molecule becomes positively charged. The negative and positive charges cause the benzene and 
the nearest iodine atom to be rapidly drawn to one another. The bond between the two iodine  atoms is stretched when one of them is sucked in towards the benzene, whereupon the other atom  breaks free and flies way. All this happens within 750 fs. Zewail found, however, that this is not the only way individual iodine atoms can be formed: sometimes the electron falls back onto benzene. But it is already too late for the iodine atoms: like a stretched rubber band breaking, the bond between the two atoms breaks and they fly apart. 

Zewail and his co-workers showed with femtosecond spectroscopy that the intermediate product was in fact formed, and had a lifetime of 700 fs. 
 
 

How does the reaction from the cyclobutane molecule to two ethylene molecules actually proceed? The  left-hand figure shows how the state energy varies if both bonds are stretched and broken simultaneously. 

The right-hand figure shows the case where one bond at a time breaks.  Another type of reaction studied with femtosecond technology is the light-induced conversion of a  molecule from one structure to another, photoisomerisation. The conversion of the stilbene  molecule, which includes two benzene rings, between the cis- and trans- forms was observed by  Zewail and his co-workers. 

In his lecture, Professor Zewail discussed his pioneering work in chemical dynamics--observation of molecules as they form or break chemical bonds. In 1972 railroad magnate Leland Stanford employed English photographer Eadweard Muybridge and 
gave him the task of showing that a galloping horse at one point lifts all four hooves off the ground. Using a series of cameras, each with a shutter that opened for only two-thousandths of a second, Muybridge caught the horse on film in a state of flight 
and proved Stanford's contention. Dr. Zewail describes his work of catching molecules in a state of flight as being conceptually similar to Muybridge's photography. Forming and breaking of chemical bonds, however, occur on a time scale of a millionth of a billionth of a second (femtosecond) and the birth or breakage of molecules can only be observed by a technology that operates on a similar time frame. Between 1985 and 1987, using advanced laser systems, Dr. Zewail and his associates developed an exquisite set of experimental techniques and followed molecular events as they actually unfolded in femtoseconds. 

Zewail's technique uses what may be described as the world's fastest camera. This uses laser flashes of such short duration that we are down to the time scale on which the reactions actually happen - femtoseconds (fs). One femtosecond is 10-15 seconds, that is, 0.000000000000001 seconds, which is to a second as a second is to 32 million years. This area of physical chemistry has been named femtochemistry. 

 In one experiment, femtochemists showed that an energized molecule of sodium iodide oscillates with a period of 1,250 femtoseconds, and in the course of each oscillation it has a 10% chance of breaking into sodium and iodine atoms. 

“Who knows? One day the sub-Femto may be discovered as a sub-unit of the Femto. Many secrets of electrons will then be revealed,"Dr. Zewail said. Using the Femto's two-photon absorption technique, it is now possible to observe the nerve cells of the brain. Dr. Zewail expects major discoveries about the way we think and about the brain functions to be made in the near future.