Atoms and molecules have an enormously complex ‘sociology’. Ever since their discovery, scientists have been concerned with their behavior in matter – why atoms and molecules sometimes like or attract each other and sometimes don’t. This love and hate dynamic is extremely important – it determines why substances exist in different shapes and phases, and how they transform to other substances. And, like humans, the only way to find out how they behave is to watch them in action. However, the entire time span of the journey during any transformation is a billion trillion times shorter than the human lifespan. That is why for twenty-four centuries, since its conception, the atom’s motion in real time was invisible.

For atoms and molecules, the scale of time involved is awesome – its unit is the femtosecond; the prefix femto meaning 10-15 is from femton, the Scandinavian word for ‘fifteen’. A femtosecond is a millionth of a billionth of a second, a quadrillionth of a second; it is one second divided by ten raised to the power of fifteen (10-15), or 0.000 000 000 000 001 second. Put in comparative terms, a femtosecond is to a second as a second is to 32 million years. In one second, light travels about 186,000 miles (300,000 kilometers), almost from here to the moon; in one femtosecond, light travels 300 nanometers (0.000 000 3 meter), the dimension of a human hair. With femtosecond timing, the atom’s motion becomes visible.

Flashing a molecule with a femtosecond laser pulse can be compared to the effect of a stroboscope flash or the opening of a camera shutter. Thus a pulse from a femtosecond laser, combined with an appropriate detector, can produce a well-resolved ‘image’ of a molecule as it passes through a specific configuration in a process of nuclear rearrangement. The detection step is based on spectroscopic or diffraction techniques, and the measured signal can be analyzed to give information about the positions of the molecule’s atoms. Molecular structures determined at different stages of a reaction process can be treated as the frames of a motion picture, allowing the motion of the atoms to be clearly visualized. The number of frames in a molecular movie could then be as high as 1014 per second!

Because the scales of distance and time of the motions that are the subject of femtochemistry research are almost unimaginably small and the measured signals require a sophisticated apparatus and analysis of data, we have, for educational purposes, designed a simple exhibit capable of highlighting the basic concepts of femtochemistry and stop-motion stroboscopy. The exhibit gives a student or visitor a concrete and visually interesting illustration of the use of short light pulses in the study of a rapidly moving object, in this case a molecular model.

by a chopper wheel, passes through three lenses, illuminates a spinning molecular model, and is reflected by the mirror at far right to the screen at center left. Bottom: Four views of the molecular model are. From left to right, the model is (1) stationary under room light only; (2) spinning under room light only; (3) spinning under room light and pulsed laser illumination; and (4) spinning under pulsed laser illumination only.

This work was recognized by the Nobel Prize in Chemistry in 1999, awarded to the LMS Director. The Nobel presentation speech by Professor Bengt Nordén of the Royal Swedish Academy of Sciences best sums up this work. His speech reads, in part:

We chemists want to understand molecules and their intrinsic essence, and to be able to predict what happens when molecules meet – do they attach weakly to each other or do they react passionately to form new molecules? Not least, we want to understand the complicated chemistry called life. Through a revolution in knowledge, molecules today take center stage in all fields, from biology and medicine through environmental sciences, and technology.

The heart of chemistry is the chemical reaction, meaning the breaking and formation of chemical bonds between atoms. How then do chemical reactions occur? We all know that they can proceed at different rates – compare the time it takes a nail to rust with explosion of dynamite!

Science has always strived to see smaller and smaller things and faster and faster events. Since the time of Arrhenius a number of methods have been developed to measure increasingly faster reaction rates, many of them rewarded with Nobel Prizes. However, no one had, until recently, been able to observe what actually happens to the reacting molecule as it passes through its so-called transition state, a metaphor for a kind of intermediate state of the reaction in which bonds are broken and formed. This remained a misty no-man's land.

The molecule passes the transition state as fast as the atoms in the molecule move. They move at a speed of the order of 1000 m/second – about as fast as a rifle bullet – and the time required for the atoms to move slightly within the molecule is typically tens of femtoseconds (1 fs = 10-15 seconds). Only few believed that such fast events would ever be possible to see.

This, however, is exactly what Ahmed Zewail has managed to do. Twelve years ago he published results that gave birth to the scientific field called femtochemistry. This can be described as using the fastest camera in the world to film the molecules during the reaction and to get a sharp picture of the transition state. His “camera” is a laser technique with light flashes of only a few tens of femtoseconds in duration. The reaction is initiated by a strong laser flash and is then studied by a series of subsequent flashes to follow the events. The key to his success was that the first femtosecond flash or starting shot, excited all molecules in the sample at once, causing their atoms to swing in rhythm. The first experiments demonstrated in slow motion how bonds were stretched and broken in rather simple reactions, but soon studies of more complex reactions followed. The results were often surprising, and the dance of the atoms during the reaction was found to differ from what was expected. Zewail’s use of the fast laser technique can be likened to Galileo’s use of his telescope, which he directed towards everything that lit up the vault of heaven. Zewail tried his femtosecond laser on literally everything that moved in the world of molecules. He turned his telescope towards the frontiers of science.

It is of great importance to be able in detail to understand and predict the progress of a chemical reaction. Femtochemistry has found applications in all branches of chemistry, but also in adjoining fields such as material science (future electronics?) and biology. The retinal molecule is an example—a substance that you are all making use of at this very moment, namely to see with. It has been found that light causes this molecule to twist like a hinge around a well-greased bond, which sends a nerve signal to the brain. The reaction takes only 200 fs, which explains the eye's sensitivity to light.

Femtochemistry has radically changed the way we look at chemical reactions. A hundred years of mist surrounding the transition state has cleared.

… From being restricted to describe them only in terms of a metaphor, the transition state, we can now study the actual movements of atoms in molecules. We can speak of them in time and space in the same way that we imagine them. They are no longer invisible.

At LMS, this ability to probe matter with atomic resolutions in space (ångström) and time (femtoseconds) is being applied to increasingly complex systems in the physical, chemical and biological realms – in an attempt to unravel some of Mother Nature’s closely guarded secrets.

*The above text has been adapted from text on the Nobel Prize website and from the book Voyage Through Time: Walks of Life to the Nobel Prize (by Ahmed Zewail).