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.
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!
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:
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
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
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