An astonishing feat by an Egyptian scientist...
Molecules, like everything in
nature, seek the lowest possible energy state. The cyclobutane
molecule, above, top left, comprises four bonded carbon atoms. (Each
carries two hydrogen atoms.) Zewail’s Nobel Prize-winning
breakthrough showed that when excited by a laser pulse for a few
femtoseconds, the molecule goes through a two-stage transition,
center, in which two of the four carbon atoms break apart, while the
other two remain bonded. From this transition state, which lasts a
few hundred femtoseconds, the reaction’s second step goes one of
two ways: Either the second carbon bond breaks, producing two
ethylene molecules, right, or the first carbon bond re-forms, which
drops the molecule to its lowest energy state, lower left. By showing
the properties of the transition state—by opening chemical
transition states to observation and thus prediction and perhaps
manipulation—Zewail created “femtochemistry.”
Ahmed Zewail is director of the
Center for Ultrafast Science & Technology at Caltech and one of
three US Science Envoys to the Middle East. His precision array of
lasers, mirrors, prisms, molecular beams and detection equipment in
the lab known as “Femtoland” constitutes the world’s fastest
imaging technology—the world’s fastest “camera.”
Warm,
late-afternoon sunlight streams through the window of a small room in
the town of Desuq, in Egypt. The year is 1960, and a teenager
studiously attacks a long list of math and chemistry problems. While
he works, the voice of renowned Egyptian virtuoso Umm Kulthum filters
from his family’s radio. When one song ends, he turns the dial,
searching station after station to catch another of her hour-long
songs of longing, loss and love. He’s good at cracking the math
problems, and—rather than distracting him—Umm Kulthum’s music
makes long hours of study a joy and a pleasure.
The boy’s parents have high hopes
for his future. They’ve hung a sign on his door that says “Dr.
Ahmed.” And Ahmed Hassan Zewail will grow up to more than fulfill
his family’s hopes and expectations: In 1999, as a professor at the
California Institute of Technology (Caltech), he will be awarded the
Nobel Prize for Chemistry, becoming the first Egyptian—and the
first Arab—to win a Nobel in a scientific field. And in 2009, his
career will come full circle when he is named one of the first three
“scientist-diplomats” in the United States’ new Science Envoy
program, aimed at forging scientific and technological partnerships
in the Muslim world to help meet global challenges in health, energy,
the environment and water and resource management.
The love of science and math and
the love of Egyptian culture are interwoven throughout Zewail’s
life history. His 1946 birthplace was Damanhur, which, as he enjoys
pointing out, lies between Alexandria and Rosetta, two cities noted
for their importance to the world’s intellectual heritage. The
famous Rosetta Stone, which Jean-François Champollion used to decode
ancient Egyptian hieroglyphic and demotic texts, was unearthed in
Rosetta in 1799. And the renowned Library of Alexandria was the
greatest library of ancient times, containing hundreds of thousands
of papyrus scrolls. For Zewail these cities were more than aspects of
local history; they were an inspiration to scholarship.
His growing-up in Desuq, a small
town on the east bank of the Rosetta branch of the Nile, was a
magical time for Zewail. He writes lovingly of his mother, Rawhia
Dar, and father, Hassan Ahmed Zewail. His parents strongly supported
his study of science and only lightly chastised him when he missed a
point or two on a test at school. But in many ways, he notes in his
autobiography, “we had a much bigger family—the people of Desuq.
Families knew each other well, shared happy and difficult times, and
valued interdependence….” At dawn, Zewail and the other children
of Desuq rose and went to study at the nearby Sidi Ibrahim al-Desuqi
mosque—a place of learning as well as prayer.
Fascinated by how science and
technology worked, Zewail, then a student, once used an Arab
coffee-roaster to heat wood chips in a test tube in an attempt to
produce wood gas. To test whether his experiment was succeeding, he
applied a lit match to the output of his apparatus and nearly set his
bedroom aflame. On another occasion, the young Zewail decided to take
his uncle’s car out for a spin. He’d never had a driving lesson,
but he’d learned how a car operated—in theory. His drive along
the banks of the Nile barely avoided a potentially fatal plunge into
the river. Decades later, in 1999, when he was awarded Egypt’s
highest state honor, he remembered his days growing up along the
Nile. Egypt has never been far from his thoughts.
Zewail studied chemistry at the
University of Alexandria and upon graduation in 1967 was appointed as
a moeid, or demonstrator, at the university, teaching
undergraduates while conducting his own graduate studies. One of the
biggest decisions he made at this time was to leave Egypt in 1969 and
go to America to pursue a Ph.D. at the University of Pennsylvania.
America presented Zewail with
cultural and language challenges, but did not dim his vibrant
optimism. Earning his Ph.D. in chemistry in 1974, he moved west to
the University of California at Berkeley and then, in 1976, joined
the faculty at Caltech, arriving with a raft of published scholarly
papers in chemistry and physics to his credit.
At Caltech, Zewail proposed
investigating what physicists and chemists call coherence; he
contemplated experiments to study this phenomenon in single molecules
as well as among billions of them. The difference between coherent
and incoherent molecular vibration is akin to the difference between
a marching band and a random crowd of people walking down the
sidewalk: The feet of people in a crowd move in a random, incoherent
manner; a marching band displays coherence when each musician moves
his or her feet up and down in unison, in step with the music. If you
know someone is in the marching band, then you can tell how that
person’s feet are moving by watching the whole band—you don’t
have to pick out the individual. Similarly, on the atomic level, you
can study the vibrations of a molecule if it is part of a large group
of coherently vibrating molecules. And if you know how molecules
vibrate, you can begin to predict how they will react with each
other—which is the essence of the science of chemistry.
Zewail’s “camera” does not
use film: It captures a spectroscopic image that indicates molecular
masses at the moment of “exposure”—the “observation pulse.”
Varying the time lag between the start pulse and the observation
pulse is accomplished by moving mirrors by a few microns (thousandths
of a millimeter) to create the time-sequence imagery that shows the
stages of chemical reactions over femtosecond spans of time.
A
femtosecond is an excruciatingly short period of time. One
femtosecond is one millionth of one billionth of a second. That
fraction is written 10-15
in scientific notation, and the word comes from the Swedish word
femton, “fifteen.” One femtosecond is as much smaller
than one second as the thickness of a human hair is smaller than the
distance to the Moon. If you moved at the speed of light for one
femtosecond, you’d only travel 30 micrometers, or about 0.0012
inches: Femtoseconds are so short that light couldn’t get even a
third of the way across a human hair in one femtosecond.
The
fastest chemical reactions—such as the twisting of a molecule of
retinal in your eye that allows you to perceive light—take about
200 femtoseconds to occur. To observe molecular transformations at
high definition requires two laser pulses about 10 femtoseconds
apart—and adjusting that time gap by making tiny changes in the
positions of the mirrors shown in the diagram above allows scientists
to visualize every stage of the transformation.
Some of his chemistry colleagues
argued that Zewail’s experiments with coherence would never
succeed. But he pushed ahead to do experiments that others thought
theoretically impossible, perhaps because his youthful experience
crashing his uncle’s car had taught him the difference between
theory and practice. Zewail’s belief in coherence was justified in
1980 when he and his fellow researchers demonstrated coherent
vibrations in isolated molecules of the hydrocarbon anthracene. This
demonstrated the reality of coherence within molecules and put
chemists on the road to using coherence to predict chemical behavior;
it was his first major breakthrough.
Zewail wanted to see further.
Although he’d never used a laser before coming to America, Zewail
recognized that if you had a laser that produced very short pulses of
light, you could use it to watch chemical reactions actually
happening. To work, though, the pulses had to be extremely
short—only a few femtoseconds in duration, a billion times shorter
than had been achieved until then. At Bell Labs, Erich Ippen and
Charles Shank developed the first femtosecond laser and Zewail
integrated it into his apparatus.
Chemical
bonds are the glue that holds atoms together—whether the atoms make
up a crystal, a leaf or your hand. If we could see bonds forming or
bonds breaking, we’d have a much better understanding of how
chemical reactions occur. Chemists know, for example, that when they
burn charcoal, oxygen atoms bond with carbon atoms to form carbon
dioxide (CO2).
They know what the starting materials are and what the end product
is, but actually watching the reaction occur was impossible—before
Zewail.
To
study chemical bonds, you need to be fast. Bonds break and form so
very quickly that you need an extremely fast “camera” to take
stop-action pictures of two atoms approaching each other and forming
a chemical bond. This is what Zewail figured out how to do: use a
laser “camera” fast enough to take stop-action “pictures” of
atoms as they formed or broke bonds. Zewail needed to set up a very
precise molecular beam: a stream of molecules shooting out of a small
nozzle at supersonic speeds. Different stages of chemical reactions
that occur within the molecular beam will be found at different
distances from the nozzle as the molecules jet away. The high speed
of the molecular beam serves to spread the reaction out in space.
Reactions in progress can then be “photographed” using a
femtosecond laser aimed across the path of the beam, as in the
diagram at the top.
To do
this, Zewail needed a laser capable of emitting ultra-short
femtosecond pulses of laser light. Zewail’s research group at
Caltech was among the first to acquire such a laser, and he and
colleague Dick Bernstein coined the word “femtochemistry” to
describe the field of chemistry that studies the making and breaking
of chemical bonds. Scientists at Caltech and elsewhere have been
using femtochemical techniques to study biological systems, such as
how oxygen binds to hemoglobin in the blood or how retinal in the eye
changes its shape to signal a protein called opsin to trigger
excitation of the optical nerve and thus vision. Zewail predicts that
femtochemical techniques may eventually allow us to do
“laser-selective chemistry,” in which we use laser beams to
manipulate bonds to create entirely new molecules.
The first chemical reaction that
Zewail and his colleagues studied with the femtosecond laser
apparatus was the breaking of a chemical bond. In late 1986, they
aimed their femtosecond laser at the simple molecule iodine cyanide
(ICN) and watched as the bond between the iodine atom and the carbon
atom of the cyanide group stretched and then snapped. The bond broke
“little by little, the first time such a thing had ever been
witnessed in real time,” Zewail wrote. This was his second major
scientific breakthrough.
The impact of Zewail’s research
was acknowledged when he was awarded a solo Nobel Prize. The Swedish
Academy of Sciences noted that Zewail “brought about a revolution
in chemistry” by enabling us to “see the movements of individual
atoms.” The Nobel Prize committee recognized that Zewail’s work
allows us to “understand and predict important [chemical]
reactions.” Today, Zewail is the Linus Pauling Professor of
Chemistry, professor of physics and director of the National Science
Foundation Laboratory for Molecular Sciences at Caltech. He is widely
respected as the founder of the field of femtochemistry and continues
to build on the work that won him the Nobel Prize. Currently, he and
his colleagues are developing techniques for four-dimensional (that
is, the familiar three dimensions of space plus time), ultra-fast
electron microscopy and diffraction. Among many other applications,
these techniques will be used to study the folding of proteins and
their misfolding, which appears to be involved in a number of
diseases such as Alzheimer’s and in some forms of obesity.
Seeing beyond the walls of his
laboratory, Zewail frequently gives public lectures stressing the
importance of fundamental scientific research. Although declining
numbers of students choose to major in the sciences, he is optimistic
that this decades-old trend can be reversed. With one foot in America
and one in Egypt, he is perhaps uniquely situated to understand the
difficulties faced by scientists in both countries. “I am an
optimist,” he says, and at every opportunity he speaks about the
importance of the promotion of science and technology in developing
countries. “Young scientists,” he says, “shouldn’t have to
leave Egypt to dream big and to engage in frontier science and
technological advancements.” In Egypt, he has promoted the building
of a new university of science and technology on the outskirts of
Cairo. This new institution, he hopes, will grow to become Egypt’s
own Caltech.
One of the remarkable aspects of
Zewail’s autobiography, Voyage Through Time, is that it is
replete with the names of hundreds of friends and acquaintances,
ranging from Umm Ibrahim, the street vendor who sold him falafel
sandwiches during his school days, to President Hosni Mubarak of
Egypt. Over the years, he has worked with more than 300 scientists,
graduate students, postdocs and other researchers—about 10 percent
of them fellow Arabs—from around the globe. That collaboration will
only grow with Zewail’s appointment as a Science Envoy, announced
by Secretary of State Hillary Clinton in Morocco November 3, which
followed his appointment earlier in 2009 to President Obama’s
Council of Advisors on Science and Technology. The Caltech professor
called the assignment as a scientist-diplomat “a great honor,”
and added that, after years of researching the dynamics of chemical
bonds, “I look forward to helping forge new bonds among nations.”
Ahmed Zewail now lives and works
half a world away from his Egyptian birthplace. He is both Egyptian
and American. But the heat of the Egyptian sun still warms his
personality and his heart is still brightened by the music of Umm
Kulthum. In his office at Caltech, he still listens to CD recordings
of her voice as he works on a new set of problems in science—and in
the world.
Source:
Saudi Aramco World,
January/February 2010, pages 12-15.
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