Development of femtochemistry rewarded
What would a football match on TV be without "slow motion" revealing afterwards the movements of the players and the ball when a goal is scored? Chemical reactions are a similar case. The chemists' eagerness to be able to follow chemical reactions in the greatest detail has prompted increasingly advanced technology. This years laureate in Chemistry, Ahmed H. Zewail, has studied atoms and molecules in "slow motion" during a reaction and seen what actually happens when chemical bonds break and new ones are created.
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.
Femtochemistry enables us to understand why certain chemical reactions take place but not others. We can also explain why the speed and yield of reactions depend on temperature. Scientists the world over are studying processes with femtosecond spectroscopy in gases, in fluids and in solids, on surfaces and in polymers. Applications range from how catalysts function and how molecular electronic components must be designed, to the most delicate mechanisms in life processes and how the medicines of the future should be produced.
How fast are chemical reactions?
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 their 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.
What did the chemists see as the time resolution was successively improved? 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.
The contribution for which Zewail is to receive the Nobel Prize means that we have reached the end of the road: no chemical reactions take place faster than this. With femtosecond spectroscopy we can for the first time observe in 'slow motion' what happens as the reaction barrier is crossed and hence also understand the mechanistic background to Arrhenius' formula for temperature dependence and to the formulae for which van't Hoff was awarded his Nobel Prize.
Femtochemistry in practice
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!
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 ? 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 ? 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 ? apart, there is a great probability that the molecule will fall back to its ground state or decay into sodium and iodine atoms.
On the banks of the Nile, the Rosetta branch, I lived an enjoyable childhood in the City of Disuq, which is the home of the famous mosque, Sidi Ibrahim. I was born (February 26, 1946) in nearby Damanhur, the "City of Horus", only 60 km from Alexandria. In retrospect, it is remarkable that my childhood origins were flanked by two great places - Rosetta, the city where the famous Stone was discovered, and Alexandria, the home of ancient learning. The dawn of my memory begins with my days, at Disuq's preparatory school. I am the only son in a family of three sisters and two loving parents. My father was liked and respected by the city community - he was helpful, cheerful and very much enjoyed his life. He worked for the government and also had his own business. My mother, a good-natured, contented person, devoted all her life to her children and, in particular, to me. She was central to my "walks of life" with her kindness, total devotion and native intelligence. Although our immediate family is small, the Zewails are well known in Damanhur.
The family's dream was to see me receive a high degree abroad and to return to become a university professor - on the door to my study room, a sign was placed reading, "Dr. Ahmed," even though I was still far from becoming a doctor. My father did live to see that day, but a dear uncle did not. Uncle Rizk was special in my boyhood years and I learned much from him - an appreciation for critical analyses, an enjoyment of music, and of intermingling with the masses and intellectuals alike; he was respected for his wisdom, financially well-to-do, and self-educated. Culturally, my interests were focused - reading, music, some sports and playing backgammon. The great singer Um Kulthum (actually named Kawkab Elsharq - a superstar of the East) had a major influence on my appreciation of music. On the first Thursday of each month we listened to Um Kulthum's concert - "waslats" (three songs) - for more than three hours. During all of my study years in Egypt, the music of this unique figure gave me a special happiness, and her voice was often in the background while I was studying mathematics, chemistry... etc. After three decades I still have the same feeling and passion for her music. In America, the only music I have been able to appreciate on this level is classical, and some jazz. Reading was and still is my real joy.
As a boy it was clear that my inclinations were toward the physical sciences. Mathematics, mechanics, and chemistry were among the fields that gave me a special satisfaction. Social sciences were not as attractive because in those days much emphasis was placed on memorization of subjects, names and the like, and for reasons unknown (to me), my mind kept asking "how" and "why". This characteristic has persisted from the beginning of my life. In my teens, I recall feeling a thrill when I solved a difficult problem in mechanics, for instance, considering all of the tricky operational forces of a car going uphill or downhill. Even though chemistry required some memorization, I was intrigued by the "mathematics of chemistry". It provides laboratory phenomena which, as a boy, I wanted to reproduce and understand. In my bedroom I constructed a small apparatus, out of my mother's oil burner (for making Arabic coffee) and a few glass tubes, in order to see how wood is transformed into a burning gas and a liquid substance. I still remember this vividly, not only for the science, but also for the danger of burning down our house! It is not clear why I developed this attraction to science at such an early stage.
After finishing high school, I applied to universities. In Egypt, you send your application to a central Bureau (Maktab El Tansiq), and according to your grades, you are assigned a university, hopefully on your list of choice. In the sixties, Engineering, Medicine, Pharmacy, and Science were tops. I was admitted to Alexandria University and to the faculty of science. Here, luck played a crucial role because I had little to do with Maktab El Tansiq's decision, which gave me the career I still love most: science. At the time, I did not know the depth of this feeling, and, if accepted to another faculty, I probably would not have insisted on the faculty of science. But this passion for science became evident on the first day I went to the campus in Maharem Bek with my uncle - I had tears in my eyes as I felt the greatness of the university and the sacredness of its atmosphere. My grades throughout the next four years reflected this special passion. In the first year, I took four courses, mathematics, physics, chemistry, and geology, and my grades were either excellent or very good. Similarly, in the second year I scored very highly (excellent) in Chemistry and was chosen for a group of seven students (called "special chemistry"), an elite science group. I graduated with the highest honors - "Distinction with First Class Honor" - with above 90% in all areas of chemistry. With these scores, i was awarded, as a student, a stipend every month of approximately ?13, which was close to that of a university graduate who made ?17 at the time!
After graduating with the degree of Bachelor of Science, I was appointed to a University position as a demonstrator ("Moeid"), to carry on research toward a Masters and then a Ph.D. degree, and to teach undergraduates at the University of Alexandria. This was a tenured position, guaranteeing a faculty appointment at the University. In teaching, I was successful to the point that, although not yet a professor, I gave "professorial lectures" to help students after the Professor had given his lecture. Through this experience I discovered an affinity and enjoyment of explaining science and natural phenomena in the clearest and simplest way. The students (500 or more) enriched this sense with the appreciation they expressed. At the age of 21, as a Moeid, I believed that behind every universal phenomenon there must be beauty and simplicity in its description. This belief remains true today.
On the research side, I finished the requirements for a Masters in Science in about eight months. The tool was spectroscopy, and I was excited about developing an understanding of how and why the spectra of certain molecules change with solvents. This is an old subject, but to me it involved a new level of understanding that was quite modern in our department. My research advisors were three: The head of the inorganic section, Professor Tahany Salem and Professors Rafaat Issa and Samir El Ezaby, with whom I worked most closely; they suggested the research problem to me, and this research resulted in several publications. I was ready to think about my Ph.D. research (called "research point") after one year of being a Moeid. Professors El Ezaby (a graduate of Utah) and Yehia El Tantawy (a graduate of Penn) encouraged me to go abroad to complete my Ph.D. work. All the odds were against my going to America. First, I did not have the connections abroad. Second, the 1967 war had just ended and American stocks in Egypt were at their lowest value, so study missions were only sent to the USSR or Eastern European countries. I had to obtain a scholarship directly from an American University. After corresponding with a dozen universities, the University of Pennsylvania and a few others offered me scholarships, providing the tuition and paying a monthly stipend (some $300). There were still further obstacles against travel to America ("Safer to America"). It took enormous energy to pass the regulatory and bureaucratic barriers.
Arriving in the States, I had the feeling of being thrown into an ocean. The ocean was full of knowledge, culture, and opportunities, and the choice was clear: I could either learn to swim or sink. The culture was foreign, the language was difficult, but my hopes were high. I did not speak or write English fluently, and I did not know much about western culture in general, or American culture in particular. I remember a "cultural incident" that opened my eyes to the new traditions I was experiencing right after settling in Philadelphia. In Egypt, as boys, we used to kid each other by saying "I'll kill you", and good friends often said such phrases jokingly. I became friends with a sympathetic American graduate student, and, at one point, jokingly said "I'll kill you". I immediately noticed his reserve and coolness, perhaps worrying that a fellow from the Middle East might actually do it!
My presence - as the Egyptian at Penn - was starting to be felt by the professors and students as my scores were high, and I also began a successful course of research. I owe much to my research advisor, Professor Robin Hochstrasser, who was, and still is, a committed scientist and educator. The diverse research problems I worked on, and the collaborations with many able scientists, were both enjoyable and profitable. My publication list was increasing, but just as importantly, I was learning new things literally every day - in chemistry, in physics and in other fields. The atmosphere at the Laboratory for Research on the Structure of Matter (LRSM) was most stimulating and I was enthusiastic about researching in areas that crossed the disciplines of physics and chemistry (sometimes too enthusiastic!). My courses were enjoyable too; I still recall the series 501, 502, 503 and the physics courses I took with the Nobel Laureate, Bob Schrieffer. I was working almost "day and night," and doing several projects at the same time: The Stark effect of simple molecules; the Zeeman effect of solids like NO2- and benzene; the optical detection of magnetic resonance (ODMR); double resonance techniques, etc. Now, thinking about it, I cannot imagine doing all of this again, but of course then I was "young and innocent".
The research for my Ph.D. and the requirements for a degree were essentially completed by 1973, when another war erupted in the Middle East. I had strong feelings about returning to Egypt to be a University Professor, even though at the beginning of my years in America my memories of the frustrating bureaucracy encountered at the time of my departure were still vivid. With time, things change, and I recollected all the wonderful years of my childhood and the opportunities Egypt had provided to me. Returning was important to me, but I also knew that Egypt would not be able to provide the scientific atmosphere I had enjoyed in the U.S. A few more years in America would give me and my family two opportunities: First, I could think about another area of research in a different place (while learning to be professorial!). Second, my salary would be higher than that of a graduate student, and we could then buy a big American car that would be so impressive for the new Professor at Alexandria University! I applied for five positions, three in the U.S., one in Germany and one in Holland, and all of them with world-renowned professors. I received five offers and decided on Berkeley.
Early in 1974 we went to Berkeley, excited by the new opportunities. Culturally, moving from Philadelphia to Berkeley was almost as much of a shock as the transition from Alexandria to Philadelphia - Berkeley was a new world! I saw Telegraph Avenue for the first time, and this was sufficient to indicate the difference. I also met many graduate students whose language and behavior I had never seen before, neither in Alexandria, nor in Philadelphia. I interacted well with essentially everybody, and in some cases I guided some graduate students. But I also learned from members of the group. The obstacles did not seem as high as they had when I came to the University of Pennsylvania because culturally and scientifically I was better equipped. Berkeley was a great place for science - the BIG science. In the laboratory, my aim was to utilize the expertise I had gained from my Ph.D. work on the spectroscopy of pairs of molecules, called dimers, and to measure their coherence with the new tools available at Berkeley. Professor Charles Harris was traveling to Holland for an extensive stay, but when he returned to Berkeley we enjoyed discussing science at late hours! His ideas were broad and numerous, and in some cases went beyond the scientific language I was familiar with. Nevertheless, my general direction was established. I immediately saw the importance of the concept of coherence. I decided to tackle the problem, and, in a rather short time, acquired a rigorous theoretical foundation which was new to me. I believe that this transition proved vital in subsequent years of my research.
I wrote two papers with Charles, one theoretical and the other experimental. They were published in Physical Review. These papers were followed by other work, and I extended the concept of coherence to multidimensional systems, publishing my first independently authored paper while at Berkeley. In collaboration with other graduate students, I also published papers on energy transfer in solids. I enjoyed my interactions with the students and professors, and at Berkeley's popular and well-attended physical chemistry seminars. Charles decided to offer me the IBM Fellowship that was only given to a few in the department. He strongly felt that I should get a job at one of the top universities in America, or at least have the experience of going to the interviews; I am grateful for his belief in me. I only applied to a few places and thought I had no chance at these top universities. During the process, I contacted Egypt, and I also considered the American University in Beirut (AUB). Although I visited some places, nothing was finalized, and I was preparing myself for the return. Meanwhile, I was busy and excited about the new research I was doing. Charles decided to build a picosecond laser, and two of us in the group were involved in this hard and "non-profitable" direction of research (!); I learned a great deal about the principles of lasers and their physics.
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This is my BrainyGoose:
United States, IL, Chicago, English, Italian, Genry, Male, 21-25, bodybulding, swiming.