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“A Liking for the Truth: Truth and Controversy in the Work of Linus Pauling.”

February 28, 2001

Video: Keynote Address: “Timing in the Invisible,” Part 1 Ahmed Zewail

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Transcript

John Westall: It gives me great pleasure this morning to introduce the lead speaker for today's symposium, Professor Ahmed Zewail, Linus Pauling Professor of Chemical Physics and Professor of Physics at the California Institute of Technology, and Nobel Laureate in Chemistry in 1999.

No one can be more appropriate to lead off a symposium on Linus Pauling than Ahmed Zewail. While Pauling pioneered the understanding of the structure of the chemical bond, and won a Nobel Prize for that, half a century later Zewail has pioneered the understanding of the dynamics of the chemical bond, and won a Nobel Prize for that. At the risk of oversimplification, let me try to convey to you in just a few words the excitement Zewail's work has caused in the last decade of the 20th century, just as I imagine Pauling's work did in the earlier part of the 20th century.

Early work on bonding and structure can be seen in the early part of the 19th century when Berzelius provided a fairly satisfactory explanation of what has become known today as ionic bonding. By the early part of the 20th century, G.N. Lewis had provided the early framework for covalent bonding. When Pauling got involved in the late ‘20s and early ‘30s, there were still large questions unanswered about molecular geometry and the relation of chemical bonding to the emerging atomic theory. Not until Pauling's groundbreaking papers on the nature of the chemical bond in 1931 did the world see how the nature of the structure of molecules could be understood from the ground up with geometry consistent with the fundamental atomic theory.

Pauling's success was based on his skill as an experimentalist in the field of x-ray crystallography, his mastery of theory, and his ability to make the appropriate approximation at the appropriate time. A similar evolution and groundbreaking event can be seen in our understanding of our dynamics of the chemical bond. Dynamics encompasses vibrational motions that make or break the chemical bond. Dynamics is at the heart of chemical reactivity, and at the heart of chemistry itself. By the end of the 19th century, Arrhenius had related chemical reaction rates to temperature. Whereas Arrhenius had considered ensembles of molecules, in the 1930s it was Eyring and Polanyi who devised a transition state theory - that is, a state midway between reactants and products, and on a time scale related to the vibrations of the molecules. This time scale is on the order of 10 to 100 femtoseconds, and that is a very, very short time. A femtosecond is to a second as a second is to about 30 million years. [3:56]

For the rest of the 20th century, the world worked with this theory of Eyring, but no one ever dreamed of actually trying to observe one of these transition states at such a very short time scale. But that is exactly what Zewail set out to do, to observe the transition state and, in an extraordinary set of experiments in the late 1980s, he succeeded. By using an ingenious combination of experimental technology, theory, and appropriate approximations, he developed what amounts to a high speed camera to image molecules at the femtoscale time scale of chemical reactions, and was actually able to view chemical bonds as they were breaking.

The impact of this work has been tremendous. Rudolph Marcus, a Nobel Prize-winner from the early 1990s, has said of Zewail: "Zewail's work has fundamentally changed the way scientists view chemical dynamics. The study of chemical, physical, and biological events that occur on the femtosecond time scale is the ultimate achievement of over a century of effort by mankind, since Arrhenius, to view dynamics of the chemical bond as it actually unfolds in real time."

Ahmed Zewail is currently Linus Pauling Professor of Chemistry and Professor of Physics at the California Institute of Technology, and Director of the National Science Foundation Laboratory for molecular science. He received his B.S. and M.S. degrees from Alexandria University in Egypt, and his Ph.D. from the University of Pennsylvania. He was appointed to the faculty of Caltech in 1976, and in 1990 was appointed Linus Pauling Chair at the Institute. Professor Zewail has numerous honorary degrees from all over the world - he is a member of several national academies of science, he has won top awards from national chemistry and physics organizations, only one of which I'll mention, the 1997 Linus Pauling Award from the Northwest sections of the American Chemical Society, which was awarded in Portland. [6:39]

Pauling's work culminated in the Nobel Prize for chemistry in 1954 with the citation for his research into the nature of the chemical bond and its application for the elucidation of the structure of complex substances. Zewail won the Nobel Prize in chemistry in 1999 with the citation for his studies on the transition states of chemical reactions using femtosecond spectroscopy. It is now my great pleasure to introduce for today's keynote lecture Professor Ahmed Zewail. [7:20]

Ahmed Zewail: It's a great pleasure to be here and to be part of this wonderful centennial. In fact, I think with what you've heard from John I should really sit down now, as you have the whole story of why I'm going to talk today. It's really very special to me, because I do have a very special relationship to Linus Pauling, not only that I sit in his chair at Caltech, but for a variety of reasons. As a matter of fact, I fly tomorrow morning from here because at Caltech we also are celebrating the centennial of Linus Pauling, and I do chair the session in the morning.

Everybody knew about Pauling, and everybody knew about Pauling's achievements, but I think, as Steve said, there's something about Pauling in terms of his personality and his way of doing things that people really don't know about. It comes out in the biographies much better, but from my daughters to people I have seen at Caltech who are in the late stage of their lives, who just get so excited when they meet Linus Pauling. He was a fair man, a very decent human being, and the word I use around our house, he was a very civilized human being. When you sit with Linus, you can talk about the world, and you can enjoy talking to him about the world, but I always find in him civility and decency.

His scientific contributions are very well known, but I would like to make a link between some of the work that Pauling has achieved in chemistry and biology. It is appropriate to tell you that I didn't know of Pauling when I arrived at Caltech, all that I knew was his name; I arrived at Caltech in 1974. But this is one point that I want to touch on, especially since we have two historians with us today who wrote about the biography of Linus; that's his relation to Caltech. When I arrived in 1974, there were rumors around the campus that Linus was not too happy about his departure from Caltech, and of the incident that took place. Things get messed up in the public press a lot and people say things that are not really true, because I dug into some of the historical minutes of the faculty and what Lee DuBridge said. But as a young assistant professor I didn't know any better so I thought it just didn't make any sense, that Pauling, who really was and still is a giant in chemistry, was not visiting Caltech. I was fortunate to be tenured very shortly at Caltech, when I became part of the establishment, so I thought we should do something about it.

And so, I had the great honor to chair and to organize…I like this photo of Linus, actually, giving a lecture at Caltech; this was February 28, 1986, and that was his 85th birthday. I thought that this was a great occasion to bring Linus to campus. He came back and was just so excited; Linus Jr. was with us, and could see his father speaking from the heart. I gave him the stage in the evening to say whatever he wanted to say about Caltech, and he did give it to us. It was just a very special occasion, and I also organized the 90th birthday for Linus at Caltech, and I think if Linus did not come back to Caltech to share his great moments, it would have been a mistake in the history of Caltech and science. I even crowned him the Pharaoh of Chemistry, and I believe that he loved this picture. I think it's here, isn't it? It cost me about $500 to do this, because I had to go to Hollywood and try to fit his face into one of Ramses II.

If you want to learn about all of this, I also edited a book on Linus called The Chemical Bond: Structure and Dynamics, and that is really the focus of my lecture today. I want to take you from the wonderful and important era, which Jack Dunitz will be talking to you about, of structures that are static, meaning there is no way of looking at the movement of the atoms in these structures, into a case where we try to understand the dynamics of how these atoms move as a bond breaks and forms. By the way, the highest honor I received was from Linus, when he wrote to me saying he considered me his friend. Linus Jr. must recognize his father's handwriting. In 1991, he sent me a book, and wrote to me "To my friend Ahmed." I treasure this. [14:39]

In his lecture at Caltech, Linus showed the structure of sodium chloride that was believed to be by Mr. [William] Barlow in 1898. This is the structure of table salt. I am showing this because there is an analogy between the beginning of Linus' work on structural chemistry, and what we were trying to do in dynamic chemistry. Looking at the structure of sodium chloride with two atoms probably seems trivial, especially during the era of the Braggs and the work that was done with x-ray crystallography. When we started it was also with table salt, sodium chloride, sodium iodide, to try and understand the dynamics of this chemical bond, but also we received a lot of criticism in the early days, that this was trivial; these are two atoms, and not much is going to be found from this. Of course, you know the work on structure by Linus has led, for example, to the structure of hemoglobin. The picture is not as pretty as you normally see, but this is the protein structure of deoxyhemoblogin. And in fact you will see at the end of my talk that we can also look at the dynamics of proteins, DNA and the like. [16:24]

It is remarkable to me that I read very recently - I didn't know that Linus wrote this - but he received the Nobel Prize in 1954 and shortly after the award he was asked to reflect on what chemistry will be in the coming 50 years. Remember, he spent all this time studying the structure, the architecture, of molecules, but he had the vision, and I think that this was the first time that anybody has seen this Scientific American article, that "the half century we are just completing has seen the evolution of chemistry from a vast but largely formless body of empirical knowledge into a coordinated science. The new ideas about electrons and atomic nuclei were speedily introduced into chemistry, leading to the formulation of a powerful structural theory which has welded most of the great mass of chemical fact into a unified system. What will the next 50 years bring?" This is Pauling speaking 50 years ago, mid-century. "We may hope that the chemists of the year 2000 will have obtained such penetrating knowledge of the forces between atoms and molecules that he will be able to predict the rate of any chemical reaction."

So even though Pauling was immersed in static structures and their study, his eye was on the next century, and maybe the next century Nobel Prize. That is precisely what I want to talk to you about today, is how we look at the forces that control the motions of atoms and molecules, whether it's in chemical or biological systems, and can we distill some concepts out of this, just like Pauling was trying to understand hybridizations and so on, that teach us something about the dynamics of chemical bonding, and hence the forces and how to predict rates and the dynamics. [19:05]

The work I will talk about relates to the Nobel Prize that we received in 1999, and incidentally, there is an incident in history here that I think I told some of the speakers at dinner last night. When we received the Nobel Prize, Caltech had a big party, as they did for Linus, and one of my colleagues, Vince McKoy, noted that Linus was born February 28, 1901, and I was born February 26, 1946, so two days away from Linus. He received the prize in 1954, we received the prize in 1999, and both in October, so we were both almost 50 years of age when we received the Nobel Prize. This is remarkable where Caltech is concerned, because both Pauling and myself started at Caltech as Assistant Professors, they did not "buy us from the outside," as they say. I thought you would be intrigued by this: when the prize was announced, it was everywhere because our work also touches on physics, and so many of the physics magazines wrote detailed articles, and here is a distinguished one from England; it surprised me, as you will see, that England would write something like this. "Ahmed Zewail receives the 1999 Nobel Prize for Chemistry…" and it goes on to say "laser-based techniques that allow the motion of atoms inside molecules to be followed during chemical reactions." It goes on, very complimentary. And then it says "Zewail was born in Egypt in 1496." I told the Nobel committee in Sweden that it is really remarkable that it took them 500 years to give me the Nobel Prize.

Here is the journey in time. I think for the people in the audience who are not used to seeing this number, you certainly hear and you know by now… Here could be 12 or 15 billion years of the Big Bang, and then you come down to our lifespan, which is about 100 years or so - your heart beats in one second. But to go from here [present day] to there [Big Bang] is about 1015, and I am going to take you from the heart into a molecule inside the heart, or eye specifically, and you have to decrease by 15 orders of magnitude to see the beats of this molecule, as you see the beats of your heart. The timescale is fast, and I wrote a review which is probably useless, but I thought it's interesting that if you go from this age of the universe, and you count back from the age of the Earth to the human lifespan to your heart (1 second), and then you go to the microscopic world (sub-second), into how molecules rotate, vibrate, and how the electrons move. In this whole microscopic world here, we reach 10-15 or so seconds, where on the opposite end you reach 1015, and the remarkable thing is the heart is the geometric average of the two. As humans, we are in a very unique position. [23:50]

It's very difficult to study history; I thought it was very easy, but it turns out it is very difficult, and it takes some time to get the facts. I showed this in the Nobel lecture, because it gives five milestones, snapshots, of an evolution over about six millennia. The first time we know of the concept of measuring time is the calendar, which was developed in 4200 BC. In fact some historians say that it is 4240 BC, so it is almost 6000 years ago, which we knew what a year is, what a day is, what a month is. What an hour is was measured using sundials in about 1500 BC. You can use a shadow from an obelisk or a sundial and know what the time of day is. The mechanical clock was developed in Europe around 1500 AD, and all of this was still in seconds or longer, so up until then we could only measure as accurately as a second.

One of the most famous experiments in the history of measurements was done by Edward Muybridge in 1887. Mr. [Leland] Stanford, the Governor of California, was very interested in horses and hired Muybridge to find out the actual nature of the motion of the horse as it gallops. Mr. Muybridge designed this camera, which had a shutter speed of about 1/1000th of a second. This was done in Palo Alto, where there is a plaque commemorating it. In so doing, as the horse galloped the shutter was opened, and Muybridge was able to take a snapshot of the horse proving the contention of Mr. Stanford, that all four legs are off the ground at the same time. I believe this was the very beginning of fast photography and motion pictures. In the 1800s, this was a very important development.

To go into the word of molecules, and to be able to see transition states, just as Muybridge saw the transition state of the horse moving, you need an increase of about 12 orders of magnitude in time resolution, and we only can do this with these lasers, as you will see in a few minutes. This was done in 1980, and it may be called femtoscopic. [27:16]

So, why is it that this is important to chemistry and biology - the dynamics of the chemical bond? It turns out, the dynamics of any chemical bond, whether it is in a chemical system or a protein or DNA, is totally controlled by the fundamental vibrational time scale, as you heard from John. This vibrational time scale, and there is some philosophy that one can dwell on here, determined by Planck's constant and so on, but fundamentally, two atoms connected by a spring will move in a spring-motion on a time scale of 10-13 seconds, and the shortest possible time for any molecule in this universe will be 10-14 seconds. All the way from hydrogen molecules to protein molecules, the time scale will be from 10-14 seconds to 10-12 seconds. Molecules can rotate in the laboratory without you seeing it in a picosecond 10-12 to 10-9, and everything longer than this we consider in the Stone Ages; it's not interesting to us. So on that time scale you will see many interesting phenomena in chemistry and biology that happen.

This is the end of time resolution for chemistry and biology, because if you look here, even molecules that are linking undergo collisions on a time scale of 10-14 seconds. A molecule can break a bond and make a bond on this time scale as well. The eye has a molecule called rhodopsin which divides and allows you to see, and that happens in 200 femtoseconds. The way we get photosynthesis to work, and the electron to transfer inside the green plant, is on the order of femtoseconds. So this is the fundamental time scale, and if we were to understand the dynamics of the chemical bond we must understand this time scale. Luckily, that is the reason behind all of this, and to be totally truthful, this is the way we were thinking about it. We were making real approximations - what I like to call the "back of the envelope" type of calculation - and we didn't go to big computers and do quantum calculations and all of that stuff, because I always feel that there is beauty in the simplicity of science, and if we are not able to distill it into the essence of science, it seems to me that we are fuzzy about it.

What I just told you is general to any molecular system; if you think of the chemical binding energies that we have, that Pauling would have calculated with his slide ruler, and then if I activate a chemical bond, I can calculate the energy in this bond. And if I have the energy I can calculate the speed at which the atoms should move in principal. For all chemical and biological systems, you'll find that this speed is about one kilometer per second. Even in the spring that I mentioned, the two atoms would collide with each other at about one kilometer per second, or 105 centimeters per second. If we're going to look on the molecular scale to understand the dynamics of the chemical bond, we have to understand that the distance scale is about an Angstrom, or 10-8. You can see, without any fancy quantum mechanics at this point, potentials and forces and all of that, that if you put these two numbers together (105 and 10-8) you come up with a time resolution of 10-13 seconds.

Therefore, our idea is simple: if we can reach a time resolution just like x-ray crystallography and electron diffraction, if we can get the de Broglie wavelengths or the special resolution less than the bond distance, you would be able to see bonds and atoms. Here, we can also say that if our time resolution is 10-14 seconds, then we can freeze this motion, we can do what Muybridge did, because now this time scale will be shorter by a factor of ten than the time it takes the atoms to move in the molecule. More scientifically, we say that the time resolution here is shorter than the vibrational period of the molecule, or the rotational period of the molecule. And that, in its totality, defines the field of femtochemistry, and now there is something called femtobiology, and so on. [33:07]

So this is a simple picture of the problem, but for the physicists in the audience the problem is much more intriguing and much more fundamental. It is remarkable that Pauling recognized, right after Schrödinger wrote his famous wave equation in 1926 in Zurich - which we shall celebrate in April - very quickly recognized that if you think of matter as particles, you can also think of them as waves. De Broglie, with brilliant intuition, used the quantization equation and Einstein equation and came up with this very simple thesis that lambda is equal to h over p, which Einstein reviewed. So for each momentum of a particle, there is an associated wave, lambda. When people study matter as light, and we made this mistake too, they are always thinking of Schrödinger's Eigen states, the wave character of the system, wave mechanics, Schrödinger wave equation. But that is not what I am interested in, because what I am interested in is to be able to see the atoms in motion, and to understand the fundamentals of these dynamics. I don't want to go to the delocalized wave picture and calculate Schrödinger equations and come at the end without a simple picture describing the particle behavior of these atoms in motion, moving from one configuration into the other.

So how can we make matter behave in a fundamental way as a particle and not as a wave? If we look at what is known as the uncertainty principle, which is seen here, that you cannot do precise measurements of the position of the particle and momentum simultaneously. Similarly, you cannot make a precise measurement of time and energy simultaneously. We understand this from Mr. Heisenberg, there is no problem. But the chemist had difficulty understanding how we can shorten the time, Δt, because this would be at the expense of the energy, ΔE, and therefore we ruined the quantum state of the molecule, the nature of this molecule.

What was not recognized, was that this is in fact a beautiful way of locating atoms in space, because in a crude way if you think of how ΔtΔE=hbar and ΔxΔp=hbar, don't think of this alone, just think of the two together, and say that the momentum and energy are related (which they are). If you combine the two, you'll find that if you shorten the time, Δt, you can make Δx very small. That is the only way in this universe that we know of to localize atoms in space. We are not in violation of the uncertainty principle here, but if you do it cleverly, combining time and energy, you can show clearly that you will localize this nuclei to better than a tenth of an Angstrom. The chemical bond distance is about an Angstrom, so we have a factor of ten in space and should be able to see the motion of these atoms as the bond breaks and forms.

It has a long history in physics, and in fact it goes back to the 1800s - there is an experiment in 1801 by Mr. [Thomas] Young on light, not on matter. If you take two slits, with light behind them, then you will see these fringes, and it is very similar to what we can now do with matter and molecules. We can take a femtosecond pulse, which is acting like the light, and we can take all these wave functions of the molecules - this, by the way, is two atoms and a spring moving - and we can take these waves and try to do this interference experiment on this matter, and all of a sudden you see what's called a "wave packet," meaning that we localize all of these vibration states in one place, which becomes a classical picture. We can speak of a spring in this point in space, and that it is coming back, and we can see this motion.

Early in the field, it was thought that this energy is too broad and ugly, and there are occasionally objections over the uncertainty principle. Actually, without the uncertainty principle, it would be impossible to see the nuclei moving on this time scale.

This, by the way, was also of concern to the big minds in 1926. This letter was from [Ernest] Lawrence to Schrödinger, pointing out that this idea of connecting the classical mechanics of Newtons, and understanding the particle instead of the wave behavior, "you will be unable to construct wave packets." This was 1926, and in 1980 not only can you do it on chemical systems and create this wave packet, but the field has expanded so much that you can also do it on liquids, solids, gas phase, and clusters - this has been observed by many people around the globe. [40:45]

The way we do this experimentally is tricky, because there are no cameras like Muybridge's that will allow you to open and close mechanically in a femtosecond; it's just impossible. The way we do this is to use an idea of creating pulses of light in femtoseconds and then delay, as a series of pulses - so this red here is delayed from the yellow - and the first one, the yellow, will set off the clock and give us a time zero for the change. With a whole series of pulses we can "photograph" what is happening here as a function of time. We do it this way, using the speed of light, because it turns out that when we delay this pulse in relation to this one by one micron in the laboratory, it's equivalent to 3.3 femtoseconds because of the speed of light: 100,000 km per second. [41:54]

 

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