Yuan Lee

Dynamics of Chemical Reactions and Photochemical Processes (Lecture + Discussion)

Category: Lectures

Date: 30 June 2010

Duration: 44 min

Quality: SD

Subtitles: EN

Yuan Lee (2010) - Dynamics of Chemical Reactions and Photochemical Processes (Lecture + Discussion)

Every macroscopic chemical transformation, whether it is atmospheric ozone depletion or the burning of a candle, consists of millions of microscopic chemical events which involve collisions between molecules

Thank you very much for the introduction. It is a great pleasure to be here this afternoon. I will introduce to you the molecular beams method to investigate dynamical chemical reaction and for the chemical processes. Then I will spend some time to discuss the ozone hole problem that has been questioned in recent years. And we might not understand ozone hole problem very well. Well, I was born on the island of Taiwan in 1936 and when I started my elementary school the Second World War was coming to an end. During that time Taiwan was occupied by Japan, dominated by Japan. And so B29 is bombing Taiwan every day toward the end of the war. So school was dismissed. The first 2 years of my life I didn’t attend school. A wonderful time, I learned everything from nature, I didn’t have to attend the classes. And there’s a picture. This is not showing how I lived when I was 7 years old. But this was the picture painted by an artist about 250 years ago, showing that at that time mankind depended for everything on biomass provided by sunshine. So you look at here power, food, everything is provided by sunshine. That was the kind of life I went through when I was 7 years old. Then, once upon a time, certainly mankind used to be part of nature. But after the Industrial Revolution during the last 200 years things changed drastically. As you can see in this picture, almost everything here was dug up from the earth and transformed. Like steel, cement transformed by using fossil fuel. And that transformation is fast. In the United States, in 1850 90% of energy still came from wood burning but by 1930, after only 80 years, 90% of the energy came from fossil fuel. And actually the burning of the fossil fuel became the energy source for the development of human society. As you know if you burn coal, carbon to hydrogen ratio is 2 to 1. And natural gas with methane is 1 to 4, petroleum is about 1 to 2. When you burn it, more stable molecules, carbon dioxide and water, form and release lots of energy, 1.74 kilocalorie per mole. And we all know molecular collision induce the chemical change. So molecules have to collide in order to produce the product. The collision between molecules will not induce the chemical change unless you initiate it. So when you mix methane and oxygen, you have to initiate the chemical reaction. Initiation often involves the breaking of one of the chemical bonds and form residue species. Once the residue species are formed, then you can use hydrogen or add oxygen or dimerise it and the process goes on. Add oxygen, remove hydrogen and eventually it becomes carbon dioxide and water molecule. The processes are very complicated. And as a chemical kineticists we are quite interested in understanding mechanism, how chemical reaction takes place. Unless you understand how chemical reaction takes place and how fast every process is, you will not be able to optimise the combustion process or eliminate the pollutions like nitric oxide. We heard a lecture from Murad about an hour ago. Of course in the combustion process you add hydrogen and burn carefully, you can produce dimer in the combustion process. My investigation of chemical reaction is, I really want to visualise, visualise how chemical reaction takes place. Although a naked eye cannot see but I really wanted to design an apparatus so the consequences of my observation will be the same as seeing how chemical reaction takes place. So I want to go through quickly. What do I mean by seeing it or visualising it? So instead of taking a complicated combustion process, I’ll take a very simple chemical reaction to go through what I am talking about. The reaction between fluorine molecule and deuterium molecule which will produce deuterium fluoride. But as I said reaction has to be initiated. Fluorine molecule dissociating into two fluorine atoms and then a fluorine atom reacts with a deuterium molecule and form deuterium fluoride. Bond energy is only 106 kilocalorie. For deuterium fluoride it’s 138 because a stronger chemical bond is formed. So it releases energies. Then deuterium atom would react with fluorine molecule and deuterium fluoride and go through a chain reaction very rapidly due to a transform in deuterium fluoride. And what kineticists have been doing is to understand the mechanism in a specific reconnaissance so you know time dependent behaviour of the entire system. Now I want to move one step further and ask the question like this. If you have an elementary process involving fluorine atom and deuterium molecule, release 31.8 kilocalorie of energy, how the reaction takes place? Where the energy goes? For example, if you can see atoms and molecules, then you see the motional atom and molecule. There’s a collision process taking place and then you form deuterium fluoride and deuterium atoms. And that’s what we like to see, the process of how reaction transforms into a product and where the energy goes. So here you can see that the energy could go into vibrational degrees of freedom. Most of the energy could go to vibrational or some of the energy go to translational or rotational or sometime all the energy become translational and molecules go out very slowly. That’s what I want to see. Of course, when I said I want to visualise it and you say: “No, it’s impossible to see what's going on there.” But in spite of that you cannot see atoms in the molecules. In the laboratory you can control the velocity of atoms and the molecules and in the vacuum system that it collides. So you can follow the trajectories. And then you measure the scattering angle and scattering velocity. So if you can measure those, like how fluorine atoms come and form the deuterium fluoride. What is the angle, what are the velocity distributions? After observing all of those, then there’s a chance, you can visualise how the chemical reaction takes place. That’s the idea between molecule scattering and understanding what's going on. So we cannot see the collisions but we can control the beam of atoms and molecules in the vacuum system and measure angular and velocity distributions. The concept is rather easy but put into practice a little more complicated. Fluorine molecules could be similarly dissociated at low pressure. And you have a hole in the vacuum system. Fluorine molecules appear from the pinhole and with a Maxwell-Boltzmann velocity distribution an angular distribution may be cosine. You can use the velocity selector. There’s a slotted disc. Disc is slotted and with a slotted... The line is banked with a certain angle. So when this wheel spins very, very fast, only those atoms with the given velocity can go through. So this velocity selector will select a certain velocity. Atoms with different velocities will be thrown away. And with this hole we also define the directions. So I can use the velocity selector to get the beam of fluorine atom with known velocity. The deuterium molecule is easier because you can expand the molecule from high pressure through isotropic expansion, temperature goes down to almost zero. All the molecules will be moving at the same speed. So now we have the beam of known velocity colliding. So the only thing I have to do is to measure the angular velocity distribution. I say the only thing I have to do but this is not as easy. Before we carried out this experiment, nobody has been able to do that, nobody has been able to see the scattering process, measuring angular and velocity distribution. The difficulty is the following: You have a stream of atoms and molecules coming in here and the collision process will produce the product. The beam intensity is strong enough so you can produce about 10^10 product per second. But those molecules will be scattering at different angles at different velocities. And if you set a detector at a given angle only about 1 million molecules go through here. And you use selection bombardment to ionise it. Efficiency is only 1 part or 10^4 so you will get about 100 ions per second produced here. You can send it to the mass spectrometer and you have a particle counting device counting the particle one by one. Before I build this machine I used to do ion molecule reaction. I’d be observing 1 ion per minute. Cosmic radiation would be the limit. But here you have got 100 ions per second. So the experiment is doable. The only thing is when you have a stream of molecules crossing, the background gas build up might be 10^-8, 10^-9 Tau. If you have 10^-8 Tau, the density of background molecules would be 3 times 10^8, 300 million molecule pairs. If you have 300 million molecules as a background, you select a bombardment ioniser. You are going to produce 300 million ions per second because those are steady state molecules. The density I’m talking about is of ozone. You will have a couple hundred ions produced. But the background is million times more than the signal. And that was the difficulty. Before we started to do this, lots of people didn’t understand that it is the signal’s noise ratio that limits the experiment, not the signal. So this machine we built nested in differential pumping system. So we have 3 stages of differential pumping go through 1 stage, 2 stage, 3 stage. Every stage will reduce the background by 100 times. So 3 stages would reduce the background by a million times. If you reduce the background by a million times then signals and noise will be 1 to 1. And then the experiment could be done. Of course somebody will ask you: “Why don’t you put more stages, more than 4, 5, 6?” But that would not help you because some of the molecules, background molecules go straight through the pinhole and arrive at the detector. So no matter how many stages of differential pumping you put in, the straight through background molecule would be the limit. Well, I was a chemist and I have to learn physics in order to understand chemistry. But then I became an experimental physical chemist. By the time I built this machine, I had to be a mechanical engineer. I have to learn to design the machine. And in order to make this machine I’ve done many 100 of mechanical drawings and every dimension, every hole has to be specified how should be the welding sequence and how precisely all those machinery has to be done, this machine will rotate and detect the product molecule. And like doing particle counting here, scintillation type particle counting, we have to put the coating ourselves, the quantum mass spectrometer we had to assemble ourselves. So it took about 10 months from the beginning to finish. And everything was done by myself and 2 students helping. And that was a very interesting time. Particle physicists will tell you a big machine did the experiment, a billion dollar spend, Well, this is the inside of the machine, how it looks like. This is the machine in Taiwan now. Still the most sensitive machine we have in the world. Here you have the beam coming out from the chamber and crossing here and the product molecule will go into the detector. And I mention that because some of the molecules, background molecules go straight through. So no matter how many stages of differential pumping you put in, it will not help you. That’s what people tell you that no matter how many stages of differential pumping you put in, background will be limited by the density of gas in front of you and the distance. That will determine the background. But that’s not quite true. If you were to be the detector, ioniser, sitting here and you keep on asking the question. You see a molecule coming in and you ask a molecule: “How did you come in?” He said: “Well, I was produced by a collision between fluorine atom and deuterium molecule, collided here and then I was bounced into the detector and now you detected me.” Then I’d say: “You are the signal and I really want you.” Another molecule comes in and I say: “Did you get in because you’re produced by collision between fluorine and deuterium molecule?” You will say: “No, no I was bouncing around and then somehow I got in.” And if I ask the molecule: “Were you kicked in by some other gaseous molecule?” You say: “No, the density is so low. I will be bouncing 500 metres before I will hit other molecules. And I got in because I hit the surface in front of the detector and then every time I bounced into a different angle but this time I just bounce in and you don’t want me. You say, I’m a background.” It tells you that the background molecule will go straight through to the detector. But every background molecule has to hit the surface in front of you. So if I put the policeman in front to catch all the molecules which are trying to get in, then I can reduce the background even further. That is the liquid helium refrigerating system. So we put a cold panel attached to it, the background will reduce and you can do the experiment. So that’s the kind of experimental result you get. So you cross the fluorine atom and deuterium molecule and measure deuterium fluoride. Sometime molecules move slowly, fourth vibrational state, third vibrational, second and first vibrational state. From the scattering angle you also know that the fluorine atom and deuterium molecule should be co-linear because deuterium fluoride will have another atom standing in front of it, so it’s bouncing backward. So you immediately can visualise the collision between fluorine atom and deuterium molecule fluoride near co-linear configuration. Other configuration will not react and bounce out and produce the product. And the energy distribution looks like this. Of course, why does it need to be collinear? Because the potential energy barrier will be higher for other configuration and when you increase the kinetic energy, acceptance angle will become larger and larger so you see the angular distribution start to move and vibrational distribution as a functional angle will depend on how we deviate from the co-linearity, so you do understand chemical reactions. But this is a simple chemical reaction I just showed you as an example. Sometime we deal with a very complicated system. But the question is: How come you have to know in such a detail? So let me take one example. If I have deuterium fluoride heated up to 3,000° K, vibrational population distribution will be 4 to 1, So it’s 3,000°, this 6 kcal Boltzmann distribution tells you 1/4th of the molecule will be up here, 1/4th will be weaker 1, weaker 2. So now if I shine the light, throwing the photon which is in resonance for the 1 to 2 transition, there now becomes weaker 1 is more than weaker 2, more molecule absorb photon and moving upward compared to those molecules similarly emit a photon. So you shine the light, light will be absorbed and excited state will increase. And this is the way we really do infrared absorption spectroscopy to identify the molecule. Now if you make a cell, mix the fluorine and deuterium molecule and under single collision conditions, then population distribution will be much higher than 3, 2 and 1 - 3 would be the highest. So now if I shine the light, which will be in resonance to the 3 to 2 transition, then you will be finding more molecules moving downward compared to the absorption moving upward. So light will be amplified and that is the way how chemical laser operate. So one can understand whether that could be a pumping mechanism, pumping reaction for the chemical lasers. Ok, so I’ll show how one can use the beam method to understand how chemical reaction takes place. Sometime form the complex and take quite a long time in angular distribution and the forward and backward [scattering]. Now I want to give you one example. If you take a molecule, this is an energetic molecule contained in fuel, CH2 hydrocarbon, and oxidising agent O2, if you burn it and nitrogen is kind of separating oxidising agent in the fuel when you have one molecule. So when you burn this molecule, it will form N2, CO and H2O. So in a sense you have a single molecule containing both fuel and oxidising agent. O2 and hydrocarbon separate by N2 and produce lots of energy. That would be a solid rocket. You can send the rocket to the space. You don’t need extra oxygen or matching tank to do this. But when you see a relatively complicated chemical reaction, you have lots of questions. How molecule dissociated? Is it uni-molecular, bi-molecular? What is the primary process, what is the secondary process and how is the energy distributed? If you use conventional chemical methods, it’s hard. But if you were to put a molecule in the beam, then we will be able to get 2 additional informations to overcome what conventional chemical method cannot overcome. For example, if I take the RDX here and make it into a beam and you shine the laser, dissociate the molecule, infrared laser to excite the molecule and make it hot, molecule dissociates. If you were to move along with the molecule, then what you would be seeing is molecule here absorbs the energy and dissociates into pairs, M1, M2, M3, M4. We know momentum is to be conserved in the centre mass system. So in addition to conservation of mass... M1+M2 should be equal to M3+M4 ... momentum is to be conserved. So first you measure the velocity ratio, you know the mass ratio, you know the total mass, so you'll be able to identify the product. So what I’m saying is: No matter how complicated, so long as you produce the pair, if you measure the velocity, even in the ionisation process, you only observe the dot ions, you will be able to tell what's going on. And this is something conventional chemical methods will not give you. This is the conservation of linear momentum. The other thing is: You measure the velocity and then you get dynamic information. So you take a molecule like nitroethane heat it up, absorb the infrared laser. Once you reach a certain point, then it is possible that you reach enough energy to form the cyclic transition state. Hydrogen atom transfers, 2 new chemical bonds are formed and 2 old chemical bonds are broken. And so form C2H4 and HONO. If you go through the transition state, the movement of the electron is a lot faster than the nuclei’s. So here new products are formed. They are so close so energy is repulsively released. So you will be moving very fast. On the other hand if you keep on pumping, stretch the chemical bond, that would be dissociating very slowly. So by measuring the velocity and understanding the dynamic you can distinguish which are the concerted reactions, which are the simple bond ruptures. And this is important for here. If you have a concerted composition for translation energy is almost 1 eV, it’s a 20 kcal. If you have a simple bond rupture, translation energy is almost peaking to zero. It’s following the statistical theory. So that could tell you whether it’s a concerted or simple bond rupture. Then you can also tell whether ... is there really momentum matching? What are the pairs? So what you usually do is to move the detector at the given angle so given mass and measure the velocity distribution, time of flight. You see sometimes here you can look at 74 move very fast. And 1 or 2 is slow. And then you keep on analysing the velocity, mass keep on going and eventually you understand actually this molecule. A molecule like this, electron will move, so form 3 stage to NO2, those are stable molecules. Those molecules are also internally excited and go through another concerted decomposition form HONO or form N2O or H2CO. Eventually those will be burned into the final product, carbon monoxide, water and N2. So with the prime molecular beam method because of the dynamic information and the conservation of momentum, you can elucidate how things are going on. But if you go one step further and analyse what's going on, it’s rather interesting. A molecule moving from here to here takes many 100 microseconds. It makes an excited molecule, the molecule moves very slowly. And in 1 or 2 microseconds the molecules dissociate. But 1 or 2 microseconds is a long time from the point of view of molecular vibration or molecular rotation. It takes 10 picoseconds for the molecule to rotate. So if you have a microsecond you can hardly see the molecule move but during that time molecule has been vibrating a million times or 100,000 times, rotating 100,000 times. So that then distribution will be isotropic, it’s already rotated. If the distribution is isotropic then I really do not need to measure all the angular and velocity and try to get this centre mass velocity distribution and find it’s isotropic. And all the information you need is the velocity distribution. So we thought about it and said, ok, we can do the experiment like this. You have the dissociated molecule here, then shine the light. After the molecule is expanding using vacuum UV photons to ionise all the molecules, which are going through the centre mass of this sphere. Then the displacement due to the velocity in so far as you can understand what is the mass. Of course you can miss the momentum and know, which are the products you produced. Sometimes there’s so many products produced and ion distribution will be smeared out. So it will be nice if I can analyse the mass along this line at the same time completely. So what I am saying is the following: If you take the beam and irradiate it with the laser, molecules start to dissociate. Dissociating, expanding, then I will shine the laser light, vacuum UV laser. Goes through the diameter of the expanding sphere and the displacement due to internal velocity. So if I can analyse a mass along another axis then I get the complete information. Sometimes you’re lucky, the molecule goes in, one laser shot, one vacuum UV laser shot, you’ve got everything you want. So you don’t have to spend 4 years to get your PhD, maybe one afternoon to finish the whole analysis. And we did make the machine this way. So you have the beam, you have the photolysis laser dissociating from this point. The product will be expanding into a sphere and wait until the centre of mass move to here. I use a vacuum UV laser to irradiate and ionise all the products in a straight line. When you produce this ion in a straight line, then I can use the mass spectrometer to analyse the mass. This mass spectrometer is a little different. We use pulse acceleration. So before any of the ions can move out from the electrode, I will shut off the electric power. So what I am doing is: Every molecule, every ion will be accelerated with a constant time. So every ion will be accelerated to the constant momentum. Most of the time we are accelerating the ion to a constant energy. But here we accelerate to the constant momentum per sphere. If you have a molecule with a constant moment P, then energy will be inversely proportionate to the mass. So I can put up the energy analyser, apply the electric field to analyse the ion energy. Then you will be able to analyse the mass. So this machine, this is the way how the machine looks like. So this shiny stainless steel is the electrostatic energy analyser. This mini plate in the curvature will tell you what the mass is. So take one molecule and try to investigate what is going on. Many students who took radiochemistry, or any chemist will tell you that if you take a molecule like Toluene excite it by 193 nanometre laser, internal combustion is very fast. Electronic energy will become vibration energy. So electronic excitation in chemical reaction takes place very fast. After the molecule is excited one of the weakest bonds is the CH bond. Because when you produce, break the CH bond, the radical side will be on the carbon, that will be delocalised with delocalised Pi electrons. So that is really a very weak bond. And that will be dominating, the second weakest bond will break the CC bond and form C3. As many people said, I don’t believe in what teachers say. It’s not that simple. If I put the deuterium on here to form CD3, Toluene, it will dissociate to C - what do we see? Well, amazingly when you put deuterated Toluene you do see CD3, but the deuterium was replaced by a hydrogen atom. So 1 hydrogen, 2 hydrogen, 3 hydrogen from the ring. And if you look at the other counterpart, it’s the same thing. And if you compare the hydrogen substitute, the species here 15, 16 and 17, those are distribution statistical depending on the number hydrogen atom you have. But 1/3 of the product goes to this way and 2/3 goes to the other way. And what happened was: Once you excited, there’s a competition between bond rupture and isomerisation for 7-membered ring. Once you form the 7-membered ring, then hydrogen migrates very rapidly. So scrambling takes place. So in the end you are observing CHD2, CH2D, CH3. So that kind of process in aromatic hydrocarbon is taking place all the time. So that is the competition taking place. And then after going through the 7-membered ring you really see the rapid isomerisation and scrambling hydrogen. So we discovered in the ‘60s when I was a student a chemist said: “Yes, you can see the isomerisation. Shine the light closely in case 1 to 3, 2 to 4 be connected and then isomerise from ortho to meta.” - But that’s not true. What's happening is when you shine the light from the 7-membered ring, the deuterium migrates and then produces a different product. And if you use the isotope, carbon isotope you can show the same thing. It’s really scrambled into the ring. And that is also important in the heterocyclic compound. So the nitrogen is outside or you put nitrogen in here, you do see the formation of the 7-membered ring and then summarise this. Now, let me talk about the ozone hole problem. I think I want to mention that the person who made the most important contribution about ozone depletion, Sherry Rowland, is sitting there. Sherry, if you raise your hand. I think Sherry was the person who pointed out that when the chlorofluorocarbon go to the upper atmosphere, the UV photon will dissociate chlorofluorocarbon and producing chlorine atom. Chlorine atom will react with ozone and produce CO plus O2 in the upper atmosphere because the sun is radiating so ozone is dissociating. The oxygen atom in a steady state concentration, oxygen atom will react with CO to generate chlorine atom, then this will go through the cycle with help of photons. This is called ozone cycle. That’s the main reason why the ozone layer is depleted. It’s going through ozone cycle. But later on people were finding out that in the cold Antarctic lower stratosphere something is happening. Because there the temperature is low, chlorine monoxide will form dimers. When chlorine monoxide forms dimers. Absorption cross section compared with the carbon monoxide is huge, much larger. So once you form the dimer, dimer will absorb visible photon and dissociate into chlorine atom plus O2. So in addition to this chlorine cycle, CO2 will really accelerate the depletion of the ozone. So when temperature goes down there’s a hole once the chlorine monoxide forms the dimer. And ozone hole was formed. I don’t want to discuss the other thing, this is a little more complicated. But here you see what's going on, ozone with chlorine atom producing CO plus O2. Those are chlorine cycles, chlorine monoxide when it’s cold to form the dimers, COOOCl. That will absorb the visible photon and produce O2 and chlorine atom much more rapidly. That’s the main reason of the ozone hole. In order to explain the ozone hole you have to have 13 absorption cross section for COOOCl. So the sun light irradiating, visible photon will be absorbed. So that will add to the acceleration of the depletion of the ozone. But all those are well investigated and everybody thought we do understand what is going on. But there’s a paper published in the Journal of Physical Chemistry in 2007 and Pope and co-workers from Jet Propulsion laboratory of California Institute of Technology publish a paper and then Nature cited and said: “Chemists poke holes in ozone theory.” It means ozone hole, I talk about chlorine monoxide dimer absorbing the photon, produce chlorine atom, that mechanism might not be right. So that’s why they say chemists poke holes in ozone hole theory. What is the reason? If you have the COOOCl molecule as a function of wavelength, this is the absorption cross section. If absorption cross section were to be high, the depletion of ozone would be faster because it would dissociate so easily. But if absorption cross section would be lower, then it would be slow. So look at the data, Burkholder in 1990 published the paper. And if you use the numbers, the ozone hole problem could be explained, there’s no problem there. Then time goes on, Hunter and DeMore 1995. They published a paper and said cross section should be lower. And how much lower? Maybe 3 or 4 times lower and DeMore in 1990 also published the data, which is slightly lower than Burkholder. So as time goes on from 1990 to 1995, the cross section becoming lower and people become a little bit uneasy. But it’s not too bad yet. But in 2007 Pope published a paper, the most recent people. People always believe that science make progress. So the most recent paper should be most reliable. So if you believe in Pope’s data, look at the cross section, around 345 cross sections went down almost by 15 times. If the cross section were to be that small, then you cannot explain the ozone hole problem. Something’s wrong, something’s wrong with our understanding of the ozone hole. So NASA asks everybody to investigate and try to determine the absorption cross section of COOOCl to within 50% in the next 5 years. That was the expectation. So we thought we should be able to do a very different kind of experiment to pin down the cross section within a few % in a relatively short time. Why do I say that? When you measure this absorption cross section of COOOCl, COOOCl is a radical species- You don’t produce it in the pure form in the cell. You always have chlorine molecule and other background in the cell. So measurement of those cross sections often involve 2 large quantities, like 1 million 10 minus 1 million and you got the difference of 10. I’m exaggerating but you have to subtract large number from large number and you got the small number. So the uncertainly is large. So let me, before I explain to you how we did look at this cell. You have this cell containing COOOCl and together with CO and other things, chlorine molecule. If I shine the light, the light will be absorbed. Then you have the chain, the COOOCl concentration and then do the experiment again and look at the difference. That is what I was saying, that we are subtracting the 2 large numbers. But if I were to do those in the beam, then I can eliminate all the problems. What I am saying is the following. We have a cell, we have a hole here and temperatures were controlled. We make COOOCl first and turn it in the cell and it gradually warm up. We know the temperature and the molecule if use out from here, we are going to use the pulse laser to do the experiment. So we use chopper to chop the beam - this is a CW beam - we chop the beam, go through this skimmer. This is mainly for reducing the background. You use the core slit, maintain 20° K so those background gas will be absorbed. So increase the signal to noise ratio. Here we will shine the laser to look at the absorption of COOOCl. When the molecule absorbs the photon it will dissociate. So we look at the depletion of the signal. And we use the mass spectrometer to tune into the mass of COOOCl to do the experiment. It’s interesting, there are many molecules coming out but when you chop the beam... Actually the experiment is done by this molecular beam apparatus, very sensitive with a detector, which looks like this, highly differential pump. And when you chop the beam, look at the CO2 molecule because we know the molecular weight, we know the temperature, the velocity distribution should be Maxwell-Boltzmann. So this is the molecular beam arrival time. You can simulate the arrival time and measurement, exactly when you use the mass spectrometer to detect the chlorine molecule. No other contaminator coming in. Follow Maxwell-Boltzmann. And when you look at the COOOCl, it’s the same. This curve is following exactly mass of one O2, follow this peak so we know that we are really identifying single species of interest by using a spectrometer. So now what we do is shine the light so molecules will be excited. Use chlorine molecules as an example, you burn it. So subtract the signal and the previous signal. You get depletion and absorption cross section of chlorine molecule is known exactly. So you can check what should be the cross section. And then you do COOOCl and you can see the depletion of the hole. And make the comparison of chlorine molecule absorption, this is exactly known with our measurement. This I show you know, we can deplete almost everything when you irradiate. Of course you don’t want to do experiment, go beyond the general region so we burn the hole and compare it. What did you see? It’s interesting. Those points are experimental result. One was done at 200° K and another one is 250°K, there is a temperature dependence of the absorption cross section. And at these wavelengths our cross section is slightly lower than Burkholder. But here it’s almost the same and now we have completed many points. And a conclusion was that the result time goes on, many people do the experiment, more and more experiment and they are not doing better than 1990 experiment. Our data is almost in line with 1990. And what does it mean? It means UV absorption cross section of COOOCl are consistent with older ozone depletion models. There is nothing wrong - we really understood what's going on. One thing was: Cross section measurement as a functional time was deteriorating, deteriorating. And with this molecular beam method we have been able to put it in the right position. Actually this work is really mainly done by one of my former students ... (inaudible 46.33). After I went back to Taiwan I become the president of the academy. I didn’t want to run my own research group, so I worked with 4 different groups, 4 young scientists, I worked for them as a special research assistant. So we worked together and ... (inaudible) was my former student but he is my boss. So we worked together and I worked on 3 other different projects. So it’s interesting. The ozone hole problem is really solved long time ago, in 1990, there’s not any discrepancy. But when I started to talk about combustion of hydrocarbon, that caused us lots of problems in the present time. and showed that if we keep on doing that, the atmosphere is going to warm up. And at the time he was looking at it from the better side, he thought Sweden might be a much more wonderful place to live in the future when the globe warm up. But at the present time I really worry about if we keep on letting temperatures to go up, we might reach so called risk of discontinuing the weather pattern. And when that happens, then all of us will have to say good bye from the surface of the earth. And that’s something we should not let happen. So we really should work hard, really look at the global warming problem more seriously. This is coming Friday there’s a panel discussion on the energy and sustainability and that will be an interesting discussion. We have the responsibility, otherwise our grandchildren will say: We might not be too late, we know what needs to be done. So we really have to work together and hopefully Friday afternoon’s panel discussion will enlighten us a little more on the future of mankind. Anyway thank you very much for your attention. (Applause.)


Every macroscopic chemical transformation, whether it is atmospheric ozone depletion or the burning of a candle, consists of millions of microscopic chemical events which involve collisions between molecules. It has been the dream of scientists for a long time to observe and understand the details of molecular collisional processes which transform reactant molecules into product molecules with our naked eyes. During the last several decades, because of the advances in crossed molecular beams method and laser technology, especially, from the measurements of product angular and velocity distributions, it has become possible to “visualize” exact details of how chemical reactions take place through molecular collisions or through photochemical processes.

In this lecture, in addition to illustrate experimental details of crossed molecular beams method, examples will be given to demonstrate how detail information on the dynamics of chemical reactions and photochemical processes can be obtained using various molecular beam approaches. Recent investigations on controversy of photochemical processes involved in the formation of ozone hole and the new understanding of basic mechanism involved in the matrix assisted laser desorption ionization method for the analysis of biological molecules will be presented.