Scientific Method / Science & Exploration

From idea to science: Pouring concrete for an experiment and securing funds

General idea turned into specific projects.

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"From idea to science" is a process, and as such, it's become a series here. We've discussed what I perceive to be the institutional boundary conditions that constrain my academic success, then followed that with a long exposition on the science that gets me out of bed in the morning. But now it's time to get specific. This is the part that I admittedly have difficulty with: turning ideas and desires into concrete plans. Or, at least making them into plans that are acceptable to people who provide money.

Today, let's take the general idea from last time and break it up into individual, graduate-student-sized projects. In one sense, this is easy—just come up with three sets of experiments. Unfortunately, they need to tie together intellectually. It isn't necessary that the students need to work together on everything, but, thematically, they should be sufficiently related. That way, the students can assist each other when problems arise.

Getting specific also means thinking about who I am going to ask for money and how much. The European Research Council is offering up to €2 million (about $2.59 million) over five years to a few clever and competent researchers fitting a certain profile. I need to be relatively young—you must have held your PhD for less than 12 years—and you should be looking to strengthen an existing research group. There are other criteria, but those are the two that give you the essence of what they are looking for: new researchers trying to get more independence. As for the money, it sounds like a lot, but, for what I want to do, it will provide for three PhD students over the entire period.

On the upside, that I can only run three PhD research projects offers an unexpected benefit: I need to think carefully about what I really want to do.

Learning the basics

I want to understand the vibrational motions of molecules at their most intimate level. To do this, what I envision doing is taking a molecule that has one unique bond—say a single oxygen atom stuck on a ring of carbon atoms. We use a laser to pump the hell out of the stretch vibrational mode of the bond between the ring and the oxygen. Because it is the only bond of that type, and all the other vibrational modes have quite different vibrational frequencies, I have a unique starting point in my experiment. I know exactly which vibration is oscillating.

Now, as the oxygen molecule bounces around, it will start transferring energy to other atoms in the carbon ring. When it collides with other molecules, it will also transfer energy. So, after a certain amount of time passes, I take a snap-shot of the excited vibrational modes. And, I do it in such a way that I can see not only which vibrational modes are excited, but also t whether they are oscillating in a fixed phase-relationship with the original excitation pulse.
All of this will depend strongly on the environment. So there are a huge number of experiments to do, each of which could tell us a little more about what the molecule is doing.

These same experiments, but with different model compounds, can be used to tell us what is going on when a molecule attaches to a surface. For instance, if an alcohol molecule attaches to the surface of certain metals, it will break up. Now, we can use this experimental scheme to understand that break-up in more detail, by exciting vibrational modes and looking at how they shift and vanish as the alcohol breaks up into new compounds. In a sense, we use the excitation of vibrational motion as a tag to track when and how the break up occurs. And that holds a great deal of importance in understanding how metal catalysts work.

It may be sad to think that the life of a grad student can be summed up in four paragraphs. But I am pretty confident that what I have described is actually more like eight years work instead of four. I'm not too concerned about that; with a good start, new students can follow the project as long as it remains interesting.

It is also the key project for the grant. Everything we learn from these experiments will be pumped into the following two.

Shoveling molecules about and making them do cartwheels

If the first project is about understanding, the second is about control. We want to use the vibrational motion of the molecules to our advantage. To do that, we need to be able to set a certain fraction of the population vibrating in a particular way. In fact, we want to be able to drive what are called "coherent population oscillations." This basically means that, for a laser pulse of a given intensity, I can be certain that every molecule within the focus is vibrating at a particular vibrational frequency, and all the vibrations are in phase with each other.

What makes this important is that population statistics underlie optical properties. For instance, if I can make a population that's evenly divided between two vibrational states, then I prevent the molecules from emitting certain colors of light. This is an important step for improving some imaging techniques. We can image using the vibrational frequencies that we excite with a laser as labels for particular chemicals. This provides high contrast images without introducing artificial contrast enhancers. If we can prevent the molecules from emitting certain colors of light, then we can improve the image contrast.

But, better yet, we can use this process to see smaller features. I can choose my laser beam shape so that it will create equalized populations in some places but not in others. And, as I increase the laser power, the size of the area where the populations are equal increases. If I set things up right, then the image is generated from the shrinking areas where the populations aren't equal. Essentially, as I turn the laser power up, I can see smaller and smaller features in an image. That will be one of the main goals of this project: imaging at high-resolution.

This isn't the only way of controlling population dynamics, either. It's possible to crank the laser intensity up so high that it keeps the molecules vibrating in phase. Once we get to that stage, the entire molecular population will cycle rhythmically between the two states. These coherent population oscillations, otherwise known as Rabi flops, are used throughout atomic physics, but doing flops with molecules is so much harder that it has never been popular.

The nice thing is that Rabi flops can also be used as a probe for molecular behavior: where is this molecule within the light field? Does its proximity to a surface shift its vibrational resonances, and how far? All of this will show up in the spectrum of the Rabi flops.

Now, you might expect that these Rabi flops will be very short-lived. But, in atomic physics and nuclear magnetic resonance spectroscopy (think MRI), a number of techniques have been developed to preserve the coherence required for Rabi flops. One aspect of this project will be trying to apply variations of these techniques to our molecular systems to preserve  coherence and increase the precision with which we can perform spectroscopy. That will be... challenging. (In case you were wondering, "challenging" is a forbidden word in grants, because it is viewed as being synonymous with "impossible.")

Spectroscopy leads to molecules, which lead to reactions, which lead to chemistry

In the last project, I want to get into some basic chemistry. When it comes to physics, I am pretty confident of my stuff. I know where the difficulties are and I know where I am pushing boundaries. But, of chemistry, I am entirely ignorant. And I want to do some chemistry.

Previously, I described how we were going to excite the vibrational mode of an oxygen atom attached to a benzene ring then see what happened. Well, one might wonder: does the excitation of a particular vibrational mode help determine the outcome of a chemical reaction?

To explore this has been a bit difficult. The natural energy of that vibrational mode is such that we would not expect a single excited molecule in a beaker full of compound. In fact, if we were to heat the beaker, the compound would boil away before that mode would be excited in a single molecule—temperatures of about 3000K are required to get a small population with that mode excited. So, usually, no one really knows if the excitation of vibrational modes can affect the outcome of a chemical reaction.

But lasers give us a tool to create molecular populations in a highly excited vibrational state, and we can take snap shots of molecular states at a later time. So we can really get right into the dynamics of the reaction itself. This is separate from determining the statistics of a reaction like its rate or the energy released—those are a compilation of many factors. Here, we would just be looking at the reaction itself. We can tag a molecule by putting it in a vibrational state, and then look for reaction products in the vibrational spectrum. We can track both the appearance of new products, and where the vibrational energy goes as the products are produced. I think this would be very informative.

The difficulty is picking model reactions to study. Again, I'm no chemist and I'm not sure that my choices are good. I've decided, rather arbitrarily, to divide the world up a bit. There are reactions that normally just go in a single direction: two reactants form a single or pair of products. Then there are reactions that go forward and backwards: the reactants form products that then recombine to form the reactants. And there are reactions where the outcome is a pair of products, with the balance between the two determined by something like the temperature.

The job is to choose some good examples in the classes that I have created and study them. It should be possible to use vibrational modes to enhance, or suppress, the direction a reaction wants to go. So, in the first case, the reaction might be severely hindered by certain excitations and sped up by others. In the second case, the right excitation might prevent the back-reaction from occurring; in the last, we might be able to selectively choose which product dominates.

In the end, though, I want to apply this to something more relevant like catalysis. The end-point would be studies on catalytic activity when the reactants and/or products are vibrationally excited. Obvious examples include the reduction of carbon dioxide and water splitting. Although they appear simple, these are not easy reactions to understand. And, as I am rapidly learning, the catalysis community speaks an entirely different language. So even once we do the work, it will be difficult to communicate the results in a way that shows their relevance to the community that should be most interested in them.

In short, I need to very circumspect in describing this project and what I want to do with it.

Where's dessert?

So that is the three-course meal of science that I have in mind. I must admit that the last project worries me. The ideas are clearly expressed in the application, but I worry that I am simply going to walk into the trap of not knowing enough chemistry to make this one work. Luckily, I was able to convince a very good chemist to collaborate in this part of the project, so that will help. I also intend to keep the chemistry as simple as possible, mostly sticking to reactions that are well-known and easy to run. Even so, a chemist might well balk at giving me money to do this.

That's where the grant stands, but I have even more minds to convince. In the next and final installment, we'll take a look at how I justify the actual research to my bosses, my institute, and myself. Or, as I tend to think of it, it's the story of how to make a sphere look square.

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