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  • 2017-04-17 15:35:22
  • Article ID: 673096

How Do You Make a Femtosecond Light Source?

Agostino "Ago" Marinelli first met pioneering accelerator physicist Claudio Pellegrini as an undergraduate student at the University of Rome. It was 2007, a couple of years before the Linac Coherent Light Source (LCLS) came online at SLAC, and people were abuzz about free-electron laser physics.

  • Credit: SLAC National Accelerator Laboratory

Agostino “Ago” Marinelli first met pioneering accelerator physicist Claudio Pellegrini as an undergraduate student at the University of Rome. It was 2007, a couple of years before the Linac Coherent Light Source (LCLS) came online at SLAC, and people were abuzz about free-electron laser physics.

Caught up in the excitement, Marinelli pursued a PhD in accelerator physics at the University of California, Los Angeles under Jamie Rosenzweig’s mentorship. Today he is involved in research and development related to femtosecond science at LCLS.

Marinelli focuses on research at the femtosecond timescale because, he says, “it’s the fastest we can reach now with X-rays, and as an accelerator physicist, I get excited about technical things like that.”

Why did you get involved in X-ray science?

Part of it was Claudio—he’s a very charismatic character. He’s an inspiring character. The field was very interesting. I thought it was a good way to spend my PhD.

LCLS was promising so much innovation: a laser 10 billion times brighter than we had then. That sounds like something that somebody who is 24 would love to get involved in. It just sounded like something that would change science in a positive way, and I wanted to be a part of it.

What is a free-electron laser?

Free-electron lasers were invented by John Madey at Stanford in 1971; later on in the ’90s Claudio Pellegrini and collaborators proposed to extend free-electron lasers to the X-ray regime. They were the next step after synchrotron light sources.

Synchrotrons send electrons around in a circle. That gives you radiation you can use in experiments. The difference between a synchrotron and the free-electron laser is the same difference between this light [points to a ceiling light] and a laser. It’s the difference between a bunch of kids making noise and a choir.

In a synchrotron, the electrons are all doing the same thing, going around in a circle, but they are unaware of each other. They are all emitting X-rays in a random way. What makes a free-electron laser a laser is that all the electrons are emitting radiation in a coherent way. They are all synchronized.

Also, since in an FEL you are using very intense and short electron bunches, the X-ray pulses will also be very short, down to the femtosecond level.

What do you do with the free-electron laser?

We talk to the users—they’re researchers that have some science they want to study with the machine. Then we “shape” the X-rays—set up the machine in a way that’s ideal for that experiment. The LCLS accelerator is very flexible. You can do all sorts of tricks with it—like arbitrarily changing the pulse duration, varying the X-ray polarization or making multiple pulses of different colors.

Speaking of which, in 2014 the European Physical Society awarded you the Frank Sacherer Prize for your work using “two-color” beams with LCLS. What is that about?

Normally LCLS shoots 120 X-ray pulses a second. But you can also make it send two pulses of different energies, separated by a few to 100 femtoseconds. You excite your sample with the first one and probe it with the second. You have to observe it within femtoseconds after you excite it because reactions happen that fast.

Normally you would excite the sample with an external optical laser; that’s how pump-probe is done. But in molecular dynamics, if you can excite a molecule with X-rays instead of an optical laser, you can get atom specificity—you can target a specific atom in the molecule.

Each atom has a core energy level. If you know that, you can shoot the X-ray and hit only the oxygen in a molecule; oxygen is the only thing that is going to react. With two pulses at separate energies, you can target different atoms in a molecule to see which one triggers a certain reaction.

Fun fact: These pulses are called a “twin bunch,” and in 2016 Marinelli became the father of twins.

What kinds of things do you study on the femtosecond scale?

A femtosecond is close to the fundamental scale of atomic and molecular physics—so, things like chemistry.

A chemical reaction is essentially two molecules or atoms interacting in some way and sharing charge and giving away energy. Ultimately to understand that, you have to understand how charge and energy flow in a molecule. You have to understand the very fundamental motion of electrons and ions in the molecule. On the femtosecond scale, you can see the positions of the atoms rearranging as it happens.

Chemical reactions are a dynamic process. They start with something. They end with something. We want to know what happens in between.

Why?

If you want the reaction to end with something else, if you want it to end with something slightly different, you want to understand how it happens so you can make changes on purpose.

What are you most excited about now?

I’m really excited about what I’m about to do, which is this sub-femtosecond project called XLEAP. We will shape the LCLS electron beam with a high-power infrared laser and use it to generate pulses that are shorter than a femtosecond! What we will be looking at is energy and electrons moving around a molecule, which happens even faster than the atoms rearranging.

Right now we’re really blind to all of this. To me, the way I understand it is, going to that timescale, you’re peeking into the very fundamental, quantum nature of the electrons in the molecule.

If you ask me, “What is the ultimate problem it will solve for us?”—the answer is: I don’t know. In general when you’re blind to some fundamental process in nature and suddenly you can see it, my guess is something good is going to come of it.

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