Coring an Atom with an X-Ray Laser

04 July 2010

Interview with

Dr. Linda Young, Argonne National Laboratory

Chris - Now also this week, researchers in America have used a very powerful x-ray laser to strip away the electrons from an atom of neon.  But they also have been able to very carefully strip away only those electrons which are closest to the atom's centre, creating the atomic equivalent of a cored apple.  Dr. Linda Young is a distinguished fellow of Argonne National Laboratory which is just outside Chicago and she's with us to tell us how it works.  Hello, Linda.

Linda -   Hello.

Chris -   Welcome to the Naked Scientists.  Do tell us if you could, first of all, what is this laser and why is it so special?

Argon-ion and He-Ne laser beamsLinda -   This is the world's first hard x-ray free electron laser and it produces x-rays that are about a billion times more intense than any other x-ray source before.  The intensity is actually equivalent to taking all of the sun's radiation on earth and putting it into 1 square centimetre.  So it's an exceedingly powerful x-ray laser.

Chris -   And obviously, you wouldn't use this clinically, but you can use this to probe things like atoms of very, very high resolution.  So tell us how you're doing that.

Linda -   Yes, that's right.  Because this laser is so intense, and all these photons come in such small bursts, you are in fact able to capture motion on the atomic molecular scale within femtoseconds.  That's about the time that it takes for molecules to vibrate within larger structures such as proteins.  But because we had such an intense laser, and it's the first time anyone had had it, one really wants to understand how that very intense x-ray beam interacts with matter.  And so, you might think that if you take a trillion photons and focus it down to a micron or so, you couldn't control at all what's going on in matter.  But in fact, we find that we can control how the matter responds by tuning the photon energy of the x-rays, and by tuning the pulse duration in which you deposit those x-ray photons into the atom.

Chris -   So tell us about the experimental set up just briefly.  What did you actually do?

Linda -   Okay, so you take this very intense x-ray beam and you focus it down to about a square micron, into a jet of neon atoms.

Chris -   So that's a millionth of a metre we're talking here, isn't it?  A thousandth of a millimetre across.

Linda -   That's right.  And when you do that, you surround that interaction region with a number of detectors that can detect all the products of the reaction.  And so, you can detect all the ions that are produced and all the electrons that are produced, and by having these very high resolution detectors, you can tell exactly the mechanism by which the neon atom becomes stripped of its electrons.

Chris -   So you fire these very intense x-ray beams into a cloud of atoms and you spray neon, so that's a noble gas, isn't it - very unreactive.  What happens to the atoms when they hit with this very intense burst of x-rays?

Linda -   Well, that depends on what photon energy you've selected.  You can select the photon energy where you hit out the inner electrons first or you can select a photon energy where you just peel away the outer electrons.  So, depending on where you are in photon energy, you can do one or the other.

Chris -   And how does this inform our understanding of physics and our understanding of atomic structure?

Linda -   Actually, what it informs you of is how very intense x-rays interact with matter.  Before this x-ray laser was available, we were only ever able to knockout one of the inner electrons in a shot, but now with this very intense x-ray laser, you can knockout both of the inner electrons simultaneously, and that leaves you with this so-called hollow atom or cored atom.  That hollow atom has different properties than a normal atom including the possibly advantageous property that when the inner electrons are missing, then the x-ray absorption is decreased, relative to the scattering cross-section, and the scattering cross-section is what forms an image for you to make further molecular movies or images of complex molecules.  So, what I would say is that we're just exploring a new regime of x-ray interactions with matter.

Chris -   Which is of course going to give you the opportunity to begin to understand and probe whole molecules at a kind of resolution and in a way that we've never seen before.  Linda, thank you very much.  That's Dr. Linda Young who is from Argonne National Laboratory and she's published that work in the journal Nature this week.

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