Molecular Photography

By JoAnne Hewett | November 3, 2006 3:46 am

How would you take a photograph of a molecule in motion? You can’t just go to the camera shop and pick the appropriate device off the shelf. You’ve got to build it yourself! So, what type of device should you build? Well, an average length for a molecule is 1 nanometer (10-9 meters), so you would want a part of the electromagnetic spectrum with a small enough wavelength to be able to discern (i.e., scatter off) an object that size. X-rays, which have wavelengths of approximately 0.01-1 nanometers, would do the trick. And you don’t want blurry shots, so you would want the X-ray pulses to be rapid enough in order to freeze the ultra-fast molecular motion — something like 10-(9-10) seconds between pulses would work. And the X-rays should be bright (high intensity) — nothing worse than a dingy picture.

Sounds like a job for an X-ray free electron laser! The world’s first such device of this scale is being built, right here at SLAC. We had the groundbreaking ceremony a few days ago, so the project is now officially underway. Lots of dignitaries were here to shovel the dirt and give inspiring speeches. They were Department of Energy Under Secretary of Science Raymond Orbach, Congresswomen Anna Eshoo and Zoe Lofgren, Congressman Mike Honda, Stanford University Provost John Etchemendy, and SLAC Director Jonathan Dorfan. Last, but not least, the Stanford University Marching Band performed at the event. They have a reputation for being a rowdy bunch (the band, that is) — the University has gone as far as to ban them from performing at football games this year — but they were appropriately toned down (for a band, that is), and added just the right air of festivity to the event.

So, just what is this X-ray laser going to do? Literally, it will take photographs of molecules and reveal their structure. Current X-ray sources primarily give a static picture of materials averaged over relatively long time scales. This new machine will make movies of fast-moving chemical reactions, capturing each movement instantaneously in a frame; it can also determine the structure of a single molecule or small clusters of molecules, and study new states of matter called warm dense plasmas, and perform a vast array of other scientific feats. For instance, it will be possible to record time-resolved images of chemical reactions, to the point of following the change in the chemical bond as the reaction proceeds. Indeed, it seems that it can do almost anything and that everyone is interested in it. Chemists, biologists, physicists, drug companies, even historians. You name it! Which is why the DOE is plunking over roughly $400 M for the project.

At the groundbreaking event, Stanford Provost John Etchemendy made an interesting comparison. He told the story of Eadweard Muybridge, who engineered and developed photographic equipment of his time. Under the employment of Leland Stanford in 1872, he was the first to take a picture of a galloping horse. Previously, during the entire history of humankind, it was thought that a galloping horse kept at least one, if not two, legs on the ground. Muybridge’s photographs comprised the first instance that a galloping horse’s movement had been frozen. And what did humankind learn? That galloping horses have all four legs simultaneously off the ground. May not sound like much to us today, but just imagine 130 years ago.

This wonder machine is called the Linear Coherent Light Source, or LCLS. LCLS will use the last third of the linac to create tightly-focused accelerated bunches of free electrons (here, free means not bound to atoms). Bunch compressors are being installed in the linac to squeeze the electrons into 20 micrometer sized, intense,pulsed packets. These electron bunches then leave the linac and enter a series of undulator magnets. To my understanding, the undulators make the electrons wiggle and thus change course, during which they give off X-rays via synchrotron radiation. And the undulators are built in such a way as to make the X-rays coherent, with a wavelength of 1.5 Angstroms (0.15 nanometer). At the end, the world’s most intense, fastest pulsed coherent (like a laser) X-rays are beamed into a suite of experiments. Cool!

Lastly, how do us SLAC particle physicists feel about turning our laboratory over to the Basic Energy Science division of DOE? I would be remiss if I did not mention this… Well, first of all, we are tickled pink that our linac will continue to produce groundbreaking scientific results. It is just another phase in a long history of outstanding science performed at our laboratory. I must also be honest and admit that there is some apprehension that particle physics will become marginalized at the lab. However, strong leadership and a thoughtful layout for the transition will ensure that this does not occur. SLAC has a very talented particle physics workforce, and current and future particle physics projects do and will depend on it!


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