The world's most intense X-ray laser may soon be the fastest strobe-light camera ever. Two of the laser's first experiments show the device will be able to take snapshots of single molecules in motion - without destroying them first.
The laser, called the Linac Coherent Light Source, takes up a third of the two-mile-long linear accelerator at the SLAC National Accelerator Lab in Menlo Park, California. In the accelerator hall, tight bunches of electrons wriggle through a series of magnets and give off X-rays billions of times brighter than earlier X-ray sources could muster. The wavelength of these X-rays is comparable to the radius of a hydrogen atom - about one angstrom, or one ten-billionth of a meter - and each pulse can be as short as a few quadrillionths of a second.
These features make this kind of X-ray, called a hard X-ray for its ability to penetrate matter, an ideal scalpel to probe the inner workings of atoms and molecules. When the laser first flashed in April 2009, physicists dreamed of using it to make 3-D, time-lapse movies of atomic bonds breaking and proteins changing shape. Just like stop-motion photographs showed 19th-century photographers how horses run, the X-ray laser should show modern scientists how atoms interact.
There's just one potential problem: The X-rays will make the molecules explode. For imaging experiments to work, the laser's shutter will have to be faster than its detonator.
In two of the first experiments, conducted last fall and reported in two recent papers, scientists put the laser through its paces to see if simple atoms and molecules can be photographed before they are destroyed.
"Understanding how intense light, and in particular intense X-rays, interact with both atoms and molecules is critical to understanding how we're going to be able to image systems using these intense light pulses in the future," said laser physicist Roger Falcone of Lawrence Berkeley National Laboratory, a member of an advisory committee for the laser's science team but was not involved in the new studies.
In the first study, reported in the July 1 Nature, physicists blasted a neon atom with X-rays in a range of different energies. The researchers chose neon partially because it is in the second row of the periodic table, which also contains carbon, nitrogen and oxygen, the makeup of biological molecules.
"If you can understand what's going on in a second row element, you can understand how these [X-rays] will interact with biological molecules," said physicist Linda Young of Argonne National Laboratory in Illinois, a co-author of the paper.
Young and her colleagues tuned the laser to irradiate neon atoms with X-rays between 400 and 1,000 times more energetic than visible light. At energies below a certain threshold (870 electronvolts, or about 435 times more energy than is carried in a photon of visible light), X-rays knocked electrons off the neon atom's outer electron shell like overenthusiastic billiard balls knocking each other off the pool table. But at higher energies, the innermost electrons were booted out first. This process left behind a hollow atom.
This hollow atom doesn't last very long before an electron from the outer shell drops down to fill the hole. And all the electrons peel off within one ten-trillionth of a second. "The neon atom is stripped bare naked within that short amount of time," Young said. But the atom lasted long enough for Young and her colleagues to notice that, while it was hollow, the atom was more transparent to X-rays.
That's good news for future experiments to take images of atoms, Young said. X-rays can either be absorbed or scattered by an atom. But only the scattered X-rays are useful for making images, because they are the only ones that will end up on a detector at the end of the experiment. Hollow, transparent atoms let through more X-rays, which will make images easier to record.
"To image single molecules and thereby reconstruct their structure, you need to be able to collect X-rays," Young said. "We really established a framework for understanding the interaction of these X-rays with matter."
In the other experiment, published June 22 in Physical Review Letters, physicist Nora Berrah of Western Michigan University and colleagues turned the laser on a simple molecule, nitrogen gas.
Instead of changing the energy of the X-rays, Berrah's group changed the duration of the pulse. They bombarded the nitrogen molecules with X-ray pulses between 4 femtoseconds (quadrillionths of a second) and 280 femtoseconds, all of which carried energies of 1000 electronvolts.
The team found that this treatment also created hollow electrons, stripping the nitrogen atoms from the inside out. But while the longer pulses steadily pulled each electron off the molecule, the shorter pulse stopped with the innermost electrons.
This is because there is not enough time for the outer electrons to fill the holes left by the inner electrons, Berrah said. The outer electrons move down on a characteristic timescale set by nature, called the Auger clock, of about 7 femtoseconds. The 4-femtosecond pulse zips through the molecule before the outer electrons have a chance to drop down. The physicists call this process "frustrated absorption."
"This is very good news for biomolecules," Berrah said. "It's promising for single molecule imaging. We can deposit the intense radiation without damaging the molecule that we want to study."
These studies provide "increasing confidence in our ability to understand these processes," Falcone said. They will also help design the next X-ray lasers. "Understanding how light interacts with matter, both single molecules and atoms, will allow us to design parameters of next-generation machine[s] as well."
1) An artist's conception of what images of single molecules taken with the LCLS might look like. The molecule will leave a distinctive pattern of rings and spots on a detector, before it explodes.
2) The hall holding the magnets that make electrons toss off X-rays. Credit: SLAC National Accelerator Lab