X-ray cat movies; dancing electrons; photonic crystals in 3D.
Leveraging the concepts behind the paradox of Schroedinger’s cat, the Department of Energy’s SLAC National Accelerator Laboratory (SLAC) has made an X-ray movie of the internal workings of a molecule.
Specifically, SLAC has taken time-resolved femtosecond x-ray diffraction patterns from a molecular iodine sample. Then, researchers created a movie of intramolecular motion with time and space resolution of 30fs and 0.3 angstroms. An angstrom is a unit of length equal to one hundred-millionth of a centimeter.
For this, researchers used SLAC’s Linac Coherent Light Source (LCLS), a hard X-ray free-electron laser. The laser enables ultrabright and ultrashort pulses. It captures atomic-scale snapshots in quadrillionths of a second.
In this experiment, a laser hits the iodine molecule. Then, it splits into two versions of itself. The first is excited, while the other is not. Following those events, a follow-up laser scatters off both versions. They recombine to form an X-ray hologram. By stringing together these X-ray snapshots, researchers can make a movie.
Researchers, however, believe it’s incorrect to say that a small fraction of the molecule was excited and the rest was not. In quantum mechanics, every molecule is excited, at least to some degree.
The analogy is much like Schroedinger’s cat. Schrodinger’s cat was devised by Austrian physicist Erwin Schrödinger in 1935. In this paradox, a cat is placed in a box. A flask of poison is also placed in the box. If the flask is broken, the poison is released. And the cat dies.
In quantum mechanics, the cat is simultaneously alive and dead. Then, one looks into the box. The cat is either alive or dead. “This poses the question of when exactly quantum superposition ends and reality collapses into one possibility or the other,” according to Wikipedia.
This dual state, or the behavior of Schroedinger’s cat, was applied to SLAC’s experiment and its molecular movie. “Our movie, which is based on images from billions of iodine gas molecules, shows all the possible ways the iodine molecule behaves when it’s excited with this amount of energy,” said Phil Bucksbaum, a professor at SLAC and Stanford University and director of PULSE, which is jointly operated by the lab and the university. “We see it start to vibrate, with the two atoms veering toward and away from each other like they were joined by a spring. At the same time, we see the bond between the atoms break, and the atoms fly off into the void. Simultaneously we see them still connected, but hanging out for a while at some distance from each other before moving back in. As time goes on, we see the vibrations die down until the molecule is at rest again. All these possible outcomes happen within a few trillionths of a second.
“Our method is fundamental to quantum mechanics, so we are eager to try it on other small molecular systems, including systems involved in vision, photosynthesis, protecting DNA from UV damage and other important functions in living things,” Bucksbaum added.
Meanwhile, using the LCLS, SLAC provided a glimpse of how electrons dance with atomic nuclei in materials. These studies could pave the way for new materials and the ability to control them.
Researchers studied the electron-phonon interactions in lead telluride. This compound is a good thermoelectric. It generates an electrical voltage when two opposite sides of the material have different temperatures.
In addition, researchers also looked at chromium. At low temperatures, chromium has similar properties as high-temperature superconductors.
In the lab, researchers from SLAC excited electrons in lead telluride. They hit the sample with a brief pulse of infrared laser light. Then, X-rays were used to determine how this burst of energy stimulated the lattice vibrations.
Mason Jiang, a recent graduate student at Stanford, said: “Lead telluride … has two important qualities: It’s a bad thermal conductor, so it keeps heat from flowing from one side to the other, and it’s also a good electrical conductor, so it can turn the temperature difference into an electric current. The coupling between lattice vibrations, caused by heat, and electron motions is therefore very important in this system. With our study at LCLS, we wanted to understand what’s naturally going on in this material.”
Photonic crystals in 3D
The Deutsches Elektronen-Synchrotron (DESY), a Research Centre of the Helmholtz Association, has developed a method to image the inner structure of photonic crystals in 3D.
Using X-ray techniques, researchers can find the positions of the individual building blocks of a crystal. This technique does not require the need for assumptions or models.
For this, researchers used coherent X-rays from DESY’s research light source, dubbed the PETRA III. With a circumference of 2.3 km, the PETRA III is a storage ring light source. It consists of 14 beamlines, which are available for researchers.
Photonic crystals are periodic optical nanostructures. Ranging in size from 200nm to 400nm, photonic crystals are used when light must be manipulated. The inner structures of photonic crystals are in the range of the wavelength of visible light. In addition, the periodic arrangements of small particles are called colloidal crystals. It is important to understand the characteristics of colloidal crystals for many applications.
The problem? At times, optical microscopy doesn’t provide enough resolution. Electron microscopy has better resolution, but the bulk of the crystals cannot be seen with this method. X-rays can penetrate the surface and provide details about the crystal. But generally, this technique doesn’t image the crystal. The structure has to be calculated based on how X-rays are scattered in the sample.
Enter DESY’s PETRA III. This X-ray technique was used to investigate the inner structure of a colloidal crystal. The crystal was grown from silica spheres with a diameter of 230nm.
The whole crystal was illuminated using X-rays. Like in traditional crystallography, the X-rays are diffracted by the crystal lattice. This, in turn, creates a diffraction pattern on the detector. Then, the inner structure can be calculated. Researchers, in turn, can determine the inner structure of the crystal in 3D. They can also position the individual silica spheres in the sample.
“But while conventional crystallography concentrates on the positions of bright spots in the diffraction pattern, known as Bragg peaks, the coherent, laser-like illumination of the sample also produces interference patterns in between the Bragg peaks,” said Anatoly Shabalin from DESY.
Ivan Vartaniants from DESY added: “Our method opens up new ways to visualize the inner three-dimensional structure of mesoscopic materials like photonic crystals with coherent X-rays. Especially with the next generation of X-ray light sources, this technique can pave the way to analyze individual nanocrystals with atomic resolution.”
Prior Week’s Manufacturing Research Bits: Sept. 20
Crystal database; fractography; sputtering standard.