• Number 344  |
• August 22, 2011

Free-electron laser light enables research on extreme dating and efficient diesel

Ancient glaciers, which have long held secrets about early history, could someday soon reveal their stories to scientists armed with ultraviolet rays. The free-electron laser (FEL) facility at DOE's Jefferson Lab recently delivered its first beams of a rare color of UV laser light into an experimental lab, opening the door to future research studies using UV light.

There are many experimental programs that could benefit from the FEL’s new capability. Among them are a select few that require a long-sought, rare color of laser light, called vacuum ultraviolet light (VUV). The FEL recently produced VUV at levels that are 100 times brighter than that generated anywhere else.

In early December, the FEL team first measured VUV light in the form of 10 eV photons (a wavelength of 124 nanometers). This color of light is called vacuum ultraviolet because it is absorbed by molecules in the air, requiring its use in a vacuum.

The availability of tunable, high-power VUV laser light may revolutionize many lines of research that were previously inaccessible, such as radiological dating and diesel engine design.

Counting kryptons

Zheng-Tian Lu, a researcher at DOE's Argonne National Laboratory in Illinois, wants to use the FEL to improve how scientists determine the age of materials. The method, called atom trap trace analysis, is used to date geological samples beyond the age range of radiocarbon dating.

"We do ultra-sensitive trace analysis, which then could be applied to all kinds of applications in the earth sciences: dating old groundwater, dating polar ice and mapping ocean currents. We would use this kind of data to model ocean circulation and map groundwater movement. This research has implications in climate change and water resource management,," Lu explained.

Radiocarbon dating allows scientists to estimate the age of some materials up to roughly 62,000 years. Lu's method uses radio-krypton dating, which could potentially allow scientists to determine the age of materials that are between 100,000 and one million years old.

The 10 eV light from the FEL is needed to produce so-called metastable krypton atoms for use in this dating method. Metastable atoms are atoms whose electrons have been given extra energy; scientists refer to this as "exciting" the atoms.

These energetic electrons are ordinarily unstable, giving up their extra energy relatively quickly. But giving the electrons just the right amount of energy can make them metastable, so that they hang on to the extra energy a little longer.

"In order for us to do the trace analysis that we want, we can only work with atoms in the metastable level, which is about 10 eV above the ground level. With the 10 eV photons from the FEL, we can excite the atoms precisely into that metastable level with very high precision and efficiency," Lu said.

Once the atoms are in that metastable level, they're given a little additional energy by an ordinary tabletop laser to "excite" them into a still higher level. Exciting the krypton atoms this one extra step allows them to be caught in a trap and counted one-by-one. The number of krypton atoms present in a sample can reveal the age of the substance.

In this way, for instance, scientists can learn about Earth's ancient climate by dating a layer of ice extracted from old glaciers. "Polar ice has been accumulating in Greenland for hundreds of thousands of years. When the ice formed, it trapped ancient air and ancient dust particles. And now ice cores can be extracted and analyzed to learn about the history of Earth's climate," Lu explained.

Lu says that the krypton dating method has been proven using other techniques for exciting the krypton atoms. But those methods are not as efficient as the VUV laser light that could be provided by the FEL.

"The more efficient we get, then the smaller the sample, and the easier it is to apply radiokrypton dating to a wider range of applications. So, I think this VUV laser will be what they call a 'game changer'," Lu concluded.

Cleaner diesel

Another research area that may benefit from the FEL's record levels of VUV light is combustion research.

David Osborn is a principal member of the technical staff of the Combustion Research Facility at DOE's Sandia National Laboratories. He's studying how fuel is burned in engines.

"Combustion is now and will be for many years in the future a primary source of energy conversion in the United States. And so it's in our national interest to use that energy as efficiently as we can and with as few pollutants generated as we can," Osborn explained.

Osborn and his colleagues explore the chemistry of how diesel fuel burns. Simply speaking, burning diesel fuel combines with oxygen in the air to yield carbon dioxide and water. Osborn studies the complex process that enables this conversion.

"The [fuel] isn't converted in one single step to carbon dioxide and water. Instead, there are many intermediate steps, and the unstable chemicals so formed are called reaction intermediates ," he explained. "Our studies try to follow all of these intermediates, providing evidence that helps us understand why one fuel forms more pollutants than another fuel."

The research involves starting a chemical reaction inside a vacuum chamber, and then sampling the contents of the chamber as the reaction proceeds. To sample the intermediates, scientists zap individual molecules with VUV light, which removes one electron, making them positively charged. Then, an electric field grabs onto the molecules and sends them into a device called a mass spectrometer, which weighs the molecule to identify it.

"We use the 10 eV light, or we would like to use it from JLab, to turn those neutral molecules into positive ions. And then our mass spectrometer can weigh each one of these positive ions," Osborn said.

He says that the tunable FEL light is key to the process. It allows the scientists to tune to the color of laser light that will ionize some isomers but not others. The FEL light will provide significantly increased sensitivity to scientists trying to differentiate molecules that weigh the same but have vastly different chemical properties.

"Two isomers [can] have the same atoms, but they're arranged in a different order; they weigh essentially exactly the same amount; but chemically, they're very different. And observing one or the other can help you understand the chemical pathways that created them," Osborn explained. "So these could be intermediates in a reaction. And if you saw only one or only the other, you might draw different conclusions about what chemistry made them."

Another major benefit to using vacuum ultraviolet light from the FEL is that an intense beam of this color of light will allow the researchers to study reactions important in diesel engines at real-world pressures.

"Most of these reactions that we study that are related to combustion - we study them at fairly low pressures, about one-one hundredth of an atmosphere, where one atmosphere is the pressure at sea level on Earth," Osborn said. "But most combustion happens at pressures above one atmosphere. The pressures may be 10 or 20 or 30 atmospheres. The intense photons from the Jefferson Lab Free-Electron Laser should allow us to study reactions at relevant pressures - high pressures - with the same signal to noise we can now do at low pressures using synchrotron radiation."

Experimental machine

Now that several experimental areas have been identified for the new VUV FEL, the Jefferson Lab team is working to get the machine ready for these experiments. A review of the state of the experiments and of the laser took place in May. Since this is a new direction, the FEL team is currently working to secure funding so that the experiments may proceed.

"We still have a lot of work ahead of us before experiments can begin," said Gwyn Williams, FEL basic research program manager. "But we are ready to begin working with the users to get support so that we can proceed."

The Free-Electron Laser program is supported in part by the Department of Defense's Office of Naval Research, the Air Force Research Laboratory and the Joint Technology Office, the Commonwealth of Virginia and the Department of Energy's Basic Energy Sciences under contract No. DE-AC05-060R23177. Key equipment was provided by the Wisconsin Synchrotron Radiation Center and Cornell University.

Submitted by DOE's Jefferson Lab