SLAC National Accelerator Laboratory

Scientists line up unruly gas molecules for X-rays.

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It’s hard to study individual molecules in a gas because they tumble around chaotically and never sit still. Researchers at SLAC overcame this challenge by using a laser to point them in the same general direction, like compass needles responding to a magnet, so they could be more easily studied with an X-ray laser.

The experiment with SLAC’s Linac Coherent Light Source (LCLS), reported Dec. 6 in Physical Review A, is a key step toward producing movies that show how a single molecule changes during a chemical reaction. Understanding the many stages of a reaction could help scientists design more efficient, controllable reactions for important industrial processes, many of which rely on gases that react with solids.

“This is the ‘trailer’ for the molecular movie – these are the first frames,” said Daniel Rolles of the Center for Free-Electron Laser Science at DESY national laboratory in Germany, who led the experiment. “People know, theoretically, that molecules do all kinds of weird things. If you can see the changes and check the theory, then you can understand how it’s happening and have a handle on controlling it.”

In the experiment, researchers jetted a thin stream of fluorocarbon gas into the path of two intersecting lasers: an optical laser that polarized the molecules – aligning them along a common axis, like a spinning top with a slight wobble – and the LCLS X-ray laser.

Fluorocarbon molecules were chosen because their chemical makeup allows them to be polarized by the electric field of a laser and because they are somewhat complex; each features a ringed structure and a tail-like spur and contains more than a dozen atoms. This makes them a good test case for future studies of even larger molecules.

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This diagram shows the setup for an experiment at SLAC’s Linac Coherent Light Source that positioned fluorocarbon molecules along a common axis with an optical laser and then used X-ray laser pulses to explore their structural details. The molecules were first channeled into a narrow molecular beam. The optical laser and X-ray laser intersected the path of this gas beam (center). The ball-and-stick structure of the molecule is shown at the upper left. Detectors captured the fingerprint of fluorine atoms and electrons that were ejected from the molecules by the X-ray pulses, which were used to understand the original shape of the molecules. Credit: Phys. Rev. A 88, 061402(R), 2013

The X-ray laser was carefully tuned so it would eject electrons mostly from the fluorine atoms in the sample before bursting the molecules into charged fragments. Scientists measured the freed fluorine electrons and charged fluorine fragments with sensitive detectors, and sorted and analyzed this data to reconstruct the original shape and structure of the molecules. Even though each X-ray laser pulse hit many molecules, the angles of the ejected electrons revealed details about the structure of individual molecules.

The LCLS X-ray is uniquely suited for this type of atomic-scale chemical study because it allows scientists to pinpoint a particular element in a molecule that they want to study, Rolles said.

“This is element and site specific: We can pick one place in a molecule and image that environment,” he said.” It’s like singling out one type of tree that otherwise would be hidden by the forest around it.” That selectivity can allow scientists to zero in on areas of particular interest in a chemical reaction.

Laser alignment of molecules, first demonstrated in 1999, is still a very young field, and the ultrafast X-ray pulses from the LCLS could allow scientists to study changes in aligned molecules that occur in quadrillionths of a second – a far shorter timescale than possible with other research tools. While not all molecules can be aligned with lasers, the researchers note that a rich assortment of molecules is suited to the technique.

If researchers could achieve fuller, three-dimensional alignment of molecules – like stopping a spinning top with your finger and rotating it to face you in a certain way – they would have an even easier time measuring their properties and determining their structure. “You could solve the structure of an individual molecule even without prior knowledge of its shape,” said John Bozek, an LCLS staff scientist who participated in the experiment, adding that this could be useful for studying the intermediate stages of a chemical reaction.

Physicists Now Want a Very Large Hadron Collider.

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The proposed project’s accelerator ring would be 100 kilometers around and run at seven times the energy of the LHC

When Europe’s Large Hadron Collider(LHC) started up in 2008, particle physicists would not have dreamt of asking for something bigger until they got their US$5-billion machine to work. But with the 2012 discovery of the Higgs boson, the LHC has fulfilled its original promise — and physicists are beginning to get excited about designing a machine that might one day succeed it: the Very Large Hadron Collider (VLHC).

“It’s only prudent to try to sketch a vision decades into the future,” says Michael Peskin, a theoretical physicist at SLAC National Accelerator Laboratory in Menlo Park, California, who presented the VLHC concept to a US government advisory panel on 2 November.

The giant machine would dwarf all of its predecessors (see ‘Lord of the rings’). It would collide protons at energies around 100 teraelectronvolts (TeV), compared with the planned 14 TeV of the LHC at CERN, Europe’s particle-physics lab near Geneva in Switzerland. And it would require a tunnel 80–100 kilometers around, compared with the LHC’s 27-km circumference. For the past decade or so, there has been little research money available worldwide to develop the concept. But this summer, at the Snowmass meeting in Minneapolis, Minnesota — where hundreds of particle physicists assembled to dream up machines for their field’s long-term future — the VLHC concept stood out as a favorite.

Some physicists caution that the VLHC would be only a small part of the global particle-physics agenda. Other priorities include: upgrading the LHC, which shut down in February for two years to boost its energies from 7 TeV to 14 TeV; plans to build an International Linear Collider in Japan, to collide beams of electrons and positrons as a complement to the LHC’s proton findings; and a major US project to exploit high-intensity neutrino beams generated at the Fermi National Accelerator Laboratory in Batavia, Illinois. Jonathan Rosner, a particle physicist at the University of Chicago, Illinois, who convened Snowmass, says that these forthcoming projects should be the focus. “It’s premature to highlight the VLHC,” he says.

In some ways, the interest in the VLHC is a sign that particle physicists are returning to their roots, pushing to ever higher energies to find the fundamental building blocks of nature.

They will have to justify it, however. The discovery of the Higgs particle lends support to the idea that some particles have mass because they interact with a pervasive, treacle-like Higgs field. Yet many aspects of the discovery are still not understood, including why the mass of the Higgs particle is so large. One way of explaining its heaviness is through supersymmetry theory, in which known particles are coupled with heavier ones that might be observed in bigger particle colliders. Although the LHC has not detected any signs of supersymmetry, Peskin hopes that a hint may come before the end of the decade, which would help to inform the design of a larger machine.

One advocate of a bigger machine is Nima Arkani-Hamed, a theoretical physicist at the Institute for Advanced Study in Princeton, New Jersey. In December, he will help to launch an institute in Beijing called the Center for Future High Energy Physics. Part of its explicit mission, he says, is to explore the physics that a future proton collider might investigate. William Barletta, an accelerator physicist at the Massachusetts Institute of Technology in Cambridge, says that this work is crucial to identify a machine size that will maximize the science per dollar. “We won’t just give hand-waving arguments,” he says.

To build a 100-TeV machine, Barletta adds, physicists will need to develop superconducting magnets that can operate at higher fields than the current generation, perhaps 20 tesla instead of 14 tesla. One leading candidate material for such magnets is niobium tin, which can withstand higher fields but is expensive and must be cooled below 18 kelvin.

CERN is developing its own plans for a collider that is similar to the VLHC. CERN accelerator physicist Michael Benedikt is leading a study of a ‘very high energy large hadron collider’ that would pass under Lake Geneva. It would have the same key parameters as the suggested VLHC: a circumference of 80–100 km and a collision energy of 100 TeV. Benedikt suggests that construction might begin in the 2020s so that the machine could be completed soon after the LHC shuts down for good around 2035. “One would not want to end up with a huge gap for high-energy physics,” he says. He adds that it is too early to offer a price tag. But other physicists speculate that a next-generation collider would have to cost less than $10 billion for the project to be politically plausible.