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Hollow Molecules Take Center Stage

More than 35 researchers were involved in experiments that led to the first published results on hollow molecules. Here, the Atomic, Molecular and Optical instrument control room during the experiments.

Barely two months after publication of the first Linac Coherent Light Source results on hollow atoms, two papers published in Physical Review Letters last Friday unveil the first results for hollow molecules. These studies show that the unprecedented intensity of the LCLS beam can reveal detailed information about a molecule's structure and dynamics.

"The LCLS is proving its mettle as a machine for discovery in strong-field atomic and molecular physics," said Phil Bucksbaum, director of the joint SLAC-Stanford PULSE Institute for Ultrafast Energy Science and co-principal investigator on one of the papers.

Nora Berrah, professor at Western Michigan University and principal investigator on the other paper, agreed. "The LCLS is already uncovering many fundamental mechanisms in the understanding of the interaction of matter with intense photons, and it has demonstrated that it is a powerful tool that will generate many discoveries in the near future," she said.

An atom is a happening place, with electrons zipping around the core in a wildly frenetic yet orderly cloud. Each circling electron occupies its own orbit in the airspace around the nucleus, carrying with it a unique amount of energy. As a result of this uneven energy distribution, the innermost of these electrons most readily absorb X-ray photons, which can propel that electron straight out of the atom. This leaves behind a "core hole"—an empty orbit where that electron used to be.

It's possible to knock the innermost electron out of an atom using the X-ray light from a synchrotron source, such as the Stanford Synchrotron Radiation Lightsource. But with its unparalleled intensity, the LCLS beam allows multiple photon absorption within a single shot and can zap not just one but two electrons out of an atom, creating what's called a "double core hole" before other electrons cascade down to take the missing electrons' places.

"This is pretty new because synchrotrons are not intense enough to initiate this process with a single shot of X-rays," said Li Fang, a research associate in Nora Berrah's group at Western Michigan University and the first author on one of the PRL papers.

When a bunch of X-rays from the LCLS hits a molecule—two or more atoms bonded together—it can create double core holes in one of two ways: it either knocks the two innermost electrons out of one atom, something that was demonstrated in the first LCLS experiments, or displaces one of the innermost electrons in each of two atoms in the same molecule, creating a molecule with a truly hollow core.

In the papers published last Friday, researchers used two methods to study both types of double core holes in a cloud of nitrogen gas, whose molecules each contain two nitrogen atoms.

The first method observed the angles and energies of the two inner electrons that escaped the molecule, which are known as photoelectrons. The second looked at a process called Auger decay, in which an electron from a higher orbital drops down to fill the gap created when an inner electron escapes. Because the outer electrons of an atom have more energy than the inner ones, the electron that falls into the inner orbital will often transfer its excess energy to one of the outer electrons, called the Auger electron, which zips out of the molecule and into a waiting detector.

"Studying the details of photo- and Auger electrons will reveal small shifts in the structure of the molecule," said James Cryan, a graduate student at the PULSE Institute and first author on the the other PRL paper, which describes experiments led by Bucksbaum and LCLS physicist Ryan Coffee. "We can learn about the distribution of the electrons in a molecule, which is important because pretty much all of chemistry is looking at the outer electrons. They're responsible for the way the world around us works."

In these experiments, Cryan said, signals from double core holes in single atoms were clear. Signals from molecules in which each atom is missing its innermost electron were difficult to see but appear to be there, buried under background noise. "That they're hard to see doesn't mean that they're impossible to see,” he said. “We're looking forward to refining our experimental method, both beating down the background and being smarter in our measurements."

Such measurements are likely to prove very important for understanding chemical processes, such as the ones in which chlorophyll molecules convert sunlight to energy.

"This is the first time scientists have used the LCLS to study double core holes in molecules; the experiment was very exciting," said Fang. "We didn't know exactly what we would observe—there were lots of expectations and everyone was holding their breath.” But as US Secretary of Energy Steven Chu said at last week’s LCLS dedication, “We're the first people that get to flip over the rock to see what's underneath it,” Fang said.

Figuring out what's under that rock won't be easy, Cryan said, but he thinks the shared expertise of the X-ray scientists, ultrafast researchers, chemists, biologists, material scientists and many others working on experiments at LCLS will get them there.

"The LCLS is an interesting melding of many different communities," he said. "By studying the details of what exactly goes on in molecules together, we're going to better understand the way the world around us works."

This work builds upon previously published studies on the interaction of the LCLS beam with nitrogen atoms and molecules. In June, a team led by Western Michigan University physicist Nora Berrah published a paper in Physical Review Letters describing how the beam pulled the two atoms in a nitrogen molecule apart by removing the outer electrons binding the two atoms. The same paper described the beam's ability to knock electrons out of nitrogen molecules, creating molecules with one hollow atom. (This work was also highlighted on the cover of the June 25 issue of Physical Review Letters.) The production of hollow atoms was described in the July 1 issue of Nature by a team led by Argonne National Laboratory physicist Linda Young.

Li Fang's research group was led by Nora Berrah of Western Michigan University. The project was carried out in collaboration with Lawrence Berkeley National Laboratory, Universita'di Perugia, Argonne National Laboratory, the PULSE Institute for Ultrafast Energy Science, Louisiana State University, SLAC, Lawrence Livermore National Laboratory, University of Turku and the University of California Berkeley.

James Cryan's research group was led by Ryan Coffee of LCLS and Phil Bucksbaum of the SLAC–Stanford PULSE Institute. The project was carried out in collaboration with Stanford University, Uppsala University, Lawrence Berkeley National Laboratory, Western Michigan University, The Ohio State University, Louisiana State University, Imperial College London, Argonne National Laboratory, CEA-Saclay, Georgia Institute of Technology, Kansas State University and Universita'di Perugia.

—Kelen Tuttle
SLAC Today, August 25, 2010