New Technique Reveals Identity Of Near-Neighbor Atoms

September 17, 1998

BERKELEY -- In a development that holds much promise for future studies of surfaces and interfaces in solid materials, including magnetic, environmental, and biological systems, researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory have successfully tested a method that not only directly determines the identity of a specific atom in a sample, but also directly determines the identities of its neighbors. Called MARPE, for Multi-Atom Resonant PhotoEmission, this new technique was developed at Berkeley Lab's Advanced Light Source (ALS).

"This is one of those rare occasions where you go after something and it turns out much better than you expected," says the leader of the MARPE research, Charles Fadley, a physicist affiliated with Berkeley Lab's Materials Sciences Division and a professor of physics with the University of California at Davis.

Fadley is one of the world's foremost practitioners of photoelectron spectroscopy (PES), a technique in which an element in a sample is identified through the energies of the electrons it emits when excited by a beam of photons. PES is one of several soft x-ray-based spectroscopy or diffraction methods that are element-specific, meaning they can be used to determine the identity of a central atom in an atomic structure. None of these methods, however, can be used to directly determine the types of atoms that neighbor this central atom.

"MARPE is an effect in soft x-ray absorption that provides a direct probe of near-neighbor atoms," says Fadley. "It also gives scientists a new way of studying the chemical bonds between two or more different types of atoms."

Like PES and other x-ray-based spectroscopy techniques, MARPE works because all atoms have characteristic energies that bind electrons to their inner or core levels (as opposed to their outer or valence levels). These energies, the minimum needed to excite a core electron, serve as "fingerprints" that can be used to identify the atom. The MARPE effect occurs when the energy of an incoming photon beam matches a specific core-level excitation energy of a neighbor atom to the atom being directly excited by the photons. The photons "resonate" with the core level of this neighbor atom, sharply intensifying the observed photoelectron signal emitted from the central atom. This neighbor atom resonance reveals the presence and identity of the neighboring atom.

"It is like a chorus effect in which the neighboring atoms begin singing in tune and pass their collective excitement to the central atom," explains Fadley. "A preliminary theoretical analysis told us we would see this effect, but our theoretical predictions were about four times smaller than the experimental results."

The MARPE effect was first observed in solid compounds containing metal oxides, with the measurements being made at what may be the world's most extensive surface science experimental station ever to be linked to a synchrotron radiation beamline. Designed and assembled under the leadership of Fadley and ALS scientist Zahid Hussain, this station contains, among other features, a photoelectron spectrometer that can be rotated through a 60 degree angle so as to record signal intensities above a surface for a choice of photon polarizations and sample orientations. The station is located at ALS beamline 9.3.2, a bend-magnet that produces photons between 30 and 1500 electron volts in energy.

"This is an experiment that could only have been done with soft x-rays," says Fadley. The flux of the ALS beam, in combination with the unique capabilities of his experimental station, was also essential to the success of the project. "Without the rotational component of our spectrometer, we could not have recorded the two sets of data that confirmed the reality of the effect we observed."

The metal-oxide results, which were reported in the July 31, 1998 issue of the journal Science, indicated that MARPE should be sensitive to chemical bond types and distances, and to magnetic order. Since the Science paper, Fadley's group has demonstrated that the effects should also be observable through x-ray fluorescence and Auger decay, which means MARPE should be applicable to a broad range of samples. Though tested so far on single crystals, MARPE should also be observable in non-crystalline materials. New measurements at the ALS by researchers from the University of Nevada also show that similar effects occur in gas-phase molecules.

"MARPE should be applicable to any solid surface or interface containing elements that have a decent core level," Fadley says, "which means any element from beryllium on up the periodic table."

Collaborating with Fadley on this project, in addition to Hussain, were Alexander Kay and Simon Mun, both with UC Davis as well as Berkeley Lab's MSD; Elke Arenholz of U.C. Berkeley's Miller Foundation; Francisco Garcia de Abajo, a visiting scientist from the University of San Sebastian in Spain; Michel Van Hove, of Berkeley Lab's MSD; and Reinhard Denecke, now with Lund University in Sweden.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.

For further information and several illustrations, see http://www-als.lbl.gov/als/science/sci_archive/MARPE.html
-end-


DOE/Lawrence Berkeley National Laboratory

Related Photons Articles from Brightsurf:

An electrical trigger fires single, identical photons
Researchers at Berkeley Lab have found a way to generate single, identical photons on demand.

Single photons from a silicon chip
Quantum technology holds great promise: Quantum computers are expected to revolutionize database searches, AI systems, and computational simulations.

Physicists "trick" photons into behaving like electrons using a "synthetic" magnetic field
Scientists have discovered an elegant way of manipulating light using a ''synthetic'' Lorentz force -- which in nature is responsible for many fascinating phenomena including the Aurora Borealis.

Scientists use photons as threads to weave novel forms of matter
New research from the University of Southampton has successful discovered a way to bind two negatively charged electron-like particles which could create opportunities to form novel materials for use in new technological developments.

The nature of nuclear forces imprinted in photons
IFJ PAN scientists together with colleagues from the University of Milano (Italy) and other countries confirmed the need to include the three-nucleon interactions in the description of electromagnetic transitions in the 20O atomic nucleus.

Pushing photons
UC Santa Barbara researchers continue to push the boundaries of LED design a little further with a new method that could pave the way toward more efficient and versatile LED display and lighting technology.

Photons and electrons one on one
The dynamics of electrons changes ever so slightly on each interaction with a photon.

An advance in molecular moviemaking shows how molecules respond to two photons of light
Some of the molecules' responses were surprising and others had been seen before with other techniques, but never in such detail or so directly, without relying on advance knowledge of what they should look like.

The imitation game: Scientists describe and emulate new quantum state of entangled photons
A research team from ITMO University, MIPT and Politecnico di Torino, has predicted a novel type of topological quantum state of two photons.

What if we could teach photons to behave like electrons?
The researchers tricked photons - which are intrinsically non-magnetic - into behaving like charged electrons.

Read More: Photons News and Photons Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.