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.
